Abstract

Background: The insulin-like growth factor-1 receptor (IGF1R) mediates the biological actions of IGF-1 and IGF-2 and is expressed in most breast cancer cells, where receptor activation induces proliferative, cell-survival, and transforming activities. Because crosstalk exists between ER and IGF1R blockade of both pathways could increase antitumor activity. Further, IGF signaling may be enhanced in ER+ cells that have become refractory to long term estrogen deprivation (LTED) explaining resistance to aromatase inhibitor therapy. The effect of the selective IGF1R antibody (R1507) was therefore assessed in a series of breast cancer cell lines in both estrogen-dependent and estrogen-independent growth phases.Methods: The ER positive breast cancer cells lines MCF7, T47D and HCC712 were kept in phenol red-free medium containing 5% charcoal-stripped serum. Several doses of R1507 were used to evaluate effects on cell proliferation, signaling and apoptosis. Western blotting with phospho-specific antibodies was used to observe effects on cell signaling, cell growth was measured using resazurin reduction and apoptosis was quantified by flow cytometry using the TUNEL assay. STED (short-term estrogen deprived) cells were treated with R1507 between 2 and 3 weeks after estrogen depletion and LTED were treated with R1507 after 6+ months of estrogen depletion.Results: R1507 suppressed IGF1R expression and inhibited IGF-1 stimulated IGF1R and AKT phosphorylation in all cell lines. R1507 had anti-proliferative effects in MCF7 cells in both estrogen-dependent and independent growth phases and the magnitude of the effect was similar to fulvestrant. R1507 had a smaller effect on cell proliferation in HCC712 cells and no effects on proliferation were observed in T47D cells. Antibody treatment resulted in significant induction of apoptosis in MCF7 STED but MCF7 LTED cells were resistant to the apoptotic effects of IGF1R blockade.Conclusions: R1507 had an inhibitory effect on IGF1R signaling and an anti-proliferative effect in two of the three cell lines tested. In addition R1507 induced synthetic lethality in combination with acute estrogen deprivation in MCF7 cells, an effect we have recently reported for direct PI3 kinase inhibitors under the same circumstances (1). While the proapoptotic effects of R1507 were lost in the estrogen-independent growth phase of MCF7 cells, the anti-proliferative effect was not, indicating that R1507 might have efficacy in patients with endocrine therapy resistant tumors. In other cell lines the efficacy of R1507 was variable and synthetic lethality was not observed, suggesting that the development of predictive biomarkers for R1507 should be a priority. We are currently addressing the activity of R1507 in a clinical trial with two cohorts, one with acquired resistance to letrozole, where the antibody is added to continued letrozole treatment and a second group of patients with stable disease on letrozole to determine if R1507 can induce tumor regression under these circumstances.1. Crowder et al Cancer Research 69:3955-62, 2009