In vitro and in vivo effects of the mTOR inhibitors everolimus (EVE) and temsirolimus (TEM).

Abstract

466 Background: Two mTOR inhibitors are approved for treating mRCC: EVE following the failure of VEGF-targeted therapy and TEM as first-line therapy in poor-risk patients. Both agents exert their clinical effect by binding to FKBP-12, which then interacts with mTOR to inhibit its kinase activity. We compared the activity of EVE and TEM in in vitro and in vivo models. Methods: EVE and TEM binding to mTOR was assessed using time-resolved fluorescence resonance energy transfer. Inhibition of cell proliferation in A549, NCI-H460, and MCF7 human tumor cell lines was assessed by methylene blue protein staining. Phosphorylation of the downstream mTOR target pS6 was assessed via immunohistochemistry in A549 cells. Antitumor activity of EVE (oral) and TEM (intraperitoneal) at doses between 0.1 and 2.5 mg/kg once daily was assessed in vivo in A549, KB-31, KB-8511, and HCT-116 human tumor xenograft models. Results: The binding efficiency of TEM for mTOR was reduced 10-fold compared with that of EVE (EC50, 56 nM vs 6 nM; p < 0.01). EVE demonstrated 6- to 7-fold greater inhibition of cell proliferation than TEM (IC50, 1.0 nM vs 6.5 nM in A549 [p < 0.001], 0.7 nM vs 4.7 nM in NCI-H460 [p < 0.01], and 19.4 nM vs 150 nM in MCF7 [p < 0.001]). Complete inhibition of pS6 phosphorylation in A549 cells at 24 hours was achieved with 6.7 nM EVE, but required 20 nM TEM. In all xenograft models, EVE and TEM showed a dose-response relationship over the range of 0.1-2.5 mg/kg/day. EVE was significantly more potent than TEM in the A549 model (EC50, 0.11 mg/kg vs 0.51 mg/kg; p = 0.002); no appreciable differences between EVE and TEM were observed in the KB-31, KB-8511, and HCT-116 xenograft models. However, correcting for drug exposure suggests increased potency of EVE over TEM. Conclusions: Compared with TEM, EVE had a higher affinity for the molecular target of FKBP-12. This was consistent with more potent antitumor activity in vitro and in vivo. Whether these data would translate into a better therapeutic index for EVE is unknown. However, the results suggest that these mTOR inhibitors may not be clinically interchangeable.