Synergistic effect of Rapamycin and Casodex in inhibition of cell growth and survival as well as AR transcriptional activity in androgen dependent and independent prostate cancer cells

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Recurrent prostate cancer is treated with androgen deprivation, which includes suppression of androgen production alone or in combination with androgen receptor (AR) antagonists (including flutamide or bicalutamide (Casodex)). Patients usually respond initially to such treatment but frequently relapse indicative of the development of hormone refractory prostate cancer. The treatment options for such patients are limited, and while Phase I trials reveal 20-24% increased survival with docetaxel, options to prevent the progression of this disease have not been identified. Prostate cancer is characterized by frequent deletion or loss of heterozygosity in the tumor suppressor PTEN leading to activation of downstream components of the P13K pathway, including the Akt and mTOR kinases. The mTOR inhibitor rapamycin has been reported to inhibit metastatic prostate tumor growth and angiogenesis in in vivo mouse models and is currently in Phase I clinical trials. While rapamycin inhibited the growth of both androgen dependent and independent prostate cancer, it also has been shown to increase AR levels and AR transcriptional activity in vitro, resulting increased expression of prostate specific antigen (PSA), a serum biomarker which is commonly used to track changes in prostate cancer patients. In contrast, the antiandrogen Casodex has been shown to inhibit tumor growth as well as PSA expression in androgen dependent tumors, whereas androgen independent tumors were refractory to Casodex treatment. Since the effects of these two drugs on PSA levels are opposing in vitro, we investigated the effects of using the two in combination. The androgen dependent cell line LNCaP and its androgen independent sublines C4-2 and LNCaP-AI were treated with 100 nM rapamycin, 10 μM Casodex or both. Rapamycin substantially inhibited proliferation in all cells examined, whereas Casodex inhibited the growth of LNCaP cells by 50% but did not affect the growth of the androgen in dependent sublines. However, in combination, the two together had a synergistic effect and inhibited the growth of both androgen dependent and independent lines to a greater extent than either alone. While neither drug caused a substantial increase in apoptosis rates, the combination of rapamycin and Casodex caused substantial amount of apoptosis even in androgen independent cells. In addition, rapamycin increased AR and PSA levels in all 3 cell lines, whereas in combination with Casodex, AR and PSA levels were decreased in all cells examined. These results suggest that Casodex and rapamycin in combination may be of better therapeutic value than each drug individually.