Potent and selective inhibitors of Akt kinases slow the progress of tumors in vivo

Abstract

The Akt kinases are central nodes in signal transduction pathways that are important for cellular transformation and tumor progression. We report the development of a series of potent and selective indazole-pyridine based Akt inhibitors. These compounds, exemplified by A-443654 (K i = 160 pmol/L versus Akt1), inhibit Akt-dependent signal transduction in cells and in vivo in a doseresponsive manner. In vivo, the Akt inhibitors slow the progression of tumors when used as monotherapy or in combination with paclitaxel or rapamycin. Tumor growth inhibition was observed during the dosing interval, and the tumors regrew when compound administration was ceased. The therapeutic window for these compounds is narrow. Efficacy is achieved at doses f2-fold lower than the maximally tolerated doses. Consistent with data from knockout animals, the Akt inhibitors induce an increase in insulin secretion. They also induce a reactive increase in Akt phosphorylation. Other toxicities observed, including malaise and weight loss, are consistent with abnormalities in glucose metabolism. These data show that direct Akt inhibition may be useful in cancer therapy, but significant metabolic toxicities are likely dose limiting. [Mol Cancer Ther 2005;4(6):977 -86]

Akt inhibitor A-443654 induces rapid Akt Ser-473 phosphorylation independent of mTORC1 inhibition

“…We have shown that A-443654 inhibits the phosphorylation of the Akt substrates GSK3a/b, mTOR and TSC2 in a dose-dependent manner (Luo et al, 2005). At the same concentrations, we now find that A-443654 induces phosphorylation of Akt Ser-473, perhaps reflecting a homeostatic feedback mechanism by which the cell attempts to maintain Akt activity.…”

“…The preponderance of data implicating Akt as a valid cancer target led us to generate small-molecule inhibitors of this kinase as potential therapeutics. We have previously described a pan-Akt inhibitor, A-443654, that inhibits the phosphorylation of Akt substrates and demonstrates toxicitylimited efficacy in human-xenografted murine tumor models (Luo et al, 2005). During our studies, we observed that A-443654 induces rapid and robust phosphorylation of Akt Ser-473, indicative of a compensatory, but futile, upregulation of Akt activity when the kinase is pharmacologically inhibited.…”

“…Induction of Akt Ser-473 phosphorylation by A-443654 is time-and concentration dependent A-443654 is a potent inhibitor of all three Akt family kinases (Luo et al, 2005). We have shown that A-443654 inhibits the phosphorylation of the Akt substrates GSK3a/b, mTOR and TSC2 in a dose-dependent manner (Luo et al, 2005).…”

Drug discovery approaches targeting the PI3K/Akt pathway in cancer

“…Although several possibilities were considered (for example, a conformational change in the structure of the compound-bound Akt that prevents Ser-473 dephosphorylation or changes in Akt relocalization), further studies will be required to delineate the mechanism(s) involved in this interesting observation. A-443654 shows some level of selectivity against other members of the AGC family: PKA, 40-fold; PKCg, 150-fold; PKCd, 200 and PDK1, >120 000-fold (Luo et al, 2005). A structural comparison of A-443654 bound to Akt and PKA demonstrated that the compound adopts a binding conformation in Akt that differs from that with PKA (Davies et al, 2007).…”

“…Promising Akt inhibitors have also been obtained from indazole-pyridine-based derivatives (Luo et al, 2005). Exemplified by A-443654 (compound 25 , Figure 4; Ki ¼ 160 pM for Akt1), these compounds are reversible, ATP competitive inhibitors, which decrease the phosphorylation of Akt downstream targets in cells (for example, GSK3a/b, FOXO3, TSC2 and mTOR) and in vivo in a dose-dependent manner.…”