Combination of CP-751871, a human monoclonal antibody against the IGF-1 receptor, with rapamycin results in a highly effective therapy for xenografts derived from childhood sarcomas

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Combination of CP751871, a Human Monoclonal Antibody Against the IGF-1 Receptor, with Rapamycin Results in Highly Effective Therapy for Xenografts Derived from Childhood Sarcomas.
 Raushan T. Kurmasheva, Claire Boltz, Doris Phelps, Christopher L. Morton, Peter J. Houghton. Department of Molecular Pharmacology, St. Jude Children’s Research Hospital, Memphis, TN 38105, U.S.A.
 Background: Under growth factor deficient conditions, rapamycin a specific inhibitor of mTORC1 signaling, induces apoptosis selectively in cells with dysfunctional p53 (Hosoi et al. Cancer Res 59:886, 1999). Apoptosis is blocked uniquely by insulin like growth factors 1 and 2. Consequently, we have evaluated the combination of rapamycin and an antibody that prevents signaling through the IGF-1 receptor (IGF-1R).
 Methods: The sensitivity of rhabdomyosarcoma (RMS) (4) and Ewing sarcoma (EWS) (5) cell lines to CP751871 was determined by Alamar Blue staining after 4 days treatment. The ability of CP751871 to block IGF-1-mediated rescue from rapamycin treatment was determined by ApoAlert assay. Secreted levels of IGF-1 and IGF-2 were determined by ELISA assay. Levels of IGF-1R, Akt, S6 and their phosphorylated forms was determined by western blot. In vivo testing was performed against subcutaneous sarcomas in CB17/scid^-/- mice. CP751871 was administered twice weekly (0.5 mg/mouse) and rapamycin was administered daily for 5 days on consecutive weeks. Tumor volume was measured weekly.
 Results: Maximum growth inhibition (range for RMS 39.2-52.0%; range for EWS: 5.5-64%) for all cell lines was obtained at 1 μM. CP751871 induced only minor decrease in phospho-Akt, mTORC1 signaling. Rapamycin induced phosphorylation of both Akt (S⁴⁷³) and IGF-IR (Y¹¹⁴⁶) in RMS cells that were both inhibited by CP751871. Of interest, both rapamycin and CP751871 significantly increased IGF-1 but not IGF-2 secreted in medium by EWS cells. However, CP751871 effectively blocked IGF-1-mediated rescue from rapamycin-induced apoptosis in vitro. CP751871 markedly downregulated IGF-1R levels and pAkt (>70%) within 24 hr in 9/11 childhood sarcoma xenografts. We evaluated the antitumor activity of CP751871, rapamycin and the combination against 2 in vivo models that are IGF-1-driven, and harbor p53 mutations: EW-5 (Ewing sarcoma) and OS-9 (osteosarcoma). In the combination groups, IGF-1R was completely downregulated at day 7 of treatment, whereas it was still detected in CP751871 treated tumors. At day 7, p-Akt levels had returned to control levels in combination treated tumors, whereas it was suppressed in tumors from mice receiving CP751871 alone. Notably, whereas tumors progressed on CP751871 or rapamycin as single agents, the combination induced maintained complete regressions of all tumors.
 Conclusions: CP751871 significantly retarded the proliferation of most sarcoma cell lines. In vivo, CP751871 retarded tumor growth, as did rapamycin. In two IGF-1-driven xenograft models the combination induced complete regressions of established tumors. Our studies need to be extended to additional sarcoma models, and potentially could be valuable for identifying pharmacodynamic markers that correlate with responsiveness to this combination.