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Aim Cabazitaxel (CBZ), a novel tubulin-binding taxane, improves overall survival in metastatic castration-resistant prostate cancer (mCRPC) that progresses during or after docetaxel treatment. We have designed a phase II study to evaluate the efficacy and safety of CBZ as a weekly schedule for ‘unfit’ mCRPC patients after docetaxel failure. Methods In this single arm phase II study. CBZ was weekly administered in 1-hour infusion on days 1, 8, 15 and 22, every 5 weeks at 10 mg/m2 to eligible ‘unfit’ patients; oral prednisone (5 mg) was administered twice a day. Circulating tumour cells (CTCs) were also collected. New treatment scheme was considered effective if at least 65% of patients met a clinical benefit criteria based on prostate-specific antigen (PSA)-progression-free survival (PFS) values at week 12. Results Seventy patients (median age: 73.9 years) were enrolled; overall, 71.4% had an Eastern Cooperative Oncology Group Performance Status (ECOG-PS) of 2; and 84%, 16% and 11% had bone, liver and lung metastases, respectively. Objective partial response or stable disease was achieved in 61% of patients, while PSA responses of ≥50% and ≥80% were observed in 34.8% and 10.6%, respectively. The median PSA-PFS was 4.8 months; and 68.6% of patients had no progression at week 12. The most frequent grade 3/4 toxicities were neutropenia (2.8%), leukopenia (5.7%) and thrombocytopaenia (9%); no cases of febrile neutropenia were reported. Early CTC response was significantly correlated with PSA-PFS. Conclusions CBZ/prednisone administered weekly to ‘unfit’ mCRPC patients appears to be as effective as classical standard 3-week scheme (TROPIC study) but with significantly lower toxicities and better tolerance. Early CTC response appears to be valuable as an early end-point of therapeutic efficacy.