Abstract:TriN2755 is an alkylating antineoplastic agent for intravenous (IV) use, carrying the triazene group as the cytotoxic principal. Using a standard 3 + 3 design, a phase I study was performed in tumour bearing dogs to determine the maximum tolerated dose (MTD), the dose limiting toxicity (DLT), and pharmacokinetic (PK) profile ofTriN2755. Thirty dogs were included in the study.TriN2755 was administered over 20 min on two consecutive weeks per month for a total of three cycles. The starting dose was 25 mg kg−1and theMTDwas 74.6 mg kg−1. Three dogs experiencedDLT, which was characterized by gastrointestinal adverse events. The PKs ofTriN2755 and its main metabolites in plasma and sputum are described in a two‐compartment model. The response rate for 19 of 30 dogs was 47.3% (six partial remission, three stable disease) and the median progression‐free interval (PFI) for the responders was 47 days (range: 21–450 days).