Abstract 928: The novel clinical candidate AT13148 is an oral multi-AGC kinase inhibitor with potent pharmacodynamic and antitumor activity and demonstrates a mechanism of action distinct from AKT inhibitors

The AGC kinase AKT is a key component of the phosphatidylinositol 3-kinase (PI3K) pathway, which is frequently deregulated in cancer, making AKT a target of major therapeutic interest. However, PI3K signaling through both AKT-dependent and AKT-independent mechanisms involving other AGC kinases, such as p70S6K, PKA, SGK and ROCK, is important in a range of cancers. Hence, the pharmacological inhibition of these multiple AGC kinases may increase response rates and minimize clinical resistance compared with targeting AKT alone. The clinical drug candidate AT13148 is a multi-AGC kinase, ATP-competitive inhibitor, identified utilizing high-throughput X-ray crystallography and fragment-based lead discovery techniques. Screening of this oral small molecule against a panel of kinases at 10μM revealed >80% inhibition of the structurally related AGC kinases AKT, PKA, ROCK2, p70S6K, MSK, RSK1/2, and SGK. We demonstrate that AT13148 has antiproliferative activity in a range of in vitro models harboring relevant genetic abnormalities, including PTEN, KRAS, PIK3CA and HER2 aberrations. AT13148 caused substantial blockade of AKT, p70S6K, PKA, ROCK and SGK substrate phosphorylation and induction of apoptosis in both a concentration and time-dependent manner in cancer cells with clinically relevant genetic defects both in vitro and in vivo. Antitumor efficacy in HER2-positive, PIK3CA-mutant BT474 breast, PTEN-deficient PC3 human prostate cancer and PTEN-deficient MES-SA uterine tumor xenografts was demonstrated. We show for the first time that induction of AKT phosphorylation at serine 473 by AT13148, as reported for other ATP competitive inhibitors of AKT, is not a therapeutically relevant reactivation step for this compound. We used gene expression microarray studies to characterize the underlying molecular mechanisms of action of AT13148 and the selective AKT inhibitor CCT128930, and observed the induction of upstream regulators including insulin receptor substrate-2 and PIK3IP1 due to compensatory feedback loops, consistent with blockade of AKT signaling. These studies also showed that AT13148 and CCT128930 have distinct molecular effects in cancer cells: AT13148 had a predominant effect on apoptosis genes and caused a greater apoptotic phenotype, while CCT128930 modulated genes in the network regulating cell cycle. This finding emphasizes the functional differences of AT13148 as a multi-AGC kinase inhibitor in contrast to a more AKT-selective inhibitor. In view of the potential mechanistic advantages detailed above, and the potent antitumor activity observed at well tolerated doses against established human tumor xenografts with clinically relevant genetic drivers, the clinical utility of such an AGC kinase inhibitor strategy will now be assessed in a first-in-human Phase I trial of AT13148. 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 928. doi:1538-7445.AM2012-928