Sprout fires back at the FDA after its female libido drug endures a rejection

The FDA has shrugged off Sprout Pharmaceuticals' female libido drug, further delaying the years-long quest to develop a Viagra equivalent for women. But the tiny drugmaker--which rounded up angel backing on a promise that they could quickly mount a new application for approval after Boehringer Ingelheim abandoned the therapy--is appealing the rejection, arguing that the agency is misunderstanding how to measure efficacy.

Sprout's drug, flibanserin, failed to impress regulators on its way to a rejection earlier this year, the company disclosed, but COO Cindy Whitehead said she believes the treatment has satisfied the requirements for approval, and now Sprout has filed a formal dispute with the FDA in hopes of getting another shot.

Sprout picked up flibanserin from Boehringer Ingelheim in 2011 after the pharma giant was subjected to a stinging rejection at the hands of the FDA. The biotech gained $20 million from angel investors last year. Its application, Sprout says, piled on new data on about 3,000 patients, and the company handed in results from more than 11,000 women that demonstrated the drug as well-tolerated and effective in boosting the frequency of satisfying sexual experiences, its chief efficacy endpoint.

In flibanserin's pivotal results, women on the drug averaged 2.5 satisfying instances over 28 days, compared  to 1.5 for those on placebo. While Sprout calls that a clinically meaningful difference, the FDA was unmoved, especially in light of the drug's risks of fatigue and nausea. Still, Whitehead maintains that the agency is failing to understand what constitutes a positive effect for women with hypoactive sexual desire disorder.

"What the division characterizes as a modest effect in their risk/benefit evaluation, we see as meaningful effect on the hallmark characteristics of the condition," Whitehead said in a statement. "It is the basis of our dispute. It is true that flibanserin does not cause hypersexuality, but by that definition, modest would be what we're aiming for."

The agency received Sprout's request for dispute resolution on Dec. 3, the company said, and management expects to get a response sometime next quarter.

Sprout hasn't given up on flibanserin, but the drug's rocky developmental path typifies the industry's struggle to get a female libido drug to market. And while Sprout has taken the treatment further than its prior owner, both ran into trouble with defining efficacy and laying out endpoints, a common pratfall for drugs targeting female sexual desire.

By contrast, Pfizer ($PFE) investigators developing Viagra needed only to demonstrate a physiological effect on erectile dysfunction. Mapping out a measurement of clinical effect on female desire has thus far been more nuanced.

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