Keywords

1 Introduction

Cancer is one of the most devastating diseases; it generates profound emotional, financial, and physical stress both to the patients and family members. Besides, the cost of cancer treatment has a strong impact on the economy of any country. Current treatments include surgical resection, chemotherapy, radiation, and immunotherapy. However, many patients have a very poor response to current treatments and/or acquire resistance leading to cancer relapse. Medical research has been looking for more effective and specific drugs to improve the quality of life of cancer patients (Hoelder et al. 2012). One of the most novel approaches in cancer research is to study the role of ion channels and transporters as potential therapeutic targets for anticancer therapy. Since the recognition that ion channels and transporters play an important role in the carcinogenesis process, there has been great scientific interest in discovering new treatments using these genes and proteins as novel tools in oncology. Actually, several compounds targeting ion channels and transporters demonstrate promising potential to be used in cancer patients (Arcangeli and Becchetti 2010; Litan and Langhans 2015). The anticancer potential of these compounds is enhanced when different therapeutic approaches are considered, for instance, by its combination with antineoplastic drugs, immunotherapy, or other molecules targeting essential processes in cancer development including oxidative stress or metabolic pathways. Here, we review and discuss most of the current knowledge suggesting novel therapeutic approaches for cancer therapy by targeting ion channels and transporters. The expression and activity of ion channels and transporters in cancer have been reviewed in detail in several excellent articles of these series. Thus, before going into details of the topics of this review, first we will provide a general and brief panorama of ion channels and transporters in cancer.

1.1 Ion Channels and Transporters in Cancer

Potassium (K+) channels are some of the most studied and deregulated channels in malignancies. The voltage-gated K+ channels Kv10.1 (EAG1) and Kv11.1 (HERG) have been implicated in the pathogenesis of various cancers (Asher et al. 2010; Serrano-Novillo et al. 2019). Kv10.1 channel ectopic expression is associated with malignant transformation, tumor development, metastasis, and poor prognosis; channel overexpression has been observed in most of the human tumors (Pardo et al. 1999; Gavrilova-Ruch et al. 2002; Gessner and Heinemann 2003; Farias et al. 2004; Camacho 2006; Hemmerlein et al. 2006; Queiroz et al. 2006; Pardo and Stühmer 2008; Garcia-Becerra et al. 2010; Asher et al. 2011; Ortiz et al. 2011; Liu et al. 2015; Martinez et al. 2015; Serrano-Novillo et al. 2019). Inhibition of either its expression or activity decreases cancer cell proliferation both in vitro and in vivo (Pardo et al. 1999; Gomez-Varela et al. 2007; Garcia-Quiroz et al. 2014; Chavez-Lopez et al. 2015). Kv11.1 channel altered expression increases cell proliferation, angiogenesis, invasiveness, migration, and lymph node dissemination and decreases cell differentiation (He et al. 2020; Jehle et al. 2011). Overexpression of Kv11.1 channels has been observed in a variety of neoplastic tissues including endometrial, colorectal, esophageal, pancreatic, gastric, ovarian, breast, thyroid, and brain cancers, as well as leukemias (Jehle et al. 2011; Lastraioli et al. 2015a, b; Iorio et al. 2018; Lastraioli et al. 2019; Iorio et al. 2020; He et al. 2020). In gastric tumors, these channels participate in the PI3K/Akt-dependent pathway that induces hypoxia-inducible factors (HIF) and vascular endothelial growth factor (VEGF) to promote cancer progression (Crociani et al. 2014). Interestingly, Kv11.1 is also aberrantly expressed in human gastric dysplasia samples, representing a potential novel marker for progression toward gastric cancer (Lastraioli et al. 2019). In pancreatic ductal adenocarcinoma (PDAC) cells, Kv11.1 activity is essential to induce cell migration by modulating the f-actin organization (Manoli et al. 2019). In addition, Kv11.1 channels may serve as prognostic factors and potential targets for cancer treatment (He et al. 2020; Lastraioli et al. 2015b). Channel blockade reduces proliferation and migration and induces apoptosis in cancer cell lines and tissues (Roy et al. 2008; Jehle et al. 2011; Lastraioli et al. 2015). Interestingly, activation of Kv11.1 also promotes anticancer effects. In SKBr3 or MDA-MB-231 mammary gland adenocarcinoma cell lines, prolonged stimulation of Kv11.1 with the diphenylurea compound NS1643 triggered a senescence-like phenotype, arresting the cell cycle in the G0/G1 phase (Lansu and Gentile 2013). Besides, NS1643 treatment (6 mg/kg) of MDA-MB-231 cell-derived breast cancer xenografts generated significantly smaller tumors, expressed lower levels of Ki67, and showed increased expression of the senescence markers p21waf/cip and p16INK4A compared with untreated mice; these NS1643-treated animals did not show cardiac function alterations (Fukushiro-Lopes et al. 2018). Likewise, NS1643 treatment of the B-RAF-dependent melanoma cell line A375 (that expresses Kv11.3 channels but not Kv11.1), significantly reduced cell proliferation. This antiproliferative effect included lowering the expression of cell cycle promoters (cyclin E, cyclin D, and phosphorylated WEE1), as well as increasing senescence markers (p21waf and p16INK4A) and autophagy markers (phosphorylation of ULK1 and LC3-II), suggesting that activation of Kv11.3 generates tumor suppression (Perez-Neut et al. 2016).

Similarly, high expression of Kv1.3 channels is detected in a great number of human malignancies including breast, colon, and prostate cancer (Comes et al. 2013; Huang and Jan 2014), and blockade of these type of channels inhibits cancer cell proliferation by arresting the cell cycle in the G1 phase (Teisseyre et al. 2015). Likewise, the expression of ATP-sensitive K+ (KATP) channels has been observed in multiple malignancies, including bladder, gastric, and cervical cancer, as well as in glioma and hepatocellular carcinoma (Monen et al. 1998; Wondergem et al. 1998; Malhi et al. 2000; Qian et al. 2008; Huang et al. 2009; Núñez et al. 2013; Vazquez-Sanchez et al. 2018). Because K+ channels have a high potential to be targeted in cancer diagnosis and treatment, several patents have been filed concerning these channels as tools for diagnostic or therapeutic purposes in oncology (D’Amico et al. 2013).

Calcium ions participate as second messengers in cellular homeostasis like gene transcription, proliferation, migration, autophagy, and apoptosis (Bootman et al. 2001; Harr and Distelhorst 2010; Varghese et al. 2019). Some of the most studied calcium channels in cancer are those from the ORAI family and the TRP (transient receptor potential) Ca2+ channel superfamily. ORAI channels are located in the plasma membrane and interact with the stromal interaction molecules (STIMs) located in the endoplasmic reticulum (ER). ORAI1 and ORAI3 isoforms are overexpressed in breast cancer (Lis et al. 2007; Azimi et al. 2014); in prostate cancer, these isoforms confer apoptosis resistance (Dubois et al. 2014). The TRP family consists of seven subfamilies: TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPA (ankyrin), TRPML (mucolipin), and TRPN (NOMPC-like); they are permeable to monovalent and divalent cations and are expressed in a variety of cell types, including sensory neurons (Clapham 2003). These channels are altered in various cancers favoring carcinogenesis mainly by dysfunction in Ca2+ signaling pathways (Miller and Zhang 2011; Nielsen et al. 2014). Changes in the expression of several TRP channels have been implicated in prostate, breast, and lung cancer progression, as well as in ovarian cancer differentiation (Zeng et al. 2013; Azimi et al. 2014; Deliot and Constantin 2015). The SERCA-ATPase pump responsible for reloading the sarco/endoplasmic reticulum with Ca2+ and regulating cytosolic free Ca2+ has been associated with different tumors including gastric, colon, prostate, lung, and breast cancers (Denmeade and Isaacs 2005; Korošec et al. 2006; Dang and Rao 2016; Izquierdo-Torres et al. 2017). SERCA3 is downregulated or absent in colon, gastric, breast, and lung cancers (Gélébart et al. 2002; Papp and Brouland 2011; Arbabian et al. 2013), whereas SERCA2 is overexpressed in colon cancer and correlates with metastasis and decreased survival in patients (Chung et al. 2006). Voltage-gated calcium (Cav) channels are also involved in the development and progression of diverse types of cancer (Wang et al. 2015a; Martinez-Delgado and Felix 2017). These channels are organized into three subfamilies: (1) L-type, (2) P/Q-, N- and R-type, and (3) T-type channels (Gao et al. 2000, 2001; Buchanan and McCloskey 2016). Several Cav’s channels are overexpressed in a variety of cancers including leukemia, sarcomas, brain, colorectal, gastric, lung, ovarian, pancreas, breast, uterus, and prostate cancer (Wang et al. 2015a; Taylor et al. 2008). Upon activation of L-type channels, gene regulation can be addressed through the activation of transcription factors such as cAMP-response-element-binding protein (CREB), nuclear factor of activated T cells (NFAT), and downstream of the regulatory element antagonist modulator (DREAM); these transcription factors favor cancer cell proliferation, invasion, and metastasis (Shankar et al. 2005; Barbado et al. 2009; Mancini and Toker 2009; Xiao et al. 2010). The blockade of T-type channel expression or activity reduces cancer cell proliferation and induces apoptosis (Bertolesi et al. 2002). Interestingly, Ca2+ channel blockers approved for the treatment of other conditions may be repurposed to treat some cancers (Buchanan and McCloskey 2016). Actually, the use of Ca2+ channel blockers for the treatment of hypertension, epilepsy, and other conditions may be inversely correlated with prostate cancer (Fitzpatrick et al. 2001; Debes et al. 2004). Lastly, voltage-gated sodium channels have been mainly associated to the metastatic potential of several cancers (Arcangeli and Becchetti 2010; Litan and Langhans 2015).

One of the major problems in cancer treatment is chemoresistance produced partly because of drug extrusion by ATP-binding cassette (ABC) transporters. Although the etiology of multidrug resistance (MDR) is multifactorial, the most common mechanism in the majority of resistant cell lines involves the overexpression of P-glycoprotein (Silva et al. 2015). Other transporters related to drug efflux are multidrug resistance-associated protein1 (MRP-1) and multixenobiotic resistance (MXR) (Xue and Liang 2012). Interestingly, some ion channels and transporters have been associated with therapy resistance by diverse mechanisms; in accordance, ion channel inhibitors restore chemotherapy sensitivity of different cancer cells (Kischel et al. 2019).

In summary, searching for high-efficacy therapies modulating the activity and/or expression of ion channels and transporters is a very active and promising field in cancer. Table 1 shows some examples of the potential therapeutic, diagnostic, and/or prognostic use of ion channels and transporters in cancer including some clinical trials in cancer patients.

Table 1 Examples of the potential clinical use of ion channels and transporters in cancer
Full size table

A major opportunity for cancer treatment comes by taking advantage of the molecular mechanisms associating ion channels and transporters with cancer. Relevant cellular processes involved in cancer progression including oxidative stress, immune response, and mitochondrial activity, as well as chemoresistance, have been associated with the different roles of ion channels and transporters in tumor progression. Therefore, novel therapeutic approaches may be suggested by simultaneously targeting ion channels and transporters and the cell processes or molecular mechanisms involved. These approaches should provide better and potentiated effects of cancer therapies. Following, we will go into details of the novel therapeutic approaches suggested by a number of groups, based on the participation of ion channels and transporters in cancer.

2 Association of Oxidative Stress with Ion Channels and Transporters in Cancer: Friends and Foes

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) participate in the regulation of metabolism, gene transcription, protein posttranslational modifications, ion transport, cell differentiation, proliferation, and migration, among other processes (Birben et al. 2012; Tochhawng et al. 2013). When ROS/RNS rises beyond their physiological levels, oxidative stress is produced (Chio and Tuveson 2017) potentially leading to DNA mutations, gene transcription alterations, protein oxidation, lipid peroxidation, ion transport alterations, mutagenesis, and cell death (Rani et al. 2016; Poprac et al. 2017). Cancer cells display increased metabolic activity that leads to ROS/RNS overproduction, to counteract this oxidative stress; they have larger pools of antioxidants (Denicola et al. 2011; Harris et al. 2015; Beatty and Gladney 2015; Sullivan et al. 2016). In fact, the use of antioxidants in cancer is controversial, since they may either prevent tumor growth and genomic instability or favor tumor progression and migration (Bjelakovic et al. 2007; Klein et al. 2011; Porporato et al. 2014; Sayin et al. 2014; Le Gal et al. 2015; Harris et al. 2015; Prasad et al. 2017). For instance, in mouse models of B-RAF- and K-RAS-induced lung cancer, treatment with the antioxidants N-acetylcysteine (NAC) and vitamin E increased tumor cell proliferation and reduced survival by reducing ROS levels which leads to the reduction of p53 expression (Sayin et al. 2014). In melanoma, oxidative stress decreases metastasis in vivo; it is melanoma metastatic tumor cells overproduce glutathione and NADPH antioxidants, resisting the damage caused by oxidative stress and promoting metastasis (Piskounova et al. 2015). On the other hand, moderate to high levels of ROS/RNS in cancer cells promote initiation, proliferation, survival, and angiogenesis (Roderick and Cook 2008; Trachootham et al. 2009; Gorrini et al. 2013; Harris et al. 2015; Sullivan et al. 2016; Chio and Tuveson 2017).

Chemotherapy drugs like doxorubicin cause cell death by increasing the production of ROS/RNS, so, pro-oxidant drugs are also currently studied as anticancer options (Kong et al. 2000; Gorrini et al. 2013; Noh et al. 2015; Vilema-Enríquez et al. 2016). Actually, the strategy of delivering and augmenting the concentration of H2O2 in tumors has been proposed for lung cancer (Vilema-Enríquez et al. 2016). Although various therapeutic approaches targeting the redox status in cancer cells have been proposed, clinical results remain elusive (Tong et al. 2015). Ion channels/transporters can be oxidized by direct interaction with ROS/RNS, particularly by H2O2, via their sulfhydryl groups and cysteine residues, or indirectly by altering signaling pathways that are involved in their regulation, expression, or function (Ramírez et al. 2016). Oxidative stress can increase [Ca2+]i inducing protein phosphorylation and gene transcription, contributing to cancer cell survival (Roderick and Cook 2008). Depending on the duration, intensity, and type of oxidant, oxidative stress may cause either influx of Ca2+ into the cytosol via different channels and transporters in the plasma membrane or efflux of Ca2+ from the endoplasmic reticulum (ER), which in turn can cause Ca2+ overload that may lead to disruption of the mitochondrial metabolism and cell death (Ermak and Davies 2002). Thus, further research is needed to take advantage of the potential anticancer effects of oxidative stress and redox status. In this regard, the modulation of ion channels and transporters by ROS/RNS may bring a new therapeutic opportunity. Next, some potential candidates for this approach are discussed.

2.1 The SERCA-ATPase Pump and the Plasma Membrane Ca2+ ATPase

SERCA inhibitors have been proposed as an anticancer therapy since its blockage generates ER stress that leads to the activation of apoptotic pathways (Denmeade and Isaacs 2005). In breast cancer, the antioxidant and anti-inflammatory compound resveratrol induces the expression of SERCA3 decreasing cell viability (Izquierdo-Torres et al. 2017). Curcumin (a SERCA inhibitor) causes apoptosis by inducing ER stress in ovarian and thyroid cancer cells (Seo et al. 2016; Zhang et al. 2018). The blockage of SERCA with thapsigargin induces sustained elevation of [Ca2+]i also leading to apoptosis in cancer cells (Denmeade and Isaacs 2005). Since the SERCA pump is widely expressed, the more specific thapsigargin-based prodrug mipsagargin has been used in a hepatocarcinoma (HCC) phase II clinical trial; the prodrug altered the tumor vasculature reducing tumor blood flow in HCC sites (Mahalingam et al. 2019).

The plasma membrane Ca2+ ATPase (PMCA) is responsible for pumping Ca+2 to the extracellular space and maintain [Ca2+]i homeostasis. The platinum (II) complex [Pt(O,O′-acac)(γ-acac)(DMS)] decreased PMCA activity and induced higher levels of ROS by activating NADPH oxidase and mitochondrial ROS production in the chemotherapeutic-resistant breast cancer cell line MCF-7 (Muscella et al. 2011). Silencing of PMCA2 and PMCA4 combined with a Bcl-2 inhibitor (ABT-263) mediated cell death in MDA-MB-231 breast cancer cells (Curry et al. 2012, 2016).

2.2 ORAI Channels

Store-operated Ca2+ entry (SOCE) is the main mechanism for the entrance of Ca2+ in the cells; it is mediated by the STIMs Ca2+sensors in the ER and the ORAI channels in the plasma membrane, both interact to restore the depletion of Ca2+ from the ER (Xie et al. 2016). SOCE inhibitors have antitumor activity in vitro, and some compounds have been studied in clinical trials (Chen et al. 2019). In fact, SOCE is necessary to induce cytotoxicity of cisplatin in non-small cell lung cancer cells, and depletion of STIM1 reduces the oxidative stress promoted by cisplatin (Gualdani et al. 2019). ORAIs and STIMs have been correlated with proliferation, apoptosis resistance, migration, and metastasis of many tumors (Fiorio Pla et al. 2016). ROS target ORAI channels, modulating [Ca2+]i, and H2O2 blocks Orai1 and Orai2, but not Orai3 because it lacks a cysteine residue at position 195 (Bogeski et al. 2010). Immune and cancer cells have a different Orai1/Orai3 isoform ratio in the cell membrane; this may alter Ca2+ signaling in oxidative stress because Orai1 can be blocked by H2O2 (Frisch et al. 2019). Orai3 is overexpressed and correlated with chemotherapy resistance in breast cancer cells (Hasna et al. 2018), besides Orai1 interacts with Kv10.1 channels and the secretory pathway Ca2+ ATPase (SPCA2) mediating a store-independent calcium entry (SICE) necessary to promote cell survival; interestingly the three proteins are overexpressed in aggressive tumor tissues (Peretti et al. 2019). Furthermore, Orai1 and Orai3 can interact with TRPC6 causing translocation of Orai channels to the plasma membrane; reduction of TRPC6 expression significantly inhibited SOCE in MCF-7 and MDA-MB-231 breast cancer cells (Jardin et al. 2018). Treatment with the phenolic compound (−)-oleocanthal downregulates TRPC6 channel expression reducing cell viability and migration of MCF-7 and MDA-MB-231 cells (Diez-Bello et al. 2019).

2.3 Members of the TRP Channel Family

One of the most studied families of ion channels in oxidative stress is the TRP family, among them TRPC5, TRPV1, and TRPA1 channels are directly activated by ROS and/or RNS by modification on their cysteine residues (Takahashi and Mori 2011); TRPM2 and TRPM7 may be activated via ROS-signaling pathways (Simon et al. 2013), although TRPM2 can also be directly activated by H2O2 in some cell types including microglia and pancreatic ß cells (Kühn et al. 2005).

In most nonmalignant cells, TRPM2 channels participate in a variety of cellular processes including insulin release, inflammatory response, and cell migration; and they are considered as redox sensors that induce Ca2+ influx leading to cell death by intracellular Ca2+ overload (Lange et al. 2009; Sumoza-Toledo et al. 2011; Faouzi and Penner 2014). TRPM2 channels have also been found in the nucleus, but its role is unclear (Zeng et al. 2010; Hopkins et al. 2015; Zhao et al. 2016). H2O2 can mediate TRPM2 activation via mitochondrial ADPR release, which can bind directly to the NUDT9-H domain of the channel (Hara et al. 2002). However, in some cancers, activation of TRPM2 by moderate levels of ROS has been considered as a protective mechanism for the ongoing growth and survival (Chen et al. 2013; Blake et al. 2017). In vitro and in vivo studies demonstrate that TRPM2 supports cancer cell survival; for instance, in neuroblastoma cells the activation and expression of the full-length TRPM2 (TRPM2-L) channel protects cell viability by modulating the expression of the hypoxia-inducible factor (HIF)-1/2α; activation of Src, Pyk2, and CREB; and increasing the levels of forkhead box transcription factor 3a (FOXO3a) and superoxide dismutase 2 (Chen et al. 2013, 2014; Hirschler-Laszkiewicz et al. 2018). In xenografts of neuroblastoma cells, tumor growth was decreased by expressing the dominant-negative isoform TRPM2-S that inhibits the functional TRPM2-L (Chen et al. 2014; Bao et al. 2016). In gastric cancer cells, expression of TRPM2 is necessary to induce migration and invasion through the PTEN/Akt signaling pathway (Almasi et al. 2019a), and PTEN downregulation is correlated with advanced stages of gastric cancer (Zhu et al. 2013). Silencing TRPM2 in lung cancer cells (A549 and H1299) increases ROS/RNS levels, induces G2/M arrest, activates JNK signaling pathway, and in SCID mice xenografts reduces cell migration and tumor growth (Almasi et al. 2019b). In prostate cancer, melanoma, and lung cancer, overexpression of the long noncoding TRPM2-AS (an antisense transcript for TRPM2 channel) has been correlated with increased proliferation and poor prognosis in patients (Orfanelli et al. 2008, 2014; Huang et al. 2017). Interestingly, inhibition of TRPM2 increases ROS, causes mitochondria dysfunction, impairs autophagy, and promotes sensitivity to chemotherapy in some cancer cells (Chen et al. 2014; Koh et al. 2015; Bao et al. 2016; Almasi et al. 2018). Thus, combining chemotherapeutic agents with TRPM2 inhibitors is a promising therapeutic approach, although possible side effects need further analysis because these channels participate in important physiological processes including protection against cardiac ischemia-reperfusion (Miller et al. 2014), activation of the immune response (Yamamoto et al. 2008), and insulin secretion from pancreatic B cells (Togashi et al. 2006).

In the case of TRPC5 channels, overexpression generates Ca2+ signals that activate NFATC3 (nuclear factor of activated T cells 3) which upregulates the synthesis of P-glycoprotein inducing chemotherapeutic drug efflux in adriamycin-resistant breast cancer cells (Ma et al. 2012). Extracellular vesicles released from breast cancer cells increase ROS which in turn activates autophagy and stimulates the release of growth-promoting factors in human mammary epithelial cells (HMECs) (Dutta et al. 2014). Interestingly, extracellular vesicles containing TRPC5 have been found in peripheral blood of breast cancer patients that underwent chemotherapy, suggesting a manner to transfer TRPC5 channels to other cells (Ma et al. 2014).

TRPA1 channels are activated by ROS by targeting cysteine residues in the intracellular site and are upregulated by NRF2, a transcription factor involved in protection against oxidative stress (Mukhopadhyay et al. 2011; Schaefer et al. 2013; Takahashi et al. 2018). Activation of TRPA1 generates Ca2+ influx stimulating proliferation pathways like RAS-ERK, PI3K/AKT, and mTOR, as well as triggering anti-apoptotic pathways. In xenograft tumor models, TRPA1 induces resistance to carboplatin (which induces ROS), and the inhibition of TRPA1 reduces tumor growth and increases chemotherapy sensitivity (Takahashi et al. 2018). In another context, TRPA1 is expressed in C-fiber nerves, and activation of the channel by chemotherapy drugs induces peripheral neuropathy; short-term treatments with antagonists have been suggested as a strategy for preventing peripheral neuropathy induced by chemotherapy (Trevisan et al. 2013). Furthermore, mice treated with doxorubicin and HC-030031 (a TRPA1 inhibitor) generated protection against doxorubicin cardiac injury (Wang et al. 2018b).

TRPV1 is also modulated by oxidizing agents potentiating its activity in neuronal tissues (Susankova et al. 2006; Özdemir et al. 2016). Combinations of antioxidants with chemotherapeutics (for instance, melatonin with doxorubicin or selenium with cisplatinum) in MCF-7 breast cancer cells promoted ROS production and apoptosis; this mechanism was due in part by inhibiting TRPV1 (Koşar et al. 2016; Sakallı et al. 2017). In contrast, a combination of the antioxidant alpha-lipoic acid (ALA) and cisplatinum increased TRPV1 activation resulting in increased ROS production, depolarization of the mitochondrial membrane, and apoptosis (Nur et al. 2017).

2.4 Chloride Intracellular Channel Protein 1 (CLIC1)

CLIC1 is considered a sensor and effector of oxidative stress; it is expressed in the nucleus and cytosol, but upon oxidation, a disulfide bond in cysteine residues of the CLIC1 monomer is formed, and it translocates to the plasma membrane as an active chloride channel (Littler et al. 2004). CLIC1 is overexpressed in various tumors including gastric, colon, and lung cancers, contributing in cell cycle progression, proliferation, migration, and invasion (Chen et al. 2007; Petrova et al. 2008; Averaimo et al. 2010; Wang et al. 2011). In the highly metastatic colon cancer LOVO cells and the SGC-7901 human gastric cancer cell line treated in hypoxic and reoxygenating conditions, CLIC1 channel expression is increased; inhibition of CLIC1 decreases ROS production and p-p38 MAPK/p-ERK levels, as well as reduces MMP-2 and MMP-9 protein levels which inhibits cell migration and invasion (Wang et al. 2014a; Zhao et al. 2015). CLIC1 silencing promotes apoptosis and decreases proliferation in human gallbladder cancer (He et al. 2018). Metformin inhibits CLIC1 reducing glioblastoma stem cell proliferation and invasiveness, compared to normal mesenchymal stem cells (Gritti et al. 2014). CLIC1 is a promising pharmacological target in stress-related diseases, including cancer, where CLIC1 increases tumorigenic and metastatic potential (Peretti et al. 2015).

2.5 Amino Acid Transporter SLC7A11

Metabolic reprogramming occurs in cancer cells to acquire the necessary nutrients to sustain their biosynthetic and bioenergetic processes, which also increases oxidative stress. The cystine/glutamate antiporter solute carrier family 7 member 11 (SLC7A11, also called xCT) imports a cysteine molecule coupled with the efflux of one glutamate molecule (Koppula et al. 2017). SLC7A11 regulates intracellular redox balance by maintaining intracellular levels of glutathione and inhibiting ferroptosis, protecting the cells from oxidative stress-induced cell death (Lewerenz et al. 2013; Zheng et al. 2019). SLC7A11 promotes cancer growth and drug resistance (Lewerenz et al. 2013), and in response to oxidative stress, the proto-oncogene K-Ras stimulates SLC7A11 transcription upregulating glutathione levels in the tumor cells (Lim et al. 2019). Inhibitors of this transporter have antitumor effects by altering the entrance of cysteine necessary for glutathione (GSH) synthesis (Robe et al. 2009; Takeuchi et al. 2014; Shitara et al. 2017). Sulfasalazine, a nonselective blocker of SLC7A11, has been studied as an anticancer drug alone or in combination with other anticancer therapies in animal models and clinical trials, but more selective inhibitors are needed to reduce high adverse effects in humans (Guan et al. 2009; Lo et al. 2010; Takeuchi et al. 2014; Peretti et al. 2015; Sehm et al. 2016; Shitara et al. 2017).

Thus, the diverse association between oxidative stress and ion channel and transporters represents a very important opportunity for cancer therapy. However, the specific channel and transporter inhibitors, as well as the particular anticancer drugs concomitantly used, should be carefully considered. High levels of ROS are also produced in the mitochondria, and ion channels and transporters of this organelle have been also associated with cancer and proposed as targets for therapy.

3 Mitochondrial Ion Channels and Transporters in Novel Potential Therapies for Cancer

The mitochondria play many important cellular functions including ATP and ROS production, apoptosis, as well as Ca2+ homeostasis (Sharma et al. 2019). Dysfunction of this organelle has been correlated with several diseases including cancer, where the involvement of several ion channels and transporters has been studied (Bachmann et al. 2018; Leanza et al. 2018).

3.1 Voltage-Dependent Anion Channels

The voltage-dependent anion channel (VDAC) transports several ions (K+, Na+, and Ca2+), organic anions, ATP, ADP, Pi, and some metabolites depending on the state of the channel across the outer mitochondrial membrane (OMM) (Camara et al. 2017). These channels interact with members of the Bcl-2 family and with hexokinase, regulating apoptosis and with IP3R for the passage of Ca2+ from the endoplasmic reticulum (Mazure 2017; Leanza et al. 2018; Sharma et al. 2019). VDACs are overexpressed in different types of cancers where their expression is related to abnormal proliferation (Shoshan-Barmatz and Ben-Hail 2012). The interaction of hexokinase with VDAC favors cellular glycolysis which is of great relevance for cancer cells; methyl jasmonate (MJ) is an inhibitor of hexokinase-2 that prevents the interaction of hexokinase with VDAC on the mitochondrial membrane and has anticancer effects. The research into new analogs of MJ should help to find new agents against different types of cancer (Sucu et al. 2019). Furthermore, erastin leads to VDAC opening and induces mitochondria dysfunction, increases ROS, inhibits GSH synthesis, decreases glycolysis, and also induces non-apoptotic cell death by ferroptosis in some types of cancers (Yagoda et al. 2007; Dixon et al. 2012; Maldonado et al. 2013).

3.2 Mitochondrial Permeability Transition Pore

Mitochondrial permeability transition pore (mPTP) is a nonspecific channel located on the inner membrane of the mitochondria (IMM). Its prolonged activation depolarizes the mitochondrial membrane and generates ROS, leading to cell death. Thus, drugs that induce mPTP activation in tumor cells have gained great interest (Zoratti and Szabò 1995; Bernardi et al. 2015). Icaritin is an active natural ingredient of the Chinese plant Epimedium that decreases the mitochondrial membrane potential by opening mPTP, leading to necrosis and decreasing proliferation in colorectal cancer (CRC) cells. In accordance, mPTP blockers such as sanglifehrin A, cyclosporin A, and bongkrekic acid, as well as siRNA targeting mPTP decreased the cytotoxic effect of icaritin on CRC cells (Zhou et al. 2016). Similarly, the gold (III)-dithiocarbamate AUL12 contributes to mPTP opening and tumor cell death and shows very low systemic toxicity in vivo (Rasola and Bernardi 2014). Interestingly, various compounds that target the mitochondrial machinery are currently being studied in clinical trials (Suh et al. 2013).

3.3 Mitochondrial Calcium Uniporter

High levels of mitochondrial Ca2+ lead to the activation of the mitochondrial Ca2+ uniporter (MCU) triggering apoptosis (Mammucari et al. 2017). MCU also participates in the proliferation, invasion, and redox signaling in some types of cancers (Vultur et al. 2018). For instance, in triple-negative breast cancer cells, MCU silencing reduces the production of mitochondrial ROS and HIF1-α, impairing cell motility (Tosatto et al. 2016). Actually, MCU overexpression has been linked to lymph node migration, poor prognosis, and breast tumor size (Tang et al. 2015; Yu et al. 2017). Moreover, hepatocellular carcinoma progression and metastasis are associated with overexpression of the MCU-regulator 1 (MCUR1) protein (Jin et al. 2019a), and the anticancer properties of minocycline and doxycycline have been suggested to be related to their inhibitory effect on MCU (Cui et al. 2019). Finally, the thiourea derivative KB-R7943 inhibits MCU reducing Ca2+ release in HeLa cervical cancer cells (Santo-Domingo et al. 2007).

3.4 Uncoupling Protein 2

The uncoupling protein (UCP) is a proton (H+) transporter located on the IMM (Berry et al. 2018). It has been suggested that UCP2 participates in the regulation of cell survival by reducing ROS and mitigating oxidative stress (Cannon et al. 2006; Baffy 2010). UCP2 is upregulated in different tumors, including hepatocellular carcinoma, colorectal, pancreatic, and thyroid cancer (Baffy 2010). UCP2 protects the cells from oxidative stress and prevents the apoptotic effects of different drugs (Derdak et al. 2008). The UCP2 inhibitor genipin reduces cell proliferation, enhances the response to chemotherapy, reverses chemotherapy resistance in some cancer cell lines, and reduces tumor growth in vivo (Mailloux et al. 2010; Dalla Pozza et al. 2012; Pons et al. 2015; Shanmugam et al. 2018). On the contrary, UCP2 expression in melanoma is associated with T-cell tumor infiltration, higher antitumor response, and prolonged survival (Cheng et al. 2019). Also, induced overexpression of UCP2 in melanoma cells generates an immunostimulatory microenvironment by producing chemokines and cytokines, enhancing CD8+ T-cell infiltration in the tumor microenvironment, and suppressing tumor progression. Furthermore, the expression of UCP2 sensitizes melanoma cells against anti-programmed cell death 1 (PD-1) treatment (Cheng et al. 2019). Immune checkpoint-block therapy (like anti-PD-1) is a novel way to fight cancer, and targeting UCP2 expression may convert this immune therapy more efficient for some cancers.

Therefore, targeting different mitochondrial ion channels and transporters should be considered to design novel anticancer therapies (Leanza et al. 2018). Regarding the immune system, ion channels and transporters are becoming an attractive field in onco-immunology.

4 Ion Channels and Transporters in Cancer Immunotherapy

The participation of the immune system is fundamental for the recognition and elimination of tumor cells. Different immune cells infiltrate the tumor microenvironment activating the immune response, for instance, by CD4+T or CD8+T cells. These cells bind directly to the MHC class I molecules presented by the tumor cells inducing the release of cytokines and cytotoxic granules, killing tumor cells (Ostroumov et al. 2018). However, cancer cells evade the immune response by several mechanisms that include defective antigen presentation, repression of T-cell activation, and production of immune-suppressive cytokines (Vinay et al. 2015; Liubomirski et al. 2019). In accordance, several types of immunotherapies are used in clinical practice including immune checkpoint inhibitors, immune system modulators, monoclonal antibodies, vaccines, and CAR T-cell therapy (Khalil et al. 2016).

4.1 Ion Channels and Leucocytes at a Glance

Some ion channels are implicated in the activation, differentiation, proliferation, chemotaxis, and migration of leucocytes (Feske et al. 2015). Lymphocyte function depends on ion channel-mediated Ca2+ signaling induced by antigen recognition. Briefly, activation of lymphocytes by binding of the antigen to the TRC (T cell) or BCR (B cells) receptor activates PLCγ1 in T cells and PLCγ2 in B cells increasing the formation of IP3. Then, the IP3 receptor (IP3R) is activated releasing Ca2+ from the endoplasmic reticulum. The depletion of Ca2+ from the ER activates either STIM1 or STIM2 subunits to oligomerize with IP3R in the ER and interacting with the ORAI channels in the plasma membrane forming functional CRAC channels. These channels allow the entrance of Ca2+ from the extracellular space into the lymphocyte. To maintain the electrical driving force for Ca2+ influx, activation of KCa3.1 channels by Ca2+ and activation of KV1.3 channels by membrane depolarization are required. Following calcium influx, the calcineurin-NFAT pathway is activated increasing the transcription of genes associated with proliferation, cytokine production, and cytotoxicity (Panyi et al. 2014; Feske et al. 2015; Chiang et al. 2017). The blockage of these potassium channels has been proposed as a therapeutic strategy for immunosuppression in a variety of conditions including chronic inflammation, autoimmune diseases, and immunologic-derived cancers (Lam and Wulff 2011). Kv1.3 is also expressed in the IMM where it participates in apoptosis by interacting with Bax and inhibiting channel activity with the subsequent elevation of ROS and release of cytochrome C (Szabó et al. 2008). Two new inhibitors of mitoKv1.3 (PCARBTP and PAPTP) induce ROS production, promote cell death in chemoresistant cells, and reduce tumor growth in melanoma and pancreatic adenocarcinoma in vivo while preserving immune cells and healthy tissues. The authors propose that the selectivity to cancer cells may be partially due to the higher expression of mitoKv1.3 in cancer cells, which hyperpolarizes the IMM and alters the redox status (Leanza et al. 2017).

4.2 Cancer Immunotherapy Targeting Ion Channels

Several approaches targeting ion channels in cells from the immune system have been used. B cells from patients with chronic lymphocytic leukemia (B-CLL) have altered redox state and overexpress Kv1.3 channels in the plasma membrane and mitochondria compared with B cells from healthy subjects. Clofazimine induced cell death by blocking Kv1.3 channels in the mitochondria and activating the intrinsic apoptotic pathway in B-CLL cells. Furthermore, healthy B/T cells or B-CLL treated with the antioxidant enzymes catalase and superoxide dismutase was resistant to apoptosis induced by clofazimine, indicating a synergic action between inhibition of Kv1.3 and ROS production (Leanza et al. 2013). Kv1.3 channels with incomplete inactivation are overexpressed in Daudi B cells. Treatment of Daudi B cells with the antihuman CD20 antibody rituximab (used in patients with non-Hodgkin’s lymphoma) downregulates Kv1.3 channels by activation of the FcγRIIB receptor, contributing to the induction of apoptosis (Wang et al. 2012). Also, in primary malignant T cells isolated from patients with Sézary syndrome, blockage of Kv1.3 inhibited activation and cell proliferation (Hu et al. 2019). Likewise, KCa3.1 is overexpressed in several cancers promoting cell proliferation, metastasis, and therapy resistance (Mohr et al. 2019). Treatment of CLL cells with clotrimazole or TRAM-34 (KCa3.1 channel blockers) decreases Ki67 expression and cell viability (Grössinger et al. 2014). Natural killer (NK) cells also express Kv1.3 and KCa3.1 in the plasma membrane. TRAM-34 increased the proliferation and degranulation levels of adherent NK cells in the presence of the leukemia cell line K562, and mice bearing K562 tumors treated with adherent NK cells and TRAM-34 formed smaller tumors (Koshy et al. 2013). Recently, it was described that radiation to the glioblastoma cell line GL-15 and primary cell cultures from tumors of patients with glioblastoma induced migration, and invasion mediated by KCa3.1channels. The blockage of KCa3.1 channels with TRAM-34 abolished the invasive phenotype of these cells (D’Alessandro et al. 2019). Besides, in the tumor microenvironment, high amounts of adenosine (ADO) are released from tumor cells in hypoxic conditions and regulatory T cells, as well as high amounts of ATP secreted from immune, stromal, apoptotic, and necrotic cells. ATP can be converted to ADO by the ectonucleotidases CD39 and CD75. In solid tumors, the excessive accumulation of ADO generates immunosuppression and failure of effector T cells to eliminate cancer cells, which is associated with tumor growth, metastasis, poor prognosis, and resistance to therapy (Allard et al. 2016). The function of KCa3.1 is inhibited by ADO in human T cells via A2A receptors, reducing T-cell migration and cytokine release (Chimote et al. 2013). In addition, ADO inhibits chemotaxis of CD8+ T cells from head and neck squamous cell carcinoma (HNSCC) patients via its A2A receptor, reducing KCa3.1 channel activity and their ability to infiltrate the solid tumor. Enhancing KCa3.1 channel activity with the agonist 1-EBIO recovers the chemotaxis ability of CD8+ T cells of HNSCC even in the presence of ADO (Chimote et al. 2018).

Adoptive T-cell transfer (ACT) therapy may be also used to target specific ion channels and transporters in cancer. ACT therapy options include tumor-infiltrating lymphocytes (TILs), T-cell receptor (TCR), and chimeric antigen receptor (CAR) therapies (June et al. 2018). KCa1.1 potassium channels (encoded by the KCNMA1 gene) have been associated with glioma, breast, prostate, and cervical cancer and express multiple splice variants (Liu et al. 2002; Bloch et al. 2007; Khaitan et al. 2009; Ge et al. 2012; Ramírez et al. 2018). Alternative splicing leads to the production of multiple mRNAs from a single gene, thus, encoding a diversity of proteins (Liu and Cheng 2013; Wang et al. 2015a). Some pathways deregulated in cancer frequently promote aberrant splicing, which in turn contributes to many aspects of tumor biology, including metabolism, apoptosis, cell cycle control, invasion, metastasis, and angiogenesis (David and Manley 2010; Wang et al. 2015a). Alternative splicing in ion channels modify their pharmacological profile, surface expression, intracellular localization, or electrophysiological properties; actually, in some instances, the splice variants lack conductive properties acting as dominant-negative subunits (Ramos Gomes et al. 2015). The gBK splice variant of KCNMA1 channels is strongly expressed in glioma cell lines and tumor tissues (Liu et al. 2002). This variant has two epitopes for T cells, namely, gBK1 and gBK2, that bind to the human leukocyte antigen HLA-A*0201 on the surface of dendritic cells (DCs). DC cells previously pulsed with gBK1 or gBK2 peptides induce cytotoxic T lymphocyte (CTL) response and cell death in glioma, gastric, lung, and breast cancer cell lines (Ge et al. 2012). Similar results were obtained with small cell lung cancer (SCLC) cell lines, where gBK-specific CTL-killing inhibits growth and stimulates IFN-γ, proposing gBK as a target for immunotherapy and vaccination in some types of cancer (Hoa et al. 2014). Since tumor cells expressed higher levels of gBK than noncancerous cells, targeting this splice variant may be a more selective therapy.

Another immunotherapy alternative is using antibodies against specific ion channels or transporters involved in cancer. The development of specific antibodies for cancer therapy has been studied for Kv10.1, Kv11.1, nfP2X7, α2δ1 subunit (isoform 5 of voltage-gated Ca2+ channels), and MRP1 (Binyamin et al. 2004; Sette et al. 2013; Zhao et al. 2013; Hartung and Pardo 2016; Gilbert et al. 2017) among other proteins. Kv10.1 is abnormally expressed in approximately 70% of all types of cancers (Hemmerlein et al. 2006). The scFv62-TRAIL antibody targeting the pore of Kv10.1 (scFc62) and linked to the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) sensitized MDA-MB435S breast cancer cells to antineoplastic drugs commonly used in the clinic including paclitaxel and doxorubicin. The combination of the antibody with doxorubicin showed also significant inhibitory effects in vivo experiments, and in prostate cancer cells, the antibody induced apoptosis only in those expressing Kv10.1 channels (Hartung et al. 2011; Hartung and Pardo 2016). Thus, this antibody-based approach provides a very selective anticancer therapy. A monoclonal antibody against Kv11.1 conjugated with TiO2 nanoparticles (Kv11.1-Mab-PEG-TiO2 NPs) was designed and tested in the pancreatic ductal adenocarcinoma cell lines MIAPaCa-2 and Panc-1. Although that treatment with the Kv11.1-Mab-PEG-TiO2 NPs did not change cell viability, other options can be considered to generate cytotoxicity like testing the photocatalytic properties of TiO2 which induce ROS production (Sette et al. 2013), or other chemotherapeutic agents linked to Kv11.1 channels. The ion channel and transporter splice variants can also be used for cancer immunotherapy, and P2X7 is an ATP-gated Ca2+ channel overexpressed in various cancers, promoting cell proliferation and invasiveness; a phase I clinical trial studying the topical administration of an antibody against the non-pore functional P2X7 (nfP2X7) variant reported reduced lesions in basal cell carcinoma, a very common skin cancer (Gilbert et al. 2017). In this way, the development of specific antibodies against malignant splice variants is emerging as a possible therapeutic approach to treat cancer. Despite that further investigation in cancer patients is needed; ion channels and transporters represent a promising alternative in cancer immunotherapy.

5 Splice Variants and Noncanonical Functions of Ion Channels in Cancer Therapy

5.1 Splice Variants

Targeting channel isoforms that are tumor-specific can provide more selectivity for drug development. In this direction, the pyrimido-indole compound CD-160130 is more effective in blocking the Kv11.1 isoform B (IC50 = 1.8 ± 0.26 mM) compared to Kv11.1 isoform A (IC50 = 13.4 ± 3.0 mM). Interestingly, leukemia cells mainly express Kv11.1 isoform B. Accordingly, CD-160130 induced apoptosis in vitro and prolonged survival in an acute myeloid leukemia mouse model at a dose of 10 mg/kg; it is worth mentioning that CD-160130 did not induce significant QT prolongation in mice and guinea pigs (Gasparoli et al. 2015).

Overexpression of different G proteins activated inwardly rectifying K+ channel 1 (GIRK1) splice variants exerts opposite actions in breast cancer cells. While GIRK1a and GIRK1c overexpression reinforces parameters associated with malignancy; overexpression of GIRK1d has the contrary effect. A segment comprising aminoacids 235–402 present in GIRK1a and GIRK1c but not in GIRK1d seems to be the responsible component for the carcinogenic effect of these channels (Rezania et al. 2016). Overexpression of GIRK1 in the primary tumor is associated with lymph node metastasis and poor prognosis (Stringer et al. 2001). In addition, in breast cancer cells, the overexpression of GIRK1 affects wound healing, invasion, cellular velocities/motilities, and angiogenesis suggesting a pathophysiological role in breast cancer (Wagner et al. 2010; Rezania et al. 2016). Alternative transcripts have been also identified for Kv10.1 channels, namely, Kv10.1a and Kv10.1b. Two shorter splice variants, E65 and E70, isolated from the human brain and cancer cell lines lack the transmembrane segments. These variants produce cytoplasmic proteins without conducting properties but reduce the current of the full-length channels when co-expressed. E65 triggers the activation of cyclin-dependent kinases in Xenopus laevis oocytes, suggesting a role in cell cycle control (Gomes et al. 2015; Ouadid-Ahidouch et al. 2016). TRPC channel splice variants play an important role in human ovarian cancer development. The nonselective TRPC channel blockers 2APB and SKF-96365 significantly inhibited the cell proliferation, while the increase of TRPC channel activity promoted the cell proliferation (Zeng et al. 2013). Some voltage-gated sodium (Nav) channels are expressed in the colon, small intestine, stomach, prostate, bladder, and breast, but the higher expression is found in the brain, as well as in skeletal and cardiac muscle. Interestingly, the neonatal splice variant of the Nav α-subunit subtype Nav1.5 (nNav1.5) displays a restricted expression pattern among tissues but is upregulated in human breast cancer. The high-level expression of this splice variant is associated with the estrogen receptor (ER) status. Thus, the nNav1.5 splice variant may be exploited both as a novel biomarker and a potential specific target for some common types of breast cancer (Yamaci et al. 2017).

5.2 Noncanonical Functions

Several splice variants may form non-conducting ion channels strongly suggesting that noncanonical functions of ion channels are also involved in carcinogenesis. For instance, Downie and colleagues developed a mutant Kv10.1 channel eliminating ion permeation and studied its oncogenic potential. This mutant fails to completely abolish xenograft tumor formation by transfected cells, strongly suggesting that the oncogenic mechanism of Kv10.1 comprises other molecular mechanisms independently of its primary function as an ion channel (Downie et al. 2008). Noncanonical functions of Cav channels are also associated with cancer. Proteolytic cleavage of the C-terminus of L-type Ca2+ channels α1C and α1D subunits (Cav1.2 and Cav1.3, respectively) produces a fragment that is translocated to the nucleus regulating the transcription of genes involved in tumor progression (Buchanan and McCloskey 2016). C-terminus cleavage of Cav1.2 channel generates the transcription factor Ca2+ channel-associated transcriptional regulator (CCAT), which regulates the expression of connexin CX31.1 and NR3, but also provides negative feedback regulating Cav1.2 channel expression (Gomez-Ospina et al. 2013). Alternatively, CCAT may result from the alternative splicing of the Cav1.2 gene (Barbado et al. 2009). The overexpression of this fragment affects also the expression of other ion channels including TRPV4 and KCa2.3, potentially leading to a cancer phenotype (Buchanan and McCloskey 2016). Electromagnetic field therapy, like tumor treating fields (TTFields), delivers a low-intensity, intermediate frequency, alternating electric field through noninvasive transducer arrays to tumor regions. This FDA-approved treatment for glioblastoma multiforme therapy disrupts mitosis and cytokinesis, stimulates calcium entry mediated by Cav1.2, and arrests the cells in the S and G1 phase of the cell cycle in glioblastoma cell lines (Neuhaus et al. 2019). Thus, the participation of ion channel splice variants in cancer and the diverse molecular mechanisms associating channels with cancer including noncanonical functions offer additional drug design and therapeutic opportunities to fight cancer. One of these opportunities arises by using current drugs prescribed for conditions different from cancer but affecting ion channels and transporters involved in cancer.

6 Repurposing Existing Drugs Targeting Ion Channels and Transporters for Cancer Therapy

Drug repurposing is an attractive strategy to reduce the cost and developing times of new antineoplastic agents. The safety, pharmacokinetics, and pharmacodynamic profile of currently used drugs are well-known; thus, these drugs may be quickly translated into phase II and III clinical studies (Oprea et al. 2011; Gupta et al. 2013; Pantziarka et al. 2014). In silico chemical genomic approaches have been used to predict drug repositioning candidates for cancer therapy based on large-scale drug-induced transcriptional signatures (Lee et al. 2016). Because several drugs used for different indications target ion channels and transporters involved in cancer, repurposing of these drugs is a very attractive and low-cost alternative to fight cancer.

6.1 Antihistamines

Histamine is involved in cell proliferation and tumor growth; thus, several antihistamines have been strongly suggested for repurposing as antineoplastic agents (Faustino-Rocha et al. 2017). Astemizole is a long-acting, non-sedating second-generation antihistamine indicated in the treatment of allergies. This drug is an antagonist of H1-histamine receptors which are present in the gastrointestinal tract, uterus, blood vessels, and bronchial muscle, among other tissues (Garcia-Quiroz and Camacho 2011). Astemizole also targets several molecules involved in cancer development including ABC transporters (P-glycoprotein) and the potassium channels Kv10.1 and Kv11.1 (Pardo et al. 1999; Ishikawa et al. 2000; Garcia-Ferreiro et al. 2004; Camacho 2006; Garcia-Quiroz and Camacho 2011). This antihistamine has antiproliferative effects in cancer cell lines from breast such as MCF-7, SUM-229PE, T-47D, and BT-474, as well as in invasive ductal breast cancer primary cultures (Ouadid-Ahidouch et al. 2001; Roy et al. 2008; Garcia-Quiroz et al. 2012, 2019). It also inhibits proliferation and increases apoptosis in several cell lines from cervical, liver, prostate, and lung cancer (Chavez-Lopez et al. 2014, 2015, 2017; Bernal-Ramos et al. 2017;), as well as in cells from leukemia (Ishikawa et al. 2000) and in keratinocytes transfected with a human papillomavirus oncogene (Diaz et al. 2009). Moreover, astemizole inhibits the Kv10.1 mRNA expression both in vitro and in vivo in breast cancer and hepatocellular carcinoma, decreasing tumor development (Garcia-Quiroz et al. 2012, 2014; Chavez-Lopez et al. 2015). The antitumor activity of astemizole has been observed in several studies in animal tumor models. The oral administration of astemizole (50 mg/kg/day) reduced the growth rate of xenografts tumors induced by implantation of Kv10.1-transfected cells or MDA-MB435S breast cancer cells (Downie et al. 2008). In a rat model, astemizole was capable to prevent hepatocellular carcinoma (HCC) development induced by the carcinogen diethylnitrosamine (Chavez-Lopez et al. 2015). The daily administration of astemizole (50 mg/kg) in drinking water inhibited tumor growth in an in vivo preclinical model using athymic mice xenografted with two different human breast cancer cell lines: T-47D and a ductal infiltrating carcinoma breast cancer-derived primary cell culture (MBCDF) (Garcia-Quiroz et al. 2014). The dose of 50 mg/kg of astemizole was sufficient to inhibit tumor growth in mice without producing noticeable adverse effects (loss of body weight, diarrhea, or alterations in physical activity) (Downie et al. 2008; Garcia-Quiroz et al. 2014). In contrast, 30 mg/kg of astemizole induced ventricular contractions in dogs and torsade de pointes in one animal (Izumi-Nakaseko et al. 2016). Astemizole was withdrawn from the US market in 1999 due to its pro-arrhythmic potential; it soon became evident that most cases of toxicity involved either overdosing, drug interaction, or subjects with predisposed cardiac disease (Paakkari 2002). At the defined daily dose of prescribed astemizole (10 mg/day), the spontaneous cardiac adverse drug reaction reported in a lapse of 10 years were 110 cases per million of doses sold (Garcia-Quiroz and Camacho 2011). These side effects are mainly attributed to the blockade of the Kv11.1 cardiac potassium channels (IC50 of 48.4 ± 3.8 nM) (Suessbrich et al. 1996; Zhou et al. 1999). It is important to mention that not all Kv11.1 channel blockers produce torsade de points, for instance, verapamil and sertindole (D’Amico et al. 2013; Gentile et al. 2016), but several anticancer drugs have a pharmacological effect on Kv11.1 (Gentile et al. 2016). Therefore, the use of Kv11.1 blockers that do not induce cardiac side effect has been suggested for cancer treatment. One of the alternative proposed approaches is using drugs that bind to a specific state of the channel, like R-roscovitine that interacts with the channel in its open state, which is longer in tumors than in cardiac cells (D’Amico et al. 2013).

A very interesting property of astemizole is that its concomitant use with other antineoplastic agents has synergistic effects. Astemizole potentiates the growth-inhibitory activity of doxorubicin in doxorubicin-resistant human leukemia cells K562/DXR by inhibiting the P-glycoprotein (Ishikawa et al. 2000). The antihistamine also synergizes the calcitriol antiproliferative effects by downregulating CYP24A1 (which inactivates calcitriol), upregulating the vitamin D receptor (VDR), and targeting Kv10.1 (Garcia-Quiroz et al. 2012). The co-administration of astemizole and calcitriol to mice xenografted with human breast cancer cells inhibited tumor growth more efficiently than each drug alone (Garcia-Quiroz et al. 2014). Likewise, in a HCC model, astemizole increased VDR expression both in vitro and in vivo, enhanced vitamin D-induced decrease in cell viability and proliferation, increased apoptosis, decreased cell migration and invasion in vitro, as well as reduced the amount and mass of tumors (Xu et al. 2018b). Furthermore, in lung cancer cells, astemizole potentiated the inhibitory effect of vinorelbine on the colony formation of NCI-H1299 and cisplatin on the colony formation of NCI-H661 cells (Ellegaard et al. 2016). The combined effect of astemizole with the epidermal growth factor receptor type 1 (EGFR) inhibitor gefitinib further repressed the proliferation, survival, and Kv10.1 expression and increased the apoptosis more than the monotherapy in the lung cancer cell lines A549 and NCI-H1974 (Chavez-Lopez et al. 2017). In the same manner, astemizole and gefitinib synergistically inhibited the proliferation of breast cancer cells expressing the targets Kv10.1 and EGFR (Garcia-Quiroz et al. 2019). In addition, astemizole acts synergistically with radiation to increase the death of prostate cancer cells through a mechanism involving autophagy (Oprea et al. 2011).

Terfenadine, a second-generation H1 receptor antagonist targets other molecules involved in cancer such as Kv11.1 (Suessbrich et al. 1996). This antihistamine induces apoptosis and inhibits tumor growth in murine models (Blaya et al. 2010). In human refractory prostate cancer cells, terfenadine upregulates and activates Bak and the cleavage of Mcl-1, leading to the loss of mitochondrial membrane potential and activation of caspase cascade resulting in DNA damage response and apoptosis (Wang et al. 2014b). Breast cancer cells resistant to HER-2/neu targeted therapy express high levels of H1 receptors and are more sensitive to terfenadine. This drug leads to Sub-G0 cell accumulation, suppresses proliferation, promotes cell motility, and triggers the activation of extracellular signal-regulated kinase (ERK), initiating the mitochondrial apoptotic pathway in basal breast cancer. Moreover, in vivo experiments showed that terfenadine (10 mg/kg) therapy reduced the tumor growth of basal and trastuzumab-resistant breast cancer cells (Fernández-Nogueira et al. 2018). The combined treatment of terfenadine with epirubicin synergistically inhibits the growth and metastatic process of chemotherapy-resistant non-small cell lung cancer (NSCLC) cells both in vitro and in vivo (An et al. 2017), and ketoconazole potentiates terfenadine-induced apoptosis in human HepG2 cells through inhibition of p450 3A4 activity (Wang et al. 2002). Terfenadine was withdrawn from the market due to the induction of prolonged QT interval in cases of overdose, inappropriate co-medications or in subjects with predisposed cardiac disease. The FDA recommended terfenadine to be replaced by its active and nontoxic metabolite fexofenadine (Berul and Morad 1995; Paakkari 2002). Another antihistamine with important antineoplastic effects is loratadine, which is associated with significantly reduced all-cause mortality among patients with non-localized non-small cell lung cancer (NSCLC) or any non-localized cancer. Astemizole showed a similar significant association with reduced mortality in patients with non-localized cancer, and ebastine shows a similar tendency. Interestingly, submicromolar concentrations of these antihistamines sensitized NSCLC cells to chemotherapy and reverted multidrug resistance in NSCLC, breast, and prostate cancer cells (Ellegaard et al. 2016). Similar results with antihistamines were observed in ovarian cancer patients (Verdoodt et al. 2019).

6.2 Imipramine

Imipramine is a tricyclic antidepressant indicated for symptom relief of depression and other conditions including panic and obsessive-compulsive disorders, bulimia, and nocturnal enuresis; it acts by blocking the sodium-dependent serotonin and norepinephrine transporters reducing reuptake and increasing their concentration in the synaptic cleft (Gillman 2007). In addition, imipramine inhibits the current through Kv10.1 channels in a voltage-dependent manner and reduces the proliferation of cancer cells (Gavrilova-Ruch et al. 2002; Garcia-Ferreiro et al. 2004; Gomez-Varela et al. 2006). This drug also promotes apoptosis in the ovarian cancer cells SK-OV-3 (Asher et al. 2011). In brain cancer patients, the effect of imipramine is associated with the channel abundance; thus, the antidepressant improves the survival rate better in patients with moderate Kv10.1 expression (Martinez et al. 2015). These findings suggest that personalized therapy with this tricyclic antidepressant based on the expression of Kv10.1 channels may be used for brain malignancies. Besides, in 2013, Jahchan and colleagues used bioinformatic tools to identify potential candidate drugs for the treatment of small cell lung cancer from FDA-approved drugs and identified imipramine as a potential candidate. Imipramine at 20 μM decreased survival in H82, H69, and H187 human small cell lung cancer (SCLC) cells and Kp1, Kp2, and Kp3 mouse SCLC cells. In vivo, imipramine (25 mg/kg) inhibited the growth of SCLC allografts (mouse SCLC cell line Kp1), xenografts (human SCLC cell line H187), and one primary patient-derived xenograft (human SCLC tumor NJH29). This drug was effective also in cisplatin-resistant SCLC cells, suggesting that imipramine may be used as second-line therapy for SCLC patients who become refractory to cisplatin/etoposide (Jahchan et al. 2013; Kale et al. 2015). Imipramine also has cardiovascular side effects including orthostatic hypotension, atrioventricular conduction delay, reduced heart rate variability in response to exercise, tachycardia, syncope, and arrhythmias particularly observed in patients with concurrent cardiovascular disease or at high doses of treatment. This may be explained because imipramine blocks several neuronal and cardiac K+, Na+, and Ca2+ channels whit IC50 values ranging from 1 to 30 μM; its IC50 in cloned Kv11.1 channels is 3.4 ± 0.4 μM, and the complete blockage is achieved with 30 μM (Teschemacher et al. 1999; Garcia-Ferreiro et al. 2004).

To increase the antineoplastic effects, imipramine has been co-administered with other compounds. The combination of imipramine with doxorubicin enhanced the anti-invasive effect, whereas a combination with ticlopidine suppressed ATG7, a member of the autophagy survival signaling, resulting in cell death (Abdelaleem et al. 2019). The combined treatment of imipramine and radiotherapy in prostate cancer did not enhance the radiosensitivity of DU145 cells; unexpectedly, the treatment of imipramine alone was more effective (Barlaz Us et al. 2019). Several studies have evaluated the effect of imipramine blue, which is an organic triphenylmethane dye synthesized from imipramine and 4,4′-diethylaminobenzophenone. This compound was suggested because gentian violet (another triphenylmethane dye) also exhibits anticancer properties. This imipramine analog inhibits the invasion of glioma cells both in vitro and in vivo and enhances the efficacy of doxorubicin (Munson et al. 2012). In addition, imipramine blue inhibits breast cancer growth, progression, and metastasis (Rajamanickam et al. 2016); moreover, it has antineoplastic effects on head and neck cancer (Yang et al. 2016), Burkitt lymphoma (Klingenberg et al. 2014), as well as on acute (Metts et al. 2017) and chronic myeloid leukemia (Laidlaw et al. 2016).

6.3 Calcitriol

The endogenous synthesis of calcitriol begins in the skin by the action of ultraviolet radiation from sunlight but takes place mainly in the kidney and has been reported in other tissues such as skin, prostate, intestine, pancreatic islets, lymph nodes, brain, colon, and the mammary gland, where local calcitriol synthesis takes place (Deeb et al. 2007; Glowka et al. 2019). The coupling of calcitriol with the VDR allows dimerization with the retinoid receptor X (RXR); this heterodimer translocates to the nucleus and binds to VDR response elements (VDREs) in the promoter of target genes inducing gene expression.

Calcitriol acting via VDR promotes cytodifferentiation and apoptosis, modulates oncogene expression, and inhibits cell proliferation and migration, reducing or preventing cancer progression. Another potential antiproliferative mechanism of this secosteroid is its ability to downregulate Kv10.1 expression in cell lines (SUM-229PE and MCF-7) and primary cultures from breast cancer (Garcia-Becerra et al. 2010; Garcia-Quiroz et al. 2012), as well as in cervical (SiHa, HeLa) and prostate (PC-3) cancer cells and in syncytiotrophoblasts from normal human placenta (Avila et al. 2010). Kv10.1 repression by calcitriol in cervical cancer cells occurs at the transcriptional level and involves a functional nVDRE (negative-VDRE) in the Kv10.1 promoter (Cazares-Ordonez et al. 2015). Calcitriol also decreases Kv10.1 expression and tumor growth in vivo of the xenografted breast cancer cell lines T-47D and HCC-1806 and the MBCDF breast cancer primary culture (Garcia-Quiroz et al. 2014, 2016). The antineoplastic effect of calcitriol has also been observed in melanoma, pancreatic, prostate, and colorectal cancer, as well as in hepatocellular carcinoma. In fact, a large number of epidemiological studies have demonstrated an association between low circulating levels of the calcitriol precursor calcidiol, with higher risk to develop colorectal and breast cancer and hepatocellular carcinoma (Diaz et al. 2015).

In addition, the antineoplastic effects of calcitriol are potentiated in breast cancer in vitro and in vivo, by combining it with other antineoplastic agents including the natural compounds curcumin and resveratrol (García-Quiroz et al. 2019). Besides, the combination of calcitriol with the receptor tyrosine kinase inhibitors gefitinib, lapatinib, and neratinib is more effective to inhibit the growth of breast cancer cell lines in comparison with each compound alone (Segovia-Mendoza et al. 2015, 2017). Furthermore, the combinations of calcitriol or its analogs with chemotherapeutic agents such as antimetabolites, platinum compounds, or taxanes improve the antineoplastic effects in different types of cancer (Abu El Maaty and Wölfl 2017). Thus, calcitriol is an endogenous natural anticancer factor targeting ion channels and promising antineoplastic agent.

6.4 Clarithromycin

Clarithromycin is a macrolide antibiotic drug having a broad spectrum of antimicrobial activity for gram-positive and gram-negative organisms, atypical pathogens, and some anaerobes (Peters and Clissold 1992). Interestingly, in colorectal cancer, this macrolide modulates the PI3K/Akt pathway by targeting Kv11.1, modulating autophagic flux, and triggering apoptosis. This drug preferentially binds to Kv11.1 channels in their closed state and inhibits the formation of a macromolecular complex between the channel and the p85 subunit of PI3K, impairing this signaling pathway (Petroni et al. 2020). Additionally, clarithromycin targets the P-glycoprotein, which is overexpressed in different kinds of tumors and confers resistance to chemotherapy (Vermeer et al. 2016). This drug also enhances the cytotoxic effect of 5-fluorouracil both in vitro and in vivo (Petroni et al. 2020).

6.5 Fluoxetine

Fluoxetine is a selective serotonin reuptake inhibitor, initially intended for the treatment of depression; however, nowadays it is also prescribed to treat other conditions like obsessive-compulsive disorders (Wong et al. 1995). Interestingly, fluoxetine is also a non-torsadogenic Kv11.1 inhibitor successfully used in glioblastoma therapy without obvious cardiotoxicity and the added benefit of treating depression (Pointer et al. 2017). Kv11.1 channel blockers reduced glioblastoma cell proliferation and improved survival in patients who received one or more Kv11.1 blockers but only if their tumors exhibited high Kv11.1 expression levels (Pointer et al. 2017), which represents another example of the potential use of channel expression levels for personalized therapy.

6.6 Glibenclamide

Glibenclamide is a second-generation sulphonylurea, used for the treatment of non-insulin-dependent diabetes mellitus; this drug binds to the sulphonylurea receptor (SUR1) expressed in pancreatic B cells and blocks KATP channels, leading to insulin release (Payen et al. 2001). KATP channels are composed of at least two types of subunits, an inwardly rectifying K+ channel (Kir6.x) and a regulatory subunit SUR. SUR1 belongs to the ATP-binding cassette (ABC) protein superfamily. Glibenclamide inhibits the activity of various ABC transporters and multidrug resistance proteins (MRPs). In the human lung cancer cells GLC4/Sb30 that overexpress MRP1 and are resistant to the anticancer drugs doxorubicin and vincristine, glibenclamide (0.39–100 μM) inhibited MRP1 activity in a dose-dependent manner reverting drug resistance (Payen et al. 2001). This drug (0.5–200 μM) also decreased cell viability and induced apoptosis in the gastric cancer cell line MGC-803 by activating mitochondrial death pathways related to ROS generation, activation of JNK, and inhibition of Akt (Qian et al. 2008). Whereas in the breast cancer cell line MDA-MB-231, the sulphonylurea (10–50 μM) inhibited cell growth and induced G0/G1 arrest (Núñez et al. 2013). Glibenclamide (150 μM) also decreased the proliferation of several cervical cancer cell lines; the higher the expression of Kir6.2 subunit in the cervical cancer cells, the higher the inhibitory effect of the drug. The overexpression of the Kir6.2 subunit was also observed in cervical tumor tissues; therefore, glibenclamide is a potential therapy for this type of cancer (Vazquez-Sanchez et al. 2018). Interestingly, the combined treatment of glibenclamide with CoCl2 decreased the expression of metalloproteinase-9 (MMP-9) and inhibited the growth in highly metastatic breast cancer cells (Rong et al. 2013). The antitumor effect of glibenclamide has been also observed in preclinical studies in melanoma (Suzuki et al. 2012), bladder carcinoma (Wondergem et al. 1998), prostate (Abdul and Hoosein 2002), and liver cancer (Malhi et al. 2000). The antineoplastic effects of glibenclamide may be explained by its ability to block KATP channels, ABC transporters, and MRPs and decrease the expression of MMP-9 (Payen et al. 2001; Rong et al. 2013).

6.7 Verapamil

Verapamil is an L-type Ca2+ channel blocker classified as a class IV antiarrhythmic agent that also blocks Kv11.1 currents (Zhang et al. 1999). This drug also exhibits anticancer effects attributed to its combined inhibitory activity against potassium and Ca2+ channels (Kale et al. 2015). Verapamil has antiproliferative effects on the breast cancer cells HT-39 both in vitro (IC50 = 10 μM) and in vivo (3.5 mg/day) (Taylor and Simpson 1992), as well as in prostate cancer (Rybalchenko et al. 2001), melanoma (Huber et al. 1989), and neuroblastoma (Schmidt et al. 1988) and in a nude mouse model of meningiomas (Jensen and Wurster 2001). Interestingly, verapamil overcomes the vincristine resistance both in vitro and in vivo in P388 leukemia cells (Yusa and Tsuruo 1989), doxorubicin-resistant myeloma (Durie and Dalton 1988), and vinblastine-resistant pediatric tumors (Cairo et al. 1989). In a prospective study in 99 patients with anthracycline-resistant metastatic breast carcinoma, verapamil given in conjunction with chemotherapy increased survival (Belpomme et al. 2000). In a randomized trial of 72 patients with advanced non-small cell lung cancer (NSCLC), verapamil plus chemotherapy (vindesine/ifosfamide) improved patient outcome (Millward et al. 1993). The reversal mechanism of MDR by verapamil is because the antiarrhythmic drug interacts with specific binding sites on the P-glycoprotein (Yusa and Tsuruo 1989); however, the clinical use of this agent has been hampered because of the unacceptable toxicity and side effects at the doses required to modulate the P-glycoprotein (Arora et al. 2005). Thus, the synthesis of new analogs of verapamil deserves further investigation.

6.8 Nifedipine and Mibefradil

Nifedipine is a potent L-type Ca2+ channel blocker indicated as an antihypertensive drug from several years ago and has an acceptable safety profile. In vitro studies showed that nifedipine reduces the mitogenic effect of endothelin-1 by blocking Ca2+ channels in lung cancer cells (Kale et al. 2015). In endometrial carcinoma cells, nifedipine induced autophagy through Beclin1 and the mTor pathway (Bao et al. 2012). In addition, the Ca2+ channel blockers nifedipine, mibefradil, and tetrandrine modulated the androgen receptor-mediated gene expression and induced cytotoxicity in LNCaP, LAPAC-4, and C4-2 androgen receptor-positive prostate cancer cells (Loughlin 2014). The antitumor effect of cisplatin was enhanced by nifedipine in cisplatin-sensitive human glioblastoma U-87MG cells and cisplatin-resistant U87-MG-CR cells both in vitro and in vivo (Kondo et al. 1995), as well as in lung carcinoma cells (Onoda et al. 1988). However, the potential use of nifedipine as antineoplastic is controversial because it cannot be used in hypotensive cancer patients. Nevertheless, the alternate dosing systems like the continuous release system developed by Bayer may help to control the blood Ca2+ levels and avoid rapid hypotension (Kale et al. 2015). Mibefradil, a T-type channel blocker was approved as an antihypertensive drug by the FDA in 1997 but voluntarily withdrawn from the market by Roche Laboratories in 1998 after reports of dangerous and even fatal interactions with at least other 25 drugs, including antibiotics, antihistamines, and anticancer drugs (SoRelle 1998). This drug has important antineoplastic effects in glioblastoma (Keir et al. 2013), breast cancer, and retinoblastoma (Bertolesi et al. 2002). Holdhoff and colleagues designed a phase I study to determine the safety and the maximum tolerated dose of mibefradil when given sequentially with temozolomide in recurrent high-grade gliomas. The study enrolled 27 patients; mibefradil followed by temozolomide was well tolerated; and the lack of toxicity and response in some patients warrants further investigation (Holdhoff et al. 2017). Besides, mibefradil regulates the gating of Kv10.1 channels inducing an apparent inactivation, probably by binding to the voltage sensor domain (Gómez-Lagunas et al. 2017), which adds a new potential mechanism of the anticancer effects of this drug.

6.9 Celecoxib

Celecoxib has been used as an anti-inflammatory, analgesic, and antipyretic drug, but it also has antineoplastic properties. The mechanism of action of celecoxib as an antineoplastic agent has been not sufficiently investigated (Toloczko-Iwaniuk et al. 2019). This drug decreases the proliferation of rat pheochromocytoma PC12 cells in a dose-dependent manner by blocking Cav-mediated currents (Zhang et al. 2007). The clinical efficacy and safety of celecoxib have been evaluated in combination with chemotherapy in metastatic or postoperative recurrent gastric patients, which offers more clinical benefits (Guo et al. 2019). A case report described that HCC practically disappeared in a patient after 8 months of treatment with celecoxib and pentoxifylline (Jimenez-Luevano et al. 2018). The combination of celecoxib with antineoplastic agents as capecitabine could be a good option for patients with thymic carcinoma (Wood et al. 2018). In addition, the combination of the anti-inflammatory drug with erlotinib may be efficacious for patients with advanced non-small cell lung carcinoma and wild-type EGFR (Jin et al. 2019b). Preclinical and clinical studies have demonstrated promising results of the role of celecoxib in the treatment and prevention of some cancers such as colon, breast, prostate, and head and neck (Toloczko-Iwaniuk et al. 2019). Whether calcium channels are involved in all these effects remains elusive.

6.10 Bromocriptine

Bromocriptine is an ergot and dopamine D2 receptor agonist used to treat Parkinson’s disease, acromegaly, hyperprolactinemia, galactorrhea, and diabetes mellitus. The drug is active also against prolactinomas and growth hormone-producing adenomas. This drug reduces tumor mass in 80–90% of patients with microadenomas and in 70% of patients with macroadenomas (Seo et al. 2018). Prolactin constitutes a growth factor for breast cancer cells, is associated with poor prognosis, and reduced efficacy of antitumor therapies in metastatic breast carcinoma. A clinical study evaluated the effect of taxotere versus taxotere plus bromocriptine in metastatic breast cancer patients pretreated with anthracyclines. The results suggested that the inhibition of prolactin secretion by antiprolactinemic drugs such as bromocriptine might enhance the efficacy of chemotherapy for metastatic breast cancer (Lissoni et al. 2002). More recently, bromocriptine (0.001–100 μM) was proved to inhibit drug-resistant tumor cells in a hormone-independent manner. The combination of bromocriptine with either doxorubicin or paclitaxel resulted in a synergic effect in the MDR P-glycoprotein overexpressing CEM/ADR5000 leukemic cells (Seo et al. 2018).

Thus, several approved drugs originally prescribed for other indications may be repurposed for cancer therapy because of their antineoplastic properties acting on ion channels or transporters. This approach should accelerate the development of clinical trials, especially for poor prognosis cancers. Toxins targeting ion channels and transporters represent an additional alternative to fight cancer.

7 Therapeutic Potential of Animal Venoms Against Channels and Transporters in Cancer

Over ten million of active peptides and proteins are estimated to be present in animal venoms; in many cases small amounts of the venom are sufficient to kill either preys or predators and microbial invaders (Wulff et al. 2019). Several venom components (salts, nucleotides, biogenic amines, enzymes such as phospholipase, hyaluronidase, L-amino acid oxidase, metalloproteinase, serine protease, mucoproteins, peptides, and proteins) possess antineoplastic effects via regulating the expression or activity of ion channels (Ding et al. 2014; Chen et al. 2018; Wulff et al. 2019).

7.1 Scorpion and Spider Venom Peptides as Antineoplastic Agents

Chlorotoxin is one of the most abundant peptides from the Leiurus quinquestriatus hebraeus deathstalker scorpion venom and exhibits great specificity for gliomas and tumors of neuroectodermal origin blocking glioma-specific chloride ion channels with high affinity and MMP-2, decreasing cells invasion (Lyons et al. 2002; McFerrin and Sontheimer 2006). Chloride channels are either absent or in low abundance in healthy tissues and in tumors of non-glial origin; however, their expression increases as gliomas progress and are crucial in tumor cell invasion and migration; chlorotoxin potently blocks these channels (Dardevet et al. 2015). Another target of chlorotoxin on the surface of glioma cells is MMP-2, which is upregulated in gliomas and related cancers but is not expressed in the normal brain. Chlorotoxin binds to MMP-2, inhibits its catalytic activity in a dose-dependent manner, and reduces its surface expression by inducing its internalization (Deshane et al. 2003). This toxin also targets other cancer cells including those from melanoma, small cell lung carcinoma, neuroblastoma, medulloblastoma, Ewing’s sarcoma, and pheochromocytoma (Dardevet et al. 2015). A chlorotoxin:CY5.5 bioconjugate that emits near-IR fluorescent signal was developed as a contrast agent with the potential to improve intraoperative detection and resection of malignancies. This bioconjugate demonstrated preferential accumulation in a wide variety of tumors, including prostate and intestinal cancer, and sarcoma (Veiseh et al. 2007). In addition, a chlorotoxin-modified doxorubicin-loaded liposome delivery system for targeting gliomas was developed to improve chemotherapeutic efficacy. The liposomes enhanced the cellular uptake by the murine (C6) and human glioma (U87MG and 251MG) cell lines and mice brain microvascular endothelial cells (BMECs), which increased drug cytotoxicity. The encapsulated doxorubicin enhanced the targeting efficiency to subcutaneous and intracranial gliomas improving the antitumor efficacy and lowering blood toxicity (Xiang et al. 2011). Furthermore, a platinum (IV) complex was conjugated to chlorotoxin in order to deliver cisplatin to cancer cells (Graf et al. 2012). In vivo assays showed that the iodine 125- and 131-labeled chlorotoxin specifically bind to brain tumor cells, making this peptide a promising candidate in radiotherapy of the postsurgical brain tumors (Srairi-Abid et al. 2019). Several chlorotoxin-based clinical trials for cancer therapy and diagnoses have been developed (Xiang et al. 2011; Pennington et al. 2018). The synthetic version of chlorotoxin reached phase III clinical trials under the name of TM-601 (Srairi-Abid et al. 2019). The FDA approved the iodine-131 radioconjugate of synthetic chlorotoxin (131I-TM-601) for glioma therapy and diagnostics (Dardevet et al. 2015).

KAaH1 is a peptide from Androctonus australis scorpion blocking Kv1.1 and Kv1.3 channels with an IC50 of 5 and 50 nM, respectively (Srairi-Abid et al. 2005). Both channels are expressed in U87 (glioblastoma), MDA-MB-231 (breast cancer), and LS174 (colon adenocarcinoma) cell lines. KAaH1 inhibited the migration of the three cell lines. KAaH2 is slightly active only on Kv1.1 channels and inhibits U87 cell proliferation probably via either the EGFR signaling pathway or other K+ channels (Aissaoui et al. 2018). Kv10.1 and Kv11.1 channels could be potential candidates for the KAaH2 effect since EGFR forms a multimeric complex with the Kv11.1 channel (Aissaoui et al. 2018), and EGFR regulates Kv10.1 currents (Wu et al. 2012). Margatoxin is a toxin from Centruroides margaritatus scorpion venom that inhibits human lung adenocarcinoma A549 cell proliferation by selective inhibition of Kv1.3 channels. This toxin increases the expression of P21Waf1/Cip1 and decreases that of Cdk4 and cyclin D3; in vivo, the toxin reduced the tumor volume when injected into the tumor (Jang et al. 2011). Moreover, margatoxin also inhibits the proliferation of the weakly metastatic rat prostate cancer cell line AT2, but not that of the strongly metastatic prostate cancer cell line Mat-LyLu (Fraser et al. 2000). Although different studies were focused mainly on the implication of Kv1.3 in regulating cell proliferation, several reports suggested that the mechanism could be also extrapolated for other K+ channels subtypes (Srairi-Abid et al. 2019). Iberiotoxin is a peptide from Mesobuthus tumulus scorpion venom that inhibits Ca2+-activated K+ channels and decreases the human malignant glioma cell number in a dose- and time-dependent manner. The toxin arrested glioma cells in the S phase of the cell cycle, which eventually led to cell death. The expression of these Ca2+-activated K+ channels is upregulated in human glioma, and the expression levels increase with the grade of the tumor (Weaver et al. 2004). The antiproliferative effects of iberiotoxin have been also observed in 1321N1 astrocytomas (Basrai et al. 2002) and PC-3 prostate cancer cell lines (Bloch et al. 2007). Charybdotoxin is another scorpion toxin inhibiting Ca2+-activated K+ channels that slow the migration of melanoma cells in a dose-dependent manner (Schwab et al. 1999). The scorpion peptide κ-hefutoxin 1 inhibits the Kv10.1 channel in a dose-dependent manner (IC50~26 μM) (Moreels et al. 2017b). Other toxins targeting Kv11.1 channels are ErgoTx from the Centruroides noxious scorpion venom, BeKm-1 from the scorpion Buthus eupeus, and BmTx3 from the scorpion Butus martensi. The analgesic-antitumor peptide (AGAP) from scorpion venom is an inhibitor of voltage-gated sodium channels and decreases proliferation and migration of glioma cells (Zhao et al. 2011; Chen et al. 2018).

Psalmotoxin is a specific acid-sensing ion channel (ASIC1) blocker from the Psalmopoeus cambridgei tarantula venom. This toxin inhibits cation currents in malignant astroglioma and glioblastoma multiforme cells, arresting the cell cycle in the G0/G1 phase and upregulating p21 and p27 protein expression due to a reduction of the phosphorylation of ERK1/2 (Wu et al. 2019). Aa1a and Ap1a spider venom peptides are gating modifiers of the Kv10.1 channel. Ap1a peptide is more selective (>30-fold) for Kv10.1 than for Kv11.1 (Ma et al. 2018). Several peptides from the Chilean tarantula Grammostla rosea venom blocks Kv11.1 channels currents transiently expressed in CHO cells in a reversible manner (Wanke and Restano-Cassulini 2007).

7.2 Blarina brevicauda Saliva Peptides, Snake Venoms, and Anemone Toxins with Antineoplastic Effects

Soricidin is a novel paralytic peptide found in the saliva of the northern short-tailed shrew Blarina brevicauda that modulates TRPV6 channels, which are highly expressed in ovarian, breast, prostate, colon, and thyroid cancers, as well as certain leukemias and lymphomas. Actually, the overexpression of these channels is associated with tumor development and progression, and its inhibition decreases cancer cell proliferation and promotes apoptosis (Pennington et al. 2018; Wulff et al. 2019). SOR-C13 and SORC-27 are two shorter peptides derived from the C-terminus of soricidin and bind to TRPV6 channels with high affinity targeting tumor sites in mice bearing ovarian or prostate tumors; thus, these peptides may be used as either drug carriers or diagnostic agents in TRPV6-enriched tumors (Bowen et al. 2013). In an ovarian cancer xenograft mouse model, daily i.p. injection of SORC-13 and SORC-27 inhibited tumor growth (Xue et al. 2018). A phase I study of SORC-13 in patients with advanced tumors of epithelial origin showed stable disease in 54.5% of the patients (ranging from 2.8 to 12.5 months) without drug-related serious adverse events. The best response in this study was a 27% reduction in a pancreatic tumor with a 55% reduction in the levels of the tumor marker CA19-9 (Fu et al. 2017a, b).

Snake venom components inhibit cell proliferation and promote cell death; the mechanisms of action include increasing Ca2+ influx, cytochrome C release, and modified protein expression. These venom toxins also prevent metastasis, promote toxicity and free-radical generation, inhibit nucleic acid synthesis, decrease the expression and activity of matrix metalloproteinase, and inhibit integrins preventing migration and invasion of cancer cells, as well as angiogenesis (Chen et al. 2018). APETx1 and APETx4 toxins from the sea anemone Anthopleura elegantissima block Kv11.1 (IC50 = 34 nM) and Kv10.1 (IC50 = 1.1 μM) channels. APETx1 modifies the voltage dependence of Kv11.1, while APETx4 seems to keep Kv10.1 channels in a closed state (Diochot et al. 2003; Moreels et al. 2017a). APETx4 induces a concentration-dependent cytotoxic and proapoptotic effect in various cancer and noncancer cell lines (Moreels et al. 2017a). The large diversity of venom components and their multiple antineoplastic effects makes them excellent candidates to establish new therapeutic and specific strategies with fewer side effects in cancer treatment. Finally, we will discuss how ion channels and transporter-based nanomedicine can be used to fight cancer.

8 Ion Channel and Transporter-Based Nanomedicine in Cancer Therapy

The medical applications of nanotechnology led to the emergence of nanomedicine with the idea of inserting nanorobots into patients to treat several diseases, including cancer (Freitas 2005). Then some nanomaterials including liposomes, nanofibers, polymeric micelles, magnetic, inorganic, and polymeric nanoparticles have been used to decrease the side effects of chemotherapy by increasing its specificity, direct targeting, easy absorption, and sustained release and reducing drug degradation (Xie et al. 2019; Yang et al. 2019). Additional advantages of drug carrier systems over traditional chemotherapy include better solubility of hydrophobic drugs, higher stability, and improved blood half-life of the therapeutic agent. Nevertheless, drug delivery is complicated due to multiple physical barriers that limit diffusion and tumor penetration of the released drugs (Sun et al. 2008b). However, nanoparticles do not necessarily have to penetrate into tumor cells because the antineoplastic efficacy could be improved if the delivered drugs target cell membrane proteins involved in cancer like ion channels and transporters. For instance, nanoprobes composed of polyethylene glycol (PEG)-coated iron oxide nanoparticles were functionalized with chlorotoxin and the fluorescent molecule Cy5.5 targeting glioma tumors with high affinity, high resolution, and good therapeutic effect (Veiseh et al. 2005, 2009; Sun et al. 2008a). Chlorotoxin was also conjugated in nanoparticles with the chemotherapeutic agent methotrexate, demonstrating preferential accumulation and increased cytotoxicity in tumor cells, as well as prolonged nanoparticle retention within tumors (Sun et al. 2008b). A cancer cell-specific magnetic nanovector functionalized with siRNA and chlorotoxin was developed with the aims of efficient siRNA delivery and noninvasive monitoring by magnetic resonance imaging (MRI). The nanovector demonstrated both increased siRNA internalization by tumor cells and intracellular trafficking toward enhanced knockdown of targeted gene expression. In addition, this nanovector enhanced MRI contrast in vitro, potentially enabling monitoring of the treatment in vivo. The elevated specificity and potency of this nanovector system make it a potential gene therapy approach for malignant tumors (Veiseh et al. 2010).

Drug resistance has been also addressed by nanomedicine. Functionalized nanoparticles deliver and concentrate drugs at the plasma membrane where ABC transporters are located and are saturated with either the antineoplastic drug, ABC transporter blockers, or inhibitors (Xue and Liang 2012). Mesoporous silica nanoparticles were used to deliver doxorubicin and P-glycoprotein siRNA to the drug-resistant cancer cell line KB-V1 (derived from HeLa cervical cancer cells). The dual delivery of doxorubicin and siRNA in KB-V1cells increased the intracellular and intranuclear drug concentration to levels exceeding free doxorubicin or the drug being delivered by mesoporous in the absence of siRNA co-delivery (Meng et al. 2010). Wang and colleagues observed similar results; the co-delivery of doxorubicin and MDR1-siRNA by mesoporous silica nanoparticles-polymerpolyethylenimine improved oral squamous carcinoma treatment in vitro and in vivo (Wang et al. 2018a). In breast cancer, the dual delivery system of doxorubicin and siRNA against P-glycoprotein resulted in synergistic inhibition of tumor growth (Meng et al. 2013). Similar results were observed using doxorubicin/P-glycoprotein siRNA-loaded nanomicelles (Suo et al. 2016) and RGD peptide (arginine-glycine-aspartic acid)-modified liposomes containing also doxorubicin and P-glycoprotein siRNA (Xue and Liang 2012).

Photothermal therapy (PTT) converts near-infrared light (NIR) stimulation into local mild heat that can stimulate a photosensitizer to control biological processes in a remote and noninvasive manner. Photothermal transducers of graphene oxide linked to a calmodulin-binding peptide (rGO-P) have been designed to activate or inactivate potassium channels based on their binding to Ca2+-calmodulin (Chai et al. 2018). Kv10.1 channel activity is inhibited by [Ca2+]i through the binding of Ca2+-calmodulin (Lörinczi et al. 2016). HEK-293 cells transfected with Kv10.1, treated with rGO-P, and stimulated with NIR irradiation displayed an open state of Kv10.1 channels because the channel cannot be closed via binding of Ca2+-calmodulin, suggesting a new strategy to regulate ion channels involved in cancer like Kv10.1 (Chai et al. 2018). The application of nanomaterials to PTT is emerging as a new strategy for cancer therapy and showing encouraging results in vivo (Zou et al. 2016).

A number of studies suggest that targeting ion channels and transporters is a feasible approach to achieve therapeutic efficacy and tumor selectivity in cancer treatment. Table 2 summarizes some of these findings, and Fig. 1 shows the diversity of the potential anticancer therapeutic options based on ion channels and transporters.

Table 2 Examples of potential ion channel and transporter-based anticancer therapies
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Fig. 1
figure 1

Novel therapeutic opportunities targeting ion channels or transporters in cancer. (a) Inducing higher ROS production in cancer cells by blocking ion channels or transporters may convert cancer cells more sensitive to chemotherapy. (b) Immunotherapy against channels overexpressed in cancer cells offers a more selective approach. (c) Toxins targeting ion channels and transporters may also help to improve selectivity and the antitumor response. (d) Identifying and investigating new uses of approved drugs hold potential and many advantages for anticancer therapy. (e) New technologies like nanoparticles and phototherapy targeting ion channels and transporters are potential more selective anticancer therapies

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9 Conclusions

Cancer is a multifactorial disease; therefore, several issues should be considered to find the most promising anticancer therapy for each patient. Specific inhibitors of ion channels and transporters including drugs, antibodies, or toxins, concomitantly used with regulators of the redox status and enhancers of the immune response, should improve the therapeutic outcome. Moreover, repurposing drugs targeting ion channels and transporters combined with currently used anticancer treatments should enhance the antitumor response. This drug repurposing approach adds several anticancer mechanisms to the treatment, may decrease the side effects by lowering the administrated doses, and should decrease treatment costs, as well as accelerating the development of clinical trials. The specificity of many toxins targeting ion channels and transporters should allow the directed and specific treatment of tumors. Exploiting ion channel and transporter-based approaches to fight cancer and the use of nanotechnology and nanomedicine should improve the current therapies for the benefit of cancer patients.