** Shares of drugmaker TG Therapeutics TGTX.O rise 3% to $42 premarket

** Co says experimental use of its multiple sclerosis drug Briumvi in myasthenia gravis, a rare disease causing muscle weakness and fatigue, showed 82% of patients improved daily activity ability

** Says average symptom score improved by more than four points in 11-patient early-stage study; treatment was generally well tolerated

** TGTX says has started mid-stage trial testing Briumvi as maintenance therapy to sustain symptom control

** Says about 120 patients to be enrolled; responders to existing drug efgartigimod will get Briumvi or placebo

** Adds it aims to expand Briumvi, already approved for multiple sclerosis, into new indication

** As of last close, stock down ~19% YTD