RecruitingDrug trialStudy to Assess Safety and Efficacy of Vactosertib in Adolescents and Adults With Recurrent, Refractory or Progressive OsteosarcomaMP-VAC-209 is a Phase I/II, open label, single arm, multi-center study to assess safety, tolerability, and antitumor activity of vactosertib as a single agent in adolescents and adults with recurrent, refractory, or progressive osteosarcoma. Vactosertib is given orally, twice a day, to people 12 years of age and older who meet the criteria for study enrollment.