Tesaro‘s (NASDAQ: TSRO) ovarian cancer drug, niraparib, will begin competing for prescriptions earlier than expected. Niraparib, which will be sold under the brand name Zejula, notched FDA approval on Monday, more than three months ahead of its June 30 decision date. The early approval sets Tesaro up for a fierce battle for market share with AstraZeneca (NYSE: AZN) and Clovis Oncology (NASDAQ: CLVS), both of which market drugs that work similarly to Zejula.