DUBLIN – Shares in PTC Therapeutics Inc. dropped as much as 13% during premarket trading Feb. 6 on eagerly awaited 12-month data for its oral spinal muscular atrophy (SMA) drug risdiplam, which is licensed to Genentech. Although patients with type 2 or type 3 SMA demonstrated statistically significant improvements in motor function, the level of that improvement evidently disappointed some investors.