There is no approved second-line systemic therapy option for malignant pleural mesothelioma (MPM), but targeting angiogenesis is an area of investigation. PF-03446962 is a fully human antibody against activin receptor-like kinase 1, which is commonly expressed in tumor vasculature. We performed a multicenter, open label, single-arm, two-stage phase II study of PF-03446962 in patients with MPM and progressive disease after platinum-based chemotherapy. In total, 17 patients were enrolled, but no partial or complete responses were observed. The trial did not meet the prespecified response criterion for moving to the second stage. There were only three grade 3 (G3) or higher nonhematological toxicities observed (G3 hypertension [n=2] and G3 fatigue [n=1]) and just one episode of G3 lymphopenia. In conclusion, PF-03446962, despite being generally well tolerated, failed to demonstrate efficacy in the treatment of advanced MPM as a single agent. There are no plans for further investigation of this agent in MPM.