Pfizer Inc. is quitting a pair of phase II studies testing its experimental Duchenne muscular dystrophy (DMD) drug domagrozumab after finding the anti-myostatin candidate offered boys with DMD no more benefit than a placebo during a stair-climbing test. The candidate was one of the 15 programs the company had identified as potential blockbuster medicines. The company's early stage gene therapy candidate for DMD remains in development.