The Insulin-like growth factor-1 receptor (IGF-1R) is overexpressed in a variety of tumors including breast, prostate and myeloma. Thus, IGF-1R and its downstream signaling effectors are good candidates for molecular-based targeted antitumor therapies. Indeed, protein inhibitors of IGF-1R signaling and IGF-1R blocking antibodies are undergoing clinical trials. Herein, the molecular basis for antibody-mediated IGF-1R signal inhibition has been investigated in a hematopoietic cell line model, FDC-P1, that has been rendered interleukin-3 independent in a ligand-dependent manner through retroviral-mediated expression of IGF-1R (FD/IGF-1R). Furthermore, the ability of an anti-IGF-1R antibody to synergize with signal-transduction pathway inhibitors and induce apoptosis was determined. The αIGF-1R antibody, A12, was capable of arresting IGF-1 or insulin-induced FD/IGF-1R cell proliferation in the G1 phase of the cell cycle and resulted in apoptotic induction. A12 effectiveness could be potentiated through combination treatment with small molecule inhibitors of the Ras/Raf/MEK/ERK or PI3K/Akt/mTOR pathways. These results validate the use of the FD/IGF-1R cells to evaluate the effectiveness and mechanisms of targeted IGF-1R therapeutic strategies. © 2006 Nature Publishing Group All rights reserved.