In the 1980s, researcher Mac Hadley was studying peptides proposed to help skin tan with minimal sun exposure, intending to dodge the cancer-causing effects of UV radiation. The University of Arizona professor had taken to testing the experimental compounds by injecting them into himself, which is not exactly a standard scientific protocol, but it's hard to argue with results. Hadley's unconventional experiments spawned a line of research that has now yielded a drug—one that's meant not to darken complexion, but to treat female sexual dysfunction.

The new drug, called bremelanotide, is a derivative of melanotan II, one of Hadley's tanning peptides. But whatever bronzing Hadley had anticipated the day he injected melanotan II must have paled in comparison to the drug's : an eight-hour erection that withstood even the application of cold packs. This was entirely unexpected since melanotan II (not to be confused with the sleep-regulating hormone melatonin) is a peptide that targets melanocortin receptors in the body, which weren't yet known to be involved in human sexual function.

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Three decades later, the Food and Drug Administration (FDA) is now reviewing the results of bremelanotide's Phase 3 , the last type of human trial done before a drug is submitted for approval. The developers also recently submitted a New Drug Application to the NDA (which is a major step, since it signals that the sponsoring company feels the drug is totally ready).

With the agency's blessing, bremelanotide—likely brand name "Rekynda"—would be one of only two pharmaceuticals approved by the FDA to treat female hypoactive sexual desire disorder (HSDD), a state of abnormally dampened interest in sex. Bremelanotide will also be used for people with Female Sexual Interest and Arousal Disorder (FSAID), a newer term that will come to encompass HSDD as well.

So who's a fit for this new get-it-on drug?

HSDD is thought to be the most common sexual dysfunction in women, and it affects all age groups. Having HSDD isn't the same as being asexual or simply having a low sex drive; HSDD is only diagnosed when low sexual desire causes distress—basically, people with HSDD want to want sex.

"Let's say you go to your favorite restaurant, and no matter how much you want to feel hungry, you just can't," says clinical psychologist Sheryl Kingsberg, Ph.D., a researcher in the field of women's sexual health who has worked on bremelanotide's clinical trials. "You just have no appetite, and that's a loss for you because you wanted to enjoy that food." Similarly, with HSDD, a person wants to have sexual desire and fantasies, but can't—and that can be profoundly upsetting.

"What I see is that many women feel distressed. They want their libido to be what it was. They feel less healthy with a diminished libido," says Stacy Tessler Lindau, M.D., a gynecologist whose clinic focuses on helping women with cancers and some other conditions recover their sexual function after treatment. Other women, she says, are primarily motivated to seek help because they see their low libido as a problem in their relationships. "They love their partner, and they want their partner to feel that they desire them back," she says. And in many cases, women seek help for a combination of these factors.

According to Lindau, there's often an underlying issue that can be addressed, such as exhaustion, untreated depression, or pain during sex. Having relationship problems can also harm libido, as can having a partner who is experiencing sexual difficulties, such as erectile dysfunction. Once those issues are resolved, whether by prescribing antidepressants or providing relationship counseling—or whatever else may be needed in a given case—sex drive often increases.

It's good to have options.

Kingsberg is a proponent of psychotherapy—she does it for a living, after all. "But sometimes my best psychotherapy just isn't enough, and I like those patients to be able to have options if they need them," she says. That's where medications such as bremelanotide or the existing FDA-approved drug for HSDD, flibanserin (brand name "Addyi"), would come in. It's important to develop new treatments because while flibanserin may be useful for some, no medication will help everyone, and bremelanotide works in a completely different way than flibanserin does.

Bremelanotide's usefulness in treating HSDD is thought to come down to dopamine signaling. Dopamine is a brain chemical often described as a "pleasure" or "reward" neurotransmitter, but these simplistic definitions obscure the way dopamine really works. How and where dopamine signaling in the brain occurs determines its effects. A found that bremelanotide seems to act on certain melanocortin receptors in a particular part of a brain region called the hypothalamus, which increases dopamine signaling specifically in that area. In people who respond to the drug, this is believed to be the trigger for increased sexual desire.

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Not only does flibanserin work via a different mechanism, but it's also used differently. Flibanserin is an oral drug taken every day over the long term, whereas bremelanotide is an on-demand drug taken using an automatic, EpiPen-like injector 30–45 minutes before a person hopes to have a sexual experience. While some may prefer consistent dosing, others may like the idea of taking a drug only when they need it.

And there are some serious downsides to existing treatment.

Another reason to be excited about the prospect of this new drug is that, at least in the United States, flibanserin has been a bit of a flop. It's often expensive, and doctors and pharmacists must be trained before they can prescribe or dispense it. Flibanserin also carries a black box warning, the strictest cautionary note the FDA assigns to drugs, stating that patients must not drink alcohol while taking the daily medication.

Although some researchers have challenged the idea that the drug should carry these restrictions, and it's now sold without them in Canada, flibanserin's chance of market success—and thus its likelihood of being prescribed to people who may benefit from it—has undoubtedly been damaged.

Some doctors aren't thrilled with flibanserin, in any case. Lindau says that while she educates her patients about all their treatment options, including flibanserin, she has never prescribed the medication, nor has any patient pressed for it.

She's withholding judgment on bremelanotide, saying that until she thinks there's good evidence that it's safe and effective, she won't be incorporating it into her practice. "Many women wish there were a magic bullet for this problem, but there just isn't," Lindau adds, though she says there would be a market for any drug that proved safe and effective for HSDD. And it's worth noting that bremelanotide, like flibaneserin before it, is still targeting the brain—it doesn’t increase blood flow to the genitalia.

It's still too early to say whether bremelanotide will join established treatments such as therapy as a top choice for HSDD, but the fact that this drug is being pursued at all is promising. Research on non-male sexuality has lagged behind—and perhaps research on new medications indicates that's finally changing.

The pill's debut in the 1960s was billed as the start of the sexual revolution, but Kingsberg points out that while the introduction of the pill was a critical step forward in that it allowed women to have control over their reproduction, it didn't address many other factors that contribute to women's ability to have fulfilling sexual lives. Let's hope the fact that researchers are investing time and funding in women's sexual desire means we won't have to wait for another researcher to self-inject experimental substances to find the next treatment option.

is a freelance science writer with a canine obsession and a Ph.D. in molecular and cell biology. Follow her on Twitter @.

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