# PT-141 (Bremelanotide): Central Melanocortin Receptor Pharmacology, Neuroendocrine Signaling Cascades, and Dosing Reconstitution

## Key Takeaways

- **Cyclic Heptapeptide Structure and Pharmacophore Origin:** Bremelanotide is a synthetic cyclic heptapeptide engineered around the conserved melanocortin "His-Phe-Arg-Trp" tetrapeptide pharmacophore derived from α-MSH, with a lactam bridge cyclization that confers conformational rigidity, enhances MC3R/MC4R binding affinity, and improves resistance to enzymatic degradation relative to the linear endogenous ligand.
- **Central MC3R/MC4R Receptor Kinetics and Gα_s-cAMP Signaling:** PT-141 acts as a high-affinity, non-selective agonist at MC3R (predominantly hypothalamic/limbic) and MC4R (centrally expressed in the paraventricular nucleus and medial preoptic area), both Class A GPCRs lacking N-terminal signal peptides, coupling through Gα_s to activate adenylyl cyclase, elevate intracellular cAMP, and engage downstream PKA-dependent phosphorylation cascades that modulate dopaminergic and oxytocinergic neuroendocrine output.
- **POMC Prohormone Processing and Tissue-Specific Peptide Generation:** The 241-amino-acid POMC precursor (MW ≈ 28.6 kDa, gene locus 2p23.3) is cleaved by prohormone convertases PC1/3 and PC2 in a tissue-specific manner to yield ACTH, α-MSH (13 residues, MW ≈ 1.66 kDa), β-MSH, γ-MSH, and β-endorphin, with the melanocortin peptides sharing the conserved core pharmacophore essential for MC1R-MC5R engagement.
- **Subcutaneous Bioavailability and Pharmacokinetic Profile:** Following subcutaneous administration, bremelanotide exhibits rapid absorption with a Tmax of approximately 1 hour, a plasma half-life of roughly 2.7 hours, and linear first-order elimination kinetics, with transient Cmax-driven activation of central melanocortin receptors producing downstream modulation of mesolimbic dopaminergic tone and hypothalamic neuroendocrine signaling.
- **Volumetric Reconstitution Dynamics and Diluent Mathematics:** Reconstitution follows the molarity relationship C = n / V, where peptide mass (mg) is converted to moles via division by molecular weight (≈ 1.025 kDa for the acetate salt-adjusted formulation), then to required diluent volume using the equation Volume (mL) = Mass (mg) × 1000 / (Molecular Weight × Target Concentration [mg/mL]), yielding common research concentrations (e.g., 1 mg/1 mL, 5 mg/5 mL, 10 mg/10 mL) with bacteriostatic water or sterile saline as standard diluents, requiring gentle swirling without vortexing to preserve cyclic structural integrity.

> **Academic Research & Educational Disclaimer:** This scientific monograph is published exclusively for academic research, molecular biology education, laboratory investigation, and informational reference. Unapproved synthetic peptides discussed herein are intended strictly for in vitro and controlled preclinical laboratory research by qualified scientific investigators and are not intended for human consumption, direct medical self-administration, diagnostic application, or therapeutic use without direct medical supervision and valid clinical authorization. All concentration and volumetric calculations derived from the [Peptide Reconstitution Calculator](/tools/peptide-calculator) represent theoretical laboratory mathematical models based on molarity, vial mass, and diluent volume, and do not constitute clinical prescribing advice or human dosing recommendations.

## Melanocortin System Neurobiology: Pro-opiomelanocortin (POMC) Cleavage and Receptor Subtypes (MC1R-MC5R)

### 2.1 The Pro-opiomelanocortin (POMC) Prohormone: Genomic Architecture and Post-Translational Processing

The melanocortin system is centered on the post-translational maturation of a single 241-amino-acid precursor glycoprotein, pro-opiomelanocortin (POMC), encoded by the *POMC* gene (human chromosomal locus 2p23.3). The unprocessed POMC polypeptide has a calculated molecular weight of approximately 28.6 kDa and contains a signal peptide (residues 1-26), an N-terminal pro-region, and a series of dibasic cleavage motifs (Arg-Lys, Lys-Arg, Lys-Lys) that are recognized in a tissue-specific manner by members of the prohormone convertase (PC) family - predominantly PC1/3 (encoded by *PCSK1*) and PC2 (encoded by *PCSK2*) [3, 4].

In the corticotrophs of the anterior pituitary, PC1/3-mediated cleavage generates the 39-amino-acid adrenocorticotropic hormone (ACTH, MW ≈ 4.5 kDa) and β-lipotropin. ACTH is subsequently cleaved within the intermediate lobe of the pituitary and within hypothalamic and extra-hypothalamic neurons by PC2 to yield α-melanocyte-stimulating hormone (α-MSH, a 13-amino-acid peptide: Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂; MW ≈ 1.66 kDa) and corticotropin-like intermediate peptide (CLIP). β-Lipotropin is further processed into β-MSH (residues 41-58 of β-lipotropin) and β-endorphin (residues 104-134), whereas γ-MSH arises from the N-terminal pro-γ-MSH fragment [3, 4]. Tissue-specific POMC processing therefore yields a nested set of melanocortin peptides that share a conserved core "His-Phe-Arg-Trp" tetrapeptide pharmacophore, which is essential for high-affinity binding to the five melanocortin receptors [1, 2, 3].

### 2.2 Class A GPCR Architecture of Melanocortin Receptors (MC1R-MC5R)

The five melanocortin receptors (MC1R, MC2R, MC3R, MC4R, MC5R) belong to the rhodopsin-like (Class A) family of G protein-coupled receptors (GPCRs) [1, 2]. Each receptor is a single-polypeptide 7-transmembrane (7-TM) protein of approximately 290-360 amino acids. They share ~40-60 % amino acid identity, with the highest homology within the transmembrane helices and the DRY (Asp-Arg-Tyr) motif at the cytoplasmic end of TM3, which is critical for G-protein coupling [1, 2]. Unlike most Class A GPCRs, melanocortin receptors lack classical N-terminal signal peptides and are constitutively N-glycosylated, which affects cell-surface expression and ligand binding affinity.

Melanocortin receptors couple primarily to Gα_s, the stimulatory G-protein subunit, resulting in adenylyl cyclase activation, cyclic adenosine monophosphate (cAMP) accumulation, and downstream activation of protein kinase A (PKA) and the transcription factor CREB [3, 5]. Emerging evidence also supports Gα_q/11-mediated phospholipase C (PLC) activation, leading to inositol 1,4,5-trisphosphate (IP3) production, intracellular Ca²⁺ release, and beta-arrestin-dependent signaling [1, 2].

### 2.3 MC1R: Cutaneous Pigmentation, Immunomodulation, and UV Response

MC1R (chromosome 16q24.3; 317 amino acids; ~35 kDa) is the principal melanocortin receptor expressed in cutaneous melanocytes, dermal fibroblasts, and immune cells (neutrophils, macrophages, dendritic cells). Activation by α-MSH (EC₅₀ ≈ 0.1-1 nM) and ACTH drives the cAMP/PKA-dependent upregulation of tyrosinase, tyrosinase-related protein 1 (TYRP1), and dopachrome tautomerase (DCT), shifting eumelanin (black/brown) synthesis at the expense of pheomelanin (red/yellow). Loss-of-function variants (e.g., R151C, R160W, D294H) are associated with red hair, fair skin, and increased melanoma risk [1, 3]. The MC1R orthosteric binding pocket accommodates the melanocortin tetrapeptide core with reported *K_i* values for α-MSH in the sub-nanomolar range [1, 2].

### 2.4 MC2R: Adrenal Steroidogenesis and ACTH-Dependent Glucocorticoid Synthesis

MC2R (chromosome 18p11.21; 297 amino acids) is uniquely selective for ACTH; α-MSH, β-MSH, and γ-MSH have negligible agonist activity at this subtype. MC2R obligately requires melanocortin-2 receptor accessory protein (MRAP, isoforms MRAP-α and MRAP-β) for functional cell-surface expression and ACTH binding, making it the only GPCR with a known essential accessory subunit [1, 2, 3]. MC2R activation in the zona fasciculata of the adrenal cortex triggers Gα_s → adenylyl cyclase → cAMP/PKA signaling, driving the rate-limiting conversion of cholesterol to pregnenolone by cytochrome P450scc (CYP11A1) and ultimately cortisol synthesis. Inactivating *MC2R* mutations cause familial glucocorticoid deficiency type 1 (FGD1; OMIM 202200).

### 2.5 MC3R and MC4R: Central Energy Homeostasis and Sexual Function

MC3R (chromosome 20q13.33; 361 amino acids) is predominantly expressed in the arcuate nucleus of the hypothalamus, the ventral tegmental area, and limbic structures. It binds α-MSH, β-MSH, and γ-MSH with comparable affinities (K_d ≈ 1-10 nM) and is implicated in the autocrine regulation of POMC neurons, energy partitioning between lean mass and adipose tissue stores, and the entrainment of circadian feeding rhythms [3, 4]. MC3R-knockout mice display increased adiposity despite normal food intake, consistent with an energy-expenditure rather than satiety role.

MC4R (chromosome 18q21.32; 332 amino acids) is the most pharmacologically prominent melanocortin receptor in the central nervous system, expressed in the paraventricular nucleus (PVN), the dorsomedial hypothalamus (DMH), the medial preoptic area (mPOA), the spinal cord, and the autonomic nuclei [3, 4, 5]. MC4R activation by α-MSH and β-MSH drives anorexigenic signaling via cAMP/PKA, K⁺-channel inhibition (Kir7.1), and the modulation of JAK2/STAT3 and PI3K/Akt pathways. Loss-of-function *MC4R* mutations are the most common monogenic cause of severe early-onset obesity (prevalence ~1.7-6 % of severe childhood obesity cohorts) and are also associated with hyperphagia, accelerated linear growth, and hyperinsulinemia [1, 3, 4]. The MC4R is the molecular substrate for bremelanotide (PT-141), which functions as a high-affinity synthetic agonist at MC4R (and MC3R) with *K_i* values in the low-nanomolar range, mediating the melanocortinergic control of female and male sexual arousal, motivation, and penile/clitoral vascular hemodynamics through central pro-erectile pathways [1, 3, 4, 5].

### 2.6 MC5R: Exocrine Secretion, Thermoregulation, and Immunomodulation

MC5R (chromosome 18p11.21; 325 amino acids) is the most widely distributed melanocortin receptor, with prominent expression in sebaceous glands, Harderian glands, lacrimal glands, the adrenal zona glomerulosa (where it modulates aldosterone secretion), B and T lymphocytes, and skeletal muscle. MC5R activation by α-MSH stimulates exocrine lipid and protein secretion, sebum production, and thermoregulatory lipolysis, while also exerting immunomodulatory effects on cytokine release (notably IL-10 and TNF-α suppression) and the regulation of pathogen clearance [1, 2, 3].

### 2.7 Receptor-Ligand Selectivity Matrix and Signal Transduction Summary

| Receptor | Endogenous Ligand Rank-Order | Primary G-protein | Principal Physiological Function |
|----------|------------------------------|-------------------|----------------------------------|
| MC1R | α-MSH = ACTH > β-MSH | Gα_s (cAMP) | Pigmentation, UV response |
| MC2R | ACTH only | Gα_s (cAMP) | Adrenal glucocorticoidogenesis |
| MC3R | γ-MSH = α-MSH ≥ β-MSH | Gα_s/Gα_q | Energy partitioning, rhythmicity |
| MC4R | α-MSH = β-MSH > γ-MSH > ACTH | Gα_s/Gα_q | Appetite, sexual arousal, autonomic tone |
| MC5R | α-MSH > ACTH > β-MSH | Gα_s (cAMP) | Exocrine secretion, immunity |

These overlapping selectivities and shared cAMP-coupled mechanisms underscore the integrative pharmacology of the melanocortin axis [1, 2, 3, 4, 5]. For clinical and research applications requiring the preparation of synthetic melanocortin agonists such as bremelanotide, practical peptide handling - including the accurate volumetric reconstitution of lyophilized solids into defined molar stock concentrations - can be performed using the interactive Peptide Reconstitution Calculator under /tools. This resource supports the precise calculation of diluent volumes (typically bacteriostatic water or sterile saline) required to achieve target molarities (e.g., μM or mg/mL), enabling reliable dose-response and receptor-binding studies at MC3R/MC4R.

## Chemical Structure: Cyclic Heptapeptide Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH and Resistance to Proteolysis

### Molecular Architecture of Bremelanotide (PT-141)

Bremelanotide, internationally designated by its developmental nomenclature PT-141 and USAN-approved generic name bremelanotide, is a synthetic cyclic heptapeptide whose complete linear amino acid sequence, when fully expanded before macrocyclization, reads: *N*-acetyl-L-norleucyl-L-α-aspartyl-L-histidyl-D-phenylalanyl-L-arginyl-L-tryptophyl-L-lysine. In standardized IUPAC-condensed peptide nomenclature, this is written as **Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH**, where "Ac-" denotes an *N*-terminal acetyl cap, "cyclo[...]" indicates a side-chain-to-side-chain cyclization through an amide (lactam) bond between the β-carboxyl group of the aspartyl residue and the ε-amino group of the C-terminal lysyl residue, and "-OH" specifies a free C-terminal α-carboxyl moiety [6, 7, 8, 9, 10]. The molecule has a calculated monoisotopic molecular weight of 1025.2 g/mol (average molecular weight ~1025.18 Da), a value confirmed experimentally by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) profiling of pharmaceutical-grade lyophilized product and of forensic seized-material analyses [8, 9].

Chemically, bremelanotide is structurally a **core fragment of α-melanocyte-stimulating hormone (α-MSH)**, the endogenous tridecapeptide (SYSMEHFRWGKPV-NH2) that is the principal physiological agonist of the central melanocortin system. Bremelanotide preserves the pharmacophoric "message" segment of α-MSH - specifically the central **His-D-Phe-Arg-Trp** tetrapeptide that constitutes the minimal melanocortin receptor recognition motif - while discarding the flanking residues that contribute little to receptor binding but render the native hormone exquisitely susceptible to aminopeptidase, carboxypeptidase, and endopeptidase degradation [6]. The synthesis therefore represents a deliberate exercise in medicinal chemistry optimization: retaining the agonist "message" while eliminating protease-vulnerable peptide bonds, replacing the endogenous Met residue with the non-natural norleucine (Nle) isostere to remove the oxidation-prone thioether, and constraining the backbone into a rigid β-turn through side-chain-to-side-chain macrocyclization [6, 7].

### Position-by-Position Structural Deconstruction

| Position | Residue | Stereochemistry | Functional Role |
|----------|---------|-----------------|-----------------|
| 1 (N-cap) | Ac-Nle | L | N-terminal acetylation blocks aminopeptidase recognition; Nle replaces Met to abolish sulfoxide formation |
| 2 | Asp | L | β-carboxyl side chain forms the lactam bridge to Lys⁷ |
| 3 | His | L | Imidazole ring participates in MC3R/MC4R H-bond networks; contributes to pH-sensitive receptor activation |
| 4 | D-Phe | **D** | The single D-amino acid substitution that enforces the type II β-turn and confers protease resistance |
| 5 | Arg | L | Guanidinium side chain forms critical salt bridges with conserved Asp residues in TM3/TM7 of MC4R |
| 6 | Trp | L | Indole ring stacks against aromatic residues in the receptor binding pocket; the most conserved melanocortin pharmacophore element |
| 7 | Lys | L | ε-amino group forms the other terminus of the lactam bridge to Asp² |

The internal **lactam bridge** - an amide bond linking the β-COOH of Asp² with the ε-NH₂ of Lys⁷ - creates a 20-atom macrocycle that, while not as conformationally restrictive as a disulfide (which PT-141 notably does *not* contain), substantially reduces backbone flexibility. NMR and molecular dynamics studies of homologous melanocortin cyclic lactam analogues have demonstrated that this constraint nucleates a stable **type II' β-turn** centered on the D-Phe⁴-Arg⁵ segment, which is the precise geometry required for optimal engagement of the melanocortin-4 receptor (MC4R) orthosteric pocket [6, 8].

### Stereochemical Engineering: The D-Phe⁴ Substitution

The single most consequential chiral modification in bremelanotide is the inversion of phenylalanine at position 4 from the naturally occurring **L-configuration to the D-configuration**. In linear α-MSH, the analogous L-Phe residue adopts a backbone dihedral angle (φ, ψ) that is sterically compatible with an extended conformation but does not strongly favor turn geometry. By substituting D-Phe, the local peptide backbone is forced into the right-handed helical/turn region of Ramachandran space, nucleating a β-hairpin-like turn that positions the upstream His³ and downstream Arg⁵ residues in the canonical spatial register required for high-affinity MC3R/MC4R binding [6, 7].

Equally important, **D-amino acids are essentially invisible to the stereospecific active sites of mammalian endopeptidases and exopeptidases**, which have evolved exclusively to cleave L-peptide bonds. The peptide bond between D-Phe⁴ and Arg⁵ is therefore a non-scissile substrate for trypsin-like, chymotrypsin-like, and neutral endopeptidase (neprilysin) activity, which together constitute the principal degradation pathways for circulating α-MSH and other linear melanocortin agonists [6, 8, 10]. The D-Phe substitution therefore functions simultaneously as a **conformational switch** (inducing the bioactive β-turn) and as a **proteolytic dead zone** (blocking cleavage at the most vulnerable peptide bond in the pharmacophore).

### Resistance to Proteolysis: Quantitative Comparison with Linear α-MSH

Linear α-MSH possesses a plasma half-life on the order of only **1-3 minutes** in humans and rodents, owing to rapid degradation by angiotensin-converting enzyme (ACE), neprilysin (NEP, CD10), dipeptidyl peptidase IV (DPP-IV), and a suite of aminopeptidases. Bremelanotide, by contrast, is essentially refractory to these enzymes. The internal lactam bridge removes the N-terminal and C-terminal exopeptidase targets entirely (there is no free α-amino terminus because of the Ac-Nle cap, and the macrocycle leaves no accessible N-terminal scissile bond within the ring), while the D-Phe⁴ substitution provides a mid-chain cleavage block [6, 8]. This combined stabilization translates into a subcutaneous absorption and degradation profile sufficiently robust to support a clinically viable plasma exposure window following a single 1.75 mg intranasal or subcutaneous dose, with measurable drug concentrations detectable for several hours post-administration [6, 10].

The proteolytic resistance of bremelanotide has been empirically verified by MALDI-TOF-MS analyses of pharmaceutical and seized peptide products, in which the intact cyclic heptapeptide molecular ion at m/z 1025.2 [M+H]⁺ is recovered without detectable truncation or hydrolytic fragments, confirming both the structural integrity of the lactam macrocycle and the absence of degradative peptide bonds in the ring [8, 9]. These mass-spectrometric fingerprints are now part of the standard analytical reference panels used to authenticate PT-141 preparations, and they serve as a direct biochemical readout of the molecule's resistance to hydrolysis under physiological conditions [8, 9].

### Practical Implications for Laboratory Handling and Reconstitution

The cyclic heptapeptide architecture of bremelanotide directly informs laboratory handling protocols. Because the molecule is conformationally constrained and lacks free thiols (no disulfide bonds), there is no requirement for reducing-agent-containing reconstitution buffers. However, because the lactam bridge is an amide bond - and therefore chemically stable across a wide pH range but susceptible to prolonged exposure to extreme acidic or basic conditions - reconstitution is best performed in sterile bacteriostatic water or 0.9% sodium chloride for injection, with the lyophilized powder allowed to dissolve gently at room temperature without vigorous vortexing, which can mechanically shear peptide aggregates [8, 9].

For research and clinical users calculating dosing from a known mass of lyophilized peptide, the **Peptide Reconstitution Calculator** available at zubairkhalid.com/tools/peptide-calculator can be used to convert the desired mass (mg) of bremelanotide (MW 1025.2 g/mol) into a molar quantity and then into the corresponding injection volume for any target final concentration in μg/mL or mg/mL. Typical reconstitution workflows dissolve a 10 mg vial of bremelanotide in 2 mL of diluent to yield a 5 mg/mL stock, from which subsequent dilutions are prepared according to the dosing protocol specified in the relevant pharmacological study [6, 10]. The tool ensures that the molarity (μmol/L), mass/volume (mg/mL), and unit (IU) representations remain internally consistent, which is particularly important when bridging between murine experimental doses (typically μg/kg) and human clinical doses (typically 1.0-1.75 mg per intranasal or subcutaneous administration) [6, 10].

### Summary of Structure-Activity Rationale

The synthetic design of bremelanotide therefore exemplifies the convergence of three complementary medicinal chemistry strategies: **(1)** truncation and modification of the parent α-MSH sequence to retain only the essential melanocortin pharmacophore (His-D-Phe-Arg-Trp); **(2)** macrocyclization through an Asp²-Lys⁷ lactam bridge to constrain the backbone into the bioactive β-turn conformation; and **(3)** stereochemical inversion at Phe⁴ to simultaneously enforce turn geometry and block proteolytic cleavage [6, 7, 10]. The result is a 1025.2 Da cyclic heptapeptide with high affinity and agonist efficacy at MC3R and MC4R, remarkable resistance to plasma and tissue proteases, and physicochemical stability compatible with lyophilized storage, intranasal delivery, and subcutaneous injection in both investigational and clinical contexts [6, 7, 8, 9, 10]. This structural foundation underlies all downstream neurochemical, signaling-cascade, and pharmacodynamic properties discussed in subsequent sections.

## Central Nervous System Mechanisms: Hypothalamic MC3R/MC4R Activation, Dopaminergic Transmission, and Nitric Oxide Pathways

### Receptor Binding Profile and Melanocortin Receptor Selectivity

PT-141 (bremelanotide; molecular formula C₅₀H₆₈N₁₄O₁₀; molecular weight approximately 1025.18 g/mol) is a synthetic heptapeptide analogue of α-melanocyte-stimulating hormone (α-MSH), originally derived from the cyclic lactam core of melanotan-I with strategic N-terminal acylation and backbone cyclization to confer metabolic stability and central bioavailability [11, 14]. Its primary pharmacodynamic footprint involves high-affinity agonist binding at two class A G protein-coupled receptors (GPCRs) within the central nervous system: the melanocortin-3 receptor (MC3R) and the melanocortin-4 receptor (MC4R) [13, 14]. In radioligand displacement assays using transfected HEK293 cell membranes, bremelanotide demonstrates sub-nanomolar binding affinity, with reported inhibition constants (Kᵢ) of approximately 1.3 nM at hMC4R and 5.0 nM at hMC3R, while exhibiting markedly reduced affinity at MC1R, MC2R, and MC5R isoforms [14]. This receptor selectivity profile is pharmacologically critical because MC2R is exclusively the adrenocorticotropin (ACTH) receptor coupled to glucocorticoid synthesis, whereas MC1R mediates cutaneous melanogenesis; bremelanotide's low intrinsic activity at these receptors explains its absence of significant tanning or corticosteroid-modulating effects at clinical doses [11, 13].

Functionally, bremelanotide operates as a biased agonist at MC3R/MC4R, preferentially stabilizing active-state receptor conformations that couple to Gαs heterotrimeric G proteins, with downstream activation of adenylyl cyclase, elevation of intracellular cyclic adenosine monophosphate (cAMP), and subsequent protein kinase A (PKA)-dependent phosphorylation of effector substrates including cAMP response element-binding protein (CREB) [12, 13, 14]. Recent cryo-EM and β-arrestin recruitment assays have demonstrated that MC4R can also signal through Gαq/11-phospholipase C (PLC) cascades, generating inositol trisphosphate (IP₃)-mediated intracellular calcium release; bremelanotide appears to retain Gαs potency while displaying comparatively lower β-arrestin-2 recruitment, a signaling bias potentially relevant to sustained receptor activation in hypothalamic neurons [13].

### Hypothalamic Microcircuitry: mPOA, PVN, and VTA Integration

The melanocortinergic neuroanatomical substrate for bremelanotide's central action is concentrated in three interconnected hypothalamic and midbrain nuclei: the medial preoptic area (mPOA), the paraventricular nucleus (PVN), and the ventral tegmental area (VTA) [12, 15]. Within the mPOA, MC4R is densely expressed on kisspeptin-neurokinin B-dynorphin (KNDy) neurons and on glutamatergic projection neurons that innervate the periaqueductal gray (PAG); agonism at these receptors increases neuronal firing rates by approximately 30-60% in ex vivo slice preparations, an effect that is abolished in MC4R-knockout mice [12, 15]. The PVN harbors the highest density of MC4R expression within the diencephalon, particularly on parvocellular oxytocinergic and corticotropin-releasing hormone (CRH) neurons, while the VTA expresses MC3R on dopaminergic A10 neurons projecting to the nucleus accumbens and medial prefrontal cortex [12].

Bremelanotide's capacity to cross the blood-brain barrier (BBB) is a defining pharmacological feature distinguishing it from earlier melanocortin agonists; whereas α-MSH is largely excluded by peptide transport constraints, bremelanotide's lipophilic acetyl-norleucine modification confers sufficient passive diffusion and active transport to achieve measurable cerebrospinal fluid concentrations within 15-30 minutes of subcutaneous administration [11, 14, 15]. Preclinical microdialysis studies in rodent models have demonstrated that subcutaneous bremelanotide (1-10 mg/kg) elevates extracellular dopamine concentrations in the nucleus accumbens shell by 120-180% above baseline within 45 minutes, an effect that is dose-dependently attenuated by pre-administration of the selective MC4R antagonist HS024 [12, 15]. This dopaminergic signal is transduced through D1-type receptors on medium spiny neurons of the mesolimbic reward circuit, where it converges with oxytocin release to potentiate pro-sexual behavioral responses [15, 16].

### Oxytocinergic Recruitment and Autonomic Integration

Concurrent with mesolimbic dopamine elevation, bremelanotide administration triggers oxytocin (OT) release from magnocellular and parvocellular neurons of the PVN, projecting to the spinal cord intermediolateral cell column, the nucleus tractus solitarius, and the dorsal motor nucleus of the vagus [15, 16]. Oxytocin binding to OTR (oxytocin receptor; a Gαq/11-coupled GPCR) in these autonomic loci induces PLC-mediated IP₃ accumulation and intracellular Ca²⁺ mobilization, facilitating parasympathetic outflow that governs vascular smooth muscle relaxation in genital erectile tissue [16]. Plasma oxytocin concentrations in human subjects receiving intranasal bremelanotide (10-20 mg) have been observed to rise by 40-90% over baseline within 60 minutes, correlating with subjective arousal scores in controlled crossover studies [15, 16].

The fifth International Consultation on Sexual Medicine (ICSM 2024) recommendations highlight bremelanotide's combined hypothalamic-autonomic mechanism as uniquely suited to centrally-mediated sexual desire disorders, in contrast to peripherally-acting phosphodiesterase type 5 (PDE5) inhibitors that target only the vascular endpoint [16]. The melanocortin pathway thus occupies an integrative position upstream of both the mesolimbic reward system and the spinal-autonomic reflexes, allowing bremelanotide to modulate psychogenic (central) arousal in addition to reflexive (peripheral) vasocongestion [12, 16].

### Neuronal Nitric Oxide Synthase (nNOS) and the NO-cGMP-PKG Axis

A third, mechanistically distinct limb of bremelanotide's central signaling cascade involves the activation of neuronal nitric oxide synthase (nNOS; NOS1) in hypothalamic and limbic neurons, with consequent generation of nitric oxide (NO) and downstream activation of soluble guanylyl cyclase (sGC), elevation of cyclic guanosine monophosphate (cGMP), and protein kinase G (PKG)-dependent phosphorylation of vascular and neural effectors [15]. In the mPOA and PVN, MC4R-mediated cAMP/PKA signaling phosphorylates nNOS at Ser¹⁴¹² (the primary activation site), increasing its catalytic rate (V_max) by approximately 2-fold and enhancing NO production [15, 16]. This NO then diffuses as a gaseous neurotransmitter to neighboring neurons and into adjacent vascular beds, where it triggers smooth muscle relaxation via cGMP-mediated reduction in intracellular Ca²⁺ concentration and dephosphorylation of myosin light chain [15].

The convergence of melanocortin signaling on NO pathways is bidirectional: NO, in turn, exerts negative feedback on MC4R signaling through S-nitrosylation of cysteine residues in the receptor's C-terminal tail, which may contribute to the tachyphylaxis observed with repeated high-dose bremelanotide administration [13, 15]. This autoregulatory loop has been proposed as a rationale for the clinical "as-needed" dosing schedule (typically 1-2 mg subcutaneously 15-60 minutes prior to anticipated sexual activity, not exceeding one dose in 24 hours or 8 doses per month), which avoids persistent receptor desensitization [11, 16]. For investigators and clinicians preparing bremelanotide for parenteral administration, the Peptide Reconstitution Calculator at /tools/peptide-calculator provides precise volumetric calculations for reconstituting the lyophilized acetate salt with bacteriostatic water for injection (BWFI), minimizing peptide loss from inaccurate dilution steps.

### Integrated Neuroendocrine Cascade and Clinical Correlates

In aggregate, bremelanotide's central mechanism can be summarized as a hierarchical neurochemical cascade: (1) BBB penetration and MC3R/MC4R binding with Kᵢ values in the low-nanomolar range; (2) Gαs-cAMP-PKA activation in mPOA, PVN, and VTA neurons; (3) increased mesolimbic dopamine release in the nucleus accumbens and oxytocin release from PVN projections; (4) nNOS phosphorylation and NO-cGMP-PKG signaling in hypothalamic and autonomic nuclei; and (5) functional integration of these signals into coordinated autonomic, endocrine, and behavioral outputs governing sexual arousal [11, 12, 13, 14, 15, 16]. The half-life of bremelanotide in plasma is approximately 2.7 hours, with the C_max achieved at ~1 hour post-subcutaneous injection and the terminal elimination phase reflecting both renal excretion of intact peptide and proteolytic cleavage by hypothalamic and peripheral peptidases [11, 16]. This pharmacokinetic profile, combined with the engagement of multiple parallel arousal pathways, underlies bremelanotide's distinctive clinical positioning as the first FDA-approved (2019) centrally-acting agent for hypoactive sexual desire disorder (HSDD) in premenopausal women, and its broader investigational applications in male sexual dysfunction and motivational/affective disorders [11, 15, 16, 17].

## Pharmacokinetics, Blood-Brain Barrier Permeation, and Clinical Indications (HSDD & Erectile Dysfunction)

## Pharmacokinetics, Blood-Brain Barrier Permeation, and Clinical Indications (HSDD & Erectile Dysfunction)

## Overview of Bremelanotide (PT-141 / Vyleesi) as a Therapeutic Agent

Bremelanotide, marketed under the trade name **Vyleesi**, is a synthetic heptapeptide analog of α-melanocyte-stimulating hormone (α-MSH) and is the first melanocortin receptor agonist approved by the U.S. Food and Drug Administration (FDA) for the treatment of **acquired, generalized hypoactive sexual desire disorder (HSDD) in premenopausal women** [18, 19]. Its developmental history traces back to the melanocortin system, where native α-MSH (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂) was identified as the endogenous ligand for the melanocortin-1 through melanocortin-5 receptors (MC1R-MC5R). Bremelanotide itself is a cyclic heptapeptide (Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys)-OH) with a molecular weight of approximately 1025.2 g/mol. Compared with linear α-MSH, the lactam bridge between the aspartic acid and lysine side chains confers resistance to enzymatic degradation by aminopeptidases and angiotensin-converting enzymes, while the substitution of L-methionine with norleucine (Nle) eliminates a potential site of oxidative methionine sulfoxide formation [18, 19].

## Pharmacokinetic Profile Following Subcutaneous Administration

Bremelanotide is administered as a **subcutaneous injection** in the abdomen or thigh, with the FDA-approved dosage being 1.75 mg administered on an as-needed basis, at least 45 minutes before anticipated sexual activity [18, 19, 20]. The pharmacokinetic profile of bremelanotide demonstrates a number of clinically relevant features:

- **Bioavailability:** Subcutaneous administration yields essentially **complete systemic bioavailability (~100%)**, bypassing first-pass hepatic metabolism, which is a substantial advantage over orally administered peptide analogs [18].
- **Peak Plasma Concentration (Cmax):** Following a 1.75 mg subcutaneous dose, Cmax is reached at **Tmax ≈ 1.0 hour**, with mean Cmax values in the range of 500-700 pg/mL [18, 19].
- **Elimination Half-Life (t½):** The terminal elimination half-life is approximately **2.7 hours**, reflecting rapid clearance from the systemic compartment [18, 19].
- **Metabolic Clearance:** Bremelanotide undergoes hydrolysis of the peptide bond to yield inactive metabolites; no significant cytochrome P450 (CYP)-mediated metabolism has been identified, which minimizes drug-drug interactions mediated by CYP3A4, CYP2D6, or CYP1A2 [18].
- **Renal Excretion:** The kidneys account for the majority of the parent compound and metabolite elimination, with approximately 64.8% of the administered dose recovered in urine and 22.8% in feces, predominantly as hydrolyzed fragments [18].
- **Steady-State Behavior:** Given the short half-life, no significant drug accumulation is observed with intermittent (as-needed) dosing, and formal dose adjustment is not required in patients with mild-to-moderate renal impairment, although caution is advised in severe renal impairment (eGFR < 30 mL/min) [19].

## Blood-Brain Barrier Permeation Kinetics

A central feature distinguishing bremelanotide from peripherally restricted erectile dysfunction therapies (e.g., PDE5 inhibitors such as sildenafil) is its ability to **cross the blood-brain barrier (BBB)** in pharmacologically meaningful concentrations [18, 19, 20]. Preclinical studies in rodents and primates employing radiolabeled [¹⁴C]-bremelanotide have demonstrated measurable brain uptake within 15-30 minutes of subcutaneous administration, with peak cerebrospinal fluid (CSF) concentrations achieved at approximately 1-2 hours post-injection - closely paralleling the plasma Tmax [18, 19]. The peptide is sufficiently lipophilic at the N-terminal acetylated and cyclic lactam regions to permit passive transcellular diffusion across the BBB, while its small molecular size (~1 kDa) further facilitates this permeation, as compounds below ~400-600 Da typically cross readily, and those up to ~1 kDa can still achieve meaningful CNS penetration if lipophilicity is adequate [18, 20].

Once in the central nervous system, bremelanotide engages **MC3R and MC4R** populations concentrated in the hypothalamic paraventricular nucleus (PVN), the medial preoptic area (MPOA), the ventromedial hypothalamus (VMH), and the spinal cord autonomic nuclei - regions that integrate sexual arousal, motivational state, and autonomic genital response [19, 20]. Functional imaging studies in humans have shown that subcutaneous bremelanotide modulates activity in brain regions associated with reward processing and sexual motivation, including the insula, cingulate cortex, and ventral striatum [18, 19].

## Mechanism of Central Melanocortin Receptor Activation

The pharmacodynamic action of bremelanotide is mediated primarily through **MC4R** (with secondary MC3R contribution) activation in the hypothalamus and limbic system [18, 19, 20]. MC4R is a Gαs/11-coupled GPCR that, upon agonist binding, initiates a canonical signaling cascade:

1. **G-protein activation:** Bremelanotide binding to MC4R triggers guanine nucleotide exchange on the Gαs subunit, leading to dissociation of the Gαs-GTP complex from the Gβγ dimer.
2. **Adenylyl cyclase stimulation:** Gαs-GTP activates membrane-bound adenylyl cyclase (AC), catalyzing the conversion of ATP to cyclic adenosine monophosphate (cAMP).
3. **cAMP/PKA signaling:** Elevated cytosolic cAMP activates protein kinase A (PKA), which phosphorylates downstream effectors including cAMP response element-binding protein (CREB), modulating gene transcription.
4. **PLC/IP3/Ca²⁺ arm:** Concurrently, MC4R coupling to Gαq/11 activates phospholipase C (PLC), generating inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), with subsequent IP3-mediated Ca²⁺ release from endoplasmic reticulum stores.
5. **β-arrestin recruitment:** Like most GPCRs, MC4R undergoes β-arrestin-mediated desensitization and internalization following sustained agonist exposure, providing intrinsic negative feedback.

Receptor binding affinity studies have shown bremelanotide's Kᵢ values at MC4R in the low nanomolar range (~1-5 nM), with somewhat lower potency at MC3R (~10-20 nM) and minimal activity at MC1R, MC2R, and MC5R [18, 19]. This selectivity profile underlies both its pro-sexual efficacy (MC4R-rich hypothalamic centers) and its favorable side effect profile (low melanotropic cutaneous pigmentation and minimal adrenal axis perturbation) [19].

## Clinical Indications: HSDD and Erectile Dysfunction

### Hypoactive Sexual Desire Disorder (HSDD)

The **RECONNECT study program** - comprising two identically designed Phase 3 randomized, double-blind, placebo-controlled clinical trials (RECONNECT 1 and RECONNECT 2) - evaluated the efficacy and safety of subcutaneous bremelanotide 1.75 mg in premenopausal women with acquired, generalized HSDD [18, 19]. The co-primary endpoints were the change from baseline in the **Desire subscale of the Female Sexual Function Index (FSFI-D)** and the **Female Sexual Distress Scale-Desire/Arousal/Orgasm (FSDS-DAO) Item 13** score. Across both studies, bremelanotide produced statistically significant improvements versus placebo, with the greatest therapeutic benefit observed in the subset of patients using bremelanotide 4-6 times per month. Improvements were generally maintained over the 6-month open-label extension phases, supporting sustained clinical benefit [18, 19].

### Erectile Dysfunction (Investigational)

Although bremelanotide is not FDA-approved for male erectile dysfunction (ED), earlier intranasal formulations (PT-141 intranasal) were investigated for this indication in Phase 2 trials, demonstrating significant increases in erectile activity as measured by RigiScan and IIEF scores [18, 20]. The development of the intranasal formulation was ultimately discontinued in favor of the subcutaneous route due to inconsistent intranasal bioavailability and higher rates of flushing and nausea. However, mechanistic extrapolation supports the use of bremelanotide in ED through MC4R-mediated activation of oxytocinergic neurons in the PVN, which project to the spinal cord and augment parasympathetic outflow to the corpus cavernosum [19, 20].

## Cardiovascular Considerations: Transient Blood Pressure Modulation

A clinically important pharmacodynamic feature of bremelanotide is its **transient blood pressure elevation** observed within the first several hours after administration, mediated by peripheral MC4R activation on vascular smooth muscle and the renal microvasculature [18, 19]. In Phase 1 studies, mean systolic blood pressure increases of 2-6 mmHg and diastolic increases of 1-3 mmHg have been documented, with peak effects occurring around 1-2 hours post-dose (coincident with Cmax) and returning to baseline by approximately 6-8 hours. This effect is generally modest and well tolerated but has prompted the FDA to contraindicate Vyleesi in patients with uncontrolled hypertension or pre-existing cardiovascular disease [18, 19]. The mechanism is thought to involve MC4R-mediated sympathetic activation and modulation of nitric oxide signaling in vascular endothelium.

## Practical Considerations: Reconstitution and Dosing

Bremelanotide is supplied as a **lyophilized powder in single-dose vials**, requiring reconstitution with 0.9% sterile sodium chloride prior to subcutaneous administration. Standard reconstitution protocols call for the addition of a defined diluent volume to achieve the target peptide concentration (typically 1 mg/mL or 0.5 mg/mL depending on manufacturer), and clinicians and patients are encouraged to use a peptide reconstitution calculator - such as the interactive tool available at **/tools/peptide-calculator** - to perform accurate molarity calculations. For example, using a molecular weight of 1025.2 g/mol for bremelanotide free base, reconstitution of 5 mg of lyophilized peptide in 5 mL of bacteriostatic water yields a concentration of 1 mg/mL (≈ 975 µM). Proper attention to aseptic technique, storage at 2-8 °C post-reconstitution, and use within the manufacturer's specified in-use stability window is essential to maintain peptide integrity and minimize aggregation or hydrolysis of the cyclic lactam bridge [18, 19].

## Summary

Bremelanotide represents a paradigm-shifting, centrally acting melanocortin receptor agonist with well-characterized pharmacokinetics (subcutaneous bioavailability ~100%, Tmax ~1.0 h, t½ ~2.7 h), demonstrable blood-brain barrier permeation, and validated clinical efficacy in HSDD through MC3R/MC4R engagement in hypothalamic and limbic circuits. Its transient blood pressure effects and rapid metabolism support an on-demand, patient-controlled dosing paradigm, and laboratory best practices - including use of a peptide reconstitution calculator - are essential for accurate, reproducible dosing in both research and clinical settings.

## Lyophilized Formulation, Osmolality, Reconstitution with Bacteriostatic Water, and Storage Guidelines

The pharmaceutical presentation of PT-141 (Bremelanotide) as a lyophilized (freeze-dried) solid constitutes the most thermodynamically stable format for the heptapeptide acylated derivative of α-MSH, and its proper handling governs the maintenance of melanocortin receptor binding integrity across its shelf life. Bremelanotide possesses the cyclic core sequence Ac-Nle-Asp-His-D-Phe-Arg-Trp-Lys-NH₂, in which the lactam bridge formed between the Asp side chain and the Lys ε-amino group imposes conformational rigidity essential for high-affinity engagement of the MC3R and MC4R subtypes. Because the molecule contains a single free α-amino group (acetylated, therefore protected), one primary guanidino group on the Arg side chain, two imidazole-bearing His residues, and a Lys ε-amine engaged in the lactam ring, the net charge state across physiological pH is constrained, with a predicted isoelectric point in the mildly acidic-to-neutral range owing to the predominance of carboxyl-terminus and Asp carboxylate over the lone Arg. Lyophilization from a volatile buffer matrix (typically mannitol or sucrose as a bulking agent at 2-5% w/v) stabilizes this conformation by immobilizing the peptide in an amorphous glassy state, with residual moisture strictly maintained below 3% to prevent Maillard-type condensation reactions and Asn/Asp-driven succinimide formation during subsequent storage [21].

The aqueous solubility profile of bremelanotide is governed by the balance between the hydrophobic aromatic cluster (D-Phe, Trp) and the polar/charged residues (Asp, His, Arg, Lys). At acidic pH (3.0-5.0), protonation of the His imidazole and partial neutralization of carboxylate side chains increases apparent solubility, which is the rationale for the acidic formulation vehicles used during the fill-finish process prior to lyophilization. Reconstituted osmolality must be carefully evaluated when the lyophilized cake is dissolved, because a 10 mg vial of bremelanotide reconstituted in 2 mL of bacteriostatic water produces a solution that is hypotonic relative to plasma osmolality (target 285-295 mOsm/kg); this is clinically inconsequential for subcutaneous depot administration in volumes ≤1 mL, but becomes relevant for intranasal or intravenous routes where isotonicity is mandated to prevent mucosal irritation or hemolysis. Practitioners performing compounding calculations frequently consult the interactive **Peptide Reconstitution Calculator (/tools/peptide-calculator)** to determine molarity, mass per vial, and diluent volumes required to achieve a target concentration in mg/mL or μM, particularly when designing subcutaneous or intranasal dosing regimens from research-grade vials [21].

Bacteriostatic water for injection (BWFI), the standard diluent for multi-dose bremelanotide vials, consists of sterile water for injection containing 0.9% (w/v) benzyl alcohol as a preservative, yielding a final benzyl alcohol concentration of approximately 9 mg/mL. The antimicrobial efficacy of benzyl alcohol is greatest against Gram-positive bacteria and is pH-dependent, with optimal activity between pH 5.0 and 7.0; bremelanotide solutions reconstituted in BWFI fall within this window. The benzyl alcohol content also exerts a mild local anesthetic effect at the injection site and provides a vehicle osmolality of approximately 0 mOsm/kg (essentially pure water), so osmolality of the final peptide solution is dictated almost entirely by the peptide mass dissolved per unit volume. The critical reconstitution procedure requires the slow introduction of diluent down the side of the vial (not directly onto the lyophilized cake) to avoid foaming of the amphipathic peptide, followed by gentle swirling or rolling - vigorous vortexing must be avoided because the shear stress and air-water interface can denature the lactam-bridged conformation and lead to adsorption losses. Sterile normal saline (0.9% NaCl) is an acceptable alternative diluent when a near-isotonic preparation is desired; however, the chloride concentration slightly stabilizes the cationic guanidino group of Arg through Debye-Hückel ionic atmosphere effects [21].

Stability of the lyophilized vial is exceptional when stored at -20 °C ± 5 °C in the dark, with documented retention of >95% purity for 24 months and acceptable potency retention beyond 36 months in validated stability studies. The dominant degradation pathway in the solid state is residual moisture-driven deamidation of the C-terminal amide (relatively minor in cyclic heptapeptides) and, more importantly, photo-oxidation of the Trp indole ring when the vial is exposed to UV or visible light above 420 nm. Bremelanotide is therefore supplied in amber Type I borosilicate glass vials to filter out the UV-A and UV-B wavelengths that catalyze 3-OH kynurenine formation on the Trp side chain. Storage at 2-8 °C is acceptable for short-term use (≤3 months) but accelerates molecular mobility within the amorphous matrix relative to -20 °C; conversely, room temperature storage (20-25 °C) for extended periods is contraindicated because the glass transition temperature of the mannitol/sucrose matrix typically lies near 30 °C, and excursions above this temperature can induce collapse of the cake, exposing peptide to residual moisture [21].

Upon reconstitution, the peptide enters solution and becomes susceptible to the full spectrum of aqueous-phase degradation pathways, including oxidation, hydrolysis, and microbial proliferation. The recommended refrigerated storage of reconstituted vials at 2-8 °C for up to 30 days balances two competing constraints: (i) the kinetic retardation of peptide hydrolysis and oxidation at low temperature, and (ii) the preserved antimicrobial efficacy of benzyl alcohol in the BWFI vehicle. Beyond 30 days, cumulative degradation can exceed 10% even under optimal refrigeration, with the Trp residue representing the primary chromophore of concern; tryptophan oxidation to N-formylkynurenine introduces a 320 nm-absorbing species that shifts the UV spectrum and reports non-receptor-binding degradation products. Some compounding pharmacies have extended the in-use period to 60 days when supported by validated stability-indicating HPLC-MS testing demonstrating ≥90% main peak area, but such extensions should not be assumed without laboratory confirmation [21].

Reconstitution volumes should be calculated based on the dosing regimen. For subcutaneous administration of 1 mg bremelanotide, reconstituting a 10 mg vial with 2 mL of BWFI yields a 5 mg/mL stock; dosing 0.2 mL delivers 1 mg. Practitioners frequently use the **Peptide Reconstitution Calculator (/tools/peptide-calculator)** to cross-check these volumetric arithmetic relationships, particularly when working with research-quantity vials of 10 mg, 25 mg, or 50 mg and titrating to non-standard doses for individualized MC3R/MC4R titration protocols. From a molecular-weight perspective, bremelanotide (free base, heptapeptide lactam, with N-terminal acetylation and C-terminal amidation) has a calculated monoisotopic molecular weight of approximately 1025.2 Da and an average molecular weight of 1025.18 g/mol; this knowledge allows direct interconversion between molarity and mg/mL concentration (1 mM = 1.025 mg/mL; conversely 1 mg/mL = 975.6 μM), enabling accurate construction of dose-response curves at the MC3R (Kᵢ ≈ 2.0 nM) and MC4R (Kᵢ ≈ 0.06-0.5 nM), where stock concentrations should be prepared at least 1000× above the working Kᵢ to ensure robust ligand availability at the receptor [21].

In summary, the formulation chemistry of lyophilized bremelanotide requires strict control of residual moisture, avoidance of UV exposure, and maintenance of the cold chain at -20 °C for long-term storage. Reconstitution with 0.9% benzyl alcohol-preserved bacteriostatic water into the 2-8 °C refrigerated environment supports a functional in-use stability window of 30-60 days, during which the cyclic heptapeptide retains its capacity to activate the Gαs-coupled MC3R/MC4R signaling cascade with the downstream cAMP-PKA and MAPK/ERK effector arms intact, provided that aseptic technique is preserved throughout the multi-dose vial usage period [21].

## Syringe Unit Conversion, Microgram-to-Milligram Dosing, and Peptide Calculator Guidance

### Foundational Units, Concentrations, and the Language of Peptide Dosing

Before any volumetric manipulation of lyophilized PT-141 (bremelanotide, Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂; molecular weight 1,025.2 g/mol) can occur, the researcher must internalize the canonical unit relationships that govern peptide reconstitution. Research-grade PT-141 is most commonly supplied as a 10 mg lyophilized cake in a 2 mL borosilicate glass vial. The arithmetic that follows is deceptively simple but absolutely unforgiving of rounding errors, because a 1,000 mcg research dose (the most widely cited exploratory dose in melanocortin signaling investigations) becomes only 100 mcL at the extreme of dilution and 20 units on a U-100 insulin-style syringe at the conservative end [22].

The cardinal relationships are:

- **1 mg = 1,000 mcg (µg)**
- **1 mL = 1,000 mcL (µL)**
- **1 mL = 100 units** on a U-100 syringe (1 unit = 0.01 mL = 10 mcL)
- **Concentration (mcg/mL) = Total peptide mass (mcg) ÷ Total reconstitution volume (mL)**
- **Injection volume (mL) = Desired dose (mcg) ÷ Stock concentration (mcg/mL)**

These five identities are the only mathematical primitives required to navigate every clinical-style reconstitution scheme encountered in melanocortin research workflows.

### Reference Reconstitution Scheme: 10 mg Vial with 2.0 mL Bacteriostatic Water

The most frequently documented reconstitution in published PT-141 research-adjacent protocols proceeds as follows [22]:

| Parameter | Value |
|---|---|
| Vial content (lyophilized PT-141) | 10 mg = 10,000 mcg |
| Diluent (0.9% benzyl alcohol-preserved bacteriostatic water) | 2.0 mL |
| Final concentration | 10,000 mcg ÷ 2.0 mL = **5,000 mcg/mL** |
| Concentration per syringe unit (U-100) | 5,000 mcg ÷ 100 units = **50 mcg/unit** |
| Volume of a 1,000 mcg research dose | 1,000 mcg ÷ 5,000 mcg/mL = **0.20 mL** |
| Syringe units corresponding to 1,000 mcg | 0.20 mL × 100 units/mL = **20 units** |

Thus, drawing the meniscus exactly to the "20" tick mark on a 1 mL U-100 syringe delivers 1,000 mcg of bremelanotide when the 10 mg vial has been brought to 2.0 mL total volume. This single calculation is the cornerstone of every downstream comparative dosing argument because it converts a peptide mass - a unit that has no intuitive physical referent - into a tactile measurement aligned with the graduations etched on the barrel of an insulin syringe.

### Syringe Tick-Mark Translation Table

The following conversion table is constructed using the identical 5,000 mcg/mL reference concentration described above, allowing researchers to translate between microgram targets, milliliter volumes, and U-100 syringe markings with a single visual reference [22]:

| Research Dose (mcg) | Volume (mL) | U-100 Syringe Tick Marks (units) |
|---:|---:|---:|
| 100 mcg | 0.02 mL | 2 units |
| 250 mcg | 0.05 mL | 5 units |
| 500 mcg | 0.10 mL | 10 units |
| 750 mcg | 0.15 mL | 15 units |
| **1,000 mcg** | **0.20 mL** | **20 units** |
| 1,250 mcg | 0.25 mL | 25 units |
| 1,500 mcg | 0.30 mL | 30 units |
| 2,000 mcg | 0.40 mL | 40 units |

Each row is internally consistent: dose ÷ 5,000 mcg/mL yields milliliters, and milliliters × 100 units/mL yields tick marks. The same arithmetic template generalizes to any reconstitution ratio by substituting the appropriate numerator in the concentration formula.

### Generalizing the Dilution Formula to Arbitrary Vial Sizes and Diluent Volumes

The reference scheme is only one point in a continuous family of valid reconstitutions. A 5 mg vial brought up in 1.0 mL of bacteriostatic water yields an identical 5,000 mcg/mL working concentration, while a 10 mg vial brought up in 3.0 mL produces 3,333 mcg/mL - a configuration in which a 1,000 mcg dose corresponds to 0.30 mL (30 units) rather than 0.20 mL (20 units). Researchers can therefore titrate syringe volumes upward by increasing diluent volume, which is occasionally desirable when working with viscous preparations or when sub-unit precision on the syringe barrel is inadequate.

The generalizable master equation for any PT-141 reconstitution is:

> **C (mcg/mL) = (Vial mass in mg × 1,000 mcg/mg) ÷ Diluent volume in mL**

> **Injection volume (mL) for a target dose D = D (mcg) ÷ C (mcg/mL)**

> **Syringe tick marks = Injection volume (mL) × 100**

When C × syringe volume = D, the dilution is internally validated. Researchers should perform this cross-check before any downstream experimental step.

### Step-by-Step Walkthrough Using the Peptide Reconstitution Calculator

Dr. Zubair Khalid's interactive **Peptide Reconstitution Calculator**, available at `/tools/peptide-calculator`, was designed precisely to externalize the arithmetic described above and to remove the cognitive load of repeated dimensional analysis. For a researcher holding a 10 mg PT-141 vial and intending to deliver a 1,000 mcg dose, the workflow proceeds as follows [22]:

1. **Enter vial peptide mass.** The calculator's first input field accepts the vial content in milligrams; for the canonical protocol, enter "10."
2. **Enter diluent volume.** The second input accepts the volume of bacteriostatic water in milliliters; for the canonical protocol, enter "2.0." The tool internally computes and displays the working concentration as **5,000 mcg/mL**.
3. **Enter target dose.** The third input accepts the desired research dose in micrograms; enter "1,000."
4. **Read the outputs.** The calculator returns three synchronized outputs: (a) the injection volume in milliliters (**0.20 mL**), (b) the equivalent volume in microliters (**200 mcL**), and (c) the U-100 syringe tick-mark equivalent (**20 units**).
5. **Cross-validate.** Researchers should mentally confirm that dose ÷ concentration = volume and that volume × 100 = syringe units. The calculator performs this validation internally, but the cognitive habit reinforces methodological rigor.

The tool's true utility emerges when alternative reconstitution schemes are explored. Substituting "3.0" mL of diluent immediately recalculates the working concentration to 3,333 mcg/mL and the corresponding 1,000 mcg dose volume to 0.30 mL (30 units). This dynamic recalculation allows researchers to optimize for syringe precision (favoring higher diluent volumes that spread the dose across more tick marks) or for injection comfort (favoring lower diluent volumes that minimize subcutaneous volume).

### Peptide-Calculator-Mediated Optimization of Sub-Unit Precision

A recurring methodological concern in low-dose peptide work is **sub-unit precision**: U-100 syringes are reliably graduated to the single-unit mark only between the 10-unit and 90-unit range, and accuracy degrades near the syringe hub and tip. Researchers targeting doses below 500 mcg at high stock concentration may find themselves attempting to draw fractional units (e.g., 5 units for a 250 mcg dose from a 5,000 mcg/mL stock) where meniscus parallax introduces meaningful volumetric error.

The Peptide Reconstitution Calculator at `/tools/peptide-calculator` solves this dilemma by recommending diluent volumes that place the target dose squarely in the syringe's most accurate central region. For a 250 mcg dose, the tool can be queried iteratively - increasing the diluent volume from 2.0 mL to 4.0 mL shifts the working concentration to 2,500 mcg/mL, expanding the 250 mcg dose volume from 5 units to 10 units, which sits precisely on a major graduation mark [22].

### Molar Equivalents and Stoichiometric Considerations

Because PT-141 is a heptapeptide with a molecular weight of 1,025.2 g/mol, every 1,000 mcg dose corresponds to 0.976 µmol (approximately 1 µmol) of bremelanotide - a value that occasionally matters when comparing receptor occupancy against published MC3R and MC4R binding constants (typically in the low-nanomolar range). Researchers conducting receptor-occupancy modeling can use the calculator outputs as direct inputs into downstream molarity calculations:

> **Moles of peptide = Mass (g) ÷ Molecular weight (g/mol)**
>
> **Molarity of stock = Moles ÷ Volume (L)**
>
> **Final bath concentration = (Stock molarity × Injected volume) ÷ Bath volume**

These identities are essential when translating between in vivo mass dosing and in vitro molar exposure, and the Peptide Reconstitution Calculator is compatible with downstream molar conversion when its microgram outputs are back-converted using the published molecular weight of bremelanotide.

### Methodological Caveats and Best Practices

Several practical constraints should accompany every reconstitution calculation. First, **lyophilized peptide is rarely 100% recoverable**; a 1-3% loss to vial surfaces, stopper adsorption, and transfer dead volume is standard, meaning the working concentration is slightly lower than the theoretical value. Second, **bacteriostatic water should be added slowly and down the side of the vial** to avoid foaming of the peptide cake, which can denature surface-localized residues. Third, **reconstituted PT-141 should be used within the published refrigerated stability window** (typically 14-30 days depending on the source certificate of analysis) and should never be subjected to freeze-thaw cycles, as the cyclic lactam bridge of bremelanotide is susceptible to conformational perturbation. Fourth, researchers should **always draw and expel the reconstituted peptide through the syringe needle several times** before final dose measurement, equilibrating the dead-space hold-up volume against the calibrated barrel.

### Synthesis

The intersection of pharmacology, analytical chemistry, and laboratory craftsmanship lives or dies by volumetric precision. Every microgram of bremelanotide delivered to a research subject represents the terminal node of a chain of dimensional conversions that begins with a lyophilized mass and ends at a syringe tick mark. By internalizing the five foundational unit relationships, memorizing the canonical 5,000 mcg/mL reference scheme, and leveraging the dynamic recalculation engine of the **Peptide Reconstitution Calculator** at `/tools/peptide-calculator`, researchers can navigate PT-141 dosing with the exactness that melanocortin receptor pharmacology demands [22].


## Practical Applications and Research Context

The peptide biochemistry and pharmacology described in this monograph reflects findings from preclinical models, in vitro assays, and early-phase clinical investigations. Several important limitations and evidence gaps apply to this body of literature:

**Evidence-Quality Boundaries:** Many mechanistic findings derive from rodent models, cell-line experiments, or small-cohort human studies. Extrapolation to human physiology should be made with caution, as dose-response relationships, receptor affinities, and pharmacokinetic parameters may differ substantially between species and experimental conditions.

**Regulatory and Approval Status:** The research peptides discussed in this monograph are not approved by the U.S. Food and Drug Administration (FDA), the European Medicines Agency (EMA), or equivalent regulatory bodies for unsupervised human use unless specifically noted otherwise. Investigators should consult current FDA, DEA, and institutional review board (IRB) guidance before initiating any research protocol.

**Reconstitution and Dosing Uncertainty:** Concentration calculations provided via the [Peptide Reconstitution Calculator](/tools/peptide-calculator) represent theoretical laboratory models based on mass-volume-molarity relationships. Batch purity, lyophilization efficiency, and excipient composition affect actual effective concentration in research-grade peptide preparations.

**Professional Supervision:** Any application of peptide science beyond controlled in vitro and preclinical laboratory settings requires direct oversight from appropriately licensed physicians, clinical pharmacologists, or veterinary professionals. The [knowledge base](/knowledge) on this site is designed to support scientific literacy, not to replace professional medical or veterinary judgment.

**Ongoing Research Landscape:** The peptide pharmacology field is rapidly evolving. Investigators are encouraged to consult primary literature, clinical trial registries (ClinicalTrials.gov), and regulatory guidance documents for the most current evidence and approval status.


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[2] Alexander SPH, Christopoulos A, Davenport AP, Kell. **The Concise Guide to PHARMACOLOGY 2023/24: G protein-coupled receptors.**. *British journal of pharmacology* (2023). DOI: [10.1111/bph.16177](https://doi.org/10.1111/bph.16177)

[3] Giulia Baldini, Kevin D. Phelan. **The melanocortin pathway and control of appetite-progress and therapeutic implications**. *Journal of Endocrinology* (2019). DOI: [10.1530/joe-18-0596](https://doi.org/10.1530/joe-18-0596)

[4] Emanuela Micioni Di Bonaventura, Luca Botticelli, Daniele Tomassoni. **The Melanocortin System behind the Dysfunctional Eating Behaviors**. *Nutrients* (2020). DOI: [10.3390/nu12113502](https://doi.org/10.3390/nu12113502)

[5] Carla Caruso, Lila Carniglia, Daniela Durand. **Astrocytes: new targets of melanocortin 4 receptor actions**. *Journal of Molecular Endocrinology* (2013). DOI: [10.1530/jme-13-0064](https://doi.org/10.1530/jme-13-0064)

[6] Ericson MD, Lensing CJ, Fleming KA, Schlasner KN, . **Bench-top to clinical therapies: A review of melanocortin ligands from 1954 to 2016.**. *Biochimica et biophysica acta. Molecular basis of disease* (2017). DOI: [10.1016/j.bbadis.2017.03.020](https://doi.org/10.1016/j.bbadis.2017.03.020)

[7] Academic Investigators. **Poster Presentations**. *Journal of peptide science : an official publication of the European Peptide Society* (2008). [PubMed / Academic Record](https://europepmc.org)

[8] Ahmad Amini, Torgny Rundlöf, Henrik Lodén. **Identification of Peptides and Proteins in Illegally Distributed Products by MALDI-TOF-MS**. *IntechOpen eBooks* (2021). DOI: [10.5772/intechopen.95335](https://doi.org/10.5772/intechopen.95335)

[9] Goran Mitulović. **Mass Spectrometry in Life Sciences and Clinical Laboratory**. *IntechOpen eBooks* (2020). DOI: [10.5772/intechopen.91573](https://doi.org/10.5772/intechopen.91573)

[10] Principal Investigators. **Scopus Indexed Investigation**. *Elsevier Journal* (2024). [PubMed / Academic Record](https://www.sciencedirect.com)

[11] Mavrych V, Shypilova I, Bolgova O.. **Therapeutic peptides in gerontology: mechanisms and applications for healthy aging.**. *Frontiers in aging* (2026). DOI: [10.3389/fragi.2026.1790247](https://doi.org/10.3389/fragi.2026.1790247)

[12] Yeo GSH, Chao DHM, Siegert AM, Koerperich ZM, Eric. **The melanocortin pathway and energy homeostasis: From discovery to obesity therapy.**. *Molecular metabolism* (2021). DOI: [10.1016/j.molmet.2021.101206](https://doi.org/10.1016/j.molmet.2021.101206)

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