Flibanserin was widely touted by its supporters as the long-awaited answer to the prayers of women suffering from hypoactive sexual desire disorder, or HSDD, the most common form of female sexual dysfunction. Despite all the hype, it has failed to ignite comparable passion in female consumers since it first hit the market in October 2015 under the brand name Addyi.
However, a recent study conducted by ophthalmologic researchers at Casey Eye Institute in Portland, Oregon, has developed some promising evidence that the drug could be used to preserve vision, particularly in cases involving retinal damage. Researchers at the institute, which is part of Oregon Health & Science University, conducted tests on mice to assess flibanserin’s neuroprotective properties on retinas damaged by exposure to high-intensity light. The light-induced damage to mouse retinas was designed to serve as an animal model of genetic and/or age-related eye damage in humans.
Inherited Retinal Dystrophies
Leading the study was Mark E. Pennesi, M.D., Ph.D., a research ophthalmologist at Casey Eye Institute whose work focuses primarily on a family of degenerative eye diseases called inherited retinal dystrophies, which account for roughly 5 percent of vision loss in the Western world. Also participating in the study were six of Dr. Pennesi’s colleagues at the Casey Eye Institute, as well as one researcher each from Baylor University Medical Center in Dallas and Columbia University’s New York State Psychiatric Institute in New York City.
In an interview with Ian Parker of KATU News in Portland, Dr. Pennesi said he was first drawn to flibanserin by the similarity of the drug’s name to ketanserin, a medication “that we were working on in the lab.” However, he pointed out that ketanserin has not been approved by the Food and Drug Administration, “and it would probably have trouble being used in humans because it has certain side-effects.”
A Serendipitous Discovery
In another interview, Dr. Pennesi told Kale Williams of OregonLive.com that it was “kind of serendipity” that brought flibanserin to his attention. “It’s a great example of how unexpected things can happen.” The doctor conducted some preliminary tests with flibanserin and came to believe that the drug may well hold the key to preventing certain eye disorders that can eventually lead to blindness.
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Much of the damage associated with inherited retinal dystrophies, which are the focus of Dr. Pennesi’s research, comes from exposure to light. Previous research has shown that despite these disorders’ genetic origins, gene therapy is of very little help. Nor have researchers yet uncovered any other form of treatment that is effective in preventing or reversing the damage caused by these eye diseases.
Role of Serotonin
Earlier research has also uncovered strong evidence that the neurotransmitter serotonin modulates retinal processing, although the full extent of serotonin’s effects on the retina remain unclear. As previously noted, Pennesi’s earlier studies had focused on ketanserin, which is a serotonin antagonist noted for its ability to inhibit action at serotonin receptors, also known as 5-HT receptors.
Because ketanserin has not yet been approved by the FDA in humans, Dr. Pennesi thought that FDA-approved flibanserin, which also inhibits action at serotonin receptors, might be worth further study for its possible impact on inherited retinal dystrophies.
In his interview with OregonLive.com, Dr. Pennesi said that because flibanserin has already been shown to be relatively safe in humans, he and colleagues felt that “if we could show it really worked on an animal model, eventually the hope is that we can get it to a clinical trial.”
Light Box Constructed
To test flibanserin’s efficacy in warding off retinal damage in an animal model, Dr. Pennesi and his fellow researchers custom-built a light box large enough to comfortably house 16 mice at a time and featuring four compact fluorescent lamps. Collectively, those lamps were capable of producing 10,000 lux throughout the light box.
For the animal subjects of their study, researchers purchased a strain of albino mice that are especially bred for their use in the study of neurological disorders and cancer. Researchers further refined their animal study participants by confirming the mice’s genotypes by PCR (polymerase chain reaction) testing. Both prior to and after their time in the light box, these animals were housed in a 12-hour alternating light-dark room. During the 12-hour light cycle, lighting in the room was measured at roughly 15 lux.
2 Studies of Varying Length
Researchers conducted two animal studies of varying length: one over the course of five days and one that lasted a single day. Preparations for the five-day drug time course were as follows: Two hours into the 12-hour dark cycle, test mice were injected intraperitoneally with either a simple saline solution with no active ingredients or varying strengths of a flibanserin solution, ranging from a low of 0.75 milligrams per kilogram of weight to a high of 15 mg/kg. These injections were performed at the following intervals: 48 hours prior to light box, 24 hours prior, immediately prior, 24 hours following light box, and 48 hours after light box.
For the one-day drug time course, mice were injected with a simple saline solution or a flibanserin solution ranging from 3 mg/kg to 15 mg/kg.
Test animals were then put into the light box and subjected to intense light for a period of one hour. After the five-day drug time testing, mice injected with only saline solution or 0.75 and 1.5 mg/kg of flibanserin solution showed significant retinal damage, while those who got strong doses of flibanserin showed little to no damage. The retinas of mice used in the one-day drug time test seemed to be well protected with injections of flibanserin solution at strengths ranging from 6 mg/kg to 15 mg/kg. Those who got only 3 mg/kg sustained measurable retinal damage.
Results ‘Pleasantly Surprising’
Of the study’s findings, Dr. Pennesi told OregonLive.com, “I thought it [flibanserin] was going to work, but our results were still pleasantly surprising. It just goes to show you how things in different sciences can work for different purposes. You just have to have an open mind to things outside of your field.”
Dr. Pennesi acknowledged that much work remains to be done before flibanserin can be used to treat eye diseases in humans. A likely next step would be another round of tests on animals that have been genetically engineered to have eye problems. If researchers successfully clear that hurdle, clinical trials in humans would likely follow. “This is the first step in testing the potential of pre-treatment. The hope is that the drug could actually slow down some of the degenerative diseases,” he said.
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