Canadian biotechnology company Cardiome saw its share price tumble yesterday after the US drug regulator asked for more safety data for its atrial fibrillation drug Kynapid (vernakalant hydrochloride).

Specifically, the Food and Drug Administration has asked for more information on an undisclosed risk identified in a subset of patients, as well as a safety update on the drug’s usage. Both companies say they will “work closely” with the agency to address all the issues raised in the approvable letter, but point out that a complete response is likely to take “several months” even if, as anticipated, an additional clinical trial may not be required.

Vernakalant selectively blocks ion channels in the heart that are known to be active during episodes of atrial fibrillation. The intravenous formulation is the initial focus and has completed three Phase III clinical trials, as well as an open-label safety study in recent-onset AF patients.

Meanwhile, positive results from a Phase 2b study of the oral version were announced in July, sending Cardiome’s shares shooting up 35%. This gain was lost yesterday, however, with the stock sinking 30% on the Toronto Stock Exchange to C$9.

Kynapid is partnered with Japan’s Astellas under an agreement signed in 2003 for North America. Cardiome retains worldwide rights to oral vernakalant for the prevention of AF recurrence and all rights to the intravenous formulations outside of Canada, US and Mexico.