Introduction

PI3K/AKT/mTOR signaling pathway participates in a wide range of cell biological processes such as cell growth, cell survival, etc. In the PI3K/AKT/mTOR signaling pathway, PI3K stands for phosphatidylinositide 3-kinases (s), AKT for a serine/threonine kinase or protein kinase B, and mTOR for mammalian target of rapamycin. The majority of cancers show hyperactivation of this pathway [1]. mTOR, a 289-kDa serine-threonine protein kinase, acts as a central regulator of cell proliferation, growth, metabolism, survival, and autophagy [2]. The signaling pathway of this mediator gets activated in a variety of cellular physiological processes including tumorigenesis, angiogenesis, insulin resistance, and T cells activation [3]. Besides, mTOR as a downstream kinase of phosphatidylinositol 3-kinase/AKT (PI3K/AKT) signaling pathway is closely paired with neoplastic disorders [4]. This pathway is a key cascade downstream of different protein kinases such as epidermal growth factor receptor (EGFR) family members (Fig. 1) [5]. It is well known that mTOR plays a major role in the inactivation of the translation suppressor eukaryotic initiation factor 4E-binding protein 1 (4E-BP1, PHAS1) and activates the ribosomal p70S6 kinase through phosphorylation [6]. To that end, mTOR is a promising option for the clinical development of effective molecular targeted therapies. Disruption of mTOR and other protein molecules in this signaling pathway often takes place in various human cancers and type 2 diabetes [7]. Moreover, tumor cells are more susceptible to mTOR inhibitors in comparison with normal cells [8]. Different reports suggested the correlation of mTOR signaling pathway hyperactivity with colorectal cancers [9], adenocarcinomas [10], glioblastomas [11], and astrocytomas [12], which leads to their higher sensitivity to mTOR inhibitors.

PI3K/AKT/mTOR signaling pathway

PI3Ks, as a family of intracellular lipid kinases, phosphorylate the 3′–OH functional group of phosphatidylinositides (PIs). Three different classes of PI3Ks are discovered. The most prominent one is class 1, which converts membrane-bound PI-(4,5)- bisphosphate [PI(4,5)P2; PIP2] to PI-(3,4,5)- trisphosphate [PI(3,4,5)P3; PIP3]. PIP3 is considered a secondary messenger that triggers the recruitment and activation of kinases mainly those that possess pleckstrin homology (PH) domain, as an example PI3K-dependent kinase-1 (PDK1) [13]. Phosphatase and tensin homolog (PTEN) as a naturally occurring negative regulator of PI3K/AKT/mTOR pathway effectively suppresses tumor (Fig. 1). PTEN can dephosphorylate PIP3; hence, it determines the signaling duration of PIP3. PTEN shows mutation or deletion in a wide range of human cancers; and in some cancers, it is epigenetically silenced [14]. The downstream kinase of PI3K is AKT [a serine/threonine kinase or protein kinase B (PKB)]. The complete activation of AKT happens when PDK1 and mTORC2 phosphorylate its T308 and S473 residues. The activated AKT phosphorylates a wide range of other enzymes including glycogen synthase kinase 3 (GSK3), tuberous sclerosis 2 (TSC2), caspase 9, and PRAS40 (AKT1S1) [15]. To that end, AKT is involved in a vast range of cellular activities such as cell proliferation, differentiation, apoptosis, angiogenesis, and metabolism. mTOR is a downstream kinase of PI3K/AKT which belongs to the PI3K-related kinase family (PIKK). This mediator is involved in at least two structurally different multi-protein complexes namely mTORC1 and mTORC2 [16, 17]. mTORC1 consists of mTOR (the catalytic subunit), Raptor (regulatory-associated protein of mTOR), mLST8 (mammalian lethal with Sec13 protein 8 also known as GβL), Deptor (DEP-domain-containing mTOR-interacting protein), and PRAS40 (proline-rich AKT substrate 40 kDa) [16, 18]. The mTORC2 includes mTOR, mLST8, Deptor, Rictor (rapamycin-insensitive companion of mTOR), mSIN1 (mammalian stress-activated protein kinase interacting protein 1 or MAPKAP1), and Protor-1 (protein observed with Rictor-1) [17]. mTORC1 can be regulated by various environmental signals such as hormones, growth factors, nutrients, and cellular stress, in which other mediators such as RHEB (RAS homolog enriched in brain), guanosine triphosphatases (GTPase), and TSC1/2 display a crucial role [19]. Moreover, the PI3K/AKT signaling pathway is a critical upstream regulator of mTORC1 [20]. mTORC1 is a rapamycin and nutrient-sensitive complex which can influence the cell-growth and survival by increasing the biosynthesis of protein. Protein synthesis can be regulated by mTORC1 through phosphorylation of S6K1 and 4E-BP1 (Fig. 1) [21]. mTORC2 is also called the rapamycin-insensitive complex and can be involved in the regulation of the cytoskeletal organization. It can also play a role in the regulation of cell survival and proliferation by phosphorylation and activation of AKT while the exact downstream of mTORC2 is unclear [22]. It is worth noting that Tti1 is another positive regulatory protein for the mTOR complexes. Tti1 interacts with telomere maintenance 2 (Tel2) and regulates the stability of phosphatidylinositol 3-kinase-related protein kinase family members (Fig. 1) [23].

Fig. 1
figure 1

The PI3K/AKT/mTOR signaling pathway: ligand binding or ionizing radiation triggers the activation of PI3K, which leads to the phosphorylation of phosphotatidylinositol-4,5-bisphosphate (PIP2) and production of phosphatidylinositol-3,4,5-trisphosphate (PIP3). PTEN antagonizes the kinase activity of PI3K by dephosphorization of PIP3. As a phosphate source PIP3 transfers a phosphate group to AKT and activates it. AKT has different regulatory sites that can be phosphorylated by the mTORC2 complex. AKT as a kinase phosphorylates different downstream targets such as mTORC1 which eventually results in the cell growth, survival, and proliferation

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Ionizing radiation, radioresistance, and PI3K/AKT/mTOR signaling pathway

Radiation resistance is a fundamental problem in radiotherapy. Understanding the mechanisms and signaling pathways of radioresistance plays a vital role in the development of strategies to overcome tumor radioresistance in cancer therapy. These strategies include at least two different approaches such as radiotherapy and chemotherapy (for instance mTOR signaling pathway inhibitors). The rapid development in biomedical science through advanced techniques like genomics and proteomics caused a huge amount of attention toward radioresistance related signaling pathways. The theory of radiation therapy is that DNA damages [such as single-strand breaks (SSBs) and double-strand breaks (DSBs)] result in apoptosis and subsequently tumor regression [24]. However, activation of some major pathways such as PI3K/AKT/mTOR signaling pathway can increase tumor cell survival even after radiation and jeopardizes the efficacy of the therapy procedure. In the presence of radiation, this signaling pathway induces tumor cell proliferation, growth, and survival. Different studies suggest that PI3K/AKT pathway controls the cell cycle progression and cell proliferation through translation of specific mRNA transcripts taking advantage of its downstream mediator mTOR. For instance, mTOR increase the expression of receptor tyrosine kinases such as EGFR (Fig. 1) [25, 26].

Strategies to inhibit PI3K/AKT/mTOR signaling pathway

A wide range of small molecules has been introduced as PI3K/AKT/mTOR signaling pathway inhibitors. These compounds fall into two major groups including single inhibitors that only inhibit PI3K, AKT, or mTOR signaling mediators and dual inhibitors that simultaneously inhibit PI3K and mTOR signaling proteins. The inhibition of PI3K/AKT/mTOR signaling pathway mediators make cancer cells more susceptible to radiotherapy, anti-EGFR treatment, and other receptor tyrosine kinase (RTK) inhibitor therapies.

ATP-competitive mTOR kinase inhibitors

A new Group of mTOR inhibitors, that bind to the ATP binding region of mTOR and suppress the kinase-dependent signaling activities of mTOR complexes, are known as ATP-competitive mTOR kinase inhibitors (TKIs). The advantage of these compounds is that they not only block the kinase activity of both mTORC1 and mTORC2, but also inhibits the feedback activation of the PI3K/AKT pathway [27]. These inhibitors include WYE-354 [28], WYE-687 [29], PP242 [30], Torin 1 [31], and KU-0063794 [32] which display a potent inhibition of mTOR pathway. Like rapamycin derivatives, TKIs can decrease protein translation, attenuate cell proliferation, and block angiogenesis in several cancer cell lines. In fact, cell proliferation inhibition by TKIs inhibitors is more potent than rapalogs [33].

Dual mTOR/ PI3K inhibitors

The interaction of mTOR with the PI3K signaling network triggers the development of dual mTOR/ PI3K inhibitors. These inhibitors target the PI3Kα, β, and γ isoforms as well as the ATP-binding sites of both mTORC1/mTORC2; hence, they suppress the PI3K/AKT/mTOR signaling pathway. Blocking the biological function of PI3K/mTOR signaling pathway components has been shown to inhibit tumor cell proliferation. Apoptosis and autophagy induction has also been observed in cell treatment with these compounds [34].

Rapamycin and its analogs (rapalogs)

Rapamycin, a potent mTORC1 inhibitor, leads to the inhibition of cell growth, cell proliferation, and cell cycle progression. However, due to the slight aqueous solubility and stability, its application is limited for cancer treatments [35]. Therefore, different rapamycin analogs (rapalogs) with more desirable pharmaceutical properties have been developed, such as CCI-779 (Temsirolimus), RAD001 (Everolimus), AP23573 (Deforolimus), SAR943 (32-deoxorapamycin) or ABT-578 (Zotarolimus) for cancers [36, 37], allergic inflammation [38, 39] or coronary stent implantation [40]. Preclinical studies confirmed their antiproliferative effect on several types of cancer. Furthermore, clinical studies have revealed favorable anticancer effectiveness in certain cancers [37, 41]. The mechanism of rapamycin action is not completely understood; however, it is admitted that rapamycin and its analog strongly bind to the FKBP12-rapamycin binding (FRB) domain and inhibits the mTORC1 assembly and its kinase activity [42]. In contrast to mTORC1, rapamycin and its analog have a much lower affinity toward mTORC2; however, prolonged drug exposure has been demonstrated to inhibit mTORC2 formation. Unlike mTORC1 inhibition, prolonged rapamycin treatment indirectly inhibits mTORC2. Rapalogs finally exert their impacts by causing changes in multiple downstream pathways including cell proliferation and angiogenesis. A possible mechanism for the antiproliferative impacts is the prevention of 4E-BP1 and S6K1 phosphorylation by mTORC1 [43].

The combination of mTOR inhibitors and radiation

mTOR inhibitors combined with radiotherapy can be favorable agents for cancer therapy [44, 45].

Rapamycin

Rapamycin is a macrolide compound that binds to FKBP-12 (FK506 binding protein 12 which plays a vital role in immunoregulation, essential cellular processes including membrane protein folding and cellular trafficking). Rapamycin, the allosteric inhibitor of mTOR, prevents major phosphorylation of proteins that are involved in the process of transcription, translation, and cell cycle regulation [46]. A great number of cancer cells with mutations in the PI3K/AKT-mTOR pathway are susceptible to the cell growth blocking induced by rapamycin. Rapamycin inhibited tumor growth in glioma xenografts treated with fractionated stereotactic radiotherapy. The local control of tumor cells can also be improved via the anti-angiogenesis effects induced by rapamycin during radiation therapy. Rapamycin significantly increased the effectiveness of fractionated radiation therapy in U87 (glioma cell line) xenograft models [47]. Dumont et al. have demonstrated that rapamycin combined with 177Lu-RM2 (37 MBq, RM2 is a DOTA-conjugated gastrin-releasing peptide receptor antagonist) resulted in prolonged survival in comparison with single agents (Table 1) [48]. A combination of rapamycin treatment and 137Cs (6.4 Gy/min) was investigated in a glioblastoma multiforme (GBM) model using U87 xenograft models. The treatment with low-intensity radiation (16 Gy/18 days) did not have considerable effects on tumor growth rates (regrowth delay of − 0.2 ± 4.6 days) compared with control in the xenograft models. However, rapamycin in combination with radiation therapy considerably improved the delayed regrowth (19.1 ± 6.3 days) in comparison with rapamycin alone; Hence, combination treatment with rapamycin and radiation was noticeably more  effective than both radiation alone and rapamycin [49]. Rapamycin at different concentrations (1–8 nM) plus radiation (dose rate of 300 cGy/min) was studied in glioblastoma cells. This study suggests that the pre-treatment of U87, T98, and A172 glioblastoma cells with rapamycin decreased survivin expression and clonogenic capacity which leads to more radiosensitivity of these cell lines to radiotherapy (Table 1) [50]. A phase I/II clinical study on rectal cancer patients evaluated the feasibility of short-course radiotherapy (SCRT) in conjugation with rapamycin. Phase I of this study in 13 patients determined that the maximum tolerated dose (MTD) of rapamycin is 6 mg. Phase II of the clinical study evaluated the combination of rapamycin (6 mg) and radiotherapy in a population of 31 rectal cancer patients. This study recommends that SCRT and rapamycin combination is feasible and safe and resulted in tumor size shrinkage, while due to the small sample size no significant complete pathological response was observed. However, positron emission tomography (PET) scans following phase II treatment confirmed a significant (p < 0.05) reduction in tumor size (Table 2) [51].

Table 1 Preclinical studies investigating the effect of combined PI3K/AKT/mTOR inhibitors and radiation on different cell lines
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Temsirolimus

Temsirolimus (also called CCI-779) is the first FDA-approved mTOR inhibitor for advanced renal cell cancer treatment. Meanwhile, its clinical development also includes research studies in different tumors [52]. Ekshyyan et al. investigated the temsirolimus capability to radio-sensitize head and neck squamous cell carcinoma (HNSCC) in both in vitro and in vivo models [53]. Temsirolimus combined with radiation therapy was far more impressive in tumor cell growth inhibition in comparison with cisplatin plus radiation in murine xenografts. Studies displayed that radiation in the presence of temsirolimus sharply reduced radiation-induced activation of the mTOR signaling pathway, enhanced apoptosis, and decreased the number of blood vessels in tumors [54]. The radiosensitivity of A549 cells was studied under both normoxia and hypoxia conditions by using temsirolimus. A549 cells revealed a radio-resistance of 5.1 Gy under normoxia and 14.2 Gy under hypoxia, as the D10 values, the oxygen enhancement ratio (OER) was also 2.8. The A549 cell line survival rate with temsirolimus under hypoxia conditions significantly decreased in comparison with those in normoxia. The D10 values were 4.8 and 5.4 Gy in normoxia and hypoxia conditions, respectively (OER = 1.1). Temsirolimus suppressed the expression of mTOR in both normoxia and hypoxia. The HIF-1α expression also decreased under hypoxia by temsirolimus. Therefore, temsirolimus could overcome the cancer radio-resistance caused by hypoxia [55].

Everolimus

Everolimus, also known as RAD001, is the 40-O-(2-hydroxyethyl) analog of rapamycin that acts similarly to rapamycin as a mTOR inhibitor. Cao et al. studied the capability of everolimus to improve radiation therapy effects on two CaP (prostate cancer) cell lines, PC-3 and DU145, and demonstrated that both cell types became more sensitive to radiation after everolimus therapy; however, PC-3 cells showed higher radiation sensitivity [56]. YU et al. evaluated the effects of everolimus treatment after irradiating the SCC4 cells with ionizing radiation. RAD001 improved the radio-sensitivity of SCC4 cells treated using 6 Gy radiation without any effect on cell death. Everolimus inhibited the mTOR, and S6, as well as 4E-BP1 phosphorylation and reduced the tumor cells’ clonogenic survival after irradiation. Moreover, the combination of everolimus (300 nM) and 6 Gy radiation considerably suppressed SCC4 cell growth in comparison with both 6 Gy radiation alone and everolimus (30 nM) combined with 6 Gy radiation [57]. The effect of the combination of Z-DEVD (caspase-3 inhibitor), everolimus, and 137Cs (1.8 Gy/min) was studied on H460 (a human large-cell lung carcinoma line) cell line. In comparison with radiation alone, the combination treatment substantially delayed tumor growth (32 vs. 23 days). The combination treatment resulted in a sharp reduction of the proliferation index (by reducing proliferative marker Ki67 levels) compared with radiation therapy alone (13 ± 2 vs. 38 ± 1.5). These findings revealed that a combination of Z-DEVD and everolimus can enhance the radiosensitivity in H460 cells (Table 1) [58].

Table 2 Clinical trials of combined PI3K/AKT/mTOR inhibitors and radiation on different patients
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The effect of everolimus in combination with radiation and fluorouracil (5-FU) on rectal cancer patients was evaluated during the phase I/II clinical trial. The MTD for everolimus was 10 mg. the patients were treated with everolimus14 days before and during chemoradiotherapy (four weeks). No significant complete pathological response was observed (Table 2) [59].

Everolimus in combination with radiation was studied during a phase-I clinical trial for the treatment of locally advanced prostate cancer. The study determined that MTD of everolimus is 7.5 mg/day, while due to the minor side effects (Dose-limiting toxicity occurred in two out of fifteen patients) the administration of a smaller dose (5 mg/day was) recommended [60]. In parallel with this study other reports also mentioned that everolimus is well tolerable below 10 mg/day (Table 2) [61].

A clinical trial phase-I investigated the combination of everolimus, cisplatin, and pelvic radiotherapy for the treatment of locally advanced cervix cancer. In this study patients received everolimus 7 days prior and during the radiotherapy or brachytherapy (2.5, 5, and 10 mg/day) and the MTD was determined to be 5 mg/ day (Table 2) [62].

A trial phase-I and two clinical trials phase-II investigated the administration of everolimus for the treatment of glioblastoma multiform (GBM). All these studies applied a treatment regimen composed of everolimus, radiotherapy and, temozolomide. Clinical trial phase-I indicated that MTD of everolimus is 10 mg/day (70 mg/week) (Table 2) [63]. A phase-II clinical trial indicated that everolimus does not improve the efficacy of standard chemoradiotherapy with radiotherapy/temozolomide. Moreover, the study indicated the administration of 70 mg/week (MTD = 10 mg/day) of everolimus leads to moderate toxicity [64]. Another similar clinical trial phase-II indicated that the addition of everolimus (10 mg/day, 70 mg) to standard chemoradiotherapy does not improve progression-free survival (Table 2) [65].

The feasibility of everolimus combination with thoracic radiotherapy followed by consolidation chemotherapy in untreated non-small-cell lung cancer (NSCLC) was also investigated. The patients were treated with 50 mg/week of everolimus (according to other studies). The treatment appears to be safe and feasible while the authors recommend close observation for pulmonary toxicity [66].

Fig. 2
figure 2

The chemical structure of PI3K/AKT/mTOR signaling pathway inhibitors

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Voxtalisib (SAR245409, XL765)

Voxtalisib is a dual PI3K/mTOR inhibitor (Fig. 2). The effect of voxtalisib has been evaluated against a panel of glioblastoma multiform (GBM) cell lines and xenografts in combination with temozolomide [67]. A phase-I clinical trial evaluated the effect of voxtalisib on patients with high-grade glioblastoma in combination with temozolomide, with or without irradiation. The administration of voxtalisib was evaluated to be safe with MTD of 90 mg/day and moderate inhibition of PI3K/mTOR signaling pathway (Table 2) [68].

Dactolisib (NVP-BEZ235)

Dactolisib, an imidazo[4,5-c]quinoline analog, is a dual PI3K/mTOR inhibitor and inhibits radiation-induced DSBs repair by inhibition of DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and ataxia-telangiectasia mutated (ATM) in GBM cell lines [69]. Cerniglia et al. studied the effect of dactolisib in head and neck cancer cells (Fig. 2) [70]. Dactolisib impressively increased radio-sensitivity in HNSCC cells and weakened the DNA repair processes by decreasing the DNA-dependent protein kinase, catalytic subunit (DNA-PKCS) phosphorylation. Dactolisib therapy induces autophagy and the combination of this compound with radiation in the presence of autophagy inhibitors enhanced cell death (Table 1) [71].

The studies also suggest that dactolisib sensitize colorectal cancer (CRC) cell lines (HCT116, SW 620, and HT29) to radiation. In the presence of 5 Gy radiation (up to three hours), the phosphorylated AKT and mTOR were gradually increased which resulted in radioresistance. The combination of radiation and dactolisib increased the radiosensitivity of all the CRC cell lines. The study suggests that dactolisib reduces cell proliferation by lowering the phosphorylation of DNA-dependent protein kinases (DNA-PKs) and Ribosomal Protein S6 (rpS6) (a protein involved in cell growth and proliferation) by inhibiting the AKT and mTOR phosphorylation. A significant (p-value < 0.001) reduction in tumor size was observed when an HCT116 xenograft received a combination of radiation and dactolisib [72]. Another similar study on the same CRC cell lines and xenograft indicated that prolonged administration of dactolisib after radiotherapy leads to enhanced apoptosis and inhibits DSB repair pathways (Table 1) [73].

Other reports suggest dactolisib improves radiosensitization of radioresistant prostate cancer (CaP) cell lines including (PC-3, DU145 and, LNCaP). This study concluded that dactolisib as a dual PI3K/mTOR inhibitor in combination with radiation is more effective against CaP cell lines in comparison with single inhibitors of PI3K/mTOR signaling pathway such as BKM120 and Rapamycin. After treatment with dactolisib, all the mentioned cell lines displayed lower colony formation, higher and cell cycle attest at G2/M phase. Moreover, DSB of DNA was increased and it reduced the expression level of proteins responsible for inactivation of cell cycle checkpoint and DNA repair [including non-homologous end joining (NHEJ) and homologous recombination (HR)] [74]. Other studies also confirmed that dactolisib causes cell cycle arrest at the G2/M phase and improves the radiosensitivity of PC3 and DU145 cell lines to radiation which is independent of oxygen concentration or PTEN mutation status (Table 1) [75, 76].

Breast cancer cell line (MDA-MB-231 and, MCF-7) displayed an improved radiosensitivity after treatment with dactolisib. This compound is a dual PI3K/mTOR inhibitor; hence, the observed radiosensitivity was independent of oxygen condition (hypoxia, normoxia and, reoxygenation) [77]. Wang et al. reported that dactolisib in combination with radiation increases autophagy, apoptosis, and cell cycle attest of glioma stem cells (SU-2 cell line) [78]. It is noteworthy, based on the drug and ionizing radiation schedule the observed effect of NVP-BEZ235 can vary from a strong radiosensitizer to a cytostatic agent in glioblastoma cells (including U87-MG, GaMG, and DK-MG) [79].

Gedatolisib (PF-05212384, PKI-587)

Gedatolisib is a dual PI3K/MTOR inhibitor (Fig. 2). A study claimed that gedatolisib significantly radiosensitizes head and neck squamous cell carcinoma cell lines (UMSCC1-wtP53 and UMSCC46-mtP53) and delays tumor growth of UMSCC1 xenograft in combination with fractionated radiation [80]. Another study on different head and neck squamous cell carcinoma cell lines (UT-SCC-14 and UT-SCC-15) reported that gedatolisib has no additive effect neither in vitro nor in vivo when combined with radiotherapy. However, gedatolisib can significantly reduce the phosphorylation of key components in the PI3K/mTOR signaling pathway (Table 1) [81].

Vistusertib (AZD2014)

Vistusertib is a selective inhibitor of mTOR with IC50 of 2.8 nM (Fig. 2). Kahn et al. determined the effects of AZD2014 on the radiosensitivity of glioblastoma stem-like cells (GSCs). They found that AZD2014 plus X-ray (dose rate of 2.3 Gy/min) resulted in the inhibition of mTORC1 and mTORC2 complexes. The combination of AZD2014 and radiation considerably enhanced survival in comparison with AZD2014 and radiation treatment alone. AZD2014 improved the radiosensitivity of GSC cells (CD133 and CD15 positive cells) by DNA repair inhibition (Table 1) [82].

Sapanisertib (INK128)

Sapanisertib is an experimental mTORC1 and mTORC2 inhibitor with IC50 of 1 nM (Fig. 2). Hayman et al. studied the effects of sapanisertib on the radiosensitivity of pancreatic carcinoma cells. Sapanisertib inhibited the activity of both mTOR1 and mTOR2 in 3 pancreatic tumor cells (PSN1, Panc1, and Miapaca-2). sapanisertib inhibited the DNA repair and formation of the cap complex in cancer cells sapanisertib in combination with radiation caused an enhancement of tumor radiosensitivity (Table 1) [83].

Torkinib (pp242)

Torkinib is a potent and selective mTORC1 and mTORC2 ATPcompetitive inhibitor with IC50 of 8 nM (Fig. 2). Connolly et al. evaluated the effects of mTOR inhibitors (pp242 and RAD001) in combination with and radiation in IBC xenografts. It is suggested that pp242 significantly inhibited mTOR1 and mTOR2 activity in IBC cells and blocked the upregulation of both AKT and ERK. However, RAD001 inhibited mTORC1 activity and led to AKT activation. Torkinib plus radiation showed a high degree of tumor control and improved radiosensitivity in IBC cells in vitro, in comparison with RAD001 (Table 1) [84].

Torin 2

Torin 2 is a selective mTOR ATP-competitive inhibitor (Fig. 2). Jia Luo et al.  studied the effect of Torin2 on the radio-sensitivity of MCF7 cells (breast cancer cell line). They found that the activity of mTOR was repressed in MCF-7 cells treated with 20 nM Torin 2 before irradiation with X-rays at (0–10 Gy ≤ 1 min). Torin 2 pretreatment reduced the radiation-induced AKT/mTOR pathway activation. Moreover, Torin 2 partially blocked the DNA repair by reducing the ataxia telangiectasia mutated (ATM) activation and caused growth inhibition in MCF-7 cells. It also improved the radiosensitivity of MCF-7 cells (Table 1) [85].

AZD8055

AZD8055 is a potent ATP-competitive inhibitor of mTORC1 and mTOR2 with an IC50 value of 0.8 nm (Fig. 2). Chang et al. evaluated the effect of AZD8055 on radiosensitivity of nasopharyngeal carcinoma (NPC) cells. They suggested that the combination of AZD8055 with radiation markedly enhanced the inhibitions of both cell proliferation and clonogenicity. In contrast to the application of ionizing radiation alone, AZD8055 combined with 4 Gy radiation resulted in 2.63-fold and 3.62-fold reductions in the survival fractions (SF) of the CNE1 and CNE2 cells respectively. Combining IR with AZD8055 delayed tumor growth and induced considerably far more apoptosis in comparison with IR and AZD8055 alone. The results displayed that 5 mg/kg/d AZD8055 induced good radiosensitivity for NPC (Table 1) [86].

SF1126

SF1126 is a peptidic pro-drug inhibitor of both PI3 kinase and mTOR. Shu et al. evaluated the effect of SF1126 on Glioma cells. All glioma cells (SF767, LNpuro, and LNvIII) displayed considerable improvement of radiosensitivity after treatment with SF1126. Moreover, SF1126 demonstrated direct effects on glioma tumor cells. SF1126 significantly inhibited tumor growth (Table 1) [87].

3-Methyladenine

3-Methyladenine (3-MA) is a PI3K autophagy inhibitor (Fig. 2). Chen et al. studied the effects of 3-MA on radio-sensitivity of EC9706 cells. 3-MA impressively inhibited the autophagy induced by radiation therapy. Autophagy inhibition enhanced cell apoptosis and reduced tumor proliferation and resulted in a significant increase in the radio-susceptibility of the tumor cells (Table 1) [88].

LY294002

LY294002 (2-4-morpholinyl-8-phenlchromone) is a cell-permeable inhibitor of PI3Ks (Fig. 2). It is reported that LY294002 as an inhibitor of PI3K sensitizes the PTEN mutant glioma cell line U251 MG to radiation [89]. Kao et al. evaluated the effect of LY294002 on the radiosensitivity of U251 cells. Inhibition of PI3K signaling pathway by LY294002 treatment resulted in suppression of DNA repair, which led to tumor cell death. Hence, it made the cells more sensitive to ionizing radiation (Fig. 2; Table 1) [90]. It is noteworthy that the toxicity of LY294002 makes it unfavorable for clinical trials [91]. Another study tested the effect of LY294002 in combination with irradiation on eight GBM cell lines. Among these cell lines only U87MG, MO59J, and LN18 display increased phosphorylation of AKT after irradiation (single dose, 6 Gy). The treatment of Y294002 before irradiation completely abolishes phosphorylation of AKT in the U87-MG cell line [92]. A combination of LY294002, gemcitabine hydrochloride, and ionizing radiation effectively inhibited vasculogenic mimicry formation and matrix metalloproteinase-2 mRNA expression of Panc-1 cells line. This combination was also effective against tumor growth in orthotopic Panc-1 xenografts by suppressing PI3K/MMPs/Ln-5γ2 signaling pathway [93].

PI-103

PI-103 is a potent ATP-competitive inhibitor of PI3K/AKT and mTOR (Fig. 2) [94]. Prevo et al. investigated the effect of PI-103 on the radio-sensitivity of HCT116 and DLD1 cancer cells. Findings revealed that the inhibition of PI3K causes a reduction in AKT phosphorylation. AKT activation played a major role in cell survival reduction. PI-103 combined with radiation increased delay in the G2/M phase. PI-103 was found to improve radiation therapy efficacy and radiation-induced apoptosis (Table 1). PI-103 prolongs DNA DSB repair by raising the levels of γ-H2AX and reduces cell proliferation [95]. A similar study on colon carcinoma cell lines (SW480 and SW48) revealed that treating these cell lines with a combination of PI-103 and NVP-AUY922 (Heat Shock Protein 90 (HSP-90) inhibitor) three hours ahead of radiotherapy (the treatment was continued for 24 h) effectively increases the radiosensitivity [96].

Omipalisib (GSK2126458)

Omipalisib is a potent and selective inhibitor of PI3K/mTOR (Fig. 2). Liu et al. evaluated the effect of both omipalisib and gedatolisib on the radiosensitivity of NPC cells. Omipalisib and gedatolisib combined with ionizing radiation effectively inhibited the proliferation of NPC cells. Both inhibitors increased the radiosensitivity of cells by DNA damage enhancement, delaying the G2/M phases, and enhanced cell apoptosis. The combination of omipalisib or gedatolisib with radiation considerably induced inhibition of tumor growth (Table 1) [97].

Palomid-529

Palomid-529 (8-(1-Hydroxy-ethyl)-2-methoxy-3-(4-methoxy-benzyloxy)-benzo[c]chromen-6-one) (Fig. 2), inhibits both TORC1 and TORC2. It also blocks phosphorylation of AKT at S473 and reduces tumor growth, angiogenesis, and vascular permeability in the xenograft mouse model of glioblastoma (GB) [98]. A study indicated that palomid-529 possesses anti-tumor activity against a large variety of cell lines (NCI-60 cell lines panel). This compound increased the radiosensitivity of NCI-60 cell lines panel and reduced the level of phosphorylated AKT (pAKT), matrix metalloproteinase (MMP)-2, and VEGF. The treatment of the PC-3 xenograft model with a combination of palomid-529 (20 mg/Kg) and radiation (6 Gy) led to 77.4 % tumor shrinkage while using palomid-529 (20 mg/Kg) or radiation (6 Gy) shrank  the tumor size by 42.9 and 53 %, respectively (Table 1) [99].

HS-173

Ethyl 6-(5-(phenylsulfonamido)pyridin-3-yl)imidazo[1,2-a] pyridine-3-carboxylate (HS-173) (Fig. 2), is a new PI3Kɑ inhibitor. HS-173 in combination with radiation inhibits radiation-induced DNA breaks repair, G2/M cell cycle arrest, and apoptosis in Miapaca-2 and PANC-1 pancreatic cell lines (Table 1) [100].

Conclusion

Radiotherapy reduces both tumor size and cancer recurrence. However, due to the radioresistance of several cancer cell lines, effective cancer treatment utilizes suitable adjuvants. Adjuvants utilized in radiation therapy should increase the radiosensitivity of tumor cells. Cancer cells might be resistant to apoptosis especially through PI3K/AKT/mTOR signaling pathway. Therefore, mTOR pathway inhibition could impressively weaken radioresistance and make cancer cells supersensitive to apoptosis. Noteworthy, dual inhibitors of PI3K/AKT/mTOR signaling pathway seem to be superior to single component inhibitors in radiosensitizing different cell lines. Hence, the combination of such inhibitors with radiation can be considered as more effective adjuvants to increase the sensitivity of cancer cells to radiation.