PT-141 (Bremelanotide): Melanocortin Receptor Research
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# PT-141 (Bremelanotide): Melanocortin Receptor Research
For Research Purposes Only -- Not Intended for Human or Animal Consumption
Introduction
PT-141 (Bremelanotide) is a synthetic cyclic heptapeptide that acts as an agonist at melanocortin receptors, particularly MC3R and MC4R. It is the active metabolite of Melanotan II, with the carboxyl terminus modified to remove the tanning activity. PT-141 received FDA approval in 2019 (as Vyleesi) for the treatment of hypoactive sexual desire disorder (HSDD) in premenopausal women -- making it one of the few research peptides to achieve regulatory approval.
Melanocortin System Overview
The melanocortin system consists of five G protein-coupled receptors (MC1R-MC5R) activated by melanocortin peptides derived from proopiomelanocortin (POMC). The receptors have distinct tissue distributions and functions:
- MC1R: Expressed in melanocytes; mediates skin pigmentation - MC2R: Expressed in adrenal cortex; mediates ACTH-stimulated cortisol synthesis - MC3R: Expressed in hypothalamus; involved in energy homeostasis and sexual function - MC4R: Expressed widely in the CNS; mediates appetite regulation, sexual function, and autonomic control - MC5R: Expressed in exocrine glands; involved in sebaceous gland function
PT-141's primary pharmacological targets are MC3R and MC4R in the central nervous system.
Mechanism of Action
PT-141 activates MC3R and MC4R in the hypothalamus and other CNS regions. The proposed mechanism for its effects on sexual function involves:
- MC4R activation in the hypothalamic paraventricular nucleus (PVN)
- Downstream activation of oxytocin neurons in the PVN
- Oxytocin release and activation of dopaminergic pathways in the mesolimbic system
- Increased sexual motivation and arousal through dopaminergic reward circuitry
FDA Approval and Clinical Evidence
The FDA approved PT-141 (Vyleesi) in 2019 for HSDD in premenopausal women based on two Phase III clinical trials (RECONNECT trials).
RECONNECT Trial Results: - Statistically significant improvement in satisfying sexual events (SSEs) per month compared to placebo - Significant improvement in sexual desire scores - Significant reduction in distress related to low sexual desire
The effect sizes were modest -- the drug-placebo difference in SSEs was approximately 0.5 events per month -- but statistically significant and clinically meaningful to the affected population.
Administration: PT-141 is administered as a subcutaneous injection 45 minutes before anticipated sexual activity. It is not a daily medication.
Adverse Effects
The primary adverse effects of PT-141 are: - Nausea: The most common adverse effect, occurring in approximately 40% of patients in clinical trials - Flushing: Facial and body flushing due to vasodilation - Transient blood pressure increase: A transient increase in blood pressure occurs approximately 30 minutes after injection and resolves within 12 hours
The nausea and flushing are mediated by MC4R activation in the brainstem and peripheral vasculature, respectively.
Comparison with Melanotan II
PT-141 is structurally related to Melanotan II (MT-II), a cyclic analogue of alpha-MSH that activates all five melanocortin receptors. MT-II produces tanning (through MC1R activation), sexual effects (through MC3R/MC4R), and appetite suppression (through MC4R) simultaneously.
PT-141 was developed by modifying MT-II to remove the tanning activity while retaining the sexual effects. The modification (removal of the C-terminal amide and addition of a hydroxyl group) reduces MC1R affinity while maintaining MC3R/MC4R activity.
Research Applications
Beyond the approved HSDD indication, PT-141 has been studied for: - Male sexual dysfunction: MC4R activation promotes erection through central mechanisms; PT-141 has been studied in men with erectile dysfunction refractory to PDE5 inhibitors - Appetite regulation: MC4R is a major regulator of appetite; PT-141's MC4R activity has been studied in obesity models - Inflammation: MC3R has anti-inflammatory properties; PT-141's MC3R activity may have implications for inflammatory conditions
References
- Diamond, L.E., et al. (2004). Double-blind, placebo-controlled evaluation of the safety, pharmacokinetic properties and pharmacodynamic effects of intranasal PT-141. Journal of Sexual Medicine, 1(1), 10-17.
- Clayton, A.H., et al. (2016). Bremelanotide for female sexual dysfunctions in premenopausal women. Obstetrics & Gynecology, 128(6), 1313-1320.
- Pfaus, J.G., et al. (2004). The melanocortins and sexual function. Peptides, 25(10), 1675-1683.