WASHINGTON A pink pill designed to offer women a counterpart to Viagra failed to boost female sex drive in two studies, U.S. drug reviewers said in documents released on Wednesday.
Women who took Boehringer Ingelheim's experimental drug flibanserin also reported depression, fainting, fatigue and other problems. The drug's tolerability was "only moderate," Food and Drug Administration staff said.
An FDA advisory panel meets Friday to weigh whether to recommend approval for flibanserin. The once-a-day pill, nicknamed the "pink Viagra," is the latest attempt at a female version of Pfizer Inc's blockbuster blue pill for men.
Drugmakers have long sought to tap a potentially lucrative market for a medication to boost a woman's libido. Viagra and other male impotence drugs have failed to help women.
Boehringer is seeking FDA approval to sell its pill by prescription for treating premenopausal women with hypoactive sexual desire disorder, defined as a lack of sex drive in otherwise healthy women.
The proposed brand name is Girosa. It is not yet approved anywhere in the world.
FDA staff, in a preliminary review, outlined "major" concerns on the company's effectiveness data. In two clinical trails, the drug increased the number of sexually satisfying experiences but failed to improve sexual desire.
"Therefore, neither study met the agreed-upon criteria for success in establishing" efficacy, the reviewers wrote in a summary prepared for the advisory panel.
Fatigue, drowsiness and sedation were "commonly reported" and "it is not clear if labeling alone will be sufficient to alert women to the numerous drug interactions that exist with flibanserin," they said.
Depression, fainting and accidental injury also were more frequent with the drug than a placebo.
Nearly 15 percent of women who took the recommended flibanserin dose stopped taking it before the study ended due to possible side effects, FDA staff added.
Flibanserin acts on neurotransmitters in the brain and was originally tested as an antidepressant. FDA reviewers recommended that the drug, if approved, carry a warning required for all antidepressants about risks of suicidal thoughts and behavior in children and young adults.
Boehringer, a privately held German drugmaker, said its data showed the drug provided a meaningful benefit for women who are bothered by their low sex drive and have limited options. The number of satisfying sexual experiences rose to an average of 4.5 per month from 2.8. For placebo patients the rate jumped to 3.7. Women took the drug for six months.
The company also said an alternative analysis showed the drug did increase sexual desire, and distress about low sex drive decreased.
"We are confident in our data that flibanserin's safety and efficacy has been demonstrated," Dr. Peter Piliero, Boehringer's executive director for U.S. medical affairs, said in an interview.
Potential side effects were low and mild to moderate, Boehringer said.
The FDA will make the final decision on whether to approve the drug but usually follows panel recommendations.
Some critics question if flibanserin will appeal to women if it wins FDA clearance. They note women must take it every day to maintain the effect on brain chemistry, while men take Viagra only as needed ahead of sexual activity.
"Both of those things, that it's a chronic drug and that it works on brain chemistry, raise important safety questions," said Amy Allina, program and policy director for the National Women's Health Network.
Drugmakers have tried various approaches to boost women's sex lives. Female and male sexual arousal are triggered by the same biological mechanism, nitric oxide increasing blood flow, and arousal is strongly linked to libido in women.
But women's sex lives have proved more difficult to target with medication.
Pfizer's Viagra and other pills to treat male impotence work by widening blood vessels to increase the blood flow needed for an erection. They have failed to help women.
Procter & Gamble Co tried to win U.S. clearance to sell a testosterone patch to treat female sexual dysfunction. An FDA advisory panel voted in 2004 against recommending approval, citing a lack of evidence for long-term safety. The patch was approved in Europe and has been sold to Warner Chilcott.
Another U.S. drugmaker, BioSante, is developing a testosterone skin gel to treat a decline in libido in menopausal women. The company estimates the market for treating female sexual dysfunction in the United States tops $2 billion annually.
(Reporting by Lisa Richwine; additional reporting by Susan Heavey; Editing by Maureen Bavdek and Tim Dobbyn)