BACKGROUND: XRP6258 is a novel taxoid with a low affinity for P-glycoprotein. This multicenter phase II study assessed the activity of XRP6258 in the treatment of taxane-resistant metastatic breast cancer (MBC). PATIENTS AND METHODS: XRP6258 was administered as a 1-h i.v. infusion every 3 weeks at 20 mg/m(2) (then, in the absence of severe toxicity, at 25 mg/m(2) from cycle 2). The primary end point was the objective response rate (ORR) assessed according to response evaluation criteria in solid tumours (RECIST) guidelines. RESULTS: Seventy-one patients were enrolled. The median relative dose intensity was 0.98. The ORR was 14% (two complete, eight partial responses). Eighteen patients (25%) had stable disease of >3 months duration. At a median follow-up of 20.0 months, the median time to progression was 2.7 months, and the median overall survival 12.3 months. The most common grade 3/4 adverse events (AEs) were neutropenia (73%) and leucopenia (55%), with a low febrile neutropenia rate (3%) and infrequent grade 3/4, treatment-related, non-hematological AEs (<5% patients for any AE). Two deaths were reported, one related to study drug and one to unknown cause. CONCLUSIONS: XRP6258 was active and well tolerated in this group of MBC patients with taxane-resistant disease. These results support the further clinical development of this agent.