** Shares of drug developer Nektar Therapeutics NKTR.O rise 17.3% to $99.74 premarket

** Co says its experimental treatment to treat patchy hair loss showed signs of hair regrowth in a extension study from a mid-stage trial

** Co was testing its under-the-skin-injection, called rezpegaldesleukin, in patients with severe-to-very-severe alopecia areata

** Co says from week 36 to 52, 29% of patients on low dose of the drug and 31% of patients at high dose had at least 80% or more of their scalp covered by hair compared to none in the placebo group

** Co says results support advancement of the therapy into late-stage development

** Rezpegaldesleukin well tolerated with safety profile consistent with previously reported results

** Up to last close, NKTR more than doubled YTD