The phosphatidylinositol 3-kinase (PI3K) signaling pathway is involved in the survival, growth, metabolism, motility, and progression of cancer and is a critical pathway in cancer (21). The PI3K family of proteins catalyzes the phosphorylation of phosphatidylinositols (PtdIns) at their 3’ position and consists of classes I, II, and III. Only class IA signaling aberrations are involved in human cancers (22). The class IA PI3Ks are composed of heterodimers of regulatory subunits (p85α, p85β, p50α, p55α, and p55γ) and catalytic subunits (p110α, p110β, p110δ). Three genes encode the regulatory subunits: PIK3R1 encodes p85α (22), and PIK3R2 and PIK3R3 encode the p85β and p55γ isoforms of the p85 regulatory subunit, respectively. Three genes, PIK3CA, PIK3CB, and PIK3CD, encode the highly homologous p110 catalytic subunit isoforms p110α, p110β, and p110δ and share a similar five-domain structure. At the amino terminus, there is an adapter-binding domain that interacts with the p85 regulatory subunit, followed by a RAS-binding domain that mediates interaction with RAS.
The class I PI3Ks can phosphorylate the 3’ position of PtdIns, PI-4-P, and PI-4,5-P2 though PI-4,5-P2. This phosphorylation generates the second-messenger phosphatidylinositol (3,4,5) triphosphate (PIP3). Cytosolic proteins, such as the AKT family of protein-serine/threonine kinases, bind to PIP3 and localize to the plasma membrane in response to PI3K activation (23). In the absence of stimulated growth conditions, baseline levels of PIP3 are undetectable in mammals. The PIP3 levels at the plasma membrane are regulated by the tumor suppressor phosphatase and tensin (PTEN) homolog, whose lipid phosphatase activity converts PIP3 to PI-4,5-P2. Loss of PTEN function through inactivating mutations, deletion, chromosomal translocation, or epigenetic silencing is the second most common initiating event in cancer after p53 mutations.
Mutations or amplifications of the PI3K catalytic subunits p110α (PIK3CA) and p110β (PIK3CB), the PI3K regulatory subunits p85α (PIK3R1) and p85β (PIK3R2), and AKT (AKT1) can activate the PI3K pathway. Mutations, deletions, or epigenetic changes in negative regulators of the PI3K axis (PTEN and inositol polyphosphate-4-phosphatase, type II) may modify tumor cell sensitivity to chemotherapy or targeted therapies (24). AKT is the main effector of PI3K activation and has three isoforms: AKT1, AKT2, and AKT3. AKT signaling plays a significant role in cell hypertrophy, survival, hyperplasia, and metabolism. Mammalian target of rapamycin (mTOR) is the catalytic subunit of mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), which are distinguished by their accessory proteins, regulatory-associated protein of mTOR (RAPTOR) and rapamycin-insensitive companion of mTOR (RICTOR) (25).
Several studies focused on targeting the PI3K-AKT-mTOR pathway. Rapamycin analogues (rapalogs) have antitumor activity in various tumors and are frequently combined with other anticancer agents (26). Everolimus is approved for the treatment of subependymal giant cell astrocytoma; hormone receptor (HR)-positive, HER2 (human epidermal growth factor receptor 2)-negative breast cancer (in combination with exemestane); neuroendocrine pancreatic tumors; tuberous sclerosis–associated subependymal giant cell astrocytoma; and renal cell carcinoma (sunitinib or sorafenib refractory). Temsirolimus is approved for renal cell carcinoma. The efficacy of rapalogs combined with endocrine therapy for advanced breast cancer was evident in the BOLERO-2 trial, which showed a median PFS of 6.9 months for everolimus and exemestane versus 2.8 months for exemestane alone (27).
The first generation of class I pan-PI3K inhibitors targeted PI3Kα, PI3Kβ, PI3Kγ, and PI3Kδ. Wortmannin and LY294002 had limited activity. Ongoing studies are evaluating new pan-PI3K inhibitors with improved pharmacokinetic profiles and target specificity. Their antitumor activity is primarily cytostatic. Novel agents that inhibit both PI3K and mTOR may improve the antitumor activity of either agent.
GDC-0941 is a selective oral class I PI3K inhibitor and at high concentrations also an mTOR inhibitor. It is being investigated in clinical trials in patients with metastatic breast cancer (NCT00960960, NCT01437566) and advanced NSCLC. Initial studies of GDC-0941 demonstrated PRs in patients with melanoma and ovarian, cervical, and estrogen receptor (ER)–positive/HER-negative breast cancer (28,29,30). At the maximum tolerated dose (MTD), dose-limiting toxicities (DLTs) included grade 3 macular rash and asymptomatic T-wave inversion on electrocardiograms, grade 3 thrombocytopenia, and grade 4 hyperglycemia (29,30). Ongoing studies evaluating GDC-0941 include a phase II study in patients with untreated advanced or recurrent NSCLC treated with carboplatin/paclitaxel or carboplatin/paclitaxel/bevacizumab with or without GDC-0941 (NCT01493843). In a phase I/II study, GDC-0941 and cisplatin are being studied in patients with androgen receptor (AR)–negative, triple-negative, metastatic breast cancer, and in a phase II study, patients with advanced/metastatic breast cancer resistant to aromatase inhibitor therapy are being treated with GDC-0941 or GDC-0980 with fulvestrant versus fulvestrant alone (NCT01437566). Clinical trials are also investigating combinations of PI3K inhibitors taselisib (GDC-0032) or pictilisib (GDC-0941) with other targeted agents (eg, palbociclib, a cyclin-dependent kinase 4 and 6 [CDK4/6] inhibitor) in advanced solid tumors or breast cancer (NCT02389842).
BKM120 is an oral pyrimidine-derived pan-PI3K inhibitor with activity against all class I PI3K isoforms. A phase I study demonstrated that BKM120 was well tolerated, with a dose-dependent safety profile (31). Adverse events included hyperglycemia, rash, nausea, fatigue, and mood alterations. Hyperglycemia is a typical adverse event associated with the use of PI3K/AKT/mTOR pathway inhibitors. Another study demonstrated that in colorectal, breast, lung, and endometrial cancers treated with BKM120, two of 77 patients had a PR (triple-negative breast cancer with KRAS and p53 mutations, n = 1; and ER-positive/HER-negative metastatic breast cancer, n = 1; both had tumor PIK3CA mutations), and 58% of patients had stable disease (SD).
BAY 80-6946 is a pan–class I PI3K inhibitor with activity against PI3Kα, PI3Kβ, PI3Kδ, and PI3Kγ. A phase I study demonstrated that the MTD of BAY 80-6946 was 0.8 mg/kg intravenously weekly (3 weeks on, 1 week off). Adverse events included hyperglycemia, fatigue, nausea, diarrhea, and mucositis. Clinical benefit was reported in patients with advanced breast, endometrial, and gastric cancers.
BEZ235 is an oral, reversible, and selective inhibitor of PI3K and TORC1/2. Preclinical data demonstrated antitumor activity in melanoma, breast, CRC and sarcoma. BEZ235 suppresses cell proliferation, induces G1 cell cycle arrest, and promotes autophagy by inhibiting the activity of AKT, S6K, S6, and 4EBP1 target proteins. BEZ235 has been investigated in phase I/II clinical trials in patients with advanced cancer alone (32) or in combination with paclitaxel, trastuzumab, everolimus, or MEK162. In a phase IB study, BEZ235 combined with trastuzumab in 15 patients with HER2-positive metastatic breast cancer with altered PI3K/PTEN status was tolerable. Stable disease and PR were reported in four and one patients, respectively (33). An improved formulation of BEZ235 was used as a monotherapy or combined with trastuzumab, and SD was noted in 40% of patients with advanced cancer. The most common adverse events were nausea, diarrhea, elevated transaminases, and headache. The DLTs were fatigue, asthenia, grade 3 thrombocytopenia, and grade 3 mucositis (32). A clinical trial of BEZ235 and everolimus in advanced cancer is ongoing (NCT01628913).
In prostate cancer, PTEN loss may be associated with resistance to castration (34,35). BEZ235 causes growth arrest in PTEN-negative prostate cancers, but inhibition of the PI3K pathway leads to activation of AR signaling (inhibition of AR appears to result in promotion of PI3K activity) (35).
Other p110α Isoform-Specific Inhibitors
Other p110α isoform-specific inhibitors, such as BYL719, GDC-0032, and INK1117, are being investigated in various solid tumors. BYL719 was associated with less hyperglycemia than the pan-PI3K inhibitor BKM120 (36). In a phase I study, BYL719 induced tumor reduction in 33% of patients with ER-positive, metastatic breast cancer and a PIK3CA mutation (37). Multiple studies are investigating the role of BYL719 in solid tumors as a single agent or in combination with targeted agents and cytotoxics. Although preclinical data demonstrated that PIK3CA alterations are the best biomarkers for predicting sensitivity to BYL719, p110α inhibitors are not effective in PIK3CA-mutated cells that also have a PTEN deletion (37).
The AKT inhibitors may induce the PI3K-stimulating receptor tyrosine kinase HER3 in breast cancer cell lines and may increase IGF-1R and the insulin receptor, thereby leading to the development of escape pathways and resistance mechanisms. Combination therapies that block the feedback response may overcome resistance to AKT inhibitors. Several studies have investigated or are investigating AKT inhibitors (such as MK2206, GSK2141795, and BAY1125976) as single agents or in combination with targeted therapies or chemotherapy in specific tumor types (examples are NCT01333475, NCT01902173, NCT01979523, and NCT01915576). Other drugs, such as MSC2363318A, a dual p70S6K/AKT inhibitor, are in clinical trials (NCT01971515).
Based on the increase in tumor inhibition with combined MEK/PI3K targeting and the tolerability of drugs targeting each pathway individually, early-phase trials combining GDC-0941 (PI3K inhibitor) with GDC-0973 (MEK inhibitor) and combining BKM120 (PI3K inhibitor) with GSK1120212 (MEK inhibitor) have been completed (38,39). The latter study demonstrated promising antitumor activity in patients with KRAS-mutant ovarian cancer.
In summary, molecular alterations in the PI3K/AKT/mTOR pathway have been identified in multiple tumor types, emphasizing the critical role of this pathway in tumorigenesis and disease progression. The PI3K inhibitors, as single agents, have mostly cytostatic activity. Several escape mechanisms are involved in resistance to PI3K/AKT/mTOR inhibitors. Clinical trials are exploring the role of PI3K, AKT, or mTOR inhibitors in combination with other targeted or cytotoxic agents. In our experience, patients with molecular alterations in the PI3K/AKT/mTOR pathway treated with targeted therapies have shorter survival compared to patients with alterations in the RAS/RAF/MEK or EGFR/HER/other pathways treated with the matched targeted agents, perhaps due to less-effective therapies than those for other pathways or intrinsic resistance (unpublished data). Carefully designed clinical trials, patient selection, and the elucidation of mechanisms of response and resistance to PI3K/AKT/mTOR pathway inhibitors, including protein and phosphoprotein expression with signatures of sensitivity/resistance, may improve clinical outcomes.