PKI-402 is a selective, reversible, ATP-competitive, equipotent inhibitor of class I phosphatidylinositol 3-kinases (PI3K), including PI3K-alpha mutants, and mammalian target of rapamycin (mTOR; IC(50) versus PI3K-alpha = 2 nmol/L). PKI-402 inhibited growth of human tumor cell lines derived from breast, brain (glioma), pancreas, and non-small cell lung cancer tissue and suppressed phosphorylation of PI3K and mTOR effector proteins (e.g., Akt at T308) at concentrations that matched those that inhibited cell growth. In MDA-MB-361 [breast: Her2(+) and PIK3CA mutant (E545K)], 30 nmol/L PKI-402 induced cleaved poly(ADP-ribose) polymerase (PARP), a marker for apoptosis. In vivo, PKI-402 inhibited tumor growth in MDA-MB-361, glioma (U87MG), and lung (A549) xenograft models. In MDA-MB-361, PKI-402 at 100 mg/kg (daily for 5 days, one round) reduced initial tumor volume of 260 mm(3) to 129 mm(3) and prevented tumor regrowth for 70 days. In MDA-MB-361 tumors, PKI-402 (100 mg/kg, single dose) suppressed Akt phosphorylation (at T308) and induced cleaved PARP. Suppression of phosphorylated Akt (p-Akt) was complete at 8 hours and still evident at 24 hours. Cleaved PARP was evident at 8 and 24 hours. In normal tissue (heart and lung), PKI-402 (100 mg/kg) had minimal effect on p-Akt, with no detectable cleaved PARP. Preferential accumulation of PKI-402 in tumor tissue was observed. Complete, sustained suppression of Akt phosphorylation may cause tumor regression in MDA-MB-361 and other 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. Mol Cancer Ther; 9(4); 976-84. (c)2010 AACR.