** Shares of drug developer Mersana Therapeutics MRSN.O rise 5.38% to $1.38 premarket
** Co says U.S. FDA has granted additional fast track tag to its experimental drug XMT-1660
** The new tag is for the treatment of some patients with a type of breast cancer
** The FDA's Fast Track program is designed to facilitate the development and expedite the review of drug candidates to treat serious conditions and fill an unmet medical need
** Separately, co announced data from an early-stage trial of the drug in patients with a type of breast cancer
** Stock fell 38.36% last year
(Reporting by Sriparna Roy in Bengaluru)
((Sriparna.Roy@thomsonreuters.com;))