Abstract B139: Molecular pharmacology and antitumor activity of PHT-427 a novel AKT/PDPK1 pleckstrin homology domain inhibitor

Abstract

Abstract The pleckstrin homology (PH) domain is a highly conserved three dimensional superfold found in many high valued pharmaceutical target proteins. While for the majority of PH domain proteins PtdIns binding is weak and non-specific, a subclass of approximately 40 PH domain containing proteins show high affinity for phosphatidylinositol-3-phosphates (PtdIns-3-P) causing their translocation to the plasma membrane and/or activation. Akt and PDKP1 are two such PH domain containing serine threonine kinase members of the PtdIns-3-kinase signaling pathway that plays a critical role in activating survival and anti-apoptotic signaling in cancer cells. This pathway is constitutively activated through mutation, overexpression and loss of the tumor suppressor PTEN in many different cancer types. There is emerging evidence that Akt and PDKP1 play complimentary yet independent roles in signaling by the pathway. Attempts to develop ATP catalytic site inhibitors of Akt and PDKP1 have generally resulted in toxic agents because of inhibition of off-target kinase family members. Through a process of reiterative molecular docking and structure refinement using a proprietary computational platform, and measurement of binding affinities by surface plasmon resonance (SPR) spectroscopy, we have developed inhibitors of these proteins that bind to the PH domain. We report here the development of PHT-427 a compound that binds to the PH domain of AKT and PDKP1. To develop PHT-427 an initial pharmacophore that bound to the PH domain PtdIns binding pocket was first identified. During the docking/modeling we identified a channel whithin the PH domains of both AKT and PDPK1 that could accommodate the binding of an alkyl chain. A series of analogs with C-4 to C-16 alkyl chain length were therefore synthesized and PHT-427 (C-12 chain) was found to bind with the highest affinity to the PH domains of both PDPK1 and AKT. PHT-427 gives a transient inhibition of AKT signaling in cells and tumor xenografts and a longer lasting inhibition of PDPK1 signaling. This inhibition correlated with decreased activation of downstream signaling targets for both proteins. PHT-427 given ias an oral formulation on a twice daily schedule inhibited the growth rate of human tumor xenografts in immunodeficient mice with up to an 80% inhibition in the most sensitive tumors and showed greater activity than analogs with C4, C6 or C8 alkyl chains. Tumors with a K-Ras mutation were less sensitive to PHT-427 as it has been seen for other inhibitors of the PtdIns-3-K signaling pathway. Combination antitumor studies showed that PHT-427 has greater than additive activity with paclitaxel in breast cancer and with erlotinib in non small cell lung cancer. There was minimal toxicity of PH-427 at doses that gave antitumor activity. PHT-427 given twice daily for 5 days caused no weight loss or change in blood chemistry. In summary we have identified a novel PH domain binding inhibitor of PDPK1/AKT signaling that has minimal toxicity and antitumor activity against human tumor xenografts.. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B139.