Abstract 3555: Anti-tumor activity of CAL-101, a potent selective inhibitor of the p110Δ isoform of PI3K, in models of human glioblastoma

Abstract Phosphatidylinositol 3-kinase (PI3K) activation and Akt phosphorylation play a pivotal role in cell proliferation and survival, transducing signals from several cell surface receptors through PI3K isoforms (designated p110α, β, Δ, and γ). Our previous research has elucidated a significant role of the PI3K p110Δ (PI3KΔ) isoform in hematologic malignancies. Aberrant upregulation of the PI3K/Akt pathway is a frequently observed defect in human malignancies, including glioblastoma, but the role of the PI3KΔ isoform, has not been systematically evaluated. Selective inhibition of PI3KΔ with the orally bioavailable investigational drug, CAL-101, offers the potential for targeting PI3KΔ as a therapeutic approach to glioblastoma while avoiding undesirable side effects associated with pan-inhibitors that broadly block PI3K isoforms in multiple cell types. CAL-101 potently inhibits PI3KΔ (with an EC50 of 65 nM in a whole-blood assay) and shows >200-fold selectivity over other Class I PI3K isoforms and no activity against Class II and III PI3K family members or other PI3K-related proteins, including mTOR and DNA-PK. We examined expression levels of PI3KΔ and constitutive pathway activation in a panel of glioma cell lines (LN18, LN229, SF-539, U87MG, U138MG, and U251). PI3KΔ was consistently expressed and was functionally active, inducing high basal levels of phosphorylated-Akt (pAkt) in all tested cell lines. Treatment with CAL-101 decreased p-Akt levels in all cell lines and also reduced phosphorylation of the downstream pathway target, S6. Furthermore, PI3KΔ inhibition with CAL-101 resulted in an accumulation of cells in G1 and a decrease in the S phase population. Consistent with the effects of CAL-101 on G1 cell-cycle arrest, immunoblotting showed a decrease in cyclin D1 levels. In mice bearing U87MG xenografts, in vivo PI3KΔ inhibition resulted in significant anti-tumor effects, delaying time to tumor progression (tumor volume of >1000 mm3) by 30 days (p<0.001). Taken together, these data suggest that PI3KΔ overexpression and heightened functional activity contribute to glioblastoma proliferation, and that selective targeting of PI3KΔ may offer a novel therapeutic approach for patients with central nervous system tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3555. doi:10.1158/1538-7445.AM2011-3555