Phase 1 and dose-finding study of patritumab (U3-1287), a human monoclonal antibody targeting HER3, in Japanese patients with advanced solid tumors

Purpose Patritumab (U3-1287) is a human epidermal growth factor receptor-3 (HER3)-targeted antibody that blocks ligand-associated activation of HER3. This open-label, phase 1 and dose-finding study (ClinicalTrials.jp Identifier: JapicCTI-101262) aimed to assess the safety, pharmacokinetics, incidence of anti-patritumab antibody, recommended dose for subsequent clinical studies, preliminary efficacy, and patritumab-related biomarkers in Japanese patients with advanced solid tumors. Methods Patients received patritumab 9 or 18 mg/kg intravenously every 3 weeks until disease progression or intolerable toxicity occurred. Adverse events (AEs) were assessed according to the National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE version 4.0). Dose-limiting toxicities (DLTs) were evaluated from the initial dose to Cycle 1 Day 21. Tumor response was assessed with Response Evaluation Criteria in Solid Tumors (RECIST version 1.1). Results Nine patients received patritumab 9 mg/kg ( n = 3) or 18 mg/kg ( n = 6). Five patients were male, all patients had Eastern Cooperative Oncology Group performance status (PS) ≤ 1, and median (range) age of 67 (50–69) years. No DLTs were reported. Patritumab-related AEs reported in ≥2 patients were ALT increase (three patients), thrombocytopenia, diarrhea, stomatitis, cheilitis, rash maculo-papular and AST increase (two each). Pharmacokinetics profile was similar to the preceding US phase 1 study. Soluble HER3 concentration in serum unexpectedly increased in all patients. These changes did not correlate with clinical response. Four patients had a best response of stable disease. All patients tested had negative for anti-patritumab antibody formation. Conclusions Patritumab was well tolerated up to 18 mg/kg without DLTs in Japanese patients with advanced solid tumors. Soluble HER3 increased in all patients.