Background: Cabazitaxel (Cbz) is a novel taxane with broad in vivo efficacy in taxane-sensitive and -resistant tumors. Clinical activity of Cbz was confirmed in the Phase III TROPIC trial (NCT00417079); Cbz significantly improved overall survival compared with mitoxantrone in patients (pts) with metastatic castration-resistant prostate cancer whose disease had progressed during or after a prior docetaxel-containing regimen. Therapeutic synergism of Cbz/cisplatin (Cis) has been demonstrated in tumor-bearing mice. Methods: The primary objective in part 1 of this Phase I study (NCT00925743) was to determine the maximum tolerated dose (MTD) and dose-limiting toxicities (DLTs) of Cbz/Cis. Secondary objectives were safety, pharmacokinetics (PK) and efficacy. The primary objective of part 2 was to determine the antitumor activity at the MTD. Eligible pts were ≥ 18 yrs, ECOG PS ≤ 1, with confirmed metastatic or unresectable solid tumors for which Cis therapy was considered appropriate. A 3 + 3 dose escalation with a starting dose (level 0) of 20/75 mg/m² (Cbz/Cis), administered IV Q3W, was used. The MTD was the highest dose at which 0/3 or ≤ 1/6 pts experienced a DLT in Cycle 1. Results: Pts (N = 25; 10 in part 1; 15 in part 2) with a median age of 56 yrs were enrolled. The most frequent primary tumors were lung (n = 4), prostate, ovary and pancreas (each n = 2). 2 of 6 evaluable pts experienced a DLT at dose level 0 (Grade 3 acute renal failure; febrile neutropenia). The MTD was 15/75 mg/m² as no DLTs were observed. Eighteen pts were treated at the MTD and 60 cycles were administered (median = 3; range 1–8). The most frequent non-hematologic treatment-related adverse events (all Grade/Grade 3–4) were nausea (78%/22%), vomiting (72%/11%), fatigue (61%/17%), anorexia (67%/11%) and diarrhea (44%/0%). Incidence of Grade 3–4 neutropenia was 78% and 1 pt had febrile neutropenia. No PK interactions between Cbz and Cis were observed. Stable disease was seen in 11 pts. No objective responses were reported. Conclusions: The MTD of Cbz/Cis was 15/75 mg/m². The combination had a manageable safety profile consistent with that of a platinum/taxane combination. No PK interactions were seen. Further investigations into the combined treatment combination are planned.