Dave Mott, president and chief executive officer of AstraZeneca’s biologics business unit, MedImmune, has decided to leave the company at the end of July for personal reasons. Tony Zook, chief executive officer of AstraZeneca’s North American business and executive vice president of global marketing, has been appointed head of MedImmune on an interim basis until a permanent successor is named.     David Brennan, chief executive officer of AstraZeneca, said, “Under Dave Mott’s leadership...