Androgen ablation is the standard treatment for advanced disease. Despite initial success, approximately 70% of patients eventually progress to hormone refractory prostate cancer (HRPC). To investigate the mechanisms of this resistance we utilized human prostate cancer cells (LNCaP). Our studies suggest a compensatory signaling mechanism and cross-talk pathway between AR and PI3K/Akt/mTOR pathway occurs with androgen ablation treatment in-vitro. We also demonstrated that dual inhibition of AR and mTOR synergistically inhibited cell proliferation. The current investigation analyzed the interaction of the AR and mTOR pathways in-vitro and in-vivo. Using co-immunoprecipitation of Akt and AR, it was found that the interaction between the pathways increases 7-fold as cells progressed from a hormone sensitive to hormone resistant model. The interaction was reduced 36% with bicalutamide and 54% with our novel anti-androgen/androgen synthesis inhibitor, VN/124-1. Everolimus, an mTOR inhibitor, also reduced the interaction modestly (37%). However, bicalutamide plus everolimus reduced the interaction by 71% while VN/124-1 plus verolimus reduced the interaction by nearly 100%. An in vivo hormone dependent LNCaP xenograft investigation demonstrated that the addition of everolimus to bicalutamide resistant tumors significantly decreased growth rate and tumor volume compared to single agent bicalutamide (p=0.0001) or everolimus (p=0.04). Additional in vivo experiments investigated tumors progression following anti-androgen therapy. PSA increased in control mice from 2.9±0.2ng/nl at week 2 to 4.3±0.568ng/ml at week 3 and to 7.4±0.783ng/ml at week 4 confirming disease progression. Both bicalutamide and VN/124-1 increased IGFR, p-Akt, p-70S6K and p-P6S6 protein expression compared to control xenografts. A third xenograft investigation with hormone resistant prostate cancer cells (HP-LNCaP) demonstrated that dual inhibition with everolimus (5mg/kg/QW-PO) and VN/124-1 (100mg/kg/QD-SC) significantly (p=0.03) reduced tumor volume and growth rate compared to single agent everolimus. Furthermore, growth trends indicate that VN/124-1 plus everolimus was more effective than bicalutamide plus everolimus. In summary, the interaction between the AR and Akt pathways is decreased by dual inhibition of AR and mTOR. As tumors progress on anti-androgen therapy, there is a more rapid increase in signal transduction pathway activation compared to controls. As such, the combination of anti-androgen, such as VN/124-1, and mTOR inhibitors from the beginning significantly reduced tumor volume compared to everolimus or bicalutamide alone in HRPC xenografts. Improved understanding of the interaction between the AR and mTOR pathways reveals a better strategy for treating disease progression that may delay, re-sensitize or prevent HRPC.
99th AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA