The phosphatidylinositol-3 kinase/mTOR pathway: new agents

The phosphatidylinositol-3 kinase (PI3K) pathway is overall the most frequently mutated pathway in cancer, with mutation and/or amplification of the genes encoding the PI3K catalytic subunits p110α (PIK3CA) and p110β (PIK3CB), the PI3K regulatory subunit p85α (PIK3R1), receptor tyrosine kinases (RTKs) such as HER2 (ERBB2) and FGFR1, the PI3K activator K-Ras, the PI3K effectors AKT1, AKT2, and PDK1, and loss of the lipid phosphatases PTEN and INPP4B. PI3K is activated by growth factor RTKs and G-protein-coupled receptors (GPCRs). PI3K activates Akt, which, in turn, phosphorylates and inactivates Tuberin (TSC2), a GTPase-activating protein of the Ras homologue Rheb. Inactivation of Tuberin allows GTP-bound Rheb to accumulate and activate the mTOR/Raptor (TORC1) complex, which regulates protein synthesis and cell growth. mTOR also couples with Rictor to form the TORC2 complex, which phosphorylates and activates AKT. Class IA PI3K isoforms are heterodimeric lipid kinases that contain a p110 catalytic subunit and a p85 regulatory subunit. The three genes PIK3CA, PIK3CB, and PIK3CD encode the homologous p110α, p110β, and p110δ isozymes, respectively. Expression of p110δ is largely restricted to immune and hematopoietic cells, whereas p110α and p110β are ubiquitously expressed. p110α is essential for signaling and growth of tumors driven by PIK3CA mutations, RTKs, and/or mutant Ras, whereas p110β lies downstream of GPCRs and has been shown to mediate tumorigenesis in PTEN-deficient cells. PIK3CA mutations are the most commonly known genetic alterations of this pathway in cancer, where ≥80% occur within the helical (E542K and E545K) and kinase (H1047R) domains of p110α. Such mutations confer increased catalytic activity through different mechanisms, but both induce characteristics of cellular transformation including growth factor-independent and anchorage-independent growth, and resistance to anoikis. Several drugs targeting multiple levels of the PI3K network (that is, PI3K, AKT, mTOR) have been developed. A number of ATP-mimetics that bind competitively and reversibly to the ATP-binding pocket of p110 are in early clinical development. These include the pan-PI3K inhibitors BKM120, XL-147, PX-866, PKI-587, and GDC-0941, the p110α-specific inhibitors BYL719, GDC-0032, and INK-1117, the p110δ-specific inhibitor CAL-101, and the dual PI3K/mTOR inhibitors BEZ235, BGT226, PF-4691502, GDC-0980, and XL-765. The pan-PI3K and p110α-specific inhibitors are equally potent against oncogenic p110α mutants. The rationale for the development of isozyme-specific antagonists is to allow higher doses of anti-p110α and anti-p110β drugs to be delivered without incurring side effects caused by pan-PI3K inhibitors. Interim results from a phase I trial with the p110δ-specific inhibitor CAL-101 in patients with hematologic malignancies showed that treatment reduced P-AKT levels >90% in peripheral blood lymphocytes and induced objective clinical responses. Recently completed phase I trials with BKM120, BEZ235, and XL-147 showed that treatment partially inhibited PI3K as measured by levels of P-S6 and P-AKT in patients' skin or tumors, and 2-deoxy-2-[18F]fluoro-D-glucose uptake measured by PET. Main toxicities were rash, hyperglycemia, diarrhea, fatigue and, mood alterations. Few clinical responses were observed in patients with and without detectable PI3K pathway mutations, although screening for genetic lesions in this pathway was not comprehensive. Both allosteric and ATP-competitive pan-inhibitors of the three isoforms of AKT are also being developed. AZD5363, GDC-0068, GSK2141795, and GSK690693 are ATP-competitive compounds that have shown antitumor activity in preclinical models and recently entered phase I trials. Allosteric inhibitors such as MK-2206 bind to the AKT PH domain and/or hinge region to promote an inactive conformation of the AKT protein that is unable to bind to the plasma membrane. MK-2206 inhibits AKT signaling in vivo, and suppresses growth of breast cancer xenografts harboring PIK3CA mutations or ERBB2 amplification. Phase I data showed that treatment with MK-2206 decreases levels of P-AKT, P-PRAS40, and P-GSK3β in tumor cells, peripheral blood mononuclear cells, and hair follicles. The mTOR kinase is a component of PI3K-driven oncogenesis that functions within two signaling complexes: TORC1 and TORC2 (described above). The macrolide rapamycin and its analogs form complexes with FK506-binding protein (FKBP12). This complex then binds to mTOR and inhibits the kinase activity of TORC1 but not TORC2. Formulation problems of rapamycin prompted the development of analogs such as CCI-779 (temsirolimus), RAD001 (everolimus), AP-23573 (deferolimus), and MK-8669 (ridaferolimus). These rapalogs have shown cytostatic activity in preclinical models and clinical trials, particularly in patients with renal cell cancer, and in patients with mutations in the TSC complex (upstream of TORC1) who harbor renal angiolipomas. Compounds that target the ATP-binding cleft of mTOR (that is, OSI-027, AZD8055, INK-128), and are thus active against both TORC1 and TORC2, are also in phase I trials.