BACKGROUND: Histone deacetylase blockade can promote heat shock protein 90 (HSP90) acetylation, abrogating androgen receptor signaling. A phase II trial of the histone deacetylase inhibitor (HDACi) romidepsin was conducted in patients with progressing, metastatic, castration-resistant prostate cancer (CRPC).PATIENTS AND METHODS: A dose of 13 mg/m(2) was administered i.v. over 4 h on days 1, 8 and 15 every 28 days. The primary end point was rate of disease control defined as no evidence of radiological progression at 6 months. A sample size of 16 assessable patients in stage 1 and nine assessable patients in stage 2 was selected; progression to stage 2 required one or more patients with disease control in stage 1 (H(o) = 0.10, H(a) = 0.30; alpha and beta = 0.10).RESULTS: Thirty-five patients were enrolled. Two patients achieved a confirmed radiological partial response (RECIST) lasting > or = 6 months, along with a confirmed prostate-specific antigen decline of > or = 50%. Eleven patients experienced toxicity necessitating early discontinuation. The commonest adverse events were nausea (30 patients; 85.7%), fatigue (28 patients; 80.0%), vomiting (23 patients; 65.7%) and anorexia (20 patients; 57.1%). There was no significant cardiac toxicity.CONCLUSIONS: At the dose and schedule selected, romidepsin demonstrated minimal antitumor activity in chemonaive patients with CRPC. Further studies of improved HDACi, alone and in combination with other therapies, should nevertheless be investigated.