Abstract #2812: Hormonal therapies differentially enhance insulin receptor isoform A and erbB receptor up-regulation in response to IGF-1R inhibitor MK-0646 in vivo

Abstract

Signaling through the IGF-1 receptor has been implicated in resistance hormonal therapies that are standard therapies in estrogen-sensitive breast cancer. To determine whether hormonal therapy can synergize with IGF-1R inhibition, we employed a post-menopausal mouse model of breast cancer (MCF-7/AC-1) to investigate the in vitro and in vivo efficacy of MK-0646 in combination with either tamoxifen or the aromatase inhibitor, letrozole. In addition, to understand potential mechanisms of resistance to combined hormonal/IGF-1R-targeted therapy, we assessed IGF-1R, insulin receptor isoform and erbB receptor expression profiles at the end of treatment. MCF-7/AC-1 cells, which stably express human aromatase, were either grown in vitro for antiproliferative assays or in vivo as bilateral flank xenografts in nude, ovariectomized mice supplemented with androstenedione. Cells were then treated with vehicle, MK-0646, tamoxifen, letrozole, MK-0646 + tamoxifen or MK-0646 + letrozole for 5 days in vitro or 28 days in vivo once tumors reached approximately 300 mm³ each side (~ 4 weeks post injection). At the end of 28 days of treatment, tumors were harvested for RNA analysis and immunohistochemistry. The study endpoint for in vivo assays was time to tumor tripling (300%). Treatment of MCF-7/AC-1 cells with antibody MK-0646 (IC₅₀: 33nM) modestly inhibited cell growth, however combined treatment with MK-0646 and 4-hydroxy tamoxifen (active metabolite) (IC₅₀: 200nM) led to enhanced anti-proliferative activity. Combination of MK-0646 with letrozole (IC₅₀: 600nM) didn't appear to have any synergistic antiproliferative activity. During the four weeks of treatments in vivo, all treatment groups were effective at reducing the growth of tumors compared to control. MK-0646 + tamoxifen significantly improved (p= 0.027) antitumor activity compared to MK-0646 alone, but not tamoxifen alone (p= 0.21). MK-0646 + letrozole did not have significantly improved antitumor activity over MK-0646 (p= 0.056) or letrozole (p= 0.46). Similarly, time to endpoint was significantly increased for the MK-0646 + tamoxifen group compared to MK-0646 alone, but not tamoxifen alone. Treatment with MK-0646 + letrozole induced upregulation of insulin receptor isoform A more than 8-fold compared to approximately 4-fold induction with MK-0646 + tamoxifen and 5-fold induction with MK-0646. EGFR, HER2 and HER3 are upregulated in all MK-0646 treatment groups. In conclusion, hormonal therapies may upregulate insulin receptor A and erbB receptors as mechanisms of resistance to IGF-1R targeting. This effect appeared to be more profound with MK-0646 + letrozole, which may explain the differential activity of the hormonal agents in combination with MK-0646 in vivo.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2812.