** Shares of drug developer Artiva ARTV.O surge 102.5% to $5.57 premarket

** Co on Thursday said U.S. FDA granted fast track designation to its cell therapy AlloNK for treating refractory rheumatoid arthritis

** U.S. FDA's "fast track" tag aims to speed development and review of drugs for serious conditions with unmet medical need

** Refractory rheumatoid arthritis is a chronic joint disease

** Over 20 patients tested with AlloNK plus antibody therapy across autoimmune trials, including RA and lupus- ARTV

** Lupus is a chronic autoimmune disease where the body's immune system mistakenly attacks its own tissues and organs

** Co says safety data to highlight tolerability and ease-of-use

** Up to last close, stock down ~73%

(Reporting by Sahil Pandey in Bengaluru)

((Sahil.pandey@thomsonreuters.com))