Mac E. Hadley

SYNTHETIC MELANOTROPIC PEPTIDE INITIATES ERECTIONS IN MEN WITH PSYCHOGENIC ERECTILE DYSFUNCTION: DOUBLE-BLIND, PLACEBO CONTROLLED CROSSOVER STUDY

PURPOSE We evaluated the erectogenic properties of a new cyclic alpha-melanocyte-stimulating hormone analogue, Melanotan-II, to treat men with psychogenic erectile dysfunction. MATERIALS AND METHODS Ten men with erectile dysfunction of no known organic cause were entered in a double-blind, placebo controlled crossover study in which the erectogenic properties of Melanotan-II and a vehicle placebo were compared using real-time RigiScan monitoring. The presence, duration and rigidity of erections were recorded during a 6-hour period. RESULTS In 8 of 10 men treated with Melanotan-II clinically apparent erections developed. Mean duration of tip rigidity greater than 80% was 38.0 minutes with Melanotan-II and 3.0 with placebo (p=0.0045). Transient side effects of nausea, stretching and yawning, and decreased appetite were reported more frequently after injections of Melanotan-II than placebo but none required treatment. CONCLUSIONS Melanotan-II is a potent initiator of erections in men with psychogenic erectile dysfunction and has manageable side effects at a dose of 0.025 mg./kg.

The Proopiomelanocortin System

ABSTRACT: POMC (31,000 MW) is localized to the pituitary, brain, skin, and other peripheral sites. The particular enzyme profile present within a cell dictates the nature of the hormonal ligand (melanocortin) synthesized and secreted: melanotropic peptides (α‐MSH β‐lipotropin, λ‐MSH), corticotropin (ACTH), several endorphins (e.g., met‐enkephalin). These POMC‐derived peptides mediate their actions through typical seven‐spanning membrane receptors (MCRs; MCR1, 2, 3, 4, and 5). A specific melanocortin acting on a specific MCR regulates a particular biological response; for example, α‐MSH on MCR1 increases melanogenesis within melanocytes, ACTH on MCR2 increases cortisol production within adrenal zona fasciculata cells. Within the brain melanocortins regulate satiety (MCR4) and erectile activity (MCR?). MCRs have been localized by melanocortin macromolecular probes, for example, fluorescent to human epidermal melanocytes and also to keratinocytes, suggesting that systemic melanocortins or localized POMC products might regulate these integumental cellular elements in synchrony to enhance skin pigmentation and/or immunological responses. Superpotent, prolonged acting melanotropic peptides have been synthesized and their application in clinical medicine has been demonstrated. MCR antagonists have been used to discover and further delineate other roles of melanocortin ligands. For example, melanocortin‐induced satiety can be antagonized by a melanocortin antagonist. Defects in melanocortin ligand biosynthesis, secretion, and melanocortin receptor function can lead to a diverse number of pathological states.

Evaluation of Melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study

A pilot phase I study was conducted with a cyclic heptapeptide analog of alpha-melanocyte stimulating hormone (alpha-MSH). The lactam-bridged molecule, called Melanotan-II (MT-II), has the structure Ac-Nle4-Asp5-His6-D-Phe7-Arg8-Trp9-Lys10 alpha-MSH4-10-NH2 (MT-II) and has superpotent melanotropic activity in vitro. A single-blind, alternating day (saline or MT-II), placebo-controlled trial was conducted in 3 normal male volunteers at the starting dose of 0.01 mg/kg of MT-II. Subcutaneous injections of MT-II or saline were given daily (Monday-Friday) for 2 consecutive weeks. Two subjects were escalated by 0.005 mg/kg increments to 0.03 mg/kg and one to 0.025 mg/kg. The 0.03 mg/kg dose produced Grade II somnolence and fatigue in one of two subjects (WHO standards). Mild nausea, not requiring antiemetic treatment, was reported at most MT-II dose levels. A stretching and yawning complex appeared to correlate with the onset of spontaneous, penile erections which were intermittently experienced for 1-5 hours after MT-II dosing, depending on the MT-II dose. Two subjects had increased pigmentation in the face, upper body and buttock, as measured by quantitative reflectance and by visual perception 1 week after MT-II dosing ended. These results demonstrate that MT-II has tanning activity in humans given only 5 low doses every other day by subcutaneous injection. The recommended single MT-II dose for future Phase I studies is 0.025 mg/kg/day.

Biogenic amines and control of melanophore stimulating hormone release.

Release of melanophore stimulating hormone (MSH) from the vertebrate pars intermedia is under inhibitory control by the hypothalamus. Removal of the rat pituitary or the neurointermediate lobe of the frog (Rana pipiens) to in vitro incubation medium results in rapid uninhibited release of MSH. This secretion is inhibited by norepinephrine, epinephrine, phenylephrine, and dopamine, and the inhibition is antagonized by α-adrenergic receptor blocking agents. Isoproterenol stimulation of MSH secretion from isolated glands is blocked by pro-pranolol, a β-adrenergic receptor antagonist. These results implicate dopaminergic or classical α-adrenergic receptors (or both) in inhibition of MSH release by catecholamines, and implicate β-adrenergic receptors in stimulation of MSH release by the bioamines.

Effect of an alpha-melanocyte stimulating hormone analog on penile erection and sexual desire in men with organic erectile dysfunction

OBJECTIVES To assess the safety, erectogenic properties, and effect on sexual desire of Melanotan II, a synthetic melanotropic initiator of erection, in men with erectile dysfunction and organic risk factors. METHODS Ten subjects were enrolled in a double-blind, placebo-controlled, crossover study. Melanotan II (0.025 mg/kg) and vehicle were each administered twice by subcutaneous injection; real-time RigiScan monitoring and a visual analog were used to quantify the erections during a 6-hour period. The level of sexual desire and side effects were recorded with a questionnaire. RESULTS Melanotan II initiated subjectively reported erections in 12 of 19 injections versus only 1 of 21 doses of placebo. The mean rigidity score of the responders was 6.9 on a scale of 0 to 10. The mean duration of tip rigidity greater than 80% was 45.3 minutes with Melanotan II versus 1.9 for placebo (P = 0.047). The level of sexual desire after injection was significantly higher after Melanotan II administration than after placebo. Nausea and stretching/yawning occurred more frequently with Melanotan II, and 4 of 19 injections were associated with severe nausea. CONCLUSIONS The erectogenic properties of Melanotan II are not limited to cases of psychogenic erectile dysfunction; men with a variety of organic risk factors developed penile erections. The finding of increased sexual desire warrants further investigation of centrally acting agents on disorders of sexual desire.

α-melanotropin: The minimal active sequence in the lizard skin bioassay

alpha-Melanotropin (alpha-melanocyte-stimulating hormone, alpha-MSH) is a tridecapeptide, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. The minimal sequence of alpha-MSH required for agonism in the lizard (Anolis carolinensis) skin bioassay was determined to be Ac-His-Phe-Arg-Trp-NH2 (Ac-alpha-MSH6-9-NH2). Smaller fragments of this sequence (Ac-alpha-MSH6-8-NH2, Ac-alpha-MSH6-7-NH2, Ac-alpha-MSH7-9-NH2, and Ac-alpha-MSH7-8-NH2) were devoid of melanotropic activity. The tetrapeptide, Ac-alpha-MSH7-10-NH2, was also inactive, thus again demonstrating the importance of His at position 6 for minimal activity. The important potentiating amino acids were found to be Met-4, Lys-11, and Pro-12, since Ac-alpha-MSH4-10-NH2 was about 100 times more potent than Ac-alpha-MSH5-10-NH2, and Ac-[Nle4]-alpha-MSH4-11-NH2 was about 40 times more potent than Ac-alpha-MSH4-10-NH2 or Ac-[Nle4]-alpha-MSH4-10-NH2. Ac-alpha-MSH4-12-NH2 and Ac-[Nle4]-alpha-MSH4-12-NH2 were equipotent and about six times more potent than alpha-MSH. Since [Nle4]-alpha-MSH and Ac-[Nle4]-alpha-MSH4-13-NH2 were both equipotent but about sixfold less active than Ac-[Nle4]-alpha-MSH4-12-NH2, it is clear that valine at position 13 does not contribute to the potency of alpha-MSH, except possibly in a negative way. The minimal message sequence for equipotency to alpha-MSH appears to be Ac-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-NH2, since the analog, Ac-[Nle4]-alpha-MSH4-11-NH2, was as active as the native hormone. Ser-1, Tyr-2, Ser-3, Glu-5, and Val-13 are not important for melanotropic potency since Ac-alpha-MSH4-12-NH2 was more potent than alpha-MSH, and Ac-alpha-MSH5-10-NH2 and Ac-alpha-MSH6-10-NH2 were equipotent, being about 4,000 times less active than alpha-MSH.

Melanocortin receptor agonists, penile erection, and sexual motivation: human studies with Melanotan II.

We review our experience with Melanotan II, a non-selective melanocortin receptor agonist, in human subjects with erectile dysfunction (ED). Melanotan II was administered to 20 men with psychogenic and organic ED using a double-blind placebo-controlled crossover design. Penile rigidity was monitored for 6 h using RigiScan. Level of sexual desire and side effects were reported with a questionnaire.In the absence of sexual stimulation, Melanotan II led to penile erection in 17 of 20 men. Subjects experienced a mean of 41 min Rigiscan tip rigidity>80%. Increased sexual desire was reported after 13/19 (68%) doses of Melanotan II vs 4/21 (19%) of placebo (P<0.01). Nausea and yawning were frequently reported side effects due to Melanotan II; at a dose of 0.025 mg/kg, 12.9% of subjects had severe nausea. We conclude that Melanotan II is a potent initiator of penile erection in men with erectile dysfunction. Our findings warrant further investigation of melanocortin agonists and antagonists on penile erection.

Melanocortin peptide therapeutics: Historical milestones, clinical studies and commercialization

The melanocortins (MCs) are a family of multifunctional peptidergic hormones. Several superpotent, prolonged acting, enzymatically resistant, MC analogs have been designed and synthesized to help clarify the nature and role of MCs and their receptors (MCRs) in physiological functions. Two of these analogs, a linear peptide, melanotan I, and a cyclic truncated peptide, melanotan II (MTI and MTII, respectively) have been patented and tested clinically for studies on tanning of the skin (MTI) and for diagnosis and treatment of male erectile dysfunction (MTII). A new MTII analog, PT-141 (Palatin Technologies) has initial phase I/II trials and is scheduled to enter pivotal stage III clinical trials leading to commercialization. MTI may provide a therapeutic tan with the potential to lower the risk of skin cancer. PT-141 may improve sexual functionality in both males and females.

Melanotropin bioassays: In vitro and in vivo comparisons

A reflectance method was utilized to compare the in vitro responses in three species of frogs (Rana pipiens, R. berlandieri forrei, and R. catesbeiana) and a lizard (Anolis carolinensis) to alpha- and beta-melanotropins (alpha- and beta p-MSH). The integumental chromatic response of the three ranid species was identical, in that alpha-MSH was about 2 times more potent than beta p-MSH. The melanotropins were equipotent in the lizard skin bioassay. A remarkable feature of the Anolis skin assay is that skins from this lizard can be utilized repeatedly many times in one day with an extremely high degree of precision. The reflectance method was also used to determine the in vivo potencies of alpha-MSH and beta p-MSH in the frog, R. pipiens. Surprisingly, the melanotropins were more active in the in vivo assay than in the in vitro bioassay. The darkening response of the frogs to alpha-MSH was reversed by 6 hr, but the response to beta p-MSH persisted for more than 8 hr. When alpha-MSH was incubated in frog serum, the melanotropic activity was almost totally abolished by 30 min, whereas the melanotropic activity of beta p-MSH was evident much longer (4 hr) in the presence of the serum. In light of the observation that the melanotropic activity of alpha-MSH is rapidly lost by incubation in frog serum, it is unclear why the hormone was more active as measured in vivo and why the darkening response in vivo persisted so long.

Structure-activity relationships of melatonin and related indoleamines

Abstract In vitro studies of the skin-lightening activity of melatonin and related indoleamines were conducted on the frog, Rana pipiens . Using objective, photometric measurements of reflectance, the response of the dermal melanocytes to these compounds was determined. Melatonin consistently produced significant lightening of skins at concentrations as low as 5 × 10−11M. Other indoleamines found to possess skin-lightening activity were ranked in approximate order of potency relative to melatonin, the most potent of the compounds studied. Studies of these and a number of related, but inactive, compounds provided evidence suggesting structural requirements for compounds having activity on the melatonin receptor. Among the least potent of the active compounds, a melatonin blocking agent, N-acetylserotonin, was discovered. This prompted the synthesis of a second melatonin blocking agent, N-acetyltryptamine, which, unlike N-acetylserotonin, was shown to possess no intrinsic skin-lightening activity. The data suggest that the mechanism of blockade is by competitive inhibition at the binding site of the receptor. In addition, the data indicate that the intrinsic activity of indolic compounds on the melatonin receptor is determined primarily by the moiety substituted on the 5th carbon atom, whereas, the affinity for the receptor binding site is determined primarily by the moiety substituted on the 3rd carbon atom of the indole nucleus. Based on these findings, criteria for the identification of similar melatonin receptors in other tissues are suggested.