**8. Dual mTOR/PI3K inhibitors**

evaluated the aromatase inhibitor letrozole in combination with everolimus in patients with

Temsirolimus (Torisel®), the first rapamycin analog to be FDA approved as an anticancer drug, is an intravenous injection drug and gets converted into rapamycin *in vivo* [137]. This drug was valued with bevacizumab or in combination with chemotherapeutic agents in endometrial cancer cell lines, and results showed the increase progesterone mRNA expression and inhibition of ER mRNA expression [138,139]. Also, preliminary phase II study using temsiro‐ limus in patients with metastatic cervical cancer showed positives results [140]. Another phase II clinical study (NCT01196429) evaluates additional effects of the temsirolimus combined with paclitaxel/carboplatin therapy have been conducted in patients with stages III/IV clear cell adenocarcinoma [141]. However, some studies failed to show the efficiency of temsirolimus in patients with persistent/recurrent epithelial ovarian cancer/primary peritoneal cancer showing a modest activity of this mTOR inhibitor, and the results were insufficient to justify

Ridaforolimus (MK-8669/AP23573), a non-rapamycin prodrug, is available in both oral and intravenous formulations. This mTOR inhibitor is actively being evaluated as either mono‐ therapy or in combination with other therapies for treatment of various cancers, including sarcomas, endometrial, prostate, breast, and non-small cell lung cancer [143]. Studies had been conducted in patients with advanced endometrial cancer and clinical benefit response was reported in 33% of the patients [144]. Another phase II study using oral ridaforolimus in patients with advanced or recurrent endometrial cancer also showed partial response in 7.7%

Although clinically promising, the efficacy of rapalogs is partially limited by the negative feedback loops in the mTOR pathway. With this regard, the exclusive inhibition of the mTORC1 complex by the rapalogs compromises the S6K-1-mediated feedback loop towards IRS-1, resulting in the activation of both the PI3K/AKT and the mitogen-activated protein kinase/extracellular signal-regulated kinases (MAPK/ERK) pathways, hence promoting compensatory cell survival, and the acquisition of chemoresistant phenotype [127,146,147]. Efforts have been made to overcome the previously mentioned clinical limitation by means of developing new generation mTOR inhibitors, which inhibit the catalytic activity of both

Although rapamycin is a potent allosteric mTORC1 inhibitor with clinical applications, a second-generation ATP-competitive inhibitor have been developed, including Torin1, Torin2, PP242, PP30, KU0063794, WAY-600, WYE-687, WYE-354, XL-388, INK-128, AZD-2014, AZD8055, and OSI-027 [148-153]. The ATP-competitive inhibitors of mTOR directly inhibit the mTOR kinase activity, affecting both mTORC1 and mTORC2 complexes simultaneously and

metastatic endometrial carcinoma (NCT01068249) and breast cancer (NCT00107016).

further study in a phase III [142].

140 Updates on Cancer Treatment

mTORC1 and mTORC2 complexes.

**7. ATP-competitive inhibitors**

suppress AKT activity.

patients [145].

Scientist have explored to shed light on strategies to overcome the limitations by concomitantly targeting two molecules in the PI3K/AKT/mTOR pathway, PI3K and mTOR, whereas the resistance mTOR inhibitors cloud arise via feedback PI3K activation. This molecular knowl‐ edge have stimulated the development of new inhibitors termed dual PI3K-mTOR inhibitors that include NVP-BEZ235, XL765, BGT226, PI-103, PF-04691502, PKI-587, and GDC-0980 [164-170]. Comparing with the other types of PI3K pathway inhibitors, dual PI3KmTOR inhibitors have the possible advantage of inhibiting all PI3K catalytic isoforms, mTORC1 and mTORC2 [171]. Therefore, these inhibitors may effectively turn off this pathway completely and display best efficacy in feedback inhibition normally observed with mTORC1 inhibitors [172]. However, it is not clear that dual PI3K-mTOR inhibitors will be tolerable at doses that effectively inhibit all p110 isoforms and mTOR [171].

The potential clinical value of the dual PI3K/mTOR inhibitors have been demonstrated by their significant inhibition of cell growth, the induction of apoptosis and/or autophagy [173] in a variety of tumor cancer cells [174-176]. In addition, these inhibitors have shown powerful effects in xenograft models of breast cancer [177], pancreatic cancer [178], melanoma [179], multiple myeloma [180], and RCC [181].

In agreement, dual PI3K/mTOR inhibitors have entered clinical trials either monotherapy or polytherapy. A single agent includes BEZ235/NVP-BEZ235 (NCT00620594) and BGT226 (NCT00600275 and NCT00742105) in advanced solid tumors and breast cancer, GDC-0980 (NCT00854126, NCT00854152, and NCT01455493) in non-Hodgkin lymphoma and endome‐ trial carcinoma, and PF-04691502 (NCT00927823) and GSK2126458 (NCT00972686 and NCT01248858) in solid tumors. In combination with others agents, the treatment includes XL765 (Exelixis) with erlotinib (NCT00777699), letrozole (NCT01082068), and temozolomide (NCT00704080) in non–small cell lung cancer, breast cancer, and gliomas, respectively.

Both BEZ235 and XL765 have shown good tolerability, with adverse effects including diarrhea, anorexia, and nausea [49]. Furthermore, the combined therapy using rapamycin and dual PI3K/mTOR kinase inhibitor (PI-103) has been shown to be efficacious against human ovarian cells *in vivo* [183].



resistance mTOR inhibitors cloud arise via feedback PI3K activation. This molecular knowl‐ edge have stimulated the development of new inhibitors termed dual PI3K-mTOR inhibitors that include NVP-BEZ235, XL765, BGT226, PI-103, PF-04691502, PKI-587, and GDC-0980 [164-170]. Comparing with the other types of PI3K pathway inhibitors, dual PI3KmTOR inhibitors have the possible advantage of inhibiting all PI3K catalytic isoforms, mTORC1 and mTORC2 [171]. Therefore, these inhibitors may effectively turn off this pathway completely and display best efficacy in feedback inhibition normally observed with mTORC1 inhibitors [172]. However, it is not clear that dual PI3K-mTOR inhibitors will be tolerable at doses that

The potential clinical value of the dual PI3K/mTOR inhibitors have been demonstrated by their significant inhibition of cell growth, the induction of apoptosis and/or autophagy [173] in a variety of tumor cancer cells [174-176]. In addition, these inhibitors have shown powerful effects in xenograft models of breast cancer [177], pancreatic cancer [178], melanoma [179],

In agreement, dual PI3K/mTOR inhibitors have entered clinical trials either monotherapy or polytherapy. A single agent includes BEZ235/NVP-BEZ235 (NCT00620594) and BGT226 (NCT00600275 and NCT00742105) in advanced solid tumors and breast cancer, GDC-0980 (NCT00854126, NCT00854152, and NCT01455493) in non-Hodgkin lymphoma and endome‐ trial carcinoma, and PF-04691502 (NCT00927823) and GSK2126458 (NCT00972686 and NCT01248858) in solid tumors. In combination with others agents, the treatment includes XL765 (Exelixis) with erlotinib (NCT00777699), letrozole (NCT01082068), and temozolomide (NCT00704080) in non–small cell lung cancer, breast cancer, and gliomas, respectively.

Both BEZ235 and XL765 have shown good tolerability, with adverse effects including diarrhea, anorexia, and nausea [49]. Furthermore, the combined therapy using rapamycin and dual PI3K/mTOR kinase inhibitor (PI-103) has been shown to be efficacious against human ovarian

**stage Tumor types Reference**

colorectal,

cancer

Solid cancers, prostate,

glioblastoma, SCCHN, non-small cell lung

Non-small cell lung cancer, prostate, breast, GBM, colon

[62,64]

[65,66]

**(company) Drug target Development**

LY294002 - Pan-PI3K inhibitorPreclinical - [57,58] Wortmannin - Pan-PI3K inhibitorPreclinical - [57,59,60]

Pan-PI3K inhibitorPhase III

effectively inhibit all p110 isoforms and mTOR [171].

multiple myeloma [180], and RCC [181].

**Trade name**

(Novartis)

PX-866 (Oncothyreon) Pan-PI3K inhibitorPhase II

cells *in vivo* [183].

142 Updates on Cancer Treatment

NVP-BKM120 Buparlisib

**Inhibitor**



**Table 2.** Overview of PI3K/AKT/mTOR pathway inhibitors.

**Inhibitor**

144 Updates on Cancer Treatment

RAD001

CCI-779

INK128

**Trade name**

Everolimus (Novartis)

Temsirolimus (Wyeth/Pfizer)

PP242 ATP competitive

Torin2 ATP competitive

AZD8055 ATP competitive

OSI-027 ATP competitive

GSK795 ATP competitive

NVP-BEZ235 (Novartis) Dual mTOR/PI3K Phase I/II

MK-8669/AP23573 Ridaforolimus

**(company) Drug target Development**

Inhibits mTOR kinase by binding to FKBP12

Inhibits mTOR kinase by binding to FKBP12

Inhibits mTOR kinase by binding to FKBP12

inhibitor of mTOR

inhibitor of mTOR

inhibitor of mTOR

inhibitor of mTOR

ATP competitive inhibitor of mTORPhase I

inhibitor of mTOR

Phase I/II/III (FDA has approved for RCC, 2009)

Phase I/II/III (FDA and European Medicine Agency have approved for RCC, 2007)

Studies *in vitro* and *in vivo*

Studies *in vitro*

Phase I

Phase I

Phase I

Phase I/II/III Sarcoma, bone,

**stage Tumor types Reference**

carcinoma

Non-small cell lung cancer; advanced solid tumors, metastatic renal cell carcinoma, hepatocellular carcinoma, cervical cancer, clear cell adenocarcinoma

endometrial cancer [144,145]

tumors, lymphoma [183,184]

tumors, lymphoma [185]

Colon cancer, acute myeloid leukemia [156]

and *in vivo* Lung cancer [159]

Advanced solid

Advanced solid

Glioblastoma, advanced solid tumors.

Advanced solid tumors

Advanced solid tumors, breast cancer, prostate cancer

Metastatic renal cell carcinoma, breast cancer, melanoma, ovarian cancer, neuroendocrine tumors of the pancreatic origin (PNET), endometrial

[56,133,134,135,13 6], NCT01939418, NCT01931163

[138,139,141]

NCT02142803

NCT00620594

[134]

[94],

#### **9. Conclusions/future perspectives**

Advances in molecular research have resulted in an improved understanding of cancer biology. There is strong preclinical rationale to support the continued development of PI3K/AKT/mTOR inhibitors, especially in some genetically defined cancer subtypes that may be the most sensitive to single-agent PI3K pathway inhibitors. These include cancers with PIK3CA activating mutations, mutations in PIK3R (p85 subunit), mutations or amplifications in one of the AKT isoforms or loss of PTEN. However, rational clinical trials design with a focus in identifying a patient population most likely to benefit from this strategy is imperative to the success of single-agent therapeutics.

The combination of PI3K/AKT/mTOR inhibitors with cytotoxic chemotherapy and other biological agents such as anti-HER2 compounds, EGFR inhibitors, and antiangiogenic agents may optimize the action of those agents in different pathways that control protein translation, cell growth, migration, metastasis, and angiogenesis. The successful development of the combinations will require determining the duration, doses, and schedules of targeted therapy and how to best incorporate it into standard treatment protocols. Several clinical trials are underway to prove the clinical use of the PI3K/AKT/mTOR inhibitors. The druggability of the components of the PI3K/AKT/mTOR signaling cascade, in addition to the enlightenment of the mutational landscape of human cancers, which points to the high frequency of genetic alterations and anomalous activation of the pathway, strongly suggests that targeting its elements might represent a useful treatment strategy in the fight against cancer.
