** Shares of drugmaker Ironwood Pharmaceuticals (IRWD.O) rise 1.9% to $11.6 premarket ** Co said late on Monday the U.S. FDA has approved co’s drug Linzess as a once-daily treatment for pediatric patients aged between 6 and 17 years with functional constipation ** Linzess is developed and marketed by AbbVie (ABBV.N) and IRWD in the U.S. The drug is already available since 2012 for adults with irritable bowel syndrome with constipation ** Functional constipation is a chronic condition characterized by hard, infrequent bowel movements that are often difficult or painful to pass, affecting 6 million 6-17 years-old in U.S. - IRWD ** The approval is based on data from late-stage study that showed statistically significant improvement in patient, when compared to placebo ** Up to Monday’s close, stock had fallen ~8% YTD