This was a 52-week , randomised , double-blind study of dapagliflozin ( n = 406 ) versus glipizide ( n = 408 ) , uptitrated over 18 weeks according to tolerability and glycaemic response to a maximum of 10 and 20 mg/day , respectively , as add-on therapies to metformin ( 1500 mg/day ) with a 156-week double-blind extension period .