Background. Deregulated insulin-like growth factor signaling through the type-1 receptor (IGF-1R) appears to be common to many human cancers, including childhood solid tumors, and offers an important molecular target for therapeutics development. We have shown previously that rapamycin induces apoptosis in sarcoma cells under growth factor-free conditions of growth in vitro, but uniquely exogenous IGF-1/-2 or high concentrations of insulin can rescue cells from death. We have tested the strategy of combining rapamycin with a human antibody targeting the IGF-1R, CP-751871, against xenograft models derived from childhood sarcomas. Methods. 5 Ewing sarcoma (ES) and 4 rhabdomyosarcoma (RMS) cell lines and 13 xenograft sarcoma models were examined in this study. Growth factor (IGF-1, IGF-2, VEGF) secretion was measured by ELISA assay. Levels of IGF-1R, Akt, S6 and their phosphorylated forms were determined by Western Blot, transcripts were quantitated by RT-PCR. Xenograft tissue sections were stained for Ki67 expression. In vivo testing was performed against subcutaneous sarcomas in CB17/scid-/- mice. CP-751871 was administered twice weekly (0.5 mg/mouse) for 4 weeks and rapamycin was administered at 5 mg/kg daily for 5 days for up to 12 consecutive weeks. Results. In vitro, ES cells secrete predominantly IGF-1 whereas RMS predominantly IGF-2. However, inhibition of proliferation for ES or RMS cells under serum-containing conditions was relatively modest (max 50%) when cells were exposed to CP-751871. Combined with rapamycin, CP751871 exerted additive effects on reducing secreted VEGF. Antitumor activity for the combination of CP-751871 and rapamycin was supra-additive against 5/13 models (EW-5, OS-1, OS-2, OS-9, Rh-18). Pharmacodynamic studies (25h and 169h of treatment) showed that inhibition of IGF-1R or mTOR signaling in responding vs progressing tumors did not correlate with tumor response (regressions), although CP-751871 suppressed hyperphosphorylation of Akt (S473) induced by rapamycin treatment. Proliferation (Ki67 staining) was dramatically suppressed only by combination treatment in 2/2 responding xenografts examined (EW-5 and OS-9). CP-751871 treatment markedly suppressed tumor-derived VEGF levels and transcripts at 25h and 169h (EW-5, OS-1, OS-2, OS-9). In contrast, rapamycin had transient effects, reducing VEGF levels at 25h (ES-1, EW-5, EW-8), but with full recovery by 169h. CP-751871 suppressed VEGF recovery in combination-treated tumors, and protracted suppression of VEGF correlated with tumor responsiveness. Conclusions. Overall, combining agents that target IGF-1R and mTORC1 signaling may have significant antitumor activity against childhood sarcomas. The pharmacodynamic marker that correlated best with the antitumor activity of the rapamycin-CP-751871 combination was rapid and prolonged suppression of tumor-derived VEGF (Supported by USPHS grant CA23099).
Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2806.
100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO