PTC Therapeutics falls after mid-stage data for Huntington's disease drug

Shares of drug developer PTC Therapeutics PTCT.O fall ~20% to $40 premarket

Co says its experimental drug to treat Huntington's disease met the main goal of a mid-stage study

Huntington's is a rare neurological disease which impairs a person's ability to think, feel and move

The drug, PTC518, helped significantly lower levels of a mutated protein in blood that causes the disease - PTCT

"Stock has rebounded into the data so some may have been looking for indisputable results, but overall we believe the initial data looks to maintain the promise of the program, which over time we believe should be a win," said RBC Capital analysts

Up to last close, stock up ~11% YTD