Zynerba Pharmaceuticals Inc. shares (NASDAQ:ZYNE) plunged more than 55 percent Monday to $6.67 on news that its lead candidate, the cannabidiol (CBD) gel ZYN-002, missed the primary endpoint of a phase II trial, failing to help adult epilepsy patients with focal seizures achieve a statistically significant reduction in seizures during the 12-week treatment period of the trial, called Star 1.