Publication Date

2013-12-11

Availability

Embargoed

Embargo Period

2015-12-11

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Molecular and Cellular Pharmacology (Medicine)

Date of Defense

2013-11-26

First Committee Member

Kerry Burnstein

Second Committee Member

Vladlen Slepak

Third Committee Member

Charles Luetje

Fourth Committee Member

Zafar Nawaz

Abstract

Prostate cancer is the most commonly diagnosed non-cutaneous malignancy in U.S. men. While tumors initially respond to androgen-deprivation therapy, the standard care for advanced or metastatic disease, tumors eventually recur as castration-resistant prostate cancer. Upregulation of the insulin-like growth factor signaling axis drives growth and progression of prostate cancer by promoting proliferation, survival, and angiogenesis. Ganitumab (formerly AMG 479) is a fully human antibody that inhibits binding of IGF-1 and IGF-2 to IGF-1R. Ganitumab decreased IGF-1 induced phosphorylation of the downstream effector AKT and reduced proliferation of multiple androgen-dependent and castration-resistant human prostate cancer cell lines in vitro albeit to varying extents. We evaluated the therapeutic value of ganitumab in several pre-clinical settings including androgen-dependent prostate cancer, castration-resistant (recurrent) prostate cancer, and in combination with androgen-deprivation therapy. Ganitumab inhibited androgen-dependent human VCaP prostate cancer xenograft growth and increased tumor doubling time from 2.3±0.4 weeks to 6.4±0.4 weeks. Ganitumab inhibited growth of castration-resistant VCaP xenografts for over 11.5 weeks of treatment. In contrast, ganitumab did not have appreciable effects on the castration-resistant CWR-22Rv1 xenograft model. Ganitumab was most potent against VCaP xenografts when combined with androgen-deprivation therapy (castration). Tumor volume was reduced by 72% after 4 weeks of treatment and growth suppression was maintained over 16 weeks of treatment. However, a significant proportion of these tumors acquired resistance to ganitumab. To address mechanisms of resistance, we developed an in vitro model of ganitumab resistance, which we termed VCaP/GanR, by prolonged culture of the VCaP human prostate cancer cell in ganitumab. Acquired resistance to ganitumab in VCaP/GanR was a reproducible and stable phenotype. Unlike parental VCaP cells, VCaP/GanR did not undergo apoptosis following ganitumab treatment or serum deprivation. VCaP/GanR did not express increased levels of IGF-1R, insulin receptor, or phospho-AKT compared to parental VCaP. Increased IGF-1R, insulin receptor and phospho-AKT are associated with acquired resistance to IGF-1R inhibition in other cancers. VCaP/GanR exhibited increased levels of phosphorylated S6 indicative of high mTOR activity. However, acquired resistance to ganitumab was not dependent on increased mTOR activity in VCaP/GanR as mTOR inhibitor rapamycin failed to restore sensitivity to ganitumab. Phospho-proteomic arrays revealed alterations in several signaling cascades in VCaP/GanR compared to parental VCaP cells. Inhibitors were identified that restored the antiproliferative effects of ganitumab in VCaP/GanR. These data implicate a new mechanism of resistance to IGF-1R inhibition and support the judicious use of ganitumab in the treatment of prostate cancer.

Keywords

prostate cancer; IGF-1R; antibody therapy; resistance; castration-resistant prostate cancer; cancer therapy

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