Abstract 3741: Inhibition of PI3Kα kinase by a selective small molecule inhibitor suppresses B-cell proliferation and leukemic cell growth (2012)

Abstract Background: Phosphoinositide-3 kinase (PI3K) belongs to a class of intracellular lipid kinases that phosphorylate the 3 position hydroxyl group of the inositol ring of phosphotidylinositol. The PI3K pathway is frequently activated in human cancers and thus represents an attractive target for small molecule inhibitors. Pan-PI3K inhibitors currently in development have been associated with adverse side-effects such as insulin resistance, thus necessitating the need to develop isoform specific inhibitors of PI3K. Herein, we describe the biological and pharmacokinetic properties of RP5264, a small molecule PI3Kα inhibitors with scope to be further developed as a clinical candidate for hematological malignancies mediated by B-cells. Methods: Activity of RP5264 on individual PI3K isoforms was determined by a Homogenous Time Resolved Fluorescence assay (Millipore, Billerica, MA) with modifications. Cell based selectivity assays against β, α, or ≤ isoforms was assessed by testing the effect of the compound on PDGF, LPA, or c5a induced Akt phosphorylation in NIH-3T3 or RAW cells. Similarly, inhibition of cellular PI3Kα activity was determined in an IgM induced human B-cell proliferation as well as LPS-induced CD19 assays. Ability to arrest cell growth and induce apoptosis was also tested. Viability assays was conducted to determine the growth inhibitory effect of the compounds in leukemic cells. Pharmacokinetic behaviour of compounds in plasma after single dose oral administration was determined in female Balb/c mice. Results: RP5264 inhibited PI3Kα activity in enzyme and cell based assays with IC50 and EC50 values of 22.2 &amp; 24.3 nM respectively. The compound displayed a high degree of selectivity over the alpha (&amp;gt;1000 fold), beta (&amp;gt;30-50 fold), and gamma (&amp;gt;15-50 fold) isoforms. Additionally, the compound caused a half-maximal inhibition of human whole blood CD19 cell proliferation between 100-300 nM. Treatment of PBMC with RP5264 resulted initially in a G2/M arrest followed by subsequent increase in the number of Sub G0 cells. Viability assays demonstrated that the compound caused a significant inhibition in growth as well as Akt phosphorylation of immortalized and primary leukemic cells. Further, the compound exhibited good oral absorption with favourable pharmacokinetic properties in rodents. Conclusions: Results demonstrate the PI3K delta selective nature of RP5264 along with an ability to suppress proliferation and Akt phosphorylation in cancer cells. In vitro selectivity and potency data indicate the therapeutics potential of the compounds in hematological cancers without the deleterious effects commonly associated with the Pan PI3K inhibitors. RP5264 is poised to enter clinical development in 2012. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3741. doi:1538-7445.AM2012-3741