The maker of flibanserin --- a so-called female Viagra designed to increase libido in women by acting on brain chemicals --- is fighting the federal Food and Drug Administration's latest rejection of the mood-altering pill.

Sprout Pharmaceuticals announced today it has reached an impasse with the FDA, which has so far remains unconvinced the drug's benefits outweigh its risks, which include side effects such as fatigue, dizziness and nausea.

Sprout said it is appealing the FDA's decision earlier this year to deny approval of the pill and ask for more information about flibanserin, which is a once-daily treatment for Hypoactive Sexual Desire Disorder, or HSDD, in pre-menopausal women, the most commonly reported form of female sexual dysfunction.

After turning down the drug in 2010 when it was developed by another pharmaceuticals company, the agency rejected flibanserin this time after Sprout re-submitted the application for the drug with 14 new clinical studies and data on more than 3,000 new patients, expanding the number of subjects who completed clinical trials to more than 11,000.

According to the new application, about 5,000 subjects were treated with flibanserin in Phase III efficacy studies.

"Sprout strongly believes that it has satisfied the statutory requirements for approval of flibanserin," said Cindy Whitehead, Sprout's president and chief operating officer. "In clinical trials, flibanserin has outperformed placebo with statistical significance and, importantly, women treated with flibanserin perceived their improvement to be clinically meaningful."

Whitehead disputed the FDA's characterization of flibanserin's beneficial effects as "modest" by explaining in a company release that "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. 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 safety of flibanserfin, said Whitehead, "is similar to that of other approved drugs where effect was characterized as modest by the agency and can be managed through a combination of labeling and responsible product launch, including post-market assessments and communications. If approved, flibanserin could meet an important unmet medical need for women."

The FDA accepted Sprout's dispute resolution request Dec. 3 and is expected to issue a response to the appeal during the first quarter of next year.

Based on prior cases, however, that chance flibanserin is granted approval through the appeals process appears narrow. Out of 17 appeals the FDA considered last year, 14 were denied.

This latest rejection puts a damper on the efforts of the half-dozen or so companies focused on developing therapies for women who suffer stress due to lack of libido --- a demographic pharmaceuticals companies have been trying to crack since Viagra, the highly successful erectile-dysfunction drug that increases blood flow to male genitals, was approved in the late 1990s.

Unlike male sexual dysfunction, which is frequently linked to physical problems, a majority of female sexual issues are psychological.

So, there are a variety of different causes doctors must consider --- including relationship problems, hormone disorders, depression and mood issues caused by other medications --- before making a diagnosis of female sexual desire disorder.

That said, Sprout's flibanserin is the first drug to approach the female sexual dysfunction via brain chemistry.