Phosphatidylinositide 3‐kinases (PI3K) are a family of lipid kinases that are involved in signaling events which control a diverse number of cellular processes. The activation of the PI3K pathway by cell surface receptors is directly mediated by the class I isoforms (α, β, δ, and γ). Aberrant regulation of the PI3K signaling pathway is frequently observed in a wide range of human malignancies including gain‐of‐function mutations in PI3K p110α isoform and/or loss‐of‐function mutations in PTEN phosphatase, which is responsible for down regulation of PI3K signaling. In purified enzyme assays, CAL‐120, a dual p110β/p110δ PI3K inhibitor, was inactive against class II and III PI3K family members,the PI3K‐related protein kinases mTOR and DNA‐PK as well as an additional ∼350 protein kinases in a genome wide screen. The ability of CAL‐120 to block oncogenic transformation mediated by individual PI3K class Ia isoforms was evaluated in primary cells using viral transduction. Foci formation mediated by p110α and p110δ was inhibited at 15–200 nM whereas little or no inhibition was observed against oncogenic forms of p110α at 20‐fold higher concentrations. To further demonstrate p110 isoform selectivity, AKT phosphorylation was induced in embryonic fibroblasts with PDGF or LPA that is mediated by p110α and p110β respectively. CAL‐120 inhibited the p110β response with an IC50 of 1.2 µM whereas the p110α IC50 was greater than 20 µM. The antitumor activity of CAL‐120 was evaluated in a panel of 23 human tumor cell lines representing different tissues and PI3K pathway mutations. Constitutive PI3K pathway activation as measured by AKT phosphorylation was observed in 50% of the cell lines and was highly correlated with PTEN mutations. In all cases, CAL‐120 blocked AKT phosphorylation at concentrations of 0.1–1.0 µM. In most cases inhibition of the phosphorylation of downstream effectors Akt, GSK‐3 , and S6 ribosomal protein was also observed over this concentration range. These effects of CAL‐120 on PI3K pathway inhibition correlated with G1 cell cycle arrest leading to inhibition of tumor cell proliferation and in a number of cases induced apoptosis. Of note was a lack of PI3K pathway activation in cell lines with K‐RAS mutations and their insensitivity to CAL‐120 treatment. In mice bearing xenografts of MCF‐7 breast adenocarcinoma (p110α mutation), PC‐3 prostate adenocarcinoma (PTEN deficient), or OVCAR‐3 ovarian adenocarcinoma (no pathway mutation), oral administration of CAL‐120 significantly inhibited tumor growth or caused tumor regression in each of these models. These data are the first to demonstrate that p110β/δ inhibition in the absence of effects on p110α is an effective strategy for the treatment of solid tumors. The antitumor activity was not restricted to cells with PTEN loss and was surprisingly observed even when p110α mutations were present. Collectively, these preclinical data support clinical evaluation of CAL‐120, an oral dual p110α/p110δ inhibitor, for the treatment of patients with solid tumors. (L. U. and P.K.V. are supported by grants from the National Cancer Institute. This is manuscript number 20362 of The Scripps Research Institute).

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B136.