Japanese phase I study of cabazitaxel in metastatic castration-resistant prostate cancer

Background: We previously reported the pharmacokinetic profile and preliminary tolerability of cabazitaxel in a phase I study in Japanese patients with metastatic castration-resistant prostate cancer (mCRPC). Here we report the final safety profile and anti-tumor activity of cabazitaxel in a larger population, including all patients enrolled in the expansion cohort of the study. Methods: Japanese patients with mCRPC previously treated with docetaxel received cabazitaxel intravenously every 3 weeks plus daily prednisolone. In patients treated with the maximum tolerated dose of 25 mg/m² we evaluated adverse events including treatment-related neutropenia, prostate-specific antigen (PSA) response and objective response. Results: In total, 44 patients were treated with the maximum tolerated dose. The most frequent adverse events (any grade) were neutropenia (100 %), febrile neutropenia (54.5 %), fatigue (54.5 %), nausea (52.3 %) and diarrhea (50.0 %). There were no deaths due to treatment-related adverse events. Neutropenia with prior docetaxel did not appear to influence the probability of febrile neutropenia with cabazitaxel. Most patients received therapeutic granulocyte colony-stimulating factor (G-CSF; cycle 1: 86.4 %; cycle 2 or later: 81.8 %). In the efficacy population, two of 12 patients with measurable disease had partial response (objective response rate: 16.7 %), while 10 had stable disease. PSA response rate was 29.3 % (12/41 patients). Median time to PSA progression was 3.68 months (95 % confidence interval 1.35–4.63). Conclusions: In this heavily pretreated Japanese population, the occurrence of neutropenia and febrile neutropenia was high, suggesting G-CSF prophylaxis may be required as part of toxicity management. However, the efficacy of cabazitaxel was consistent with global studies. ClinicalTrials.gov identifier: NCT01324583.