Shares in Depomed have taken a knock after US regulators requested more data on its recently approved pain drug Gralise in order to grant special marketing exclusivity.

Gralise has been granted Orphan Drug designation by the Food and Drug Administration as a once-daily formulation of gabapentin for the management of postherpetic neuralgia, primarily because of the small patient population (around 70,000 - 100,000 patients in the US) and, importantly, a lower incidence of side effects observed in clinical trials compared to that reported for immediate release forms of the medicine.

However, in a regulatory filing this week the company said that, following the FDA’s approval of Gralise last month, the regulator asked for additional submissions/evidence to prove the clinical superiority of the drug - based on its improved safety profile - before it will grant the seven years of marketing exclusivity associated with Orphan Drug status. 

For now Depomed is remaining tight-lipped on the issue, but its stock slipped 8% on the news, marking a knee-jerk reaction on fears that new studies will be needed to provide the information requested by the FDA, though some think this unlikely.  

"Shares are down on concerns if [Depomed] will run new trials but the FDA just wants a cross-study comparison, a meta-analysis of Gralise, not any new trials," said Caris & Company analyst James Molloy, as reported by Reuters

Gralise is licensed to Abbott in the US, Canada and Mexico (via its acquisition of Solvay in 2008), but the drug giant is currently locked in a dispute with Depomed about whether it is obliged to launch and commercialise the medicine under the terms of the contract.