Purpose

A 2-part, Phase 1-2, open-label, parallel group, randomized study in patients with Castration-Resistant Prostate Cancer (CRPC) who are no longer responding to treatment with abiraterone and steroids. In Part A (Phase 1), patients will continue to receive the same doses of abiraterone and steroids they were receiving prior to study entry and will be randomized to receive 1 of 2 different treatment regimens of AT13387 in combination with abiraterone. Once the best regimen is established in Part A, based on safety and antitumor activity, patients will be randomized to the selected treatment regimen and dose of AT13387 in combination with abiraterone or AT13387 alone in Part B (Phase 2).