Abstract B142: PKI-402, a dual PI3K/mTOR inhibitor

Abstract

Abstract We have developed small molecule inhibitors of the PI3K/mTOR signaling pathway. An example is PKI-402. This compound is a reversible, ATP competitive, and equipotent inhibitor of class I PI3Ks, including the E545K and H1047R mutant forms of PI3K- , and mTOR (IC50 vs PI3K- = 1nM). Selectivity of PKI-402 was established in a screen against 236 diverse human kinases. PKI-402 caused in vitro growth inhibition of human tumor cell lines derived from breast, brain (glioma), pancreas, and non small cell lung cancer (NSCLC) tissues. In many cases IC50 values were <100 nM. In vitro, PKI-402 suppressed phosphorylation of PI3K and mTOR effector proteins, particularly p-Akt at threonine 308 (T308), at concentrations that closely matched those that inhibited tumor cell growth. In MDA361, a breast tumor line with elevated levels of Her2 receptor, and mutant PI3K- (E545K), 30 nM PKI-402 induced cleaved PARP, a marker for apoptosis. This occurred after an exposure time of 4 h. In vivo, PKI-402 displayed anti-tumor activity when administered by IV route in glioma (U87MG, PTEN), NSCLC (A549; K-Ras, STK11), and breast (MDA361: Her2+, PIK3CA [E545K]) xenograft models. A key question about PI3K/mTOR signaling inhibitors is: can they cause tumor regression in pre-clinical models? At 25, 50, and 100 mg/kg PKI-402 caused regression of MDA361 tumors. PKI-402 effect was most pronounced at 100 mg/kg (daily for 5 days [dx5], 1 round), which reduced an initial tumor volume of 260 mm3 to 129 mm3, and prevented tumor re-growth for 70 days. Tumor re-growth occurred between days 16–20 when PKI-402 was administered at 25 and 50 mg/kg (dx5, 2 rounds). However, when PKI-402 was re-administered at 100 mg/kg (dx3, 1 round) at day 37 (25 mg/kg group), large tumors (∼600 mm3) were reduced in volume by 69%. In MDA361 tumor tissue PKI-402 at 100 mg/kg (single dose) caused sustained suppression of Akt phosphorylation at both the T308 and S473 sites, and induced cleaved PARP. Suppression of p-Akt was complete at 8 h, and still evident at 24 h; while cleaved PARP was evident at both 8 h and 24 h. In vivo biomarker analysis of heart and lung tissue showed minimal effect on p-Akt and no induction of cleaved PARP by PKI-402 at 100 mg/kg. Preferential accumulation of PKI-402 in tumor tissue was observed. It may be that complete and sustained suppression of p-Akt is needed to cause tumor regression in the MDA361 and other tumor xenograft models. We are testing whether dual PI3K/mTOR inhibitors can durably suppress p-Akt, induce cleaved PARP, and cause tumor regression in a diverse set of human tumor xenograft models. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B142.