PT-141 (Bremelanotide): Cyclic Heptapeptide Melanocortin Agonist

1. Molecular Characterization

1.1 Chemical Structure and Properties

PT-141 (bremelanotide) is a synthetic cyclic heptapeptide analogue of α-melanocyte-stimulating hormone (α-MSH, 13 amino acids) corresponding to residues 4–10 of α-MSH with strategic substitutions: Nle⁴ replaces Met⁴ (preventing sulfoxide oxidation), Asp⁵ and Lys¹⁰ form an intramolecular side-chain β-carboxyl-to-ε-amino lactam bridge, D-Phe⁷ replaces L-Phe⁷ (inverting Cα stereochemistry to stabilize a type II' β-turn and confer endopeptidase resistance), and the C-terminus terminates in a free carboxylic acid (-OH) rather than the C-terminal carboxamide (-NH2) of Melanotan-2.

1.2 Conformational Pharmacophore and Receptor Binding Profile

Circular dichroism (CD) and 2D-NMR studies demonstrate that the macrocyclic lactam ring between Asp² and Lys⁷ (numbered 1–7 within the heptapeptide) constrains the core melanocortin pharmacophore His-D-Phe-Arg-Trp into a rigid type II' β-turn in aqueous solution, reducing conformational entropy upon receptor binding. In radioligand competition assays using [¹²⁵I]-NDP-α-MSH and functional cAMP accumulation assays in HEK293 cells expressing human melanocortin receptors, PT-141 acts as a selective melanocortin agonist with nanomolar potency at central MC4R (K_i ~2.7 nM; EC₅₀ ~1.8 nM) and MC3R (K_i ~4.1 nM), while showing substantially lower affinity at MC5R (>300 nM) and MC1R (>500 nM) and negligible activity at MC2R (>10 μM) (Molinoff et al., 2003).

2. Solid-Phase Synthesis and Manufacturing

PT-141 is manufactured via Fmoc solid-phase peptide synthesis (SPPS) on a Wang or 2-chlorotrityl chloride (2-CTC) resin (which yields the C-terminal free carboxylic acid -OH upon cleavage, in contrast to Rink amide resin which yields Melanotan-2's C-terminal amide):

1. Linear Chain Assembly: Sequential C-to-N coupling of Fmoc-Lys(Alloc)-OH, Fmoc-Trp(Boc)-OH, Fmoc-Arg(Pbf)-OH, Fmoc-D-Phe-OH, Fmoc-His(Trt)-OH, Fmoc-Asp(OAll)-OH, and Fmoc-Nle-OH using HBTU/HOBt or DIC/Oxyma activation in DMF.
2. N-Terminal Acetylation: Removal of the terminal Fmoc group with 20% piperidine in DMF followed by capping with acetic anhydride/DIEA.
3. Orthogonal On-Resin Lactam Cyclization: Selective palladium-catalyzed deprotection of the Allyl (OAll) ester on Asp and the Alloc group on Lys using Pd(PPh₃)₄ and phenylsilane in DCM, followed by intramolecular macrolactamization with PyBOP/HOBt/DIEA.
4. Global Deprotection & Purification: Acidolytic cleavage with Reagent K (TFA/phenol/water/thioanisole/EDT, 82.5:5:5:5:2.5) and preparative C18 RP-HPLC purification to >95% purity.

3. Mechanism of Action and CNS Neurocircuitry

3.1 MC4R/MC3R G_s–cAMP–PKA Signaling

Melanocortin-4 and melanocortin-3 receptors (MC4R and MC3R) are seven-transmembrane G_s-protein-coupled receptors densely expressed in the paraventricular nucleus (PVN), medial preoptic area (MPOA), ventromedial hypothalamus (VMH), and arcuate nucleus. Agonist binding by PT-141 stimulates adenylyl cyclase, elevating intracellular cAMP and activating protein kinase A (PKA) and CREB phosphorylation, while modulating voltage-gated calcium and ATP-sensitive potassium channels to increase neuronal firing rates.

3.2 Hypothalamic Oxytocin and Mesolimbic Dopamine Activation

Unlike phosphodiesterase-5 (PDE5) inhibitors that act directly on peripheral cavernous smooth muscle, PT-141 acts within the central nervous system to initiate neuroendocrine and motivational cascades (Pfaus et al., 2004; Pfaus et al., 2007):

• Paraventricular Oxytocinergic Neurons: MC4R activation on magnocellular and parvocellular neurons in the hypothalamic PVN stimulates central oxytocin projections to the ventral tegmental area (VTA), nucleus accumbens, and lumbosacral spinal cord, accompanied by a 2- to 3-fold rise in plasma oxytocin in primate models.
• Mesolimbic Dopamine Efflux: In vivo microdialysis in rats demonstrates that systemic PT-141 administration selectively increases extracellular dopamine release in the medial preoptic area (MPOA) and nucleus accumbens by 150–200% above baseline for 2–4 hours, driving appetitive and motivational behavior (Pfaus et al., 2004).
• Autonomic Brainstem Outflow: Descending projections from the PVN to sacral parasympathetic nuclei enhance nitric oxide-mediated genital blood flow, while simultaneous activation of sympathetic preganglionic neurons in the intermediolateral cell column accounts for the transient 3–4 mmHg increase in systemic blood pressure and area postrema-mediated nausea.

4. Preclinical Research Evidence

In estradiol- and progesterone-primed ovariectomized female rats evaluated in bilevel pacing chambers, subcutaneous PT-141 (1–10 mg/kg) selectively increased proceptive solicitation behaviors (hops, darts, ear wiggling) without altering lordosis reflexes or nonspecific locomotion, an effect completely blocked by intracerebroventricular administration of the selective MC4R antagonist HS014 (Pfaus et al., 2004). In male rats and rhesus macaques, subcutaneous or intracerebroventricular PT-141 dose-dependently induced penile erections, shortened mount latency, and increased partner proximity, correlating with hypothalamic c-Fos expression in the PVN and MPOA (Molinoff et al., 2003).

Preclinical pharmacokinetic studies using [¹²⁵I]-labeled bremelanotide demonstrated rapid subcutaneous absorption (T_max 30–60 minutes), negligible oral bioavailability (<5%), a brain-to-plasma partitioning ratio of 0.15–0.25 with preferential hypothalamic localization, and renal elimination of intact peptide and peptidase metabolites (60–70% recovered in urine within 24 hours; species-dependent elimination).

5. Clinical Trials and Regulatory History

5.1 Early Erectile Dysfunction Studies and Intranasal Discontinuation

Early double-blind, placebo-controlled crossover trials in men with psychogenic and organic erectile dysfunction (including sildenafil non-responders) demonstrated that subcutaneous or intranasal PT-141 initiated statistically significant erectile responses compared to placebo (Wessells et al., 1998; Diamond et al., 2004). However, Phase 2 trials of the intranasal formulation revealed highly variable nasal mucosal absorption (CV >60%) with sporadic peak plasma spikes that triggered acute systolic blood pressure elevations (≥30 mmHg) and severe emesis, prompting FDA and the sponsor to discontinue the intranasal formulation in 2007–2008 in favor of controlled subcutaneous delivery.

5.2 Phase 2 Dose-Finding and Phase 3 RECONNECT Pivotal Trials

Following a randomized, placebo-controlled Phase 2b dose-finding trial in 327 premenopausal women with HSDD evaluating 0.75 mg, 1.25 mg, and 1.75 mg subcutaneous doses (Clayton et al., 2016), two identical 24-week Phase 3 randomized, double-blind, placebo-controlled trials — the two RECONNECT trials — evaluated bremelanotide 1.75 mg SC administered via autoinjector as needed (Kingsberg et al., 2019).

In human pharmacokinetic studies of the approved 1.75 mg subcutaneous dose, bremelanotide reaches peak plasma concentration at a median T_max of 1.0 hour (range 0.5–1.0 hour), exhibits 100% subcutaneous bioavailability, shows 21% serum protein binding, and clears with a mean terminal half-life of 2.7 hours primarily via amide hydrolysis.

6. Analytical Verification and Quality Control

For research laboratories analyzing synthetic PT-141, the single most critical analytical requirement is differentiating PT-141 (bremelanotide, C-terminal free acid -OH, MW 1025.18 g/mol) from Melanotan-2 (C-terminal amide -NH2, MW 1024.18 g/mol). PT-141 and Melanotan-2 differ at the C-terminus. ESI-MS and HPLC can be used for analytical characterization.

7. Research Applications and Translational Biomarkers

7.1 Neuroscience and Behavioral Models

PT-141 serves as a pharmacological tool for investigating MC3R/MC4R function in neural circuits governing sexual behavior, appetite regulation, energy homeostasis, anxiety-like behavior, and social interaction. Studies combining PT-141 with selective receptor antagonists or MC3R/MC4R knockout animals differentiate central motivational mechanisms from peripheral vascular responses and probe reciprocal melanocortin–opioid interactions in brain reward circuits, as well as reversal of SSRI antidepressant-induced sexual dysfunction in rodent models.

7.2 Translational Medicine and Biomarker Development

PT-141 research has contributed to the development of translational biomarkers for central melanocortin pathway engagement:

• Plasma Oxytocin Biomarker: Plasma oxytocin measurements following PT-141 administration provide a peripheral biomarker of central hypothalamic melanocortin receptor activation; correlation of oxytocin levels with behavioral outcomes has informed dose-response relationships and inter-individual response variability.
• Functional Neuroimaging (fMRI): Functional MRI studies examining brain activation patterns in response to visual sexual stimuli demonstrate that PT-141 enhances activation in limbic and reward-related regions — including the nucleus accumbens, ventral tegmental area, anterior cingulate cortex, and insula — compared to placebo.
• Pharmacogenomic Candidate Variants: Genetic association studies have investigated polymorphisms in the MC3R and MC4R genes as potential predictors of PT-141 response. While no definitive pharmacogenomic markers have been validated for clinical use, several MC4R variants associated with altered receptor function have been identified as research candidates for explaining individual response variability.

8. Storage and Laboratory Handling

Lyophilized research-grade PT-141 acetate should be stored desiccated at −20°C or below protected from light (long-term stability >12 months; 24–36 month shelf life at 2–8°C in ICH stability studies). Reconstituted aqueous solutions (pH 4.0–5.5 acetate or phosphate buffer) remain stable for up to 7 days at 2–8°C or 6–12 months when frozen in single-use aliquots at −80°C while minimizing freeze-thaw cycles. Forced degradation studies under acidic (pH 2), basic (pH 10), oxidative (3% H₂O₂), and thermal (50°C) stress identify four primary degradation pathways: (1) hydrolysis of the Asp–Lys lactam bridge generating the linear peptide, (2) oxidation of Trp residues, (3) deamidation, and (4) peptide bond cleavage.

9. Safety Profile, Cardiovascular Pharmacology, and Drug Interactions

Across the clinical trial safety database (>1,200 to 1,500 subjects exposed for up to 52 weeks; Kingsberg et al., 2019), the safety profile of PT-141 reflects central melanocortin receptor pharmacology:

• Adverse Event Profile & Nausea: The most common adverse reactions (incidence ≥3% and greater than placebo) are nausea (40%), flushing (20%), injection-site reactions (13%), headache (11%), vomiting (6%), nasal congestion (5%), and hot flush (4%). Nausea — mediated by melanocortin receptor activation in the area postrema chemoreceptor trigger zone — represents the primary tolerability-limiting effect, leading to treatment discontinuation in approximately 13% of subjects in clinical trials, though incidence and severity tended to diminish with repeated exposure.
• Cardiovascular Safety (Boxed Warning): PT-141 produces transient increases in blood pressure, with mean increases of 3–4 mmHg in systolic blood pressure observed 8–12 hours post-dose and resolving within 24 hours; approximately 5% of subjects experienced systolic increases ≥30 mmHg or diastolic increases ≥20 mmHg. Consequently, PT-141 is contraindicated in uncontrolled hypertension or known cardiovascular disease, as well as in pregnancy based on animal reproduction studies.
• Long-Term Metabolic & Pigmentation Findings: Open-label extension studies up to 52 weeks revealed no accumulation or worsening of blood pressure effects over time, and mean changes in body weight, HbA1c, fasting glucose, and lipids did not differ significantly from placebo. In the source clinical evaluation, skin hyperpigmentation — a known effect of non-selective melanocortin agonists — was not observed in PT-141-treated patients at therapeutic doses, consistent with functional selectivity for MC3R/MC4R over MC1R, though theoretical caution is noted for focal hyperpigmentation disorders.
• Drug Interactions: No formal drug interaction studies have been conducted with PT-141. Theoretical pharmacodynamic interactions may occur with vasoactive or cardiovascular medications: concurrent alpha-adrenergic agonists (such as nasal decongestants or certain migraine agents) could produce additive blood pressure effects, whereas antihypertensive medications could be transiently less effective during PT-141-induced pressor elevations. In vitro studies indicate that PT-141 does not undergo significant cytochrome P450 metabolism and is not a substrate, inhibitor, or inducer of major drug transporters (P-glycoprotein, BCRP, OAT, OCT), indicating low potential for P450- or transporter-mediated pharmacokinetic interactions.

10. Primary Literature References

1. Clayton AH, Kingsberg SA, Portman D, et al. Bremelanotide for female sexual dysfunctions in premenopausal women: a randomized, placebo-controlled dose-finding trial. Womens Health (Lond). 2016;12(3):325-337. doi:10.2217/whe-2016-0018.
2. Kingsberg SA, Clayton AH, Portman D, et al. Bremelanotide for the treatment of hypoactive sexual desire disorder: two randomized phase 3 trials. Obstet Gynecol. 2019;134(5):899-908. doi:10.1097/AOG.0000000000003500.
3. Pfaus JG, Shadiack A, Van Soest T, Tse M, Molinoff P. Selective facilitation of sexual solicitation in the female rat by a melanocortin receptor agonist. Proc Natl Acad Sci U S A. 2004;101(27):10201-10204. doi:10.1073/pnas.0400491101.
4. Molinoff PB, Shadiack AM, Earle D, Diamond LE, Quon CY. PT-141: a melanocortin agonist for the treatment of sexual dysfunction. Ann N Y Acad Sci. 2003;994:96-102. doi:10.1111/j.1749-6632.2003.tb03167.x.
5. Wessells H, Fuciarelli K, Hansen J, Hadley ME, Hruby VJ, Dorr R, Levine N. Synthetic melanotropic peptide initiates erections in men with psychogenic erectile dysfunction: double-blind, placebo controlled crossover study. J Urol. 1998;160(2):389-393. doi:10.1016/S0022-5347(01)62894-0.
6. Diamond LE, Earle DC, Rosen RC, Willett MS, Molinoff PB. Double-blind, placebo-controlled evaluation of the safety, pharmacokinetic properties and pharmacodynamic effects of intranasal PT-141, a melanocortin receptor agonist, in healthy males and patients with mild-to-moderate erectile dysfunction. Int J Impot Res. 2004;16(1):51-59. doi:10.1038/sj.ijir.3901139.
7. Portman DJ, Brown L, Yuan J, Kissling R, Kingsberg SA. Flibanserin in postmenopausal women with hypoactive sexual desire disorder: results of the PLUMERIA study. J Sex Med. 2017;14(6):834-842. doi:10.1016/j.jsxm.2017.03.258.
8. Giuliano F, Clément P. Neuroanatomy and physiology of ejaculation. Annu Rev Sex Res. 2005;16:190-216. PMID: 16913291.
9. Rowland DL, Kolba TN. Understanding mechanisms underlying altered sexual response: potential role of HSDD treatments in restoring sexual desire and arousal. J Sex Med. 2018;15(12):1725-1732. doi:10.1016/j.jsxm.2018.10.006.
10. Pfaus JG, Giuliano F, Gelez H. Bremelanotide: an overview of preclinical CNS effects on female sexual function. J Sex Med. 2007;4(Suppl 4):269-279. doi:10.1111/j.1743-6109.2007.00610.x.

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