**Shares of drug developer Sarepta Therapeutics SRPT.O 2.1% to $19.78 in early trading

** Co says U.S. FDA has approved the start of new study to test whether adding an extra immune-suppressing drug can make its gene therapy for Duchenne muscular dystrophy safer for patients who can no longer walk

** DMD is a rare genetic disorder that causes progressive muscle weakness and loss, mostly in boys and young men

** The early stage trial enrolls ~25 patients who can no longer walk, co says

** Patients will receive co's Elevidys gene therapy plus sirolimus to lower risk of serious liver complications - RNA

** The therapy is approved for children aged four and older who can walk, but not for those unable to walk due to safety concerns - RNA

** Including session's move, stock down ~85% YTD

(Reporting by Sahil Pandey in Bengaluru)

((Sahil.Pandey@thomsonreuters.com))