While the newly approved “pink pill” has been dubbed the female Viagra, its job is a more difficult and subtle one—changing not sexual performance but desire. That is one reason why the drug, flibanserin, helped only a small fraction of women in clinical trials. Nonetheless, some scientists are optimistic that similar drugs aimed at boosting a flagging desire for sex will follow.
The pink pill, developed by Sprout Pharmaceuticals, took an indirect path to market. Originally flibanserin was tested for depression, as many drugs are, with the hope that it might work something like Prozac and other so-called selective serotonin reuptake inhibitors. But those drugs increase the brain’s supply of serotonin, which is a neurotransmitter, while flibanserin dampened it. And the decreased serotonin led to an increase in a multitasking brain chemical, dopamine, because the two substances counterbalance each other.
Scientists suspected that the increased dopamine might, among other things, enhance sexual desire. That clue came about from research on a drug called bremelanotide, which was originally developed and tested as a sunless tanning agent. Under the name Melanotan, it was designed to stimulate the body’s pigment-producing cells, known as melanocytes, with the aim of preventing skin cancer. In tests, some subjects didn’t get much of a tan but reported that they felt unusually horny, says James Pfaus, a neuroendocrinologist at Concordia University in Canada.
Pfaus was among the researchers charged with studying this surprising and possibly lucrative side effect. He found that in female rats, bremelanotide dramatically increased the frequency of solicitation—a scientific term referring to the initiation of sex, no money exchanged. Under the drug’s influence, female rats solicited males about 40 times in 30 minutes, even when their normal libidos were suppressed by depleting their female hormones.
Flibanserin also increased rat solicitations, restoring normal behavior in hormone-depleted rats that would otherwise not initiate sex at all.
In humans, flibanserin showed an effect in just 9 to 14 percent of women experiencing low sex drive, which is being called “hypoactive sexual desire disorder.” The drug increased the frequency of satisfying sexual encounters, but not much more than did a placebo.
One reason for the difference is that rats lack cultural inhibitions that we humans have, says Pfaus. “When female rats want sex they go and get it … if male 1 doesn’t give it to them they go to male 2.”
And sexual desire in humans is more complex than, say, erectile dysfunction. Pfaus compares the former to a computer glitch and the latter to a plumbing problem. The distinction was first appreciated in the 1980s, says Emory University neuroscientist Kim Wallen. Women who’d undergone medical procedures such as removal of the ovaries reported that they engaged in sex as often as they used to, but they didn’t desire it as much. But while there are many reasons for people to lose interest in sex, there may soon be many different drugs for them to experiment with. “Once the first one is out,” Pfaus says, “that opens the door for the invasion.”
Bremelanotide, which is more powerful than the pink pill, is one possibility, although it’s not yet approved for either tanning or sex. Its path to becoming a sex drug was slowed when an earlier formulation, administered as a nasal spray, caused potentially deadly blood pressure spikes in a couple of male subjects. At that point it was being tested as a Viagra-type drug – some of the male tanning subjects reported spontaneous erections. But now the focus is on using it to enhance sexual desire in women.