Bortezomib-mediated inhibition of steroid receptor coactivator-3 degradation leads to activated Akt.

Purpose: To assess the safety of administering bortezomib to patients undergoing a radical prostatectomy, to assess pathologic changes induced by bortezomib in prostate cancer specimen, and to verify alterations by the drug in proteasome protein targets.

Experimental Design: Bortezomib is a proteasome inhibitor that has shown activity in vitro and in vivo in prostate cancer. We performed a neoadjuvant clinical trial of bortezomib in men with prostate cancer at high risk of recurrence. The primary endpoints were to evaluate safety and biological activity.

Results: Bortezomib is generally safe in the preoperative setting. Antitumor activity was manifested by tumor cytopathic effect, drops in serum prostate-specific antigen in some patients, and increases in tumor apoptosis. This was associated with cytoplasmic entrapment of nuclear factor-kappaB. We found an unexpected increase in proliferation in treated tissues and in vitro. Bortezomib also increased SRC-3 levels and phosphorylated Akt, both in vitro and in treated prostate cancer tissues. Knockdown of SRC-3 blocked the increase in activated Akt in vitro. Combined treatment with bortezomib and the Akt inhibitor perifosine was more effective than either agent alone in vitro.

Conclusion: These data suggest that combined therapies targeting the proteasome and the Akt pathway may have increased efficacy.