Ridaforolimus (AP23573; MK-8669), a potent mTOR inhibitor, has broad antitumor activity and can be optimally administered using intermittent dosing regimens.

The mTOR pathway is hyperactivated through oncogenic transformation in many human malignancies. Ridaforolimus (AP23573; MK-8669) is a novel rapamycin analogue that selectively targets mTOR and is currently under clinical evaluation. In this study, we investigated the mechanistic basis for the antitumor activity of ridaforolimus in a range of human tumor types, exploring potential markers of response, and determining optimal dosing regimens to guide clinical studies. Administration of ridaforolimus to tumor cells in vitro elicited dose-dependent inhibition of mTOR activity with concomitant effects on cell growth and division. We showed that ridaforolimus exhibits a predominantly cytostatic mode of action, consistent with the findings for other mTOR inhibitors. Potent inhibitory effects on vascular endothelial growth factor secretion, endothelial cell growth, and glucose metabolism were also observed. Although PTEN and/or phosphorylated AKT status have been proposed as potential mTOR pathway biomarkers, neither was predictive for ridaforolimus responsiveness in the heterogeneous panel of cancer cell lines examined. In mouse models, robust antitumor activity was observed in human tumor xenografts using a series of intermittent dosing schedules, consistent with pharmacodynamic observations of mTOR pathway inhibition for at least 72 hours following dosing. Parallel skin-graft rejection studies established that intermittent dosing schedules lack the immunosuppressive effects seen with daily dosing. Overall these findings show the broad inhibitory effects of ridaforolimus on cell growth, division, metabolism, and angiogenesis, and support the use of intermittent dosing as a means to optimize antitumor activity while minimizing systemic effects.