Last updated: April 2026 · UK research-grade reference · For laboratory research use only — not for human consumption
Table of Contents
- 1. Overview — PT-141’s distinctive position
- 2. Molecular structure
- 3. Melanocortin receptor pharmacology
- 4. MC4R in CNS sexual circuitry
- 5. Pharmacokinetics
- 6. Vyleesi — regulatory history
- 7. RECONNECT trials — the HSDD evidence base
- 8. Erectile dysfunction research
- 9. Research beyond sexual function
- 10. PT-141 vs Melanotan-II
- 11. Safety profile and adverse events
- 12. Reconstitution, storage and stability
- 13. UK research protocol design
- 14. UK research-grade sourcing standards
- FAQ
- References
1. Overview — PT-141’s distinctive position
PT-141 (bremelanotide) is pharmacologically unique among approved sexual-function therapeutics. Unlike the PDE5 inhibitors sildenafil, tadalafil and vardenafil — which act peripherally on vascular smooth muscle in the corpus cavernosum — bremelanotide acts centrally, via CNS melanocortin-receptor activation in hypothalamic circuitry governing sexual desire and arousal. This gives it a mechanism of action applicable to conditions (female HSDD, PDE5-non-responder ED) where peripheral vasoactive approaches have no pharmacological rationale.
For UK laboratory research, PT-141 is the reference peptide for melanocortin-system investigation — CNS melanocortin receptor pharmacology, sexual-function neurobiology, and downstream metabolic and inflammatory studies where MC3R/MC4R activation has mechanistic relevance.
2. Molecular structure
PT-141 is a cyclic heptapeptide: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. Molecular formula C₅₀H₆₈N₁₄O₁₀; molecular weight 1025.19 Da.
Structural features:
- N-terminal acetyl group for stability
- Nle (norleucine) at position 4 (corresponding to α-MSH position 4)
- Cyclic lactam bridge between Asp and Lys side chains — forms the peptide loop that constrains 3D conformation
- D-Phe at position 7 — enhances receptor affinity and metabolic stability
- Conserved His-Phe-Arg-Trp (HFRW) core motif shared with α-MSH and Melanotan-II — the pharmacophore for melanocortin receptor binding
The cyclisation and D-amino acid substitutions differentiate PT-141 from linear α-MSH: the cyclic structure increases plasma stability, reduces enzymatic degradation, and provides sufficient receptor-binding half-life for practical pharmaceutical use.
3. Melanocortin receptor pharmacology
The melanocortin receptor family comprises five subtypes (MC1R–MC5R), each with distinct tissue distribution and function:
- MC1R: melanocytes; skin and hair pigmentation
- MC2R: adrenal cortex; cortisol production (ACTH receptor)
- MC3R: hypothalamus, placenta, gut; energy homeostasis, appetite
- MC4R: hypothalamus, brainstem, spinal cord; appetite, sexual behaviour, autonomic function
- MC5R: exocrine glands; sebaceous and other secretions
PT-141 binding affinity profile (Ki in nM, approximate):
- MC4R: 0.6 nM (highest affinity)
- MC3R: 2.1 nM
- MC1R: 2.8 nM
- MC5R: 11 nM
- MC2R: >1000 nM (effectively no activity at the ACTH receptor)
The combination of high MC4R and MC3R affinity with no MC2R activity is pharmacologically important: it isolates CNS hypothalamic effects from adrenal cortisol stimulation. This is a key distinction from unselective α-MSH analogues.
Signal transduction is the standard melanocortin receptor cascade: Gαs coupling → adenylyl cyclase → cAMP → PKA → downstream effector activation. Some Gαq coupling has also been demonstrated at MC4R, contributing to IP3/Ca²⁺ signalling.
4. MC4R in CNS sexual circuitry
The mechanistic basis of PT-141’s sexual-function activity is MC4R expression in specific hypothalamic and brainstem nuclei:
- Medial preoptic area (mPOA): central integrator for male sexual behaviour; MC4R activation promotes pro-erectile and pro-copulatory signalling
- Paraventricular nucleus (PVN): oxytocinergic projections to spinal cord and limbic system; MC4R activation increases oxytocin release
- Ventral tegmental area (VTA): mesolimbic dopaminergic pathway; MC4R modulates dopamine release relevant to sexual desire/arousal
- Spinal cord sympathetic preganglionic neurons: MC4R contributes to autonomic signals driving genital response
The combined upstream activation of hypothalamic-mesolimbic and direct spinal pathways explains why PT-141 produces both subjective desire/arousal effects and objective genital responses — a profile distinct from PDE5 inhibitors which only act on the peripheral vascular component.
5. Pharmacokinetics
- Plasma half-life: ~2.7 hours (SC administration)
- Tmax after SC injection: ~60 min
- Bioavailability (SC): approximately 100%
- Volume of distribution: ~25 L
- CNS penetration: limited but sufficient for therapeutic effect at approved doses
- Protein binding: moderate
- Metabolism: peptide hydrolysis (no CYP involvement)
- Elimination: primarily renal (peptide fragments)
- Onset of subjective effect: 30-60 min post-injection
- Duration of effect: 6-16 hours reported in trial diaries
The ~2.7 hour half-life combined with the extended duration of reported subjective effect suggests that CNS melanocortin receptor engagement produces downstream effects that outlast plasma peptide presence — consistent with the transcriptional and neuromodulatory mechanisms of central monoaminergic signalling.
6. Vyleesi — regulatory history
Bremelanotide, under the trade name Vyleesi (Palatin Technologies / AMAG Pharmaceuticals), received FDA approval in June 2019 for the treatment of premenopausal women with acquired, generalised hypoactive sexual desire disorder (HSDD) — defined as reduced sexual desire causing marked distress, not attributable to a medical or psychiatric condition, relationship problems, or medication effects.
Approval was based on the RECONNECT-1 and RECONNECT-2 Phase 3 trials. Vyleesi is supplied as a single-use 1.75 mg subcutaneous autoinjector for as-needed (PRN) administration 45 minutes before anticipated sexual activity. Maximum approved dose is one 1.75 mg injection per 24 hours, and 8 per month.
Vyleesi is not approved in the UK as a prescription medicine as of 2026 — MHRA approval was not pursued given the limited European commercial opportunity.
7. RECONNECT trials — the HSDD evidence base
RECONNECT-1 (NCT02333071) and RECONNECT-2 (NCT02338960) were 24-week Phase 3 RCTs enrolling 1,247 premenopausal women with HSDD, randomised 1:1 to bremelanotide 1.75 mg SC PRN or placebo.
Primary endpoints at 24 weeks:
- Change in Female Sexual Function Index (FSFI) desire domain score: bremelanotide +0.54 vs placebo +0.27 (p < 0.001)
- Change in Female Sexual Distress Scale (FSDS-DAO) item 13 (bothered by low desire): bremelanotide −0.74 vs placebo −0.42 (p < 0.001)
- Responder analysis (FSFI desire increase ≥1.2): bremelanotide 25.2%, placebo 17.0%
The magnitude of effect is modest on absolute scales but statistically robust and clinically meaningful for a treatment of HSDD — a condition with few evidence-based pharmacological options. Flibanserin (Addyi) is the only prior approved treatment, with similar magnitude of effect but a daily dosing burden and substantial tolerability issues.
8. Erectile dysfunction research
PT-141 was originally developed as an ED treatment and has substantial research data in that indication:
- Early Phase 2 trials showed efficacy in men with ED, including PDE5-inhibitor non-responders
- Intranasal formulation development was halted in 2008 due to transient blood-pressure elevations at higher intranasal doses
- Subcutaneous formulation (as used for Vyleesi) has not progressed to approval for male ED
- Research interest continues in PDE5-refractory ED populations, where central MC4R activation may provide clinical benefit not available from peripheral vasoactive therapy
A particular research interest is the combination of PT-141 with PDE5 inhibitors — the mechanisms are complementary (central arousal + peripheral vasodilation) and preliminary data suggest additive benefit in difficult ED cases.
9. Research beyond sexual function
MC4R activation has broader relevance beyond sexual function, and PT-141 is increasingly used as a research tool in:
- Appetite and energy homeostasis: MC4R is one of the most penetrant monogenic obesity genes (loss-of-function mutations cause severe early-onset obesity). PT-141 is a research tool for MC4R pharmacology in energy-balance studies.
- Inflammatory modulation: Melanocortin receptor activation has anti-inflammatory effects via cytokine modulation; PT-141 has been used in sepsis and autoimmune research models.
- Neuroprotection: MC4R activation in CNS has neuroprotective signalling in stroke and spinal cord injury models.
- Autonomic function: MC4R modulates sympathetic outflow; PT-141 is a research tool for studies of hypertension and cardiovascular autonomic regulation.
- Haemorrhagic shock: early research showed melanocortin agonism produces beneficial cardiovascular effects in models of severe blood loss.
10. PT-141 vs Melanotan-II
PT-141 is chemically derived from Melanotan-II (MT-II) — both share the cyclic heptapeptide backbone — but with key differences:
- Selectivity: PT-141 has higher MC4R/MC3R selectivity with reduced MC1R activity; MT-II has higher MC1R activity (hence MT-II’s tanning effect)
- Carboxyl terminus: PT-141 has a free C-terminal carboxyl; MT-II has a C-terminal amide
- Tanning effect: significantly reduced in PT-141 compared to MT-II, owing to reduced MC1R activity
- Clinical approval: PT-141 approved as Vyleesi (2019); MT-II never approved
- Research use: PT-141 preferred for sexual-function and MC4R research; MT-II retained for MC1R / pigmentation research
For UK laboratory research, PT-141 is the appropriate molecule for MC4R-focused studies; MT-II is the appropriate choice for MC1R-focused pigmentation biology work.
11. Safety profile and adverse events
Pooled adverse events from RECONNECT Phase 3 trials (bremelanotide 1.75 mg PRN SC):
- Nausea: 40.0% (vs placebo 1.3%)
- Flushing: 20.3% (vs 0.3%)
- Injection site reactions: 13.2%
- Headache: 11.3%
- Vomiting: 4.8%
- Transient blood pressure elevation: 6.4% (mean +1.9 mmHg systolic, peak 2-4 hours post-dose)
- Focal hyperpigmentation: 1.0% (concentration-dependent; persistent in some cases — stronger with frequent dosing)
Contraindications in clinical use: uncontrolled hypertension, known cardiovascular disease. The transient BP rise is the primary safety consideration — Vyleesi labelling contraindicates use in patients with uncontrolled hypertension.
Focal hyperpigmentation is a known melanocortin-class effect (via residual MC1R activity even in the reduced-MC1R PT-141 structure), more common with high-frequency dosing. Research protocols typically cap administration at 8 doses per month to minimise this risk.
12. Reconstitution, storage and stability
PT-141 typical research-grade vial: 10 mg lyophilised powder. Reconstitution:
- Add 2 mL bacteriostatic water (0.9% benzyl alcohol preserved) → 5 mg/mL
- At 1.75 mg per administration, 0.35 mL (35 units on a U-100 insulin syringe)
- Post-reconstitution storage: 2-8°C, use within 30-45 days
- Protect from light and freezing
- Lyophilised powder (unreconstituted) stable at −20°C for >2 years; refrigerated at 4°C for 12+ months
The cyclic structure gives PT-141 good aqueous stability — better than linear peptides of similar size. Methionine (Nle in PT-141, norleucine — actually non-oxidisable by design) means oxidation is not a significant degradation pathway.
13. UK research protocol design
Typical UK laboratory research protocols using PT-141:
- Sexual-function neurobiology research: 1.75 mg SC PRN protocol, mirroring Vyleesi approved use; 24-week observation with FSFI/FSDS or IIEF endpoints
- MC4R pharmacology research: single-dose or dose-escalation (0.5, 1.0, 1.75, 3.0 mg) with pharmacokinetic and downstream readouts
- Combination protocols: PT-141 + PDE5 inhibitor co-administration, mechanistic studies
- Neuroprotection / inflammation: typically rodent or ex vivo models rather than human protocols
- Safety monitoring: BP measurement at baseline, 2 hours post-dose, 4 hours post-dose; standard cardiovascular history exclusion
14. UK research-grade sourcing standards
PT-141 should be sourced with full documentation:
- ≥98% HPLC purity (≥99% is the emerging 2026 standard)
- Mass spectrometry identity confirmation (theoretical MW 1025.19 Da)
- Batch-specific Certificate of Analysis
- Endotoxin quantification
- Residual TFA analysis
- Cyclisation integrity confirmation (LC-MS can distinguish cyclic from linear isomers; free-carboxyl vs amide C-terminus must be confirmed)
- Lyophilised powder with cold-chain shipping
Quality-control note: PT-141’s cyclic structure is the most common site of synthesis failure. A high-quality COA should specifically address cyclisation yield and the absence of des-cyclic (linear) parent peptide. Blue colouration (suggesting copper contamination from coupling reagent residues) is occasionally observed in poorly-purified batches — an immediate visual red flag.
FAQ
Is PT-141 the same as Vyleesi?
Yes — Vyleesi is the trade name for bremelanotide 1.75 mg SC autoinjector for HSDD. PT-141 is the research/development name for the same molecule.
Is PT-141 the same as Melanotan-II?
No. PT-141 is derived from Melanotan-II but has different selectivity (reduced MC1R activity) and a different C-terminus (free carboxyl vs amide). They are distinct molecules with distinct pharmacology.
Does PT-141 cause tanning?
Minimal — residual MC1R activity produces some focal hyperpigmentation at clinical doses (1.0% of RECONNECT participants), but PT-141 does not produce the systemic tanning effect of Melanotan-II.
Can PT-141 treat erectile dysfunction?
Research interest continues but PT-141 is not approved for ED. Early Phase 2 data showed efficacy; approval pursued only for HSDD (Vyleesi, 2019). Combination with PDE5 inhibitors is a research area.
Is PT-141 legal to supply in the UK?
For research-grade laboratory use only, yes. PT-141 is not MHRA-approved as a medicine in the UK and is not for human consumption. UK research-grade suppliers supply for in vitro and preclinical research use.
What is the typical research dose?
1.75 mg SC per administration (matching Vyleesi clinical dose). Lower doses (0.5-1.0 mg) are used in dose-response mechanistic studies; higher doses (2-3 mg) have been used in ED research.
How does PT-141 compare to flibanserin (Addyi) for HSDD?
Both are approved for HSDD with similar magnitude of effect. Flibanserin is daily oral, with tolerability (sedation, hypotension) and alcohol-interaction concerns. PT-141 is PRN subcutaneous, with nausea and transient BP elevation as main side effects. Mechanism is different: flibanserin is a 5-HT1A agonist / 5-HT2A antagonist; PT-141 is a melanocortin receptor agonist.
References
- Kingsberg SA et al. Bremelanotide for the treatment of hypoactive sexual desire disorder: two randomized phase 3 trials (RECONNECT). Obstet Gynecol 2019;134:899–908.
- Clayton AH et al. Bremelanotide for female sexual dysfunctions in premenopausal women: a randomized, placebo-controlled dose-finding trial. Womens Health (Lond) 2016;12:325–337.
- Safarinejad MR. Evaluation of the safety and efficacy of bremelanotide, a melanocortin receptor agonist, in female subjects with arousal disorder. Int J Impot Res 2008;20:301–307.
- Hadley ME. Discovery that a melanocortin regulates sexual functions in male and female humans. Peptides 2005;26:1687–1689.
- Pfaus JG et al. Selective facilitation of sexual solicitation in the female rat by a melanocortin receptor agonist. Proc Natl Acad Sci USA 2004;101:10201–10204.
- Molinoff PB et al. PT-141: a melanocortin agonist for the treatment of sexual dysfunction. Ann N Y Acad Sci 2003;994:96–102.
- Shadiack AM, Sharma SD, Earle DC, Spana C, Hallam TJ. Melanocortins in the treatment of male and female sexual dysfunction. Curr Top Med Chem 2007;7:1137–1144.
- Diamond LE et al. Double-blind, placebo-controlled evaluation of the safety, pharmacokinetic properties and pharmacodynamic effects of intranasal PT-141. J Sex Med 2005;2:620–628.
- Tao YX. The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology. Endocr Rev 2010;31:506–543.
- Van der Ploeg LH et al. A role for the melanocortin 4 receptor in sexual function. Proc Natl Acad Sci USA 2002;99:11381–11386.
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Disclaimer: All peptides referenced are sold strictly for in vitro laboratory research use. Not for human consumption, veterinary use, food additive, cosmetic, or household purpose. Nothing in this article is medical advice. UK researchers are responsible for compliance with the Human Medicines Regulations 2012 and Misuse of Drugs Regulations 2001 where applicable.
