Keywords

1 Introduction

Cancer is a genetic disease causing high morbidity and mortality, with approximately 14 million new cases arising in 2012. The number of cases is expected to rise to almost 19 million by 2024. After cardiovascular disease, cancer represents the second leading cause of death in the world, having been responsible for 8.8 million deaths in 2015 (WHO 2017), which means that nearly 1 out of 6 deaths is due to this disease.

Cancer is caused by the deregulation of genes controlling cell growth, division, and death. The genetic changes that contribute to the development of cancer affect three main types of genes: proto-oncogenes, tumor suppressor genes, and DNA repair genes. These changes are sometimes called “drivers” of cancer. The final consequence is the generation of an imbalance between growth and death causing cells to proliferate and migrate toward distant organs, a phenomenon known as metastasis. Cancer may be either hereditary, may be the consequence of errors that occur during cell division, or may be due to DNA damage caused by certain environmental noxious agents such as the chemicals present in tobacco smoke or radiation such as ultraviolet rays (WHO 2005; Ferlay et al. 2015). Normal and tumor cells present a number of differences such as the lack of specialization of the latter, which causes them to divide continuously. Cancer cells have also lost their capacity to respond to the intracellular signals that regulate cell growth and apoptosis. Tumors are known to establish a relationship with the microenvironment, mainly through contact with normal cells, nearby molecules, and surrounding blood vessels that serve as nutrient source. Besides, tumor cells have developed mechanisms for immune evasion.

Cancers can be classified according to the organs or tissues where they originate. Thus, carcinomas are the most common type, which are constituted by epithelial cells; sarcomas are cancers generated in bones and soft tissues, including muscle, fat, blood vessels, lymph vessels, and fibrous tissue (such as tendons and ligaments); melanomas are made up of melanocyte precursors; and multiple myeloma is a cancer type of plasma cells.

Leukemia and lymphoma are a group of heterogeneous neoplastic disorders of white blood cells characterized by the uncontrolled proliferation and the blockage in the differentiation process of hematopoietic cells. While lymphomas are characterized by an abnormal lymphocyte proliferation developing as a solid tumor, most commonly in the lymph nodes of the neck, chest, armpit, or groin (Jaffe 2009), leukemias develop in the bone marrow and do not form solid tumors.

Leukemia and lymphoma stand at the fifth place of cancer-related deaths over the world (Armitage 2012). Leukemia includes acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML). AML is the most common acute leukemia in adult patients, which progresses rapidly and may be lethal within weeks or months. Based on the cytogenetic/genetic features of AML, and according to the 2008 revised WHO classification system, patients can be broadly classified into three different risk groups. Treatment strategies and prognoses vary among these subtypes and are further influenced by patient-specific factors (Zeisig et al. 2012).

As stated above, lymphomas involve the uncontrolled growth of lymphocytes (T cells or B cells) and include two types: Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL), the latter is subdivided into two main types depending on the cell growth rate, that is, high-grade (or “aggressive”) and low-grade (or “indolent”) NHLs.

Taking into account patient-specific characteristics, such as age and overall health status, and each tumor type, it is obvious that the treatment strategies spectrum to be applied will be broad.

Chemotherapy is routinely used for cancer treatment; however, this strategy is not always effective due to the development of cell resistance phenomena and toxic effects (Hopfinger et al. 2012). For example, 5-fluorouracil, a common chemotherapeutic agent, is known to cause myelotoxicity and cardiotoxicity. Doxorubicin, which is another widely used drug, is known to cause cardiotoxicity, nephrotoxicity, and myelotoxicity. Similarly, bleomycin, a well-known chemotherapeutic agent, is known for its pulmonary toxicity. In addition, bleomycin causes cutaneous toxicity. Cyclophosphamide, a drug used to treat many malignant conditions, has been demonstrated to cause bladder toxicity in the form of hemorrhagic cystitis, immunosuppression, alopecia, and, at high doses, cardiotoxicity (Chabner et al. 2006).

All AML subtypes, except for the acute promyelocytic leukemia (APL), are typically treated with an induction chemotherapy using a “3 + 7” combination of daunorubicin and cytarabine (Zeisig et al. 2012). Almost all APL patients can achieve complete remission after treatment with all-trans-retinoic acid, which can effectively induce APL cell differentiation (Degos and Wang 2001). Conversely, although treatment strategies for AML have substantially progressed in recent decades, effective therapies for non-APL subtypes of AML are still urgently needed.

In this context, plants have enormous potential as a source of new drugs, constituting a reservoir of natural chemicals with low toxicity chemoprotective properties against cancer. There are four classes of commercially available plant-derived anticancer agents: the vinca alkaloids (vinblastine, vincristine, and vindesine), the epipodophyllo toxins (etoposide and teniposide), the taxanes (paclitaxel and docetaxel), and the camptothecin derivatives (camptothecin and irinotecan). The mechanisms of action of these drugs include cell cycle arrest, the induction of cell differentiation, or the induction of cell death through the activation and sensitization of the intrinsic and extrinsic pathways of apoptosis, respectively. Some drugs also interact with the cytoskeleton by either inhibiting the polymerization or by inducing depolymerization of microtubules. The latter effects also lead to a cell cycle arrest and apoptosis (Balunas and Kingborn 2005; Cragg et al. 2009).

Sesquiterpene lactones are a group of plant secondary metabolites that are abundant in members of the Asteraceae family (Merfort 2011). The α, β unsaturated carbonyl function of STLs is important to exert the antiproliferative effect of these compounds. Among cytotoxic STLs, costunolide, eupatolide, parthenolide, dehydrocostuslactone, and thapsigargin can be mentioned (Fig. 13.1).

Fig. 13.1
figure 1

Chemical structure of cytotoxic sesquiterpene lactones

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In this chapter, the cytotoxic and antitumor effects and mechanisms of action of STLs will be summarized, focusing on their effects on leukemias, lymphomas, and myelomas. Besides, some clinical trials assessing the effect of this type of compounds in other tumors will also be described. The remission obtained after the co-treatment with STLs and conventional chemotherapeutics will also be addressed.

2 In Vitro Studies and Mechanism of Action

There are two mechanisms by which natural compounds can induce cytotoxicity, namely, cell death and cell differentiation.

Three types of cell death are recognized: apoptosis, necrosis, and autophagy. Apoptosis is characterized by the activation of cysteine aspartate-specific proteases (caspases), chromatin condensation, cell shrinking, and blebbing of the cell membrane. Autophagy is characterized by the formation of intracellular vacuoles, organelle degradation, and, finally, cell death. In contrast, in necrotic cells, the endoplasmic reticulum (ER), mitochondria, and cytosol continue to swell until the cell bursts and releases its contents (Krysko et al. 2003). Apoptosis is the most common mechanism of cell death by which natural compounds exert their cytotoxic effects.

Apoptosis can occur through an extrinsic pathway activated by the binding of ligands to death receptors on the cell membrane and involves caspase-8 and caspase-3 or through an intrinsic pathway activated by other stimuli involving the release of mitochondrial proteins and the activation of the caspase-9 and caspase-3. The extrinsic and intrinsic pathways are interconnected, for example, the cleavage of BH3 interacting domain death agonist (Bid) by caspase-8 results in the formation of pro-apoptotic truncated Bid (tBid), which activates the intrinsic pathway (Krysko and Vandenabeele 2009).

The mechanisms leading to the activation of apoptosis are diverse: the direct activation of caspases, the direct effect on mitochondria (cytochrome release) and consequent caspase activation, a redox imbalance and the generation of oxidative stress, a disturbance in cell cycle progression, the induction of ER stress, the activation of transcription factors that promote apoptotic proteins, the inhibition of transcription factors related to survival and proliferation such as NF-κB and STATs, and the inhibition or activation of mitogen-activated protein kinases (MAPK), which transmit extracellular signals to regulate cell growth, proliferation differentiation, and apoptosis. Three major MAPK families have been described: P-38, the extracellular signal-regulated kinase (ERK), and C-Jun N-terminal kinase (JNK). When cells are exposed to stressors, one of the major outcomes of MAPK activation is the induction of apoptosis, a process in which the transcription of the Bcl-2 family proteins, such as Bax and Bcl-2 takes place. While the inhibition of DNA methylation is related to cell differentiation and apoptosis, the inhibition of histone deacetylase 1 (HDAC1) induces apoptosis through the activation of p21 and p53 proteins.

The induction of apoptosis through the generation of oxidative stress is a mechanism by which many STLs produce either cytostatic or cytotoxic effects in tumors involving the participation of mitochondria. The mitochondrial events involve the release of apoptotic factors, including cytochrome C and second mitochondria-derived activator of caspases (Smac/Diablo). Usually, the permeabilization of the mitochondrial outer membrane leading to the release of these components is controlled by the balance between pro-apoptotic and anti-apoptotic members of the B-cell CLL/lymphoma 2 (Bcl-2) superfamily.

Over the last 5 years, many studies have been performed to assess the antiproliferative activity of STLs. It has been demonstrated that the treatment of tumor cells with STLs disrupts their redox status and causes oxidative stress, with the subsequent initiation of the intrinsic apoptosis pathway (Sun et al. 2010). For example, the treatment of tumor cells with parthenolide increases the generation of reactive oxygen species (ROS) in tumor cells through diverse mechanisms such as the reduction of the cysteine sulfhydryl group of the nonprotein antioxidant molecule glutathione (Ralstin et al. 2006), the activation of NADPH oxidase, and the downregulation of the antioxidant enzymes manganese superoxide dismutase and catalase.

Other STLs produce apoptosis by interfering with the cell cycle progression and by arresting cells in the G2/M phase (Rozenblat et al. 2008) together with an increase in the sub-G0 fraction or, conversely, by arresting the cells in the G1/S phase with the appearance of a broad sub-G1 peak (Cho et al. 2004). The activation of p53 and the subsequent inactivation of cell division control protein 2 (Cdc2) are responsible for the G2/M arrest, whereas the G1/S arrest involves the modulation of cyclin-dependent kinases and their activating partners, cyclins. Other STLs trigger apoptosis by increasing the levels of ROS, leading to the activation of either the extrinsic or the intrinsic pathway. Psilostachyin, psilostachyin C, peruvin, and cumanin exert an antiproliferative effect by inducing apoptosis on the BW5147 murine lymphoma cell line. Psilostachyin C has proved to be the most active and the less toxic pro-apoptotic compound. These effects are achieved through an impairment in the mitochondrial membrane potential and the generation of ROS. These phenomena are related to a modulation of the activity of antioxidant enzymes and by inducing cell cycle arrest in the S phase (Martino et al. 2015). Besides, psilostachyin and psilostachyin C are considered novel checkpoint inhibitors, since they cause the arrest of other cell types such as breast cancer cells (MCF-7) in the G2 phase (Sturgeon et al. 2005).

In addition, the STL EM23, isolated from Elephantopus mollis, which is a traditional herbal medicine with multiple pharmacological activities, inhibits the proliferation of human chronic myeloid leukemia (CML) K562 cells and acute myeloid leukemia (AML) HL-60 cells through the induction of apoptosis involving the translocation of the membrane-associated phospholipid phosphatidylserine, changes in cell morphology, the activation of caspases, and the cleavage of poly-(ADP-ribose) polymerase1 (PARP 1). The disruptions in the mitochondrial membrane potential observed have allowed suggesting the involvement of the mitochondrial pathway in EM23-mediated apoptosis. Mechanistic studies have indicated that EM23 caused a marked increase in the levels of ROS (Li et al. 2016).

Furthermore, five melampolide STLs (uvedalin, enhydrin, polymatin B, sonchifolin, and fluctuanin) isolated from the leaves of Smallanthus sonchifolius have been demonstrated to exert poor cytotoxic effects on peripheral blood mononuclear cells (PBMC) of healthy human subjects while being strongly cytotoxic against human T-cell acute lymphoblastic leukemia (CCRF-CEM) cells, CEM-ADR5000, and pancreatic cancer cells. These effects were achieved by triggering the generation of oxidative stress. These findings suggest that these STLs display a certain selectivity for tumor cells. It has been reported that the generation of ROS entails the induction of apoptosis in CCRF-CEM and CEM-ADR5000 cells through the activation of receptor-interacting protein kinase 1 (RIP1K). Neither apoptosis nor necroptosis was observed in human pancreatic carcinoma cell lines (MIA-PaCa-2 cells (de Ford et al. 2015)).

The dimeric STLs uvedafolin and enhydrofolin, also isolated from S. sonchifolius, have been demonstrated to have cytotoxic activity on HeLa cells with low IC50 values (2.96 and 3.17 mM, respectively) after 24 h of treatment, displaying low toxicity on normal NIH/3 T3 cells. These STLs caused cell cycle arrest in the G2/M phase and induced apoptosis through the activation of the mitochondrial pathway, with the participation of caspase-9 and caspase-3/7. In addition, the STL 2,3-dehydropsilostachyn isolated from the n-butanolic extract of Ambrosia cumanensis Kunth. has shown cytotoxic activity with different potencies in HeLa, Jurkat, and U937 cell lines. This STL was found to cause arrest in the G2/M cell cycle block (Del Socorro Jimenez-Usuga et al. 2016).

Another mechanism through which STLs activate the apoptotic intrinsic pathway is the interaction with ER through the ER stress-dependent and ER stress-independent pathways related to proteasome inhibition. The ER is responsible for the folding and the posttranslational modification of secreted proteins. When the ER becomes stressed due to the accumulation of newly synthesized unfolded or misfolded proteins, a complex intracellular signal transduction pathway, named the unfolded protein response (UPR), is activated, leading to the activation of the intrinsic pathway of apoptosis by phosphorylation and inactivation of Bcl-2, repression of Bcl-2 expression, or translocation of Bax/Bak to the mitochondria (Krysko and Vandenabeele 2009). The dehydrocostus lactone activates the ER-resident transmembrane protein sensors, which lead to the activation of a transcription factor that regulates UPR target gene expression and upregulates the pro-apoptotic transcription factor growth arrest and GADD153/C/EBP protein (Hung et al. 2010).The guaianolide thapsigargin induces ER stress by decreasing the Ca2+concentration in the ER by blocking the capacity of the sarco-/endoplasmic reticulum calcium ATPase (SERCA) pump to transfer Ca2+ from the cytosol to the ER, thus increasing the cytosolic Ca2+concentration. The latter phenomenon leads to the generation of ROS and subsequent cell death (Denmeade and Isaacs 2005).

Costunolide has antiproliferative effects on human bone osteosarcoma epithelial cells (U2OS) by inducing the loss of the mitochondrial transmembrane potential. This STL decreases the Bcl-2/Bax ratio, induces cytochrome C release, and activates caspases. All these effects are related to the generation of ROS and the ER stress-induced mitochondrial dysfunction, which activate kinases such as c-Jun N-terminal kinase (JNK) and induce apoptosis (Zhang et al. 2016).

It has also been demonstrated that the balance between apoptosis and survival can be affected by the reactivation of tumor suppressor genes. Tumor suppressor genes are silenced in tumor cells by hypermethylation. Therefore, inhibition of DNA methylation results either in the differentiation of cancer cells or in apoptosis. In this sense, the inhibition of DNA methyl transferase 1 by parthenolide results in significant hypomethylation and reactivation of histidine nucleotide-binding protein 1 (HIN-1) suppressor gene (Liu et al. 2009). Moreover, the activation of HIN-1 results in the inhibition of AKT signaling, which leads to the dephosphorylation of Bad, allowing it to bind anti-apoptotic Bcl-2 members to neutralize them. Parthenolide also affects histone acetylation by inhibiting histone deacetylase 1 (HDAC1), leading to p21 and p53 activation and subsequent apoptosis (Gopal et al. 2009).

It has also been demonstrated that rapidly proliferating cancer cells express elevated levels of transferring receptors on their cell surface and have higher amounts of intracellular iron than normal or slowly proliferating cells. In this regard, artemisinin has an endoperoxide bridge that is cleaved upon binding to Fe (II), producing toxic C-4 and seco-C-4 free radicals that destroy tumor cells. The addition of holotransferrin and iron sources to artemisinin increases its antitumor properties and targeted delivery (Nakase et al. 2009).

It is noteworthy that some STLs produce sensitization of the extrinsic pathway of apoptosis. This pathway is activated upon binding of membrane death receptors to their extracellular ligands, such as TNF and TNF-related apoptosis-inducing ligand (TRAIL). For example, the germacranolide eupatolide sensitizes cancer cells to TRAIL-induced apoptosis by downregulating the cellular FLICE-like inhibitory protein (c-FLIP). The treatment of human breast cancer cells with TRAIL in combination with subtoxic concentrations of eupatolide enhanced the TRAIL-induced cytotoxicity, as compared with the effect exerted by each STL alone (Lee et al. 2010). Dehydrocostus lactone renders human leukemia HL-60 cells susceptible to TNF-induced apoptosis by enhancing caspase-3 and caspase-8 activities (Oh et al. 2004). Parthenolide sensitizes cancer cells for TNF-induced apoptosis by downregulating anti-apoptotic genes and sensitizes hepatocellular carcinoma cells to TRAIL by inducing the expression of death receptors through the inhibition of STAT3 (Carlisi et al. 2011).

However, the most important mechanism by which STLs exert their antiproliferative effect and apoptosis involves the inhibition of NF-κB or STATs transcription factors. NF-κB and STAT3 are two important pathways involved in cell survival and proliferation that are constitutively activated in neoplastic cells in response to autocrine and paracrine factors that are produced within the tumor microenvironment (Takeuchi and Akira 2010). The NF-kB activation favors angiogenesis and tumorigenesis and plays a key role in the expression of several inflammatory genes and anti-apoptotic genes. Thus, NF-κB has been considered an important link between inflammation and cancer.

Parthenolide is one STL that is capable of inhibiting NF-κB transcription, presenting antiproliferative activity on leukemia cells. This STL can be found in different medicinal plants, particularly in feverfew (Tanacetum parthenium). It has been demonstrated that parthenolide induces apoptosis in acute myeloid leukemia (AML) stem and progenitor cells, without causing significant toxicity to normal hematopoietic stem cells (HSCs). AML growth is hierarchical and originates from leukemic stem cells (LSCs). All these findings have anointed parthenolide as the prototypical member of next-generation therapies for the elimination of LSCs (Guzman et al. 2005).

Parthenolide also suppresses the growth of the Raji cell line and activates the transcription of BZLF1 by inhibiting the activity of NF-κB. BL has been reported to be strongly associated with EBV infection. When parthenolide was used in combination with ganciclovir, the cytotoxic effect of first drug was enhanced. These data suggest that the induction of EBV-mediated lysis achieved with this drugs combination may constitute an efficient viral-targeted therapy for EBV-associated BL (Li et al. 2012). In addition, the STLs honokiol, magnolol, and parthenolide, isolated from Magnolia grandiflora, have proved to induce apoptosis and to exert cytotoxic effects in vitro on the atypical lymphocytes follicular Non-Hodgkin lymphoma cells (NHL) obtained from a patient (Marin et al. 2013). The increase in Iĸ-B and the consequent decrease in NF-κB DNA binding, the inhibition of the EGFR/PI3K/Akt signaling pathway, and the inhibition of telomerase are possible mechanisms proposed by the authors to explain this effect. The same authors had previously demonstrated that parthenolide has an antitumor activity in a lymphocyte malignancy animal model.

Another antitumor mechanism exerted by STLs involves NF-kB-driven transcription of matrix metalloproteinase 9, a crucial angiogenesis-related transcript that is suppressed by treatment with parthenolide (Oka et al. 2007) and by the germacranolide deoxyelephantopin (Huang et al. 2010). These antiangiogenic treatments reduce the occurrence of metastasis.

More recently, the chemical optimization of parthenolide on the α-methylene-γ-butyrolactone has been done to improve the potency and pharmacokinetic parameters. The modification in C1-C10 olefinic groups of parthenolide leads to the generation of the parthenolide analogues cyclopropane 4 and micheliolide, which were found to have inhibitory activity on drug resistant AML cells and low toxicity to healthy bone marrow cells. These analogues are also known to induce the generation of ROS (Kempema et al. 2015).

Not only do STLs inhibit NF-κB, but also they exert inhibition of STATs. Using the luciferase reporter system, some authors have observed that two STLs, damsin and coronopilin, isolated from Ambrosia arborescens, are capable of inhibiting NF-κB and STAT3 transcriptional activities in Jurkat and HeLa cells. The expression of NF-κB and STAT3 were induced by TNFα and IFNγ, respectively. In both cases, the IC50 values for damsin were lower than those of coronopilin, indicating that damsin is more effective than coronopilin. The authors hypothesized that the NF-κB signaling pathway can be inhibited by multiple mechanisms, including the activation of the inhibitor of NF-κB kinase (IKK) activity, the activation of the inhibitor of NF-κB (IκB) phosphorylation through IκB degradation, and DNA binding. Some STLs, such as ergolide, artemisinin, costunolide, and zerumbone, have also been reported to have inhibitory effects on the NF-κB pathway (Villagomez et al. 2013).

Leptocarpin (LTC) is a STL isolated from a native Chilean plant, Leptocarpha rivularis, which has been widely used in traditional medicine by Mapuche peoples. It decreases cell viability of cancer cell lines (such as HT-29, PC-3, DU-145, MCF7, and MDA-MB-231) by inducing caspase-dependent apoptosis and through the inhibition of the NF-kB factor, the latter effect being associated with a decreased DNA-binding capacity of this transcription factor. However, the specific mechanism underlying this effect remains unknown (Bosio et al. 2015). Four new STLs, 8α-(2′Z-tigloyloxy)-hirsutinolide, 8α-(2′Z-tigloyloxy)- hirsutinolide-13-O-acetate, 8α-(4-hydroxytigloyloxy)-hirsutinolide, and 8α-hydroxy- 13-O-tigloyl-hirsutinolide, isolated from Vernonia cinerea leaves and steams, are capable of inhibiting the proliferation of U251MG glioblastoma and MDA-MB-231 breast cancer cells by inhibiting aberrant active STAT3 (Joung Youn et al. 2014).

It has been observed that the inhibition of NF-κB and STATs by STLs sensitize tumoral cells to the action of chemotherapeutic agents. When STLs are combined with other treatments, they also sensitize the extrinsic apoptosis pathway by interfering with anti-apoptotic gene expression or by increasing the expression of death receptors. Parthenolide has potent antitumor activity against cancer stem cells (Guzman et al. 2005) and acts synergistically with the treatment of pancreatic adenocarcinoma and on a xenograft model of breast cancer when used in combination with docetaxel or sulindac, respectively (Yip-Schneider et al. 2005; Sweeney et al. 2005).

It is noteworthy that STLs are active on cells that have become resistant to many chemotherapeutic agents. The multidrug resistance is a prevailing phenomenon leading to chemotherapy treatment failure in cancer patients. In this regard, two STLs, neoambrosin and damsin, isolated from Ambrosia maritima (Asteraceae), have been studied on MDR P-gp CEM/ADR 5000 cells P-gp-overexpressing CEM/ADR5000 leukemic cells. The compounds inhibited cell proliferation by silencing the proto-oncogene tyrosine kinase (c-Src) which is a non-receptor tyrosine kinase protein that in humans is encoded by the SRC gene. This protein phosphorylates specific tyrosine residues in other proteins, for example, in tumor cells, the overexpression of c-Src leads to the constitutive activation of PI3K (phosphatidylinositol-3-kinase)/Akt-NF-kB and STATs (STAT3 and STAT5) pathways. As described above, this activation governs cancer development and progression (Saeed et al. 2015).

Sesquiterpene lactones can also exert antiproliferative effects through the inhibition of oncogenes or the inhibition of tumor suppressor gene p53 (related to apoptosis). It has been demonstrated that STLs inhibit the oncogene nucleophosmin-anaplastic lymphoma kinase (NPM/ALK) chimera, which is involved in the majority of anaplastic large cell lymphomas (ALCLs). The furanoheliangolide STL lobatin B, isolated from Neurolaena lobata, downregulates NPM-ALK, and as a consequence, an inhibition of expression of Jun B (which is a transcription factor of the tyrosine receptor kinase PDGF-Rβ) is observed (Kiss et al. 2015).

The other mechanism by which STLs induce death is necrosis. Through this mechanism, some STLs, such as the pseudoguainolide helenalin, kill apoptosis-resistant cells. Helenalin induces necrosis in three cell lines overexpressing Bcl-2 by mechanisms involving free intracellular iron, ROS, and inhibition of NF-kB. This form of necrosis has proved to be independent of receptor-interacting protein kinase 1 (RIPK1) and poly-(ADP-ribose) polymerase1 (PARP1) (Hoffmann et al. 2011).

Differentiation is another mechanism exerted by antitumoral drugs. The STL arsantin, present in Artemisia santolina, induces cell differentiation of the human promyelocytic leukemia HL-60 cell line in a concentration-dependent manner. A cytofluorometric analysis indicated that arsantin induces HL-60 cell differentiation predominantly into granulocytes by increasing the levels of protein kinase C alpha (PKCα) and protein kinase β (PKCβII) isoforms and by increasing the phosphorylation rate MAPKs in HL-60 cells. In addition, arsantin synergistically enhanced the differentiation of HL-60 cells in a dose-dependent manner when combined with low doses of 1–25-(OH)2D3 (Kweon et al. 2015).

In many cases, the antiproliferative activity of STLs in leukemic cells is shown without describing the mechanism of action. This is a case of an amino conjugate of a sulfated guaiane STL isolated from the roots of Scorzonera divaricata. This STL exhibits significant in vitro cytotoxic activity against human cancer cell lines such HL60, HeLa, HepG2, and SMMC-7721, among others (Yang et al. 2016).

In addition, the STL dehydroleucodine, isolated from Gynoxys verrucosa, a species used in traditional medicine in Southern Ecuador, has proved to have an antiproliferative effect against eight acute myeloid leukemia (AML) cell lines, with LD50 values ranging from 5.0 to 18.9 μM. Besides, this STL was active against human AML cell samples from five patients, with an average LD50 of 9.4 μM. It is noteworthy that the compound does not affect the proliferation of normal peripheral blood mononuclear cells. The authors concluded that an exocyclic methylene in the lactone ring is required for the cytotoxic activity (Ordóñez et al. 2016).

The mechanism of action of the three more studied STLs, parthenolide, artemisinin, and thapsigargin, is shown in Fig. 13.2.

Fig. 13.2
figure 2

Mechanism of action of sesquiterpene lactones on tumoral cells. The mechanism of action of the three more studied STLs is shown. STLs diffuse through the plasma membrane and selectively target the SERCA pump, high iron content and cell surface transferrin receptors, NF-kB signaling, and epigenetic mechanisms. Thapsigargin can diffuse into the SERCA pump blocking its ability to transfer Ca2+ from the cytosol into the ER. High cytosolic Ca2+ concentrations lead to the generation of ROS and subsequent cell death. Many tumoral cells have high intracellular Fe2+ contents and high levels of transferrin receptors. The cytosolic Fe2+ binds to the endoperoxide bridge of artemisinin, leading to its activation and the consequent generation of toxic ROS. Parthenolide directly inhibits DNMT1 and causes ubiquitin-mediated proteasomal degradation of HDAC1, leading to expression of HIN-1 and p21, respectively. Parthenolide directly inhibits the NF-kB p65 subunit and the IKK complex preventing IKK-mediated phosphorylation and proteasomal degradation of IkB. Artemisinin also inhibits NF-kB activity. Abbreviations: ER sarcoplasmic/endoplasmic reticulum, HDAC1 histone deacetylase 1, IKK, IkB kinase, IkB inhibitor of NF-kB, NF-kB nuclear factor-kB, ROS reactive oxygen species, SERCA, ER calcium ATPase, Ub ubiquitin. Dashed and solid lines indicate translocation and activation, respectively

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Apart from blood cell malignancies, STLs also have activity on other tumors. For example, parthenolide presents activity against breast cancer and pancreatic carcinoma (Carlisi et al. 2016; Liu et al. 2017); the prodrug of thapsigargin, mipsagargin, has proved to induce apoptosis in prostatic tumors (Mahalingam et al. 2016).

Moreover, eupatolide induces apoptosis in breast cancer cells, and artemisinin is being evaluated in clinical trials for breast and colorectal cancer (see below).

Furthermore, dehydrocostus lactone and costunolide are active on hepatocellular carcinoma and on sarcomas (SW-872, SW-982, and TE-671) (Lohberger et al. 2013). Furthermore, deoxyelephantopin exerts antitumoral activity on breast adenocarcinoma in mice (Kreuger et al. 2012; Merfort 2011). Psilostachyin, psilostachyin C, helenalin, and mexicanin I are active on different tumor cells apart from white blood cells (Martino et al. 2015; Kreuger et al. 2012; Merfort 2011; Sturgeon et al. 2005; Bujnicki et al. 2012).

3 In Vivo Studies

The “in vivo” antitumoral activity of STLs had been demonstrated in animal models. For example, goyazensolide exerts antitumoral activity in an in vivo hollow fiber assay employing HT 29 and MDA-MB 435 human tumoral cells. The fibers were implanted into immunodeficient female NCr nu/nu mice. On the contrary, the STLs 15-deoxygoyazensolide and ereglomerulide did not present antitumoral activity in the same system (Muñoz Acuña et al. 2013). Japonicone A, which is found in the traditional herb Inula japonica, is considered a promising compound. This STL has proved to have potent in vitro and in vivo antitumor activity against Burkitt’s lymphoma cells (Wang et al. 2014). Besides, it has been demonstrated that the treatment of BALB/c mice bearing Ehrlich tumors with a daily dose of the STL diacetyl piptocarphol, isolated from Vernonia scorpioides, produced a decrease on the tumor size and prevented the development of the ascitic tumor. Furthermore, the dichloromethane fraction of Moquiniastrum polymorphum subsp. floccosum, which is rich in STLs, has shown activity against Walker-256 carcinosarcoma in rats. The treatment with the fraction significantly reduced the tumor volume and weight. The fraction also modified hepatic glutathione and superoxide dismutase levels and normalized plasma glucose, alkaline phosphatase, and amylase and the nitric oxide levels in the tumoral tissue. Moreover, the fraction induced apoptosis through the upregulation of the p53 and Bax gene expression. It is noteworthy that no clinical signs of toxicity or death were observed in the rats treated with the fraction (Gomes Martins et al. 2015)

Angiogenesis and metastasis are also two important targets for antitumoral drugs in vivo. Artemisinin has proved to prevent angiogenesis by inhibiting human vein endothelial cell proliferation and vascular endothelial growth factor and receptor expression. DNA microarray analysis correlates tumor inhibition exerted by artemisinin with the reduced expression of crucial angiogenesis-related transcripts, namely, vascular endothelial growth factor, fibroblast growth factor, several matrix metalloproteinases, and hypoxia-inducing factor (Anfosso et al. 2006). Thapsigargin also inhibits microvessel formation and proliferation of human artery endothelial cells in vitro (Shukla et al. 2001). Furthermore, parthenolide has been found to inhibit the expression of matrix metalloproteinase-9 and urinary plasminogen activator and the migration of carcinoma cells in vitro, as well as osteolytic bone metastasis in vivo (Idris et al. 2009).

4 Clinical Trials

Nowadays, some STLs such as artemisinin from Artemisia annua, thapsigargin from Thapsia genus (Apiaceae), and parthenolide from Tanacetum parthenium (feverfew) and/or many of their synthetic derivatives are under evaluation in clinical trials.

These compounds have been selected for their selectivity, which is attributed to their ability to target the sarco-/endoplasmic reticulum (ER) calcium ATPase (SERCA) pump, particular proteases secreted by cancer cells, high iron content and cell surface transferrin receptors, nuclear factor-kB (NF-kB) signaling, MDM2 degradation and p53 activation, angiogenesis, metastasis, and epigenetic mechanisms.

Clinical evidence indicates that artemisinin-derived drugs are promising for laryngeal carcinoma, uveal melanoma, and pituitary macroadenoma. These drugs are in phase I–II trials against lupus nephritis and breast, colorectal, and non-small cell lung cancers (Singh and Panwar 2006). In addition, the artemisinin derivatives, artemether and artesunate, have been subjected for evaluation in cancer clinical trials (Chen et al. 2004). Besides, artesunate was then also incorporated in several clinical trials: in 2008, it was evaluated for the treatment of metastatic breast cancer (Table 13.1); in 2015, a study was undertaken to assess the effect of artesunate administered intravaginally for the HPV-positive high-grade cervical intraepithelial neoplasia (CIN2/3) (Table 13.1); and in 2016, this STL was incorporated in a phase I study involving patients with solid tumors to evaluate its effect after intravenous administration (Table 13.1). In the same year, the safety and pharmacokinetics parameters of artesunate were evaluated in a phase I dose-escalation study in patients with hepatocellular carcinoma. The study was completed in December 2016; however, the results have not been published (Table 13.1). Artesunate will also be evaluated in two clinical trials: one of the trials is not in the recruiting phase yet (March 2017), and the other is already recruiting participants (October 2016) to study the safety and effectiveness of this STL on colorectal cancer and stage II/III bowel cancer (Table 13.1). Furthermore, artemether will also be assessed in a clinical trial (not yet recruiting participants, March 2017) to study its effect on solid tumors (Table 13.1). The orally bioavailable parthenolide analogue dimethyl amino-parthenolide or LC-1 has been subjected to phase I clinical trials against acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), and other blood and lymph node cancers (Guzman et al. 2007).

Table 13.1 Sesquiterpene lactones evaluated in clinical trials
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Thapsigargin-derived drugs are currently being studied in phase I clinical trials for breast, kidney, and prostate cancer treatments. For example, a thapsigargin prodrug (G-202 mipsagargin, which consists of a thapsigargin molecule coupled with a masking peptide that is cleaved at the tumor site) is in phase I clinical trials for advanced solid tumors (Christensen et al. 2009). Mipsagargin has also been evaluated in several clinical trials for the treatment of multiform glioblastoma, advanced adult hepatocellular carcinoma, clear renal cell carcinoma, and prostatic neoplasm (Table 13.1) (Quynh Doan and Christensen 2015). For example, in a study carried out with patients affected with solid tumors [14 patients (32%) with colorectal, 9 (20%) with prostate, and 6 (14%) with hepatocellular carcinoma], the drug presented a good pharmacokinetic profile and was well tolerated. Only 12 (28.6%) of the patients presented a disease stabilization (Mahalingam et al. 2016). The clinical trials with STLs are shown in Table 13.1.

Some other STLs are currently being used for the treatment of different cancer types, for example, arglabin, isolated from Artemisia glabella (Asteraceae), is used for liver, breast, ovarian, and uterus cancer (Adekenov 2016).

5 Conclusion

Sesquiterpene lactones are very attractive compounds to be used in antitumor therapy due to the diverse mechanisms of action they exert, such as ROS generation, epigenetic modulation of gene expression, and targeting of the sarco-/endoplasmic reticulum calcium ATPase (SERCA) pump, the NF-kB signaling pathway, and the p53 pathway. Because of this, sesquiterpene lactones could be useful to treat resistant tumors. In addition, sesquiterpene lactones have proved to inhibit angiogenesis and metastasis. Promising results have also been achieved in animal models, and some sesquiterpene lactones are under study in clinical trials.

It can be concluded that sesquiterpene lactones can exert the antitumor activity mainly by inducing apoptosis through different pathways. All these considerations make these compounds very attractive to be used as antitumor drugs to be used either alone or in combination with other traditional chemotherapeutic drugs.