Background: Ketoconazole (keto), a known CYP17 inhibitor, is a traditional systemic treatment for CRPC. However, most of the published data has been in the pre-chemo setting; its efficacy in the post-chemo setting has not been as widely reported. Chemo-naive patients treated with attenuated doses of keto (200-300 mg TID) had prostate specific antigen (PSA) response rate (> 50% decline) ranges from 21%-62% and treatment was well tolerated. We hypothesized that low dose keto would likewise possess efficacy and tolerability in the CRPC post-chemo state. Methods: CRPC patients with ECOG PS 0-3, adequate end organ function, who had received at least one chemo were treated with low-dose keto (200 mg PO TID) and HC (20 mg PO q AM and 10 mg PO q PM) until progression, as defined by either RECIST or PSA rise > 50% from nadir or baseline. Primary endpoint was PSA response rate (> 50% reduction from baseline). A Simon minimax design was used. PSA response of > 25% was to be considered promising for further study (versus null rate of < 5%); 25 patients were required. Secondary endpoints included PSA response > 30%, progression-free survival (PFS), duration of stable disease, and evaluation of adverse events (AE). Results: 29 patients were accrued: median age was 71 (range 55-86) and median pretreatment PSA was 76 ng/mL (range 7-11,420 ng/ml); all had prior docetaxel-based chemotherapy. 28 patients were evaluable for response; all were evaluable for toxicity. PSA response of >50% was seen in 48% of patients and 59% of patients had a PSA response of > 30%. Median PFS was 138 days; median duration of stable disease was 123 days. 12 patients had grade 3 or 4 toxicity on treatment. Of the 17 grade 3 AEs, only 3 were considered ‘probably’ or ‘possibly’ related to treatment, while none of the 2 grade 4 AEs were considered related to treatment. Conclusions: In docetaxel pre-treated CRPC patients, low-dose keto + HC is a well-tolerated, relatively inexpensive and clinically active treatment option. PSA response with low-dose keto appears comparable to that of abiraterone in this patient context. A prospective randomized study of available post-chemo options is needed to assess comparative efficacy. Clinical trial information: NCT00895310.