Rhythm Pharmaceuticals Inc., of Boston, said the FDA expanded a previously granted breakthrough therapy designation for setmelanotide, the company's melanocortin-4 (MC4) receptor agonist, to include the treatment of obesity associated with genetic defects upstream of the MC4 receptor in the leptin-melanocortin pathway, which includes both pro-opiomelanocortin (POMC) and leptin receptor deficiency obesity.