PT-141 (Bremelanotide) and Melanocortin Receptor Signalling: A Research Overview

PT-141, later designated bremelanotide, is described in the literature as a cyclic heptapeptide analogue of the neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) that acts as an agonist at melanocortin receptors, with activity reported at MC3R and MC4R [1][2]. Reviews of melanocortin receptor pharmacology place bremelanotide within a broader ligand class spanning classical melanocortins, agouti-related peptide, and later synthetic agents such as setmelanotide and afamelanotide [3][4]. MC3R and MC4R are described as expressed primarily in the central nervous system, which is the basis for framing this compound as a centrally acting melanocortin ligand rather than a peripheral vasoactive agent [1][3]. A review of melanocortin receptor genetics catalogues receptor subtype tissue distribution and notes bremelanotide among melanocortin agonists that have received regulatory approval [5]. Early preclinical characterisation reported that administration of PT-141 to rats and nonhuman primates produced penile erections, and that systemic administration in rats increased c-Fos immunoreactivity in hypothalamic neurons, a marker of neuronal activation [1]. The same report noted that neurons in the corresponding central nervous system region take up pseudorabies virus injected into the rat corpus cavernosum, a tracing result used to argue for a central circuit connection [1]. A mechanistic review summarising animal work proposes that bremelanotide may act on presynaptic MC4R on neurons in the medial preoptic area of the hypothalamus, with downstream dopamine release described as the excitatory step [6]. Not all animal work has supported a reward-circuit mechanism. In female Syrian hamsters, most MC3R and MC4R mRNA in the mesolimbic system was localised to dopamine neurons of the ventral tegmental area, while MC4R mRNA in the nucleus accumbens was found largely in interneurons rather than in D1- or D2-receptor-expressing neurons [7]. In that animal study, bremelanotide did not alter melanocortin receptor mRNA expression in the mesolimbic dopamine system, and although sexual experience produced a conditioned place preference in the hamsters, bremelanotide treatment did not enhance sexual reward on that measure; the authors concluded the compound does not act through the VTA–nucleus accumbens reward circuit [7]. Separately, an in vitro study in human glioblastoma cell lines reported that bremelanotide reduced survivin expression and induced cell death at concentrations described as non-toxic to normal human cells, with both effects abolished by an MC3R/MC4R antagonist and cell death prevented by forced survivin over-expression [8]. Human work with PT-141 began with erectile-response studies. In healthy male subjects and in men with erectile dysfunction reporting inadequate response to sildenafil, subcutaneous PT-141 across a dose range produced erectile responses measured by RigiScan that reached statistical significance, and was reported as safe and well tolerated in both cohorts [9]. A crossover clinical trial in 19 men with erectile dysfunction found that co-administration of intranasal PT-141 with sildenafil produced a significantly greater erectile response than sildenafil alone, without new or more frequent adverse events relative to monotherapy [10]. A contemporaneous development profile described PT-141 being advanced as a nasal spray, with a completed phase IIb trial in erectile dysfunction and phase III planning [11]. In women, a randomised double-blind crossover study in 18 premenopausal participants with female sexual arousal disorder compared a single intranasal dose of bremelanotide with placebo [12]. More participants reported moderate or high sexual desire after bremelanotide than placebo (P = 0.0114), and among those who attempted intercourse within 24 hours, more reported satisfaction with their level of arousal (P = 0.0256); vaginal vasocongestion measured by photoplethysmography during erotic video viewing did not differ significantly between conditions [12]. The two phase 3 RECONNECT trials randomised 1,267 premenopausal women with hypoactive sexual desire disorder to subcutaneous bremelanotide 1.75 mg as needed or placebo for 24 weeks, reporting statistically significant increases in the Female Sexual Function Index desire domain and statistically significant reductions in desire-related distress versus placebo, with nausea, flushing, and headache each occurring in 10% or more of participants in both studies [13]. Prespecified subgroup analyses of the integrated RECONNECT population (n = 1,202) reported statistically significant changes on the same endpoints across age, weight, BMI, and baseline bioavailable testosterone subgroups, with few exceptions [14]. Safety reporting across the clinical development programme covered 3,500 subjects in 43 completed studies, with nausea (40.0% vs 1.3%), flushing (20.3% vs 1.3%), headache (11.3% vs 1.9%), and injection-site reactions described as the most common adverse events versus placebo in the integrated double-blind phase 3 data; nausea was the most common reason for discontinuation, focal hyperpigmentation occurred in more than a third of subjects after up to 16 consecutive daily dosings, and small transient blood pressure increases were observed on ambulatory monitoring [15]. A dedicated randomised, double-blind, placebo-controlled ambulatory blood pressure trial in 397 premenopausal women reported increases in ambulatory systolic blood pressure relative to placebo of 2.4–3.2 mmHg at the 1.25 mg and 1.75 mg doses in the 0–4 hour postdose window, with peak increases typically lasting under 15 minutes and accompanied by heart rate reductions of about 4.6–4.7 bpm at 1.75 mg [16]. Review and systematic-review appraisals of the trial record describe the changes on validated questionnaires as statistically significant but characterise the overall clinical effect size as modest, and note the difficulty of interpreting female sexual dysfunction trials given large placebo responses and recall-dependent outcome measures [17][18][19]. Regulatory-history reviews record bremelanotide's 2019 US approval as a melanocortin receptor agonist for acquired, generalised hypoactive sexual desire disorder in premenopausal women [2][4].