1. Molecular Characterization

1.1 Chemical Structure and Properties

PT-141, chemically designated as bremelanotide, is a synthetic heptapeptide analog of α-melanocyte-stimulating hormone (α-MSH) with the molecular formula C₅₀H₆₈N₁₄O₁₀. The compound exhibits a molecular weight of 1025.2 g/mol and represents a metabolite of the melanocortin receptor agonist melanotan II (MT-II). The peptide sequence is characterized by Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, featuring a critical lactam bridge between the aspartic acid and lysine residues that confers conformational stability and receptor selectivity.

1.2 Structural Features and Pharmacophore

The cyclic structure of PT-141 is essential for its biological activity and metabolic stability. The lactam bridge constrains the peptide backbone into a β-turn conformation that optimally presents the His-D-Phe-Arg-Trp tetrapeptide pharmacophore to melanocortin receptors. The N-terminal acetylation and norleucine (Nle) substitution enhance proteolytic resistance, particularly against aminopeptidase degradation. The incorporation of D-phenylalanine at position 7 provides additional protection from enzymatic hydrolysis while maintaining critical aromatic interactions with the receptor binding pocket.

Conformational analysis via circular dichroism spectroscopy and nuclear magnetic resonance (NMR) studies demonstrates that PT-141 adopts a stable type II' β-turn structure in aqueous solution, with the cyclic constraint reducing conformational entropy and pre-organizing the molecule for receptor binding. This structural rigidity contributes to the compound's enhanced selectivity profile compared to linear melanocortin analogs.

1.3 Receptor Binding Profile

PT-141 demonstrates nanomolar affinity for melanocortin receptor subtypes, with particular selectivity for MC3R and MC4R. Radioligand binding studies employing [¹²⁵I]-NDP-α-MSH have established K_i values of approximately 2.7 nM for MC4R and 4.1 nM for MC3R in human receptor-expressing cell lines. The compound exhibits substantially lower affinity for MC1R (K_i >500 nM) and MC5R (K_i >300 nM), which accounts for its minimal melanogenic effects compared to non-selective melanocortin agonists.

2. Synthesis and Manufacturing

2.1 Solid-Phase Peptide Synthesis

PT-141 is synthesized using standard Fmoc (9-fluorenylmethyloxycarbonyl) solid-phase peptide synthesis (SPPS) methodology on appropriate resin supports. The synthetic route employs orthogonal protecting group strategies to enable selective lactam bridge formation between the Asp and Lys side chains. The synthesis typically proceeds on a Rink amide MBHA resin with the following sequential steps:

1. Linear Chain Assembly: Sequential coupling of Fmoc-protected amino acids in the C-to-N direction using HBTU/HOBt activation chemistry. Each coupling cycle involves: (a) Fmoc deprotection with 20% piperidine in DMF, (b) amino acid activation with HBTU in the presence of DIEA, and (c) coupling for 1-2 hours with monitoring by Kaiser test.
2. On-Resin Cyclization: Following assembly of the linear heptapeptide with appropriate Asp and Lys side-chain protection (tBu and Alloc, respectively), selective Alloc deprotection is performed using Pd(PPh₃)₄ catalyst. The exposed lysine ε-amino group undergoes lactam formation with the Asp β-carboxyl group using PyBOP/HOBt activation chemistry.
3. N-Terminal Acetylation: The terminal amino group is acetylated using acetic anhydride in DMF with DIEA as base, yielding the acetyl-capped N-terminus.
4. Cleavage and Deprotection: Global deprotection and resin cleavage is accomplished using Reagent K (TFA/phenol/water/thioanisole/EDT, 82.5:5:5:5:2.5) for 3 hours at ambient temperature.

2.2 Purification and Characterization

Crude peptide is precipitated with cold diethyl ether, dissolved in 0.1% TFA, and purified by preparative reversed-phase HPLC on C18 columns using acetonitrile/water gradients containing 0.1% TFA. Typical purification achieves >95% purity as assessed by analytical HPLC with UV detection at 214 nm and 280 nm. Structural verification employs electrospray ionization mass spectrometry (ESI-MS) to confirm the expected molecular ion [M+H]⁺ at m/z 1025.2, with additional confirmation by tandem mass spectrometry (MS/MS) fragmentation analysis. Amino acid analysis following acid hydrolysis verifies the expected 1:1:1:1:1:1:1 molar ratio of constituent residues.

2.3 Manufacturing Considerations

Commercial-scale production of PT-141 requires stringent quality control measures to ensure batch-to-batch consistency. Critical quality attributes (CQAs) include peptide purity (>98% by HPLC), correct disulfide/lactam connectivity, residual TFA content (<0.1%), water content (typically 5-8% by Karl Fischer titration), and endotoxin levels (<5 EU/mg). The manufacturing process must comply with current Good Manufacturing Practices (cGMP) for pharmaceutical peptides, with validated analytical methods for release testing and stability assessment.

3. Mechanism of Action

3.1 Melanocortin Receptor Pharmacology

PT-141 functions as a potent agonist at melanocortin-3 and melanocortin-4 receptors (MC3R and MC4R), which are seven-transmembrane G protein-coupled receptors (GPCRs) that couple predominantly to G_s proteins. Upon agonist binding, these receptors activate adenylyl cyclase, resulting in elevated intracellular cyclic adenosine monophosphate (cAMP) levels. Quantitative cAMP accumulation assays in HEK293 cells stably expressing human MC4R demonstrate an EC₅₀ of approximately 1.8 nM for PT-141, with maximal stimulation achieving 180-220% of the response elicited by the endogenous ligand α-MSH.

The downstream signaling cascade involves protein kinase A (PKA) activation, which phosphorylates multiple substrate proteins including cAMP response element-binding protein (CREB). CREB-mediated transcriptional changes modulate expression of genes involved in neuronal excitability, synaptic plasticity, and neurotransmitter release. Additionally, MC4R activation influences neuronal firing patterns through direct modulation of ion channel activity, particularly voltage-gated calcium channels and ATP-sensitive potassium channels.

3.2 Neurocircuitry of Sexual Function

The therapeutic efficacy of PT-141 in treating sexual dysfunction derives from its actions on specific neural circuits within the central nervous system. MC3R and MC4R are highly expressed in hypothalamic nuclei including the paraventricular nucleus (PVN), ventromedial hypothalamus (VMH), and medial preoptic area (MPOA), regions critically involved in regulating sexual motivation and arousal. Immunohistochemical studies demonstrate dense MC4R expression on oxytocin-producing neurons in the PVN, and electrophysiological recordings confirm that melanocortin receptor activation increases oxytocin neuron firing rates.

Oxytocin release into both central synaptic regions and peripheral circulation represents a key mediator of PT-141's effects. Central oxytocin projections to limbic structures including the nucleus accumbens and ventral tegmental area enhance dopaminergic neurotransmission, thereby augmenting motivational and reward-related aspects of sexual behavior. Peripheral oxytocin promotes vasodilation and enhances genital blood flow through nitric oxide-dependent mechanisms in vascular endothelium.

3.3 Additional Neurotransmitter Systems

Beyond the oxytocin system, PT-141 influences multiple neurotransmitter pathways relevant to sexual function. Melanocortin receptor activation in the PVN stimulates release of dopamine and norepinephrine in projection regions, contributing to heightened arousal and attention to sexual stimuli. Conversely, MC4R-mediated signaling modulates GABAergic inhibitory tone, potentially disinhibiting pro-sexual neural circuits. Studies employing microdialysis in rodent models demonstrate that systemic PT-141 administration increases dopamine efflux in the nucleus accumbens by approximately 150-200% above baseline, with sustained elevation persisting for 2-4 hours post-administration.

The compound's effects on sexual function appear to be mediated primarily through central nervous system mechanisms rather than direct peripheral actions. This is supported by observations that PT-141 retains efficacy in preclinical models even when administered via routes that minimize systemic exposure, and by the relatively rapid onset of effects following subcutaneous administration, suggesting CNS-mediated mechanisms predominate over peripheral vascular effects.

4. Preclinical Research

4.1 Animal Models of Sexual Behavior

Extensive preclinical evaluation of PT-141 has been conducted in multiple species including rats, rabbits, and non-human primates. In ovariectomized female rats treated with estradiol priming, subcutaneous PT-141 administration (1-10 mg/kg) dose-dependently increased receptive behaviors including lordosis quotient and proceptive behaviors such as ear wiggling and hopping. The ED₅₀ for increasing lordosis quotient was approximately 3.2 mg/kg, with maximal effects observed at 10 mg/kg.

Male rat studies demonstrated that PT-141 (1-5 mg/kg SC) significantly reduced ejaculatory latency and post-ejaculatory interval, while increasing mount frequency and the percentage of animals achieving ejaculation within a 30-minute testing period. These effects were observed in sexually experienced males and also in sexually naive animals, suggesting the compound can facilitate both performance and motivation components of male sexual behavior. Importantly, the pro-sexual effects of PT-141 were observed in animals without concurrent administration of peripheral vasodilators, distinguishing its central mechanism from peripherally-acting agents.

4.2 Primate Studies

Studies in rhesus macaques provided critical translational evidence for PT-141's therapeutic potential. In these experiments, male macaques received subcutaneous PT-141 (0.5-2.0 mg/kg) and were then paired with sexually receptive females. Dose-dependent increases in sexual behavior were quantified using standardized ethograms, with significant increases in mounting, intromission attempts, and time spent in proximity to female conspecifics. Plasma oxytocin measurements confirmed that behavioral effects correlated with 2-3 fold elevations in circulating oxytocin concentrations, with peak levels occurring 30-60 minutes post-injection.

Female macaque studies similarly demonstrated enhanced sexual receptivity following PT-141 administration, with increased presentation behaviors and reduced rejection of male mounting attempts. These primate findings were particularly valuable given the phylogenetic proximity to humans and the complex social and motivational aspects of primate sexual behavior that more closely model human sexuality compared to rodent paradigms.

4.3 Pharmacokinetic and Distribution Studies

Pharmacokinetic characterization in preclinical species revealed that PT-141 exhibits rapid absorption following subcutaneous administration, with T_max values of 30-60 minutes across species. The compound demonstrates relatively poor oral bioavailability (<5%) due to peptidase degradation in the gastrointestinal tract, necessitating parenteral administration routes. Tissue distribution studies using radiolabeled [¹²⁵I]-PT-141 demonstrated CNS penetration, with brain-to-plasma ratios of approximately 0.15-0.25 in rats, indicating modest but pharmacologically significant blood-brain barrier permeability. The highest CNS concentrations were observed in hypothalamic regions, consistent with the known distribution of MC3R and MC4R.

Elimination half-life values ranged from 2.1 hours in rats to 4.7 hours in primates, with renal clearance representing the primary elimination route. Approximately 60-70% of an administered dose is recovered in urine within 24 hours, predominantly as intact peptide and minor metabolites resulting from peptidase cleavage. Hepatic metabolism plays a minimal role, with less than 10% of dose recovered in feces.

4.4 Safety Pharmacology

Comprehensive safety pharmacology studies assessed cardiovascular, respiratory, and CNS effects. Telemetry studies in conscious dogs demonstrated transient, mild increases in blood pressure (10-15 mmHg systolic) and heart rate (15-20 bpm increase) following PT-141 administration at supratherapeutic doses (5-10x projected human exposure). These effects were self-limiting and resolved within 3-4 hours. Respiratory parameters including rate, tidal volume, and minute ventilation were unaffected at therapeutic exposure levels.

CNS safety assessment included modified Irwin screens in rats and observational studies in primates. At therapeutic doses, no significant alterations in locomotor activity, body temperature, or behavioral parameters were observed. Higher doses (>20 mg/kg in rodents) produced stretching and yawning behaviors characteristic of melanocortin receptor activation, but these did not progress to more severe CNS effects. Notably, PT-141 did not produce skin darkening effects typical of MC1R agonists, confirming the receptor selectivity profile.

5. Clinical Studies and FDA Approval

5.1 Phase 1 Clinical Trials

Initial Phase 1 studies in healthy volunteers established the safety, tolerability, and pharmacokinetic profile of PT-141 in humans. Single ascending dose studies evaluated subcutaneous doses ranging from 0.75 mg to 20 mg, with plasma sampling conducted over 24 hours post-administration. Pharmacokinetic analysis revealed dose-proportional increases in C_max and AUC, with mean T_max of 0.5-1.0 hours and elimination half-life of approximately 2.7 hours. The compound demonstrated linear pharmacokinetics across the dose range studied, with no evidence of accumulation upon repeated dosing.

Common adverse events in Phase 1 studies included transient nausea (occurring in 40-50% of subjects at doses ≥10 mg), flushing (25-35%), and headache (20-30%). These effects were generally mild to moderate in severity and self-limiting, resolving within 4-6 hours of administration. Notably, a dose-dependent increase in blood pressure was observed, with mean increases of 5-10 mmHg systolic and 3-7 mmHg diastolic at therapeutic doses. This finding necessitated careful cardiovascular monitoring in subsequent clinical development.

5.2 Phase 2 Clinical Development

Phase 2 proof-of-concept studies enrolled premenopausal women with hypoactive sexual desire disorder (HSDD). These randomized, double-blind, placebo-controlled trials evaluated PT-141 administered subcutaneously at doses of 1.25 mg, 1.75 mg, and 2.0 mg, with dosing occurring in the home setting prior to anticipated sexual activity. The primary efficacy endpoint was change from baseline in the number of satisfying sexual events (SSEs) over a 4-week period, assessed using electronic patient diaries.

Results from the pivotal Phase 2 study (n=327) demonstrated statistically significant increases in SSEs for the 1.75 mg dose group compared to placebo. The mean change from baseline was 2.8 additional SSEs per month in the 1.75 mg group versus 1.1 in the placebo group (p<0.001). Secondary endpoints including Female Sexual Function Index (FSFI) desire domain scores and Female Sexual Distress Scale-Revised (FSDS-R) scores also showed significant improvements. Approximately 60% of women in the active treatment group reported meaningful improvement in sexual desire compared to 35% in the placebo group.

5.3 Phase 3 Pivotal Trials

The Phase 3 clinical program consisted of two identically designed, randomized, double-blind, placebo-controlled trials (RECONNECT studies) that enrolled premenopausal women with acquired, generalized HSDD. These studies excluded women with relationship problems or major psychiatric disorders and required screening testosterone levels within normal range to ensure the HSDD diagnosis reflected a primary disorder of sexual desire rather than secondary to other conditions.

RECONNECT Study 1 (n=1,267) and Study 2 (n=1,247) evaluated subcutaneous PT-141 1.75 mg administered as needed, with no more than one dose per 24-hour period and no more than eight doses per month. The co-primary efficacy endpoints were: (1) change from baseline to week 24 in the number of SSEs, and (2) change from baseline to week 24 in the FSFI desire domain score. Both studies employed a 4-week baseline observation period followed by 24 weeks of treatment.

Both pivotal trials met their co-primary endpoints, demonstrating statistically significant and clinically meaningful improvements in sexual desire and satisfying sexual activity. The effect sizes, while modest, were considered clinically relevant in the context of HSDD, a condition with limited treatment options. Subgroup analyses revealed consistent efficacy across age groups, baseline severity of HSDD, and menopausal status (premenopausal vs. perimenopausal).

5.4 Safety Profile and Adverse Events

The integrated safety database from the Phase 3 program encompassed over 1,200 women exposed to PT-141 for up to 52 weeks. The most common adverse events were nausea (40%), flushing (20%), injection site reactions (13%), headache (11%), and vomiting (6%). Most adverse events were mild to moderate in intensity and occurred within the first few hours after injection, with symptom resolution typically within 6-12 hours.

Nausea represented the most significant tolerability challenge, leading to treatment discontinuation in approximately 13% of subjects. The incidence and severity of nausea appeared to diminish with repeated dosing, suggesting development of tolerance. Pretreatment with antiemetic medications was explored but not formally recommended in the approved labeling.

Cardiovascular safety assessment revealed transient increases in blood pressure, with mean increases of 3-4 mmHg systolic blood pressure observed 8-12 hours post-dose. Approximately 5% of subjects experienced systolic blood pressure increases ≥30 mmHg or diastolic increases ≥20 mmHg. These findings led to contraindication of PT-141 in women with uncontrolled hypertension or known cardiovascular disease. The mechanism of blood pressure elevation is postulated to involve melanocortin receptor-mediated effects on sympathetic tone and possibly direct vascular effects.

5.5 FDA Approval and Regulatory Status

On June 21, 2019, the U.S. Food and Drug Administration approved PT-141 under the trade name Vyleesi for the treatment of acquired, generalized hypoactive sexual desire disorder (HSDD) in premenopausal women. This approval represented a significant milestone as only the second medication approved for this indication. The approved dosing regimen is 1.75 mg administered subcutaneously via autoinjector into the abdomen or thigh at least 45 minutes before anticipated sexual activity, with no more than one dose in a 24-hour period and a maximum of eight doses per month.

The prescribing information includes a boxed warning regarding transient increases in blood pressure and a contraindication in patients with uncontrolled hypertension or known cardiovascular disease. Healthcare providers are advised to counsel patients on proper injection technique and the expected timing of onset (effects typically manifest 30-60 minutes post-injection) and duration of action (up to 12-24 hours).

Post-marketing requirements included a cardiovascular outcomes study in women with mild to moderate cardiovascular risk factors and continued pharmacovigilance for rare adverse events. The European Medicines Agency (EMA) has not yet approved PT-141, with ongoing discussions regarding the benefit-risk profile in European populations.

6. Analytical Methods

6.1 High-Performance Liquid Chromatography

Analytical characterization of PT-141 employs reversed-phase HPLC as the primary method for purity assessment and stability testing. A typical method utilizes a C18 column (250 x 4.6 mm, 5 μm particle size) with a gradient elution system consisting of 0.1% TFA in water (mobile phase A) and 0.1% TFA in acetonitrile (mobile phase B). The gradient profile proceeds from 20% B to 50% B over 30 minutes at a flow rate of 1.0 mL/min, with UV detection at 214 nm and 280 nm. Under these conditions, PT-141 elutes at approximately 18.5 minutes with excellent resolution from potential impurities including truncated sequences, deletion peptides, and isomeric variants.

Quantitative analysis is performed using external standard calibration with peak area integration. The method demonstrates linearity over the range of 0.1-2.0 mg/mL (r² >0.999) with precision (RSD) <2% for replicate injections. Accuracy is verified by spiking known amounts of reference standard into sample matrices, with recovery typically 98-102%. The limit of quantification (LOQ) is established at 0.1 μg/mL, adequate for analysis of formulation samples and pharmacokinetic specimens.

6.2 Mass Spectrometry Methods

Electrospray ionization mass spectrometry (ESI-MS) provides definitive structural confirmation and detection of impurities. Analysis is conducted in positive ion mode with the molecular ion [M+H]⁺ detected at m/z 1025.2. High-resolution mass spectrometry (HRMS) on Q-TOF or Orbitrap instruments confirms the exact mass within 5 ppm accuracy. Tandem mass spectrometry (MS/MS) generates sequence information through collision-induced dissociation (CID), producing b-ion and y-ion fragment series that confirm the amino acid sequence and location of the cyclic constraint.

For pharmacokinetic studies, liquid chromatography-tandem mass spectrometry (LC-MS/MS) provides sensitive and specific quantification of PT-141 in biological matrices. Plasma samples are prepared by protein precipitation with acetonitrile or solid-phase extraction on C18 cartridges. Chromatographic separation employs a short C18 column (50 x 2.1 mm, 3 μm) with a rapid gradient over 5 minutes. Multiple reaction monitoring (MRM) transitions track the precursor ion [M+2H]²⁺ at m/z 513.1 to product ions at m/z 136.1 (Tyr fragment) and m/z 110.1 (His fragment). The method achieves a lower limit of quantification (LLOQ) of 0.1 ng/mL with inter-day precision <15% and accuracy of 90-110% across the calibration range of 0.1-100 ng/mL.

6.3 Peptide Mapping and Sequence Confirmation

Peptide mapping via enzymatic digestion followed by LC-MS analysis provides comprehensive structural characterization. PT-141 is subjected to digestion with trypsin, which cleaves at Arg and Lys residues, or chymotrypsin, which cleaves at aromatic residues. The resulting peptide fragments are separated by reversed-phase HPLC and analyzed by ESI-MS/MS to generate sequence coverage maps. This approach confirms the amino acid sequence, identifies the cyclization site (Asp-Lys lactam), and detects potential sequence variants or post-translational modifications.

Amino acid analysis following acid hydrolysis (6 N HCl, 110°C, 24 hours) provides quantitative determination of amino acid composition. The hydrolysate is derivatized with phenylisothiocyanate (PITC) or 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) and analyzed by reversed-phase HPLC with UV or fluorescence detection. Results are compared to theoretical values to confirm the expected 1:1:1:1:1:1:1 ratio of constituent amino acids, with allowance for degradation of Trp during acid hydrolysis.

6.4 Stability-Indicating Methods

Stability-indicating analytical methods are essential for evaluating PT-141 under various storage conditions and formulation matrices. Forced degradation studies subject the peptide to acidic (pH 2), basic (pH 10), oxidative (3% H₂O₂), and thermal (50°C) stress conditions. Degradation products are characterized by HPLC with mass spectrometric identification. Common degradation pathways include: (1) hydrolysis of the lactam bridge, generating the linear peptide; (2) oxidation of Trp and Met residues; (3) deamidation of Asn residues; and (4) peptide bond cleavage, particularly at Asp-Pro sequences.

Real-time and accelerated stability studies follow ICH guidelines, with storage at 25°C/60% RH (long-term) and 40°C/75% RH (accelerated). Samples are analyzed at 0, 1, 3, 6, 12, and 24 months for purity (HPLC), potency (bioassay or HPLC with reference standard), water content (Karl Fischer), and pH. Results inform expiration dating and storage requirements, typically indicating 24-36 month shelf life when stored at 2-8°C as lyophilized powder.

7. Research Applications

7.1 Neuroscience Research

PT-141 serves as a valuable pharmacological tool for investigating melanocortin receptor function in neural circuits governing sexual behavior, appetite regulation, and energy homeostasis. Its selective MC3R/MC4R agonism allows researchers to probe the specific contributions of these receptor subtypes to complex behaviors. Studies employing PT-141 in combination with selective antagonists or in MC3R/MC4R knockout animals have elucidated the distinct roles of these receptors in modulating sexual motivation versus performance, and in differentiating central versus peripheral mechanisms of action.

In addition to sexual function research, PT-141 has been utilized to study melanocortin system involvement in other physiological processes. Research has demonstrated that melanocortin receptor activation influences anxiety-like behaviors, social interaction, and grooming behaviors in rodents. Furthermore, the compound has proven useful in examining the interplay between melanocortin and opioid systems, as these pathways exhibit reciprocal regulation in brain reward circuits. Studies using PT-141 in combination with opioid agonists or antagonists have revealed synergistic effects on sexual behavior and provided insights into neural mechanisms of addiction and reward processing.

7.2 Translational Medicine and Biomarker Development

PT-141 research has contributed to development of translational biomarkers for sexual dysfunction and melanocortin pathway engagement. Plasma oxytocin measurements following PT-141 administration provide a peripheral biomarker of central melanocortin receptor activation. Correlation of oxytocin levels with behavioral outcomes has informed understanding of dose-response relationships and individual variability in treatment response. Additionally, functional neuroimaging studies using fMRI have examined PT-141's effects on brain activation patterns in response to sexual stimuli, revealing enhanced activation in limbic regions including the nucleus accumbens, anterior cingulate cortex, and insula in treated versus placebo groups.

Genetic association studies have investigated polymorphisms in MC3R and MC4R genes as potential predictors of PT-141 response. While no definitive pharmacogenomic markers have been validated for clinical use, research has identified several MC4R variants associated with altered receptor function that may influence individual treatment outcomes. Such investigations exemplify the potential for precision medicine approaches in sexual medicine, tailoring treatments based on genetic profiles.

7.3 Sexual Dysfunction Research Models

The availability of PT-141 has enabled development of improved animal models for studying female sexual dysfunction. Traditional rodent models focused primarily on receptive behaviors (lordosis), which have limited translational relevance to human HSDD. PT-141 research has prompted development of more sophisticated models assessing motivational aspects of sexual behavior, including partner preference paradigms, paced mating procedures, and operant conditioning models where animals perform tasks to gain access to sexual partners. These models better capture the desire and motivation components central to human sexual dysfunction.

Furthermore, PT-141 has been employed in models of drug-induced sexual dysfunction, particularly those associated with selective serotonin reuptake inhibitor (SSRI) antidepressants. Studies have demonstrated that PT-141 can reverse SSRI-induced sexual dysfunction in animal models, suggesting potential clinical utility as an adjunctive treatment for patients experiencing antidepressant-associated sexual side effects. This application remains under investigation in clinical populations.

8. Dosing and Administration

8.1 Clinical Dosing Protocols

The FDA-approved dosing regimen for PT-141 (Vyleesi) is 1.75 mg administered subcutaneously at least 45 minutes prior to anticipated sexual activity. The recommended injection sites are the abdomen or anterior thigh, with rotation of injection sites advised to minimize local reactions. Patients should not administer more than one dose within a 24-hour period, and usage should not exceed eight doses per month. This dosing schedule was established through dose-ranging studies that balanced efficacy against tolerability, particularly nausea incidence.

The onset of action typically occurs within 30-60 minutes of subcutaneous injection, with peak plasma concentrations achieved at 0.5-1.0 hours. Behavioral effects may persist for 12-24 hours post-administration, allowing for a window of opportunity for sexual activity rather than requiring precise timing. Patients are advised that effects may be more pronounced with the first few doses and that some reduction in side effects (particularly nausea) may occur with continued use, although the recommended PRN (as-needed) dosing schedule limits development of complete tolerance.

8.2 Research Dosing Considerations

In research settings, PT-141 dosing varies considerably based on species, experimental paradigm, and route of administration. Rodent studies typically employ subcutaneous or intraperitoneal doses ranging from 0.5 mg/kg to 10 mg/kg, with 1-3 mg/kg representing commonly used effective doses for sexual behavior studies. Doses are often normalized to body surface area rather than body weight when translating from preclinical to clinical studies, as this approach better accounts for differences in metabolic rate across species.

For neuroscience research involving intracerebroventricular (ICV) or site-specific microinjection, doses are substantially lower, typically in the range of 0.1-5 μg per injection. Such studies have been instrumental in mapping the anatomical sites of PT-141 action and have confirmed that behavioral effects can be elicited by direct CNS administration at doses far lower than those required for systemic administration, supporting a primarily central mechanism of action.

8.3 Special Populations

No dose adjustment is recommended for patients with mild to moderate renal impairment (creatinine clearance ≥30 mL/min), as pharmacokinetic studies demonstrated only modest increases in exposure (AUC increase <25%) in this population. PT-141 has not been studied in patients with severe renal impairment or end-stage renal disease, and use in these populations is not recommended due to potential accumulation of the peptide and increased risk of adverse effects.

Hepatic impairment does not significantly affect PT-141 pharmacokinetics, as the peptide undergoes minimal hepatic metabolism. Consequently, no dose adjustment is required for patients with hepatic dysfunction. However, clinical experience in this population is limited. The medication has not been studied in postmenopausal women, and efficacy has not been established in this population. Use during pregnancy is contraindicated, as animal reproduction studies demonstrated adverse developmental outcomes at exposures similar to or lower than therapeutic human exposure.

9. Storage and Handling

9.1 Pharmaceutical Formulation

PT-141 is commercially available as a preservative-free, sterile solution in single-dose prefilled autoinjectors. Each autoinjector contains 1.75 mg bremelanotide in a pH-controlled aqueous formulation with appropriate excipients to maintain stability and isotonicity. The formulation typically includes buffering agents (e.g., sodium phosphate or acetate buffer, pH 4.0-5.5), isotonicity modifiers (e.g., sodium chloride), and may contain stabilizers such as mannitol or trehalose to protect the peptide from aggregation and oxidative degradation.

The autoinjector design ensures accurate dose delivery and simplifies the injection procedure for patients, eliminating the need for dose measurement or needle attachment. The device is configured for single use only and should be discarded after administration, even if some solution remains in the device. This design minimizes risk of contamination and dosing errors while improving patient convenience and adherence.

9.2 Storage Requirements

Vyleesi autoinjectors should be stored refrigerated at 2-8°C (36-46°F) in the original carton to protect from light. The product should not be frozen, and if freezing occurs, the autoinjector should be discarded. Prior to use, the autoinjector may be removed from refrigeration and stored at room temperature (up to 25°C/77°F) for up to 7 days. If not used within this period, the autoinjector may be returned to refrigerated storage. However, the total cumulative time at room temperature should not exceed 7 days.

For research-grade PT-141 supplied as lyophilized powder, storage at -20°C or below is recommended for long-term stability (>12 months). The lyophilized peptide should be protected from moisture and light, preferably stored in amber vials under inert atmosphere (nitrogen or argon). Upon reconstitution in sterile water or appropriate buffer, solutions should be used immediately or stored at 2-8°C for no more than 7 days. For extended storage of reconstituted solutions, aliquoting and freezing at -80°C is advisable, with minimization of freeze-thaw cycles to prevent aggregation and loss of potency.

9.3 Handling Precautions

PT-141 solutions should be inspected visually prior to administration for particulate matter and discoloration. The solution should be clear to slightly opalescent and colorless to slightly yellow. If the solution is cloudy, discolored, or contains visible particles, it should not be used. The autoinjector should be allowed to reach room temperature (15-30 minutes) before injection to improve comfort and reduce injection site reactions.

Standard precautions for handling pharmaceutical peptides should be observed in research settings. Personnel should wear appropriate personal protective equipment including gloves and lab coats when handling PT-141 powder or concentrated solutions. Avoid generating aerosols during reconstitution or transfer procedures. In case of skin contact, wash thoroughly with soap and water. For eye contact, rinse immediately with copious amounts of water and seek medical attention if irritation persists.

10. Safety and Contraindications

10.1 Adverse Event Profile

The safety profile of PT-141 is well-characterized from extensive clinical trial experience encompassing over 1,500 treated patients. The most common adverse reactions (incidence ≥3% and greater than placebo) include nausea (40%), flushing (20%), injection site reactions (13%), headache (11%), vomiting (6%), congestion (5%), and hot flush (4%). These effects are typically transient, occurring within hours of administration and resolving spontaneously without intervention.

Nausea represents the primary tolerability-limiting adverse effect, with approximately 13% of patients discontinuing treatment due to this symptom in clinical trials. The nausea is thought to result from melanocortin receptor activation in the area postrema, a circumventricular organ lacking a complete blood-brain barrier that contains emetic chemoreceptor trigger zones. Some evidence suggests development of tolerance to nausea with repeated exposure, though the as-needed dosing regimen limits this adaptation. Prophylactic antiemetics have been explored empirically but are not formally recommended in prescribing information.

10.2 Cardiovascular Safety

PT-141 produces transient increases in blood pressure, with mean increases of 3-4 mmHg in systolic blood pressure observed in clinical trials. In approximately 5% of patients, more substantial increases (≥30 mmHg systolic or ≥20 mmHg diastolic) were observed. Blood pressure elevations typically manifest 8-12 hours after injection and resolve within 24 hours. The mechanism is postulated to involve melanocortin receptor-mediated increases in sympathetic nervous system activity.

Based on these findings, the prescribing information includes a boxed warning regarding blood pressure increases and contraindication in patients with uncontrolled hypertension (blood pressure >150/90 mmHg) or known cardiovascular disease. Blood pressure should be assessed prior to prescribing PT-141 and monitored periodically during treatment. Patients should be counseled to recognize symptoms of severe hypertension (severe headache, visual changes, chest pain) and seek immediate medical attention if these occur. Caution is advised in patients with controlled hypertension or those taking antihypertensive medications.

10.3 Contraindications and Precautions

PT-141 is contraindicated in patients with uncontrolled hypertension or known cardiovascular disease including coronary artery disease, stroke, arrhythmias, or cardiac valvular disease. It is also contraindicated in pregnancy due to evidence of fetal harm in animal reproduction studies. Women of reproductive potential should use effective contraception during treatment. PT-141 has not been studied during lactation, and the presence of bremelanotide in human milk is unknown; therefore, the developmental and health benefits of breastfeeding should be considered along with the mother's clinical need for PT-141.

Patients with known hypersensitivity to bremelanotide or any excipients should not use PT-141. Additionally, patients with focal hyperpigmentation disorders should use caution, as melanocortin receptor agonism theoretically could exacerbate such conditions, although this has not been observed in clinical trials with PT-141's selective MC3R/MC4R profile.

10.4 Drug Interactions

No formal drug interaction studies have been conducted with PT-141. However, theoretical interactions may occur with medications that affect blood pressure or cardiovascular function. Concurrent use with alpha-adrenergic agonists (e.g., nasal decongestants, certain migraine medications) could potentially produce additive effects on blood pressure. Conversely, antihypertensive medications might be less effective during the period of PT-141-induced blood pressure elevation.

PT-141 does not undergo significant metabolism by cytochrome P450 enzymes and is not expected to interact with medications metabolized by these pathways. The peptide is not a substrate, inhibitor, or inducer of major drug transporters (P-glycoprotein, BCRP, OAT, OCT) based on in vitro studies, further reducing potential for pharmacokinetic drug interactions. Nonetheless, patients taking medications with narrow therapeutic indices or complex regimens should be monitored appropriately when initiating PT-141.

10.5 Long-Term Safety Considerations

Long-term safety data extending to 52 weeks of treatment are available from open-label extension studies. These studies revealed no new safety signals with prolonged use and suggested some reduction in nausea incidence with continued exposure, consistent with development of partial tolerance. Cardiovascular monitoring in these extended studies did not demonstrate accumulation or worsening of blood pressure effects over time.

Theoretical concerns regarding chronic melanocortin receptor activation include potential effects on pigmentation, energy homeostasis, and metabolic parameters. However, clinical trial data have not substantiated these concerns at therapeutic doses of PT-141. Mean changes in body weight, glucose homeostasis markers (HbA1c, fasting glucose), and lipid parameters were not significantly different between PT-141 and placebo groups in controlled trials. Skin hyperpigmentation, a known effect of non-selective melanocortin agonists, was not observed in PT-141-treated patients, confirming the functional selectivity for MC3R/MC4R over MC1R.

11. Literature Review and References

11.1 Foundational Research

The development of PT-141 builds upon decades of melanocortin system research, beginning with the identification of melanocyte-stimulating hormones in the 1950s and subsequent discovery of their effects beyond pigmentation. Pioneering work in the 1990s demonstrated that centrally administered melanocortin peptides elicited sexual arousal and penile erection in animal models, independent of peripheral vascular mechanisms. These observations catalyzed development of synthetic melanocortin analogs with enhanced potency, selectivity, and metabolic stability.

PT-141 emerged as a metabolite of melanotan II during clinical studies of that compound for inducing cosmetic tanning. When melanotan II subjects reported enhanced sexual function as an unexpected effect, systematic investigation revealed that this activity resided primarily in the PT-141 metabolite. Subsequent research focused on developing PT-141 specifically as a therapeutic for sexual dysfunction, capitalizing on its improved safety profile compared to the parent compound.

11.2 Key Clinical Publications

The pivotal RECONNECT clinical trials establishing PT-141's efficacy for HSDD were published in peer-reviewed journals and provided the evidence base for FDA approval. These studies employed rigorous methodology including validated patient-reported outcome measures (FSFI, FSDS-R), electronic diaries for real-time assessment of sexual activity, and appropriate statistical analysis accounting for placebo response. The consistency of results across two independent trials strengthened confidence in the treatment effect and supported the conclusion of clinically meaningful benefit.

Mechanistic studies employing functional neuroimaging have provided insights into PT-141's effects on brain activity patterns. fMRI studies demonstrated that PT-141 enhances activation in reward-related brain regions including the nucleus accumbens and ventral tegmental area in response to visual sexual stimuli. These findings corroborated animal studies implicating mesolimbic dopamine circuits in PT-141's mechanism and validated the hypothesis that the compound acts centrally to enhance sexual motivation and desire.

11.3 Comparative Effectiveness Research

PT-141 represents one of only two FDA-approved medications for HSDD in premenopausal women, the other being flibanserin (Addyi). These agents differ substantially in mechanism of action, dosing regimen, and safety profile. Flibanserin is a serotonin 5-HT1A receptor agonist and 5-HT2A antagonist administered as a daily oral medication, whereas PT-141 is a melanocortin receptor agonist used on an as-needed basis via subcutaneous injection. No head-to-head comparative trials have been conducted, precluding direct efficacy comparisons.

Indirect comparisons based on placebo-controlled trial results suggest broadly similar effect sizes for both medications, with increases in satisfying sexual events of approximately 0.7-1.2 events per month above placebo. Patient preference between daily oral dosing versus as-needed injection administration likely varies based on individual factors including severity and pattern of symptoms, tolerance of side effects, and personal preferences regarding medication administration. The availability of mechanistically distinct options allows for personalized treatment selection and sequential trials if initial therapy proves inadequate or poorly tolerated.

11.4 Related Research Topics

Research on PT-141 has stimulated broader investigation into melanocortin system pharmacology and its therapeutic potential. Selective MC4R agonists are being developed for obesity treatment, capitalizing on this receptor's role in regulating energy homeostasis and satiety. Conversely, MC4R antagonists are being investigated for cachexia and muscle wasting conditions. The development of PT-141 has informed structure-activity relationship understanding that guides design of next-generation melanocortin therapeutics with optimized selectivity, potency, and pharmacokinetic properties.

Additionally, PT-141 research has contributed to evolving perspectives on female sexual dysfunction classification and treatment. The recognition of desire disorders as distinct from arousal disorders, and the appreciation for central nervous system contributions to sexual function, represent important conceptual advances. PT-141's mechanism highlights the importance of neurobiological factors in sexual function and validates pharmaceutical approaches to addressing desire disorders that were historically viewed as purely psychological in origin.

11.5 Future Directions

Ongoing research continues to explore additional applications of PT-141 and to refine understanding of optimal patient selection. Studies are investigating efficacy in postmenopausal women, patients with medication-induced sexual dysfunction (particularly SSRI-associated dysfunction), and male patients with desire disorders or erectile dysfunction. Biomarker research aims to identify predictors of treatment response, including genetic polymorphisms in melanocortin receptors, baseline oxytocin levels, and neuroimaging signatures of reward circuit function.

Formulation development efforts focus on improving ease of administration and potentially reducing side effects. Alternative delivery routes including intranasal administration have been explored, though concerns regarding cardiovascular effects when administered by this route led to discontinuation of earlier intranasal development programs. Novel formulations incorporating gradual-release technologies or combination with antiemetic agents may improve tolerability. Furthermore, research into the fundamental neurobiology of sexual desire continues to identify additional therapeutic targets that may complement or enhance melanocortin-based approaches.

12. 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. Women's Health. 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 USA. 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, et al. 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

Related Research Resources

• Melanocortin Receptor Agonists: Comprehensive Database
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