Flibanserin, FDA-approved in August 2015, is the first medication approved to treat acquired, generalized hypoactive sexual desire disorder (HSDD) in premenopausal women (Table 1). In clinical trials,1-4 the drug has shown modest efficacy in improving symptoms of low sexual desire (number of satisfying sexual events [SSEs], sexual desire, and overall sexual function). Flibanserin is not indicated to enhance sexual performance, for HSDD in postmenopausal women, or in men.
Clinical implications
Flibanserin could help premenopausal women who have distressing low sexual desire, which must be acquired and generalized:
- “Acquired low sexual desire” means that a patient had an adequate sexual desire that decreased or ceased for an unknown reason.
- “Generalized low sexual desire” means that lack of sexual desire occurs all the time and in all situations, not only with a certain partner or in some situations.
Women taking flibanserin could experience gradually increased sexual desire, increase in SSEs, and decrease of sexual distress. Flibanserin is indicated for long-term use; however, it should be discontinued after 8 weeks if the patient does not report any improvement in symptoms.
The number needed to treat with flibanserin likely would be rather large, but it is not available because of complex outcome measures in clinical trials. Flibanserin was not approved at 2 previous FDA committee hearings—mainly because of safety issues but also because of concerns about efficacy. For example, during the 2013 FDA hearing, the results presented showed statistically significant, but numerically small, treatment differences at 24 weeks compared with placebo. In an FDA responder analysis of the Phase-III trials, after accounting for the placebo effect, approximately 8% to 13% women were at least “much improved” on at least 1 of the primary outcomes.5
Flibanserin is not indicated for women whose sexual desire is due to (1) coexisting medical or psychiatric condition, (2) effects of medication or substance abuse, or (3) a relationship problem. It is unknown whether supplemental treatment would help these patients; however, it seems reasonable that combining flibanserin with psychosocial treatment, such as sex therapy or individual therapy, could be beneficial because it may be difficult to disentangle sexual dysfunction and relationship issues—2 problems that often are interwoven.
How it works
Flibanserin is a serotonin 1A receptor agonist and serotonin 2A receptor antagonist. In vitro, flibanserin demonstrated high affinity for the following 5-HT receptors:
- agonist activity at 5-HT1A
- antagonist activity at 5-HT2A, mostly in the prefrontal cortex.
Flibanserin also has moderate antagonist activities at the 5-HT2B, 5-HT2C, and dopamine D4 receptors. Flibanserin presumably acts centrally in the CNS; it has been suggested that flibanserin could rebalance neural circuitry involved in processing sexual desire by reducing serotonin activity and enhancing dopamine and epinephrine activity. The exact mechanism of how flibanserin improves sexual desire in women is unknown.
Pharmacokinetics
Flibanserin has a mean termination half-life of approximately 11 hours. It is administered once a day (50 to 100 mg) at bedtime. Steady state in healthy women was achieved after 3 days. Based on clinical observations, onset of action seems to be gradual and reaches maximum efficacy in approximately 8 weeks. Patients should discontinue the drug if no improvement is reported after 8 weeks. Flibanserin is readily absorbed from the gastrointestinal tract; however, food slows its absorption. The drug is 98% protein (mostly albumin)-bound.
Flibanserin is primarily metabolized in the liver by cytochrome P450 (CYP) 3A4 and to a lesser extent by CYP2C19. Co-administration of moderate (diltiazem, erythromycin, fluconazole, fosamprenavir, verapamil) or strong (eg, ketoconazole, clarithromycin, nefazodone, ritonavir) CYP3A4 inhibitors increases the concentration of flibanserin. This could lead to severe hypotension and syncope; therefore, co-administering flibanserin with a strong CYP3A4 inhibitor is contraindicated. Grapefruit juice is a moderate inhibitor of CYP3A4, and in a study of 26 healthy females, 240 mL of grapefruit juice increased flibanserin concentration 1.4-fold. Flibanserin is excreted though urine and feces. Flibanserin should be taken once a day at bedtime because of sedation, somnolence, and possible syncope.
Efficacy
The efficacy of flibanserin for treating HSDD was established in three 24-week, randomized, double-blind, placebo-controlled studies (Table 2). The target population in these studies was premenopausal women (mean age 36, range 19 to 55) with acquired HSDD lasting at least 6 months (mean duration, approximately 5 years). The 3 studies included 1,187 women who received flibanserin, 100 mg at bedtime, and 1,188 women who received placebo. Participants were mostly white (88.6%), and included black (9.6%) and Asian (1.5%) women. The completion rates were 69% for flibanserin and 78% for placebo. Some of the trials included arms with a lower dosage of flibanserin (25 mg and 50 mg), which are not included in this analysis.
