A phase I study of ontuxizumab, a humanized monoclonal antibody targeting endosialin, in Japanese patients with solid tumors

Background We conducted a first-in-Japanese, phase I study of ontuxizumab, a humanized, anti-endosialin monoclonal antibody, to confirm its tolerability, safety, and pharmacokinetics, and identify exploratory efficacy. Methods This was a multicenter, multiple-dose, open-label study in Japanese patients aged ≥20 years with solid tumors, including gastric cancer (GC) or advanced hepatocellular carcinoma (HCC), who had failed standard chemotherapy. The study comprised two parts: part 1 (dose-escalation; ontuxizumab 2–12 mg/kg weekly) and part 2 (cohort-expansion; 4 or 8 mg/kg weekly, or 12 mg/kg biweekly). Results Fifteen patients were treated in part 1, and 31 in part 2 (16 patients with GC and 15 with HCC). In part 1, the most common treatment-related, treatment-emergent adverse event (TEAE) was fatigue (20%); no patients had grade ≥ 3 treatment-related TEAEs. In part 2, the most common treatment-related TEAEs were constipation, malaise, hiccups, and increased bilirubin; treatment-related grade 3 TEAEs occurred in two patients with HCC. In part 1, no patients achieved a partial response, and 6/15 (40%) had stable disease (SD). In part 2, 2/15 patients (13.3%) with GC and 8/15 (53.3%) with HCC had SD. Tumor shrinkage was observed in 5/15 HCC patients (33.3%). Conclusions Ontuxizumab, up to a dosage of 12 mg/kg weekly, was generally safe and well tolerated in this population, with no dose-limiting toxicities. The maximum tolerated dose was not reached; 8 mg/kg weekly or 12 mg/kg biweekly were the recommended dosages. We observed long-term disease stabilization in GC and extraskeletal chondrosarcoma, and tumor shrinkage in gastrointestinal stromal tumor and HCC. Trial registration: NCT01773434 (ClinicalTrials.​gov).