Robert Mulroy is now CEO of Partner Therapeutics, a Boston cancer drug developer. Mulroy was co-founder and CEO of Cambridge-based Merrimack Pharmaceuticals (NASDAQ: MACK). Partner also named Debasish Roychowdhury its chief medical officer. Roychowdhury was most recently chief medical officer of San Diego-based Seragon Pharmaceuticals, which was acquired by Genentech. His experience includes senior management posts in oncology at Sanofi (NYSE: SNY) and Eli Lilly (NYSE: LLY). Recently launched Partner says it plans to acquire late-stage cancer compounds to build its drug portfolio.