Flibanserin was originally developed as an antidepressant, but in clinical trials against depression it reportedly failed to perform better than placebo. The standard for getting approved as an antidepressant is low, so this is quite an achievement.
The BBC today described flibanserin as the "Female Viagra", which is rather confusing, because it's meant to increase sexual desire, which is one thing Viagra (sidenafil) doesn't do. The reason for the Female Viagra headline is that, as Professor John Thorp says:
"It's essentially a Viagra-like drug for women in that diminished desire or libido is the most common feminine sexual problem, like erectile dysfunction is in men"Yes, one in ten women suffer apparently from "Hypoactive Sexual Desire Disorder" (HSDD) as Boehringer Pharmaceuticals helpfully informs us. And “As many as two out of every 10 women describe some degree of decreased sexual desire" according to the unfortunately named Dr Charles de Wet, Boehringer medical director for the UK.
HSDD is a diagnosis in the DSM-IV, the American Psychiatric Association's listing of psychiatric illnesses, and it's been recognised as a disorder since 1980. It is not, however, a very popular diagnosis yet. There are only 60,000 Google hits for it, as opposed to 1,600,000 for "major depression" and, er, 90,000 for "neuroskeptic". Odd for a disorder apparently plaguing at least 10% of women.
Indeed, some people say that it's no more than a label invented by psychiatrists who didn't understand women and then promoted by drug companies in order to sell drugs. This is almost certainly true, but it's also a bit simplistic, because there are people who perceive themselves as suffering from low libido, and if flibanserin really helps them, that's surely a good thing.
How is flibanserin supposed to work? According to a paper on the Pharmacology of Flibanserin, it's a serotonin receptor 5HT1A agonist and a 5HT2A antagonist. This makes it a kind of cross between the antidepressants nefazadone and buspirone. Neither of these are widely used as antidepressants because they're not considered highly effective. Flibanserin is also a weak dopamine D4 receptor partial agonist. This might underlie its aphrodisiac properties, because drugs which increase dopamine levels are known to enhance motivation and libido (or indeed cause problematic hypersexuality.) In rats and mice, flibanserin has sedative effects and enhances the effects of other sedatives. It also has antidepressant-like effects in some tests but not all. Drug geeks can click the image on the left for more details.
Now for the big question - does it actually work? Well, there have been no published clinical trials yet. At all. The trials in depression, where it failed to work, have never been published. Hmm. However, four trials in "Hypoactive Sexual Desire Disorder" were recently completed and the results were presented yesterday at a sexual medicine conference in Europe (ESSM) in the form of three posters (1,2,3). The trials were known as - groan - VIOLET, ORCHID, DAISY and DAHLIA. I probably don't have to tell you that they were all funded by Boehringer Pharmaceuticals.
The main poster is Efficacy of flibanserin 100 mg qhs as a potential treatment for Hypoactive Sexual Desire Disorder in premenopausal women which pools the data from three trials with a total of about 1,400 women. They found that taking flibanserin 100 mg every night had small beneficial effects. Relative to placebo, it increased the number of "satisfying sexual encounters" by 0.7 per month. It also improved scores on questionnaire measures of sexual function, a bit.
In any trial like this you have to ask whether there is result cherry-picking going on. Maybe they asked dozens of questions about the women's sex lives, and they're only telling us about the minority where the drug seemed to work? People often do that but in this case, the Clinical Trials Register suggests there was no funny business of that kind. It also shows that there have been no trials using 100mg which weren't included in the poster, so the trials themselves weren't cherry picked either. That's reassuring. But it looks like the effects were only significant when all three trials were pooled - one poster shows the results of the ORCHID trial alone, and most were non-significant.
What about the side effects? There's a whole poster about them. 100 mg flibanserin nightly caused 14% of patients to drop out due to side effects, vs 7% in the placebo group - so an extra 7% decided it wasn't worth it. It caused dizziness, nausea, fatigue, somnolence - and bizarrely, also insomnia. Notably, 50mg daily was much worse than 100 mg nightly, which suggests that taking this at night, rather than in the morning, is a good idea. But given what it is meant to treat, you'd want to do that anyway, right?
But this leads onto my biggest problem with these findings. It's obvious from the side effects data that this drug is a sedative - it makes you tired and sleepy. The animal data confirm this. It's much more likely to put you to sleep than it is to make you enjoy sex in any given month. Off the top of my head, I suspect its sedative properties are a result of its 5HT2A antagonism.
Any sedative can increase sexual desire, as anyone who has ever been to a bar will know. So whether this drug actually has an aphrodisiac effect, as opposed to just being a sleeping pill, is anyone's guess. To find out, you'd need to compare it to a sleeping pill, say, Valium. Or a couple of glasses of wine. Until someone does that, we don't know if this drug is destined to be the next big thing or a big disappointment.
Edit: Just noticed that Dr Petra Boynton has a fantastic post about the background to flibanserin and the manufacturer's apparent attempt to recruit her to write about HSDD.
Borsini F, Evans K, Jason K, Rohde F, Alexander B, Pollentier S (2002). Pharmacology of flibanserin. CNS drug reviews, 8 (2), 117-42 PMID: 12177684