Genetic and pharmacological antagonism of NK1 receptor prevents opiate abuse potential

Development of an efficacious, non-addicting analgesic has been challenging. Discovery of novel mechanisms underlying addiction may present a solution. Here we target the neurokinin system, which is involved in both pain and addiction. Morphine exerts its rewarding actions, at least in part, by inhibiting GABAergic input onto substance P (SP) neurons in the ventral tegmental area (VTA), subsequently increasing SP release onto dopaminergic neurons. Genome editing of the neurokinin 1 receptor (NK1R) in the VTA renders morphine non-rewarding. Complementing our genetic approach, we demonstrate utility of a bivalent pharmacophore with dual activity as a μ/δ opioid agonist and NK1R antagonist in inhibiting nociception in an animal model of acute pain while lacking any positive reinforcement. These data indicate that dual targeting of the dopaminergic reward circuitry and pain pathways with a multifunctional opioid agonist-NK1R antagonist may be an efficacious strategy in developing future analgesics that lack abuse potential.

Contribution of regional brain melanocortin receptor subtypes to elevated activity energy expenditure in lean, active rats

Physical activity and non-exercise activity thermogenesis (NEAT) are crucial factors accounting for individual differences in body weight, interacting with genetic predisposition. In the brain, a number of neuroendocrine intermediates regulate food intake and energy expenditure (EE); this includes the brain melanocortin (MC) system, consisting of MC peptides as well as their receptors (MCR). MC3R and MC4R have emerged as critical modulators of EE and food intake. To determine how variance in MC signaling may underlie individual differences in physical activity levels, we examined behavioral response to MC receptor agonists and antagonists in rats that show high and low levels of physical activity and NEAT, that is, high- and low-capacity runners (HCR, LCR), developed by artificial selection for differential intrinsic aerobic running capacity. Focusing on the hypothalamus, we identified brain region-specific elevations in expression of MCR 3, 4, and also MC5R, in the highly active, lean HCR relative to the less active and obesity-prone LCR. Further, the differences in activity and associated EE as a result of MCR activation or suppression using specific agonists and antagonists were similarly region-specific and directly corresponded to the differential MCR expression patterns. The agonists and antagonists investigated here did not significantly impact food intake at the doses used, suggesting that the differential pattern of receptor expression may by more meaningful to physical activity than to other aspects of energy balance regulation. Thus, MCR-mediated physical activity may be a key neural mechanism in distinguishing the lean phenotype and a target for enhancing physical activity and NEAT.

Development and characterisation of novel fentanyl-delta opioid receptor antagonist based bivalent ligands

BACKGROUND

Opioid tolerance is a limiting factor in chronic pain. Delta opioid peptide (DOP)(δ) receptor antagonism has been shown to reduce tolerance. Here, the common clinical mu opioid peptide (MOP)(µ) receptor agonist fentanyl has been linked to the DOP antagonist Dmt-Tic (2',6'-dimethyl-L-tyrosyl-1,2,3,4-tetrahydrisoquinoline-3-carboxylic acid) to create new bivalent compounds.

METHODS

Binding affinities of bivalents(#9, #10, #11, #12 and #13) were measured in Chinese hamster ovary (CHO) cells expressing recombinant human MOP, DOP, Kappa opioid peptide (KOP)(κ) and nociceptin/orphanin FQ opioid peptide (NOP) receptors. Functional studies, measuring GTPγ[(35)S] or β-arrestin recruitment, were performed in membranes or whole cells respectively expressing MOP and DOP.

RESULTS

The new bivalents bound to MOP (pKi : #9:7.31; #10:7.58; #11:7.91; #12:7.94; #13:8.03) and DOP (#9:8.03; #10:8.16; #11:8.17; #12:9.67; #13:9.71). In GTPγ[(35)S] functional assays, compounds #9(pEC50:6.74; intrinsic activity:0.05) #10(7.13;0.34) and #11(7.52;0.27) showed weak partial agonist activity at MOP. Compounds #12 and #13, with longer linkers, showed no functional activity at MOP. In antagonist assays at MOP, compounds #9 (pKb:6.87), #10(7.55) #11(7.81) #12(6.91) and #13(7.05) all reversed the effects of fentanyl. At DOP, all compounds showed antagonist affinity (#9:6.85; #10:8.06; #11:8.11; #12:9.42; #13:9.00), reversing the effects of DPDPE ([D-Pen(2,5)]enkephalin). In β-arrestin assays, compared with fentanyl (with response at maximum concentration (RMC):13.62), all compounds showed reduced ability to activate β-arrestin (#9 RMC:1.58; #10:2.72; #11:2.40; #12:1.29; #13:1.58). Compared with fentanyl, the intrinsic activity was: #9:0.12; #10:0.20; #11:0.18; #12:0.09 and #13:0.12.

CONCLUSIONS

The addition of a linker between fentanyl and Dmt-Tic did not alter the ability to bind to MOP and DOP, however a substantial loss in MOP functional activity was apparent. This highlights the difficulty in multifunctional opioid development.

Building a better analgesic: multifunctional compounds that address injury-induced pathology to enhance analgesic efficacy while eliminating unwanted side effects

The most highly abused prescription drugs are opioids used for the treatment of pain. Physician-reported drug-seeking behavior has resulted in a significant health concern among doctors trying to adequately treat pain while limiting the misuse or diversion of pain medications. In addition to abuse liability, opioid use is associated with unwanted side effects that complicate pain management, including opioid-induced emesis and constipation. This has resulted in restricting long-term doses of opioids and inadequate treatment of both acute and chronic debilitating pain, demonstrating a compelling need for novel analgesics. Recent reports indicate that adaptations in endogenous substance P/neurokinin-1 receptor (NK1) are induced by chronic pain and sustained opioid exposure, and these changes may contribute to processes responsible for opioid abuse liability, emesis, and analgesic tolerance. Here, we describe a multifunctional mu-/delta-opioid agonist/NK1 antagonist compound [Tyr-d-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-Bn(CF3)2 (TY027)] that has a preclinical profile of excellent antinociceptive efficacy, low abuse liability, and no opioid-related emesis or constipation. In rodent models of acute and neuropathic pain, TY027 demonstrates analgesic efficacy following central or systemic administration with a plasma half-life of more than 4 hours and central nervous system penetration. These data demonstrate that an innovative opioid designed to contest the pathology created by chronic pain and sustained opioids results in antinociceptive efficacy in rodent models, with significantly fewer side effects than morphine. Such rationally designed, multitargeted compounds are a promising therapeutic approach in treating patients who suffer from acute and chronic pain.

Spinal or systemic TY005, a peptidic opioid agonist/neurokinin 1 antagonist, attenuates pain with reduced tolerance

BACKGROUND AND PURPOSE

The use of opioids in treating pain is limited due to significant side effects including somnolence, constipation, analgesic tolerance, addiction and respiratory depression. Pre-clinical studies have shown that neurokinin 1 (NK(1) ) receptor antagonists block opioid-induced antinociceptive tolerance and may inhibit opioid-induced rewarding behaviours. Here, we have characterized a bifunctional peptide with both opioid agonist and NK(1) antagonist pharmacophores in a rodent model of neuropathic pain.

EXPERIMENTAL APPROACH

Rats were evaluated for behavioural responses to both tactile and thermal stimuli in either an uninjured, sham- or nerve-injured state. TY005 (Tyr-DAla-Gly-Phe-Met-Pro-Leu-Trp-O-3,5-Bn(CF(3) )(2) ) was delivered spinally or systemically to assess the antinociceptive effects after acute exposure. Motor skills were evaluated using the rotarod test to determine potential sedative effects. Spinal TY005 was given chronically to sham- or nerve-injured animals to determine the development of tolerance.

KEY RESULTS

Bolus injections of TY005 produced dose-dependent antinociception in non-injured animals and alleviated nerve injury-induced thermal and tactile hypersensitivities (i.e. antihyperalgesia) more effectively than morphine. Sedative effects were not evident from the rotarod test at doses that were antihyperalgesic, nor at doses threefold higher. Repeated administration of TY005 did not lead to the development of antihyperalgesic tolerance or alter sensory thresholds.

CONCLUSIONS AND IMPLICATIONS

Collectively, the data suggest that opioid agonist/NK(1) antagonist bifunctional peptides represent a promising novel approach to the management of chronic pain without the development of tolerance, reducing the need for escalation of doses and unwanted side effects associated with opiates alone.

Reduced peptide bond cyclic somatostatin based opioid octapeptides Synthesis, conformational properties and pharmacological characterization

The conformational and pharmacological properties that result from peptide bond reduction as well as the use of secondary amino acids in a series of cyclic peptides related to the mu opioid receptor selective antagonist D-Phe1-Cys2-Tyr3-D-Trp4-Orn5-Thr6-Pen7+ ++-Thr8-NH2 (IV), have been investigated. Peptide analogues that contain [CH2NH] and [CH2N] pseudo-peptide bonds (in primary and secondary amino acids, respectively) were synthesized on a solid support. Substitution of Tyr3 in IV by the cyclic, secondary amino acid 1,2,3,4-tetrahydroisoquinoline carboxylate (Tic) and of D-Trp4 with D-1,2,3,4-tetrahydro-beta-carboline(D-Tca4), gave peptides 4 and 1, respectively. Both analogues displayed reduced affinities for mu opioid receptors. Conformational analysis based on extensive NMR investigations demonstrated that the backbone conformations of 1 and 4 are similar to those of the potent and selective analogue D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 (I), while the conformational properties of the side chains of Tic3 (4) and D-Tca4 (1) resulted in topographical properties that were not well recognized by the mu opioid receptor. Peptide bond modifications were made including (Tyr3-psi[CH2NH]-D-Trp4), 3; (Tyr3-psi[CH2N]-D-Tca4), 2; and (Cys2-psi[CH2N]-Tic3), 6. These analogues showed decreases in their mu opioid receptor affinities relative to the parent compounds IV, 1, and 4, respectively. 1H NMR based conformational analysis in conjunction with receptor binding data led to the conclusion that the reduced peptide bonds in 2, 3, 5, and 6 do not contribute to the process of discrimination between mu and delta opioid receptors, and in spite of their different dynamic behaviors (relative to 1 and 4), they are still capable of attaining similar receptor bound conformations, possibly due to their increased flexibility.

Syntheses of optically pure, conformationally constrained, and highly hydrophobic unusual amino acids: 2-amino-3, 3-diarylpropionic acids*

A series of optically pure, conformationally constrained, and highly hydrophobic unusual aromatic amino acids, 2-amino-3,3-diarylpropionic acids, were synthesized via asymmetric 1,4-Michael addition reaction/azidation reactions in seven steps with overall yields of 20-30%.

Biological actions of melanocyte-stimulating hormone

Melanocyte-stimulating hormone (alpha-melanotropin, MSH) may function in a number of diverse physiological roles. MSH stimulates (1) rapid translocation of melanosomes (melanin granules) in dermal melanophores to effect rapid colour change and (2) melanogenesis in normal and abnormal (melanoma) epidermal melanocytes. Both actions involve (1) initial binding of the peptide on the melanocyte membrane, (2) transduction of signal to adenylate cyclase, and (3) increased cytosolic levels of cyclic AMP. Efforts to prepare radioiodinated MSH and analogues for radioreceptor studies using melanoma membranes and intact cells reveal that conventional iodination procedures inactivate the hormone because of oxidative and iodination effects on specific structural components of the peptide. These effects can be circumvented by the use of synthetically tailored MSH analogues. Transduction of signal from receptor to adenylate cyclase requires calcium, but prostaglandin or beta-adrenoceptor stimulation of melanophores does not. The nucleotide and metal ion requirements for mouse melanoma adenylate cyclase activity have been characterized. There is both a transcriptional and translational requirement for MSH stimulation of tyrosinase activity and melanin production in melanoma cells. Melanosome translocation within melanophores is enhanced in the absence of extracellular calcium. A model for the MSH control of melanosome movements suggests a bifunctional, but compartmentalized, role for calcium in the action of MSH.

Isomerization/Recyclization of some 5-Ethoxycarbonyl-pyrimidines

This communication reports on the investigation of a new recyclization conversion of a pyrimidine ring, which can be referred to as C-C recyclization. In this reaction the nucleophile cleaves the pyrimidine ring at the N(3)-C(4) bond, and following rotation around the single C(5)-C(6) bond the new cyclization takes place. This type of recyclization has general applicability, and takes place upon alkali treatment of substituted 4-methyl-5-ethoxycarbonyl- and 4-amino-5-ethoxycarbonyl-pyrimidines (1) which are transformed respectively to 4-hydroxy-5-acetyl- and 4-hydroxy-5-carbamoylpyrimidines (2). The obtained pyrimidyl-ketones can be readily converted to their hydrazones 7-12.

Exploring Ramachandran and chi space: conformationally constrained amino acids and peptides in the design of bioactive polypeptide ligands

Ligand binding and concomitant changes in receptor structure provide the means to target signal transduction pathways. With appropriate refinement of the ligand's interaction with the "receptor," one in theory could produce ligands that have greater therapeutic benefits. This review will discuss how, when these ligands are amino acids and peptides, the introduction of appropriate conformational constraints provides a powerful strategy for improved drug design. This review will discuss how various constraints on amino acids can provide a powerful tool for ligand design, determination of the three dimensional pharmacophore and new insights into receptor systems and information transduction. Through the use of constrained ligands, new information regarding their interaction with their "receptor" systems, and further refinement of the use of constraints, scientists can produce more beneficial drugs for mankind.

Biological and conformational study of beta-substituted prolines in MT-II template: steric effects leading to human MC5 receptor selectivity

To investigate the molecular basis for the interaction of the chi-constrained conformation of melanotropin peptide with the human melanocortin receptors, a series of beta-substituted proline analogs were synthesized and incorporated into the Ac-Nle-C[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 (MT-II) template at the His6 and D-Phe7 positions. It was found that the binding affinities generally diminished as the steric bulk of the p-substituents of the 3-phenylproline residues increased. From (2S, 3R)-3-phenyl-Pro6 to (2S, 3R)-3-(p-methoxyphenyl)-Pro6 analogs the binding affinity decreased 23-fold at the human melanocortin-3 receptor (hMC3R), 17-fold at the hMC4R, and eight-fold at the hMC5R, but selectivity for the hMC5R increased. In addition, the substitution of the D-Phe7 residue with a (2R, 3S)-3-phenyl-Pro resulted in greatly reduced binding affinity (10(3)-10(5)) at these melanocortin receptors. Macromodel's Large Scale Low Mode (LLMOD) with OPLS-AA force field simulations revealed that both MT-II and SHU-9119 share a similar backbone conformation and topography with the exception of the orientation of the side chains of D-Phe7/D-Nal (2')7 in chi space. Introduction of the dihedrally constrained phenylproline analogs into the His6 position (analogs 2-6) caused topographical changes that might be responsible for the lower binding affinities. Our findings indicate that hMC3 and hMC4 receptors are more sensitive to steric effects and conformational constraints than the hMC5 receptor. This is the first example for melanocortin receptor selectivity where the propensity of steric interactions in chi space of beta-modified Pro6 analogs of MT-II has been shown to play a critical role for binding as well as bioefficacy of melanotropins at hMC3 and hMC4 receptors, but not at the hMC5 receptor.

Extensive structure-activity studies of lactam derivatives of MT-II and SHU-9119: their activity and selectivity at human melanocortin receptors 3, 4, and 5

The melanocortin system is involved in the regulation of several diverse physiologic pathways. Recently we have demonstrated that replacing His6 by Pro6 in the well-known antagonist SHU-9119 resulted in a potent agonist at the hMC5R (EC50 = 0.072 nm) with full antagonist activity at the hMC3R and the hMC4R. We have designed, synthesized, and pharmacologically characterized a series of peptide analogs of MT-II and SHU-9119 at the human melanocortin receptors MC3R, MC4R and MC5R. All these peptides were modified at position 6 with a Pro instead of a His residue. In this study, we have identified new scaffolds which are antagonists at the hMC4R and hMC3R. Additionally, we have discovered a new selective agonist at the hMC4R, Ac-Nle-c[Asp-Pro-D-Phe-Arg-Trp-Lys]-Pro-Val-NH2 (6, PG-931) which will be useful in further biologic investigations of the hMC4R. PG-931 was about 100-fold more selective for the hMC4R vs. the hMC3R (IC50 = 0.58 and 55 nm, respectively). Some of these new analogs have exceptional biologic potencies at the hMC5R and will be useful in further efforts to differentiate the substructural features responsible for selectivity at the hMC3R, hMC4R, and hMC5R.

Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH2 induces penile erection via brain and spinal melanocortin receptors

Penile erection induced by alpha-melanocyte-stimulating hormone and melanocortin receptors (MC-R) in areas of the spinal cord and periphery has not been demonstrated. To elucidate sites of the proerectile action of melanocortin peptides, in awake male rats we administered the MC-R agonist Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH(2) (MT-II) i.c.v., intrathecal (i.th.) and i.v. and scored penile erection and yawning. Injection of the MC-R antagonist Ac-Nle-c[Asp-His-DNal(2')-Arg-Trp-Lys]-NH(2) (SHU-9119) i.c.v. or i.th. in combination with i.th. MT-II differentiated spinal from supraspinal effects. To exclude a site of action in the penis, we recorded intracavernous pressure responses to intracavernosal injection of MT-II in the anesthetized rat.I.c.v., i.th., and i.v. MT-II induced penile erections in a dose-dependent fashion. Yawning was observed with i.c.v. and i.v. MT-II, while spinal injection did not produce this behavior. Intrathecal delivery of MT-II to the lumbosacral spinal cord was more efficacious in inducing erections than i.c.v. or i.v. administration; SHU-9119 blocked the erectile responses to i.th. MT-II when injected i.th. but not i.c.v. Intracavernosal MT-II neither increased intracavernous pressure nor augmented neurostimulated erectile responses. We confirmed the central proerectile activity of MT-II and demonstrated that in addition to a site of action in the brain, the distal spinal cord contains melanocortin receptors that can initiate penile erection independent of higher centers. These results provide new insight into the central melanocortinergic pathways that mediate penile erection and may allow for more efficacious melanotropin-based therapy for erectile dysfunction.

Design of novel peptide ligands which have opioid agonist activity and CCK antagonist activity for the treatment of pain

Disease states such as neuropathic pain offer special challenges in drug design due to the system changes which accompany these diseases. In this manuscript we provide an example of a new approach to drug design in which we have modified a potent and selective peptide ligand for the CCK-2 receptor to a peptide which has potent agonist binding affinity and bioactivity at delta and mu opioid receptors, and simultaneous antagonist activity at CCK receptors. De novo design based on the concept of overlapping pharmacophores was a central hypothesis of this design, and led to compounds such as H-Tyr-DPhe-Gly-DTrp-NMeNle-Asp-Phe-NH(2) (i.e., RSA 601) which have the designed properties.

Effect of peptide conformation on membrane permeability

The effect of peptide conformational constraint on the peptide permeation across the model membranes was examined by determining the permeability of pairs of cyclic and acyclic peptides related to c[d-Pen2, d-Pen5] enkephalin (DPDPE). The peptides were cyclized by formation of an intramolecular disulfide bridge between the second and fifth residues composed of either d-penicillamine or cysteine. In each case the acyclic peptide was three to seven times more permeable than corresponding cyclic peptide. The possibility that the differences in permeability of cyclic and acyclic peptides is based on the greater conformational freedom of the acyclic peptides in the presence of membrane was examined in more detail by isothermal titration calorimetric studies of Trp6-DPDPE and its acyclic analog. The membrane binding of the acyclic peptide is a more exothermic process than binding of its cyclic Trp6-DPDPE. The transfer of acyclic peptide from water to membrane is an enthalpy driven process, whereas the transfer of the cyclic peptide is driven by entropy.

Design of novel chimeric melanotropin-deltorphin analogues. Discovery of the first potent human melanocortin 1 receptor antagonist

A number of novel alpha-melanotropin (alpha-MSH) analogues have been designed, synthesized, and assayed for bioactivity at the melanocortin-1 (MC1) receptor from Xenopus frog skin, and selected potent analogues were examined at recombinant human MC1, MC3, and MC4 receptors expressed in human embryonic kidney (HEK) cells. These ligands were designed from Deltorphin-II, by a new hybrid approach, which incorporates the hydrophobic tail and the address sequence of Deltorphin-II (Glu-Val-Val-Gly-NH(2)) and key pharmacophore elements of melanotropins. Some of the ligands designed, c[Xxx-Yyy-Zzz-Arg-Trp-Glu]-Val-Val-Gly-NH(2) [XXX = nothing, Gly, beta-Ala, gamma-Abu, 6-Ahx; YYY = His, His(3-Bom), (S)-cyclopentylglycine (Cpg); ZZZ = Phe, d-Phe; d-Nal(2')], show high potency at melanocortin receptors. One ligand, GXH-32B-c[beta-Ala-His-d-Nal(2')-Arg-Trp-Glu]-Val-Val-Gly-NH(2), the most potent of the chimeric analogues tested, displayed agonist activity at each of the MC receptor subtypes analyzed, with an EC(50) of 2 nM at the amphibian MC1 receptor. In contrast, GXH-38B-c[Gly-Cpg-d-Nal(2')-Arg-Trp-Glu]-Val-Val-Gly-NH(2) (Cpg = cyclopentyl glycine) was an antagonist with a IC(50) of 43 nM at the amphibian receptor, and among the human subtypes tested, was the most potent at the MC1 receptor subtype where it also acted as an antagonist (K(i) = 53 nM), which is the first potent antagonist discovered for the human MC1 receptor. These results provide strong evidence supporting our hypothesis that ligand scaffolds for different G-protein coupled receptors (GPCRs) can be used to design ligands for other GPCRs and to design more potent ligands to treat diseases associated with the human MC1 receptor.

Binding of agonists, antagonists and inverse agonists to the human delta-opioid receptor produces distinctly different conformational states distinguishable by plasmon-waveguide resonance spectroscopy

Structural changes induced by the binding of agonists, antagonists and inverse agonists to the cloned delta-opioid receptor from human brain immobilized in a solid-supported lipid bilayer were monitored using plasmon-waveguide resonance (PWR) spectroscopy. Agonist (e.g. deltorphin II) binding causes an increase in membrane thickness because of receptor elongation, a mass density increase due to an influx of lipid molecules into the bilayer, and an increase in refractive index anisotropy due to transmembrane helix and fatty acyl chain ordering. In contrast, antagonist (e.g. TIPPpsi) binding produces no measurable change in either membrane thickness or mass density, and a significantly larger increase in refractive index anisotropy, the latter thought to be due to a greater extent of helix and acyl chain ordering within the membrane interior. These results are closely similar to those reported earlier for another agonist (DPDPE) and antagonist (naltrindol) [Salamon et al. (2000) Biophys. J.79, 2463-2474]. In addition, we now find that an inverse agonist (TMT-Tic) produces membrane thickness, mass density and refractive index anisotropy increases which are similar to, but considerably smaller than, those generated by agonists. Thus, a third conformational state is produced by this ligand, different from those formed by agonists and antagonists. These results shed new light on the mechanisms of ligand-induced G-protein-coupled receptor functioning. The potential utilization of this new biophysical method to examine structural changes both parallel and perpendicular to the membrane normal for GPCRs is emphasized.

Synthesis and biological evaluation on hMC3, hMC4 and hMC5 receptors of gamma-MSH analogs substituted with L-alanine

To elucidate the molecular basis of the interaction of the native dodecapeptide gamma-MSH with the melanocortin receptors, we performed a structure-activity study in which we systematically replaced l-Ala in each position of this peptide. Here we report the binding affinity and agonist potency on human MC3R, MC4R and MC5R. Intracellular cAMP concentration was measured on CHO cells, and binding assays were carried out using membranes prepared from these cell lines which stably express hMC3R, hMC4R and hMC5R. Our results indicate that the last four amino acids in the C-terminal region of gamma-MSH are not important determinants of biological activity and selectivity at human melanocortin receptors. Interesting results were obtained when l-Ala was substituted for His6, Phe7, Arg8 and Trp9. For these peptides, the affinity and activity at all three human receptors (MC3R, MC4R and MC5R) decreased significantly, demonstrating that the His-Phe-Arg-Trp sequence in gamma-MSH is important for activity at these three melanocortin receptors. Similar results were obtained when Met3 was replaced with l-Ala, suggesting the importance of this position in the interaction with all three receptors. This study highlights the role played by the His-Phe-Arg-Trp sequence in receptor binding and in agonist activity of gamma-MSH.

Crystal structure of biphalin sulfate: a multireceptor opioid peptide

Biphalin is a dimeric opioid peptide, composed of two tetrapeptides connected 'tail-to-tail', that exhibits a high affinity for all three opioid receptor types (i.e. mu, delta and kappa). This study presents the X-ray crystal structure of biphalin sulfate and compares it to other opioids that interact with the same biological targets. Both halves of the molecule have a folded backbone conformation but differ significantly from one another. Residues 1-4 in biphalin, which compare well with the delta selective opioid peptide DADLE, fold into a random coil. Residues 5-8, which can be fit to the mu selective peptide D-TIPP-NH2, exhibit a fairly normal type III' beta bend. Biphalin also exhibits structural similarities with two naltrexone analogs, naltrexonazine and norbinaltorphamine, that are specific to mu and kappa receptor sites.

Fast, efficient and selective deprotection of the tert-butoxycarbonyl (Boc) group using HCl/dioxane (4 m)

Fast, efficient and selective deprotection of the tert-butoxycarbonyl (Boc) group of various amino acids and peptides was achieved by using hydrogen chloride (4 m) in anhydrous dioxane solution for 30 min at room temperature. In the cases studied in our laboratory, this protocol provided superior selectivity to deprotect Nalpha-Boc groups in the presence of tert-butyl esters and tert-butyl ethers, including thio-tert-butyl ethers, but not phenolic tert-butyl ethers.

A new approach to search for the bioactive conformation of glucagon: positional cyclization scanning

In search for the bioactive conformation of glucagon, "positional cyclization scanning" was used to determine secondary structures of glucagon required for maximal interaction with the glucagon receptor. Because glucagon is flexible in nature, its bioactive conformation is not known except for an amphiphilic helical conformation at the C-terminal region. To understand the conformational requirement for the N-terminal region that appears to be essential for signal transduction, a series of glucagon analogues conformationally constrained by disulfide or lactam bridges have been designed and synthesized. The conformational restrictions via disulfide bridges between cysteine i and cysteine i + 5, or lactam bridges between lysine i and glutamic acid i + 4, were applied to induce and stabilize certain corresponding secondary structures. The results from the binding assays showed that all the cyclic analogues with disulfide bridges bound to the receptor with significantly reduced binding affinities compared to their linear counterparts. On the contrary, glucagon analogues containing lactam bridges, in particular, c[Lys(5), Glu(9)]glucagon amide (10) and c[Lys(17), Glu(21)]glucagon amide (14), demonstrated more than 7-fold increased receptor binding affinities than native glucagon. These results suggest that the bioactive conformation of glucagon may adopt a helical conformation at the N-terminal region as well as the C-terminal region, which was not evident from earlier biophysical studies of glucagon.

Development of potent glucagon antagonists: structure-activity relationship study of glycine at position 4

We examined the functional role of glycine at position 4 in the potent glucagon antagonist [desHis(1), Glu(9)]glucagon amide, by substituting the L- and D-enantiomers of alanine and leucine for Gly(4) in this antagonist. The methyl and isobutyl side-chain substituents were introduced to evaluate the preference shown by the glucagon receptor, if any, for the orientation of the N-terminal residues. The L-amino acids demonstrated only slightly better receptor recognition than the D-enantiomers. These results suggest that the Gly(4) residue in glucagon antagonists may be exposed to the outside of the receptor. The enhanced binding affinities of analogs 1 and 3 compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide, may have resulted from the strengthened hydrophobic patch in the N-terminal region and/or the increased propensity for a helical conformation due to the replacement of alanine and leucine for glycine. Thus, as a result of the increased receptor binding affinities, antagonist activities of analogs 1-4 were increased 10-fold compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide. These potent glucagon antagonists have among the highest pA(2) values of any glucagon analogs reported to date.

Mutation W284L of the human delta opioid receptor reveals agonist specific receptor conformations for G protein activation

Intrinsic activities of different delta opioid agonists were determined in a [35S]GTPgammaS binding assay using cell membranes from Chinese hamster ovary (CHO) cells stably expressing the wild type (hDOR/CHO) or W284L mutant human delta opioid receptor (W284L/CHO). Agonist binding affinities were regulated more robustly by sodium and guanine nucleotide in W284L/CHO than in hDOR/ CHO cell membranes. The W284L mutation selectively reduced the affinity of SNC 80 while having moderate effect ((-) TAN 67) or no effect (DPDPE) on the affinities of other delta selective agonists. The mutation had opposite effects on the intrinsic activities of agonists belonging to different chemical classes. The effects of the mutation on agonist affinities and potencies were independent from its effects on the intrinsic activities of the agonists. Maximal stimulation of [35S]GTPgammaS binding by SNC 80 was 2-fold higher in W284L mutant cell membranes than in wild type hDOR/CHO cell membranes, despite lower receptor expression levels in the W284L/CHO cells. The binding affinity of SNC 80 however, was significantly reduced (15-fold and 30-fold in the absence or presence of sodium+GDP respectively) in W284L/CHO cell membranes relative to wild type hDOR/CHO membranes. Conversely, the Emax of (-)TAN 67 in the [35S]GTPgammaS binding assay was markedly reduced (0.6-fold of that of the wild type) with only a slight (6-fold) reduction in its binding affinity. The affinity and intrinsic activity of DPDPE on the other hand remained unchanged at the W284L mutant hDOR. The mutation had similar effects on the affinities potencies and intrinsic activities of (-)TAN 67 and SB 219825. The results indicate that delta opioid agonists of different chemical classes use specific conformations for G protein activation.

Design in topographical space of peptide and peptidomimetic ligands that affect behavior. A chemist's glimpse at the mind--body problem

Efforts to determine the bioactive conformations of peptide ligands for membrane-bound proteins such as G-protein-coupled receptors (GPCRs) have been particularly challenging due to the flexibility of the ligands and the lack of 3D structural information (X-ray, NMR, etc.) for integral membrane proteins. An approach to determining these conformations by conformational constraint of the backbone template (phi and psi angles) and by topographical constraint (chi(1), chi(2), etc. constraint) is outlined. Special attention is given to peptide neurotransmitter ligands that affect critical behaviors (feeding, sexual, addiction, pain, etc.). It is demonstrated that small changes in structure or a single torsional angle are sufficient to dramatically modify complex behaviors.

Development of potent truncated glucagon antagonists

In pursuit of truncated glucagon analogues that can interact with the glucagon receptor with substantial binding affinity, 23 truncated glucagon analogues have been designed and synthesized. These truncated analogues consist of several fragments of glucagon with 11 or 12 amino acid residues (1-4), conformationally constrained analogues containing the sequence of the middle region of glucagon (5-15), and truncated analogues containing the sequence of the C-terminal region (16-23). Biological assays of these analogues showed that the truncated glucagon analogues with the sequence of the C-terminal region possess significantly better binding affinity compared to the truncated analogues with the sequence of the middle region, and these analogues (17-23) demonstrated potent antagonistic activity (pA(2) values between 6.5 and 7.5). On the basis of these results, it can be suggested that glucagon interacts with its receptor with two hydrophobic patches located in the middle and the C-terminal regions of glucagon, and both hydrophobic patches are necessary for significant receptor recognition. These two hydrophobic binding motifs, located in two different regions of glucagon, appear to be the reason why the earlier attempts to obtain truncated analogues with good binding affinity did not result in any success. Long peptide hormones such as glucagon seem to require more than one binding pocket on the receptors for maximal interaction.

Solid-phase synthesis of O-linked glycopeptide analogues of enkephalin

The synthesis of 18 N-alpha-FMOC-amino acid glycosides for solid-phase glycopeptide assembly is reported. The glycosides were synthesized either from the corresponding O'Donnell Schiff bases or from N-alpha-FMOC-amino protected serine or threonine and the appropriate glycosyl bromide using Hanessian's modification of the Koenigs-Knorr reaction. Reaction rates of D-glycosyl bromides (e.g., acetobromoglucose) with the L- and D-forms of serine and threonine are distinctly different and can be rationalized in terms of the steric interactions within the two types of diastereomeric transition states for the D/L and D/D reactant pairs. The N-alpha-FMOC-protected glycosides [monosaccharides Xyl, Glc, Gal, Man, GlcNAc, and GalNAc; disaccharides Gal-beta(1-4)-Glc (lactose), Glc-beta(1-4)-Glc (cellobiose), and Gal-alpha(1-6)-Glc (melibiose)] were incorporated into 22 enkephalin glycopeptide analogues. These peptide opiates bearing the pharmacophore H-Tyr-c[DCys-Gly-Phe-DCys]- were designed to probe the significance of the glycoside moiety and the carbohydrate-peptide linkage region in blood-brain barrier (BBB) transport, opiate receptor binding, and analgesia.

Pronociceptive actions of dynorphin maintain chronic neuropathic pain

Whereas tissue injury increases spinal dynorphin expression, the functional relevance of this upregulation to persistent pain is unknown. Here, mice lacking the prodynorphin gene were studied for sensitivity to non-noxious and noxious stimuli, before and after induction of experimental neuropathic pain. Prodynorphin knock-out (KO) mice had normal responses to acute non-noxious stimuli and a mild increased sensitivity to some noxious stimuli. After spinal nerve ligation (SNL), both wild-type (WT) and KO mice demonstrated decreased thresholds to innocuous mechanical and to noxious thermal stimuli, indicating that dynorphin is not required for initiation of neuropathic pain. However, whereas neuropathic pain was sustained in WT mice, KO mice showed a return to baselines by post-SNL day 10. In WT mice, SNL upregulated lumbar dynorphin content on day 10, but not day 2, after injury. Intrathecal dynorphin antiserum reversed neuropathic pain in WT mice at post-SNL day 10 (when dynorphin was upregulated) but not on post-SNL day 2; intrathecal MK-801 reversed SNL-pain at both times. Opioid (mu, delta, and kappa) receptor density and G-protein activation were not different between WT and KO mice and were unchanged by SNL injury. The observations suggest (1) an early, dynorphin-independent phase of neuropathic pain and a later dynorphin-dependent stage, (2) that upregulated spinal dynorphin is pronociceptive and required for the maintenance of persistent neuropathic pain, and (3) that processes required for the initiation and the maintenance of the neuropathic pain state are distinct. Identification of mechanisms that maintain neuropathic pain appears important for strategies to treat neuropathic pain.

Preparation of 'side-chain-to-side-chain' cyclic peptides by Allyl and Alloc strategy: potential for library synthesis

Automated and manual deprotection methods for allyl/allyloxycarbonyl (Allyl/Alloc) were evaluated for the preparation of side-chain-to-side-chain cyclic peptides. Using a standard Allyl/Alloc deprotection method, a small library of cyclic peptides with lactam bridges (with seven amino acids) was prepared on an automatic peptide synthesizer. We demonstrate that the Guibe method for removing Allyl/Alloc protecting groups under specific neutral conditions [Pd(PPh3)4/PhSiH3)/DCM] can be a useful, efficient and reliable method for preparing long cyclic peptides on a resin. We have also manually synthesized a cyclic glucagon analogue containing 24 amino acid residues. These results demonstrated that properly controlled palladium-mediated deprotection of Allyl/Alloc protecting groups can be used to prepare cyclic peptides on the resin using an automated peptide synthesizer and cyclic peptides with a long chain.

Michael addition reactions between chiral Ni(II) complex of glycine and 3-(trans-enoyl)oxazolidin-2-ones. A case of electron donor-acceptor attractive interaction-controlled face diastereoselectivity

This study has demonstrated that the readily available and inexpensive 3-(trans-3'-alkyl/arylpropenoyl)oxazolidin-2-ones, featuring high electrophilicity and conformational homogeneity, are synthetically superior Michael acceptors over the conventionally used alkyl enoylates, allowing for a remarkable improvement in reactivity and, in most cases, diastereoselectivity of the addition reactions with a Ni(II) complex of the chiral Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone. Kinetically controlled diastereoselectivity in the corresponding Michael addition reactions between the Ni(II) complex of glycine and the oxazolidin-2-ones was systematically studied as a function of steric, electronic, and position effects of the substituents on the starting Michael acceptor. In both aliphatic and aromatic series the simple diastereoselectivity was found to be virtually complete, affording the products via the corresponding TSs with the approach geometry like. The face diastereoselectivity of the reactions between the Ni(II) complex of glycine and the 3-(trans-3'-alkylpropenoyl)oxazolidin-2-ones was found to depend exclusively on the steric bulk of the alkyl group on the starting Michael acceptor. In contrast, the face diastereoselectivity in the reactions of aromatic oxazolidin-2-ones with the Ni(II) complex of glycine was shown to be controlled predominantly by the electronic properties of the aryl ring. In particular, the additions of the Ni(II) complex of glycine with 3-(trans-3'-arylpropenoyl)oxazolidin-2-ones, bearing electron-withdrawing substituents on the phenyl ring, afforded the (2S,3R)-configured products with synthetically useful diastereoselectivity and in quantitative chemical yields, thus allowing for an efficient access to the sterically constrained beta-aryl-substituted pyroglutamic and glutamic acids.

Asymmetric synthesis of alpha,beta-dialkyl-alpha-phenylalanines via direct alkylation of a chiral alanine derivative with racemic alpha-alkylbenzyl bromides. A case of high enantiomer differentiation at room temperature

[figure: see text] This study demonstrates that the direct alkylation of a Ni(II)-complex of the chiral Schiff base of alanine with (S)-o-[N-(N-benzylprolyl)amino]- benzophenone, with racemic alpha-alkylbenzyl bromides, is a synthetically feasible and methodologically advantageous approach to the target alpha,beta-dialkylphenylalanines over previously reported methods. For the first time we report and rationalize a case of a high enantiomer differentiation process at room temperature.

Biological properties of Phe(o)-opioid peptide analogues

Biological properties of new analogues, which represent Phe(o)-propeptides of a variety of opioid peptides, are described. All Phe(o)-opioid analogues expressed both receptor binding affinities and in vitro biological activities at least at the level of the primary opioid peptides. Surprisingly, some of the propeptides expressed slightly higher activity than the primary opioid peptides. Nevertheless, no significant shift in receptor selectivity was observed, which indicate that these Phe(o)-analogues undoubtedly are propeptides. The possible role of membrane proteolytic enzymes associated with opioid receptors in transformation of propeptides is discussed.

D-Amino acid scan of gamma-melanocyte-stimulating hormone: importance of Trp(8) on human MC3 receptor selectivity

In our search for potent and receptor-selective agonists and antagonists, we report here the results of D-amino acid substitution at each position of the short peptide gamma-melanocyte-stimulating hormone (gamma-MSH). The native gamma-MSH shows weak binding at all three receptors (i.e., the human MC3, MC4, and MC5) and a selectivity of 1-2 orders of magnitude at the MC3R over the MC4R and MC5R. Sequential replacement of each residue in the gamma-MSH sequence with the corresponding D-isomer results in analogues which mostly have weaker binding affinity than the native peptide, except for two analogues. For the DTrp(8) analogue, there is an increase in binding affinity by about 1 order of magnitude (IC(50) = 6 nM) at the MC3R compared with that of the natural molecule and an increase in selectivity for the MC3R by 2 orders of magnitude compared with the activity at the MC4R and MC5R. The DPhe(6) analogue is about 10-fold more potent (IC(50) = 8.8 nM) at the MC3R compared with the native peptide but lacks subtype selectivity. Measurement of the intracellular cAMP accumulation in human MC3R, MC4R, and MC5R revealed that the native peptide shows potent activity at the MC3R (EC(50) = 5.9 nM) and is about 50-100-fold selective at this receptor compared with the MC4R and MC5R. The DArg(10) (EC(50) = 35 nM) and DPhe(11) (EC(50) = 11 nM) analogues are selective for the MC3R by 1 and 2 orders of magnitude compared with the MC4R and MC5R, respectively. The DTrp(8) compound (EC(50) = 0.33 nM) shows about 300- and 250-fold increase in selectivity at the MC3R compared with the MC4R and MC5R, respectively. Finally, the DTyr(1) peptide is selective for the MC3R (EC(50) = 12 nM) by 40-200-fold compared with the MC4R and MC5R. In general, the trend is that D-amino acid substitutions of the aromatic residues 1, 6, 8, and 11 and the basic residue Arg(10), but not Arg(7), result in an increase in MC3R selectivity over the MC4R and MC5R and only agonist activity is observed. Thus, the key residues of gamma-MSH identified in this study include the aromatic residues 1, 6, 8, and 11 and the basic residue Arg(10) (but not Arg(7)), as important for MC3 selectivity over the MC4 and MC5 subtypes. Further, the study reveals the extreme importance of DTrp at position 8 in imparting potency and selectivity since this is the most selective analogue for the human MC3R reported thus far.

Plasmon resonance studies of agonist/antagonist binding to the human delta-opioid receptor: new structural insights into receptor-ligand interactions

Structural changes accompanying the binding of ligands to the cloned human delta-opioid receptor immobilized in a solid-supported lipid bilayer have been investigated using coupled plasmon-waveguide resonance spectroscopy. This highly sensitive technique directly monitors mass density, conformation, and molecular orientation changes occurring in anisotropic thin films and allows direct determination of binding constants. Although both agonist binding and antagonist binding to the receptor cause increases in molecular ordering within the proteolipid membrane, only agonist binding induces an increase in thickness and molecular packing density of the membrane. This is a consequence of mass movements perpendicular to the plane of the bilayer occurring within the lipid and receptor components. These results are consistent with models of receptor function that involve changes in the orientation of transmembrane helices.

Improved bioavailability to the brain of glycosylated Met-enkephalin analogs

The blood-brain barrier prevents the entry of many potentially therapeutic peptide drugs to the brain. Glycosylation has shown potential as a methodology for improving delivery to the CNS. Previous studies have shown improved bioavailability and improved centrally mediated analgesia of glycosylated opioids. In this study we investigate the effect of glycosylation on the cyclic opioid peptide [D-Cys(2,5),Ser(6),Gly(7)] enkephalin. The peptide was glycosylated on the Ser(6) via an O-linkage with various sugar moieties and alignments. The peptides were then investigated for receptor binding, physiochemical attributes, in situ brain uptake in female Sprague-Dawley rats and antinociception in male ICR mice. Glycosylation resulted in a slight decrease in affinity to the delta-opioid receptor, and mixed effect on binding to the mu-opioid receptor. There was a significant decrease in lipophilicity resulting from glycosylation and a slight reduction in binding to bovine serum albumin. In situ perfusion showed that brain uptake was improved by up to 98% for several of the glycosylated peptides, and the nociceptive profiles of the peptides, in general, followed the rank order of peptide entry to the brain with up to a 39-fold increase in A.U.C.

Rational design of highly diastereoselective, organic base-catalyzed, room-temperature Michael addition reactions

Via the rational design of a single-preferred transition state, stabilized by electron donor-acceptor-type attractive interactions, structural and geometric requirements for the corresponding starting compounds have been determined. The Ni(II) complex of the Schiff base of glycine with o-[N-alpha-picolylamino]acetophenone, as a nucleophilic glycine equivalent, and N-(trans-enoyl)oxazolidin-2-ones, as derivatives of an alpha,beta-unsaturated carboxylic acid, were found to be the substrates of choice featuring geometric/conformational homogeneity and high reactivity. The corresponding Michael addition reactions were found to proceed at room temperature in the presence of catalytic amounts of DBU to afford quantitatively the addition products with virtually complete diastereoselectivity. Acidic decomposition of the products followed by treatment of the reaction mixture with NH4OH gave rise to the diastereomerically pure 3-substituted pyroglutamic acids.

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.

Conformational and topographical considerations in designing agonist peptidomimetics from peptide leads

The design of peptidomimetic ligands with agonist biological activities in vitro and in vivo has been challenging. Lofty goals have been set for this research including high potency, high receptor type selectivity, high stability in vitro and in vivo, and high efficacy in vitro and in vivo for agonists. A systematic stepwise strategy has been developed to accomplish these goals. These include determining the primary amino acid side chain residues required for molecular recognition and, in the case of agonist activity, those required for information transduction. In addition to determining the preferred backbone conformation which can serve as a template for the bioactive conformation (an alpha-helix, beta-turn, beta-sheet, etc.), a strategy has been developed to examine and determine the preferred side chain conformations in chi space (chi1, chi2, etc.). These include specific covalent and non-covalent constraints which can place the constrained side chains at highly preferred gauche (-), or gauche (+), or trans conformations. Examples are provided that illustrate this methodology and provide insight into the topographical requirements for ligand receptor interactions. Often, at this juncture one can obtain a quite precise 3D pharmacophore for the ligand, as well as high stability to agonist biodegradation and good bioavailability including the ability to cross membrane barriers. If a non-peptide ligand is desired, efforts are in progress to develop templates, and aspects of conformational design that permit assembling of all components necessary for molecular recognition and transduction. Here the proper choice of template that can place the key side chain residue in 3D space is still difficult, and thus only partial success has been achieved in terms of potent and selective ligands. A few of these approaches are presented and discussed in some detail.

Local inhibitory effects of dynorphin A-(1-17) on capsaicin-induced thermal allodynia in rhesus monkeys

Although dynorphin A-(1-17) has been characterized in vitro as a high efficacy kappa-opioid receptor agonist, functional studies of dynorphin A-(1-17) following central or systemic administration indicate the involvement of both opioid and non-opioid components. The aim of this study was to investigate whether local administration of dynorphin-related analogs can attenuate capsaicin (8-methyl-N-vanillyl-6-nonenamide)-induced nociception and what type of opioid receptor mediates the local action of dynorphin A-(1-17) in monkeys. Capsaicin (100 microg) was used to evoke a nociceptive response, thermal allodynia, which was manifested as a reduced tail-withdrawal latency in normally innocuous 46 degrees C warm water. Co-administration of dynorphin A-(1-17) (0.3-10 microg) with capsaicin in the tail dose-dependently inhibited thermal allodynia; however, both non-opioid fragments dynorphin A-(2-17) (10-300 microg) and dynorphin A-(2-13) (10-300 microg) were ineffective. Local antiallodynia of dynorphin A-(1-17) was antagonized by a small dose (100 microg) of an opioid receptor antagonist, quadazocine, applied s.c. in the tail. Pretreatment with a selective kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI), s.c. 320 microg in the tail also reversed local antiallodynia of dynorphin A-(1-17). Both locally effective doses of antagonists, when applied s.c. in the back, did not antagonize local dynorphin A-(1-17), indicating that peripheral kappa-opioid receptors selectively mediated the local action of dynorphin A-(1-17) in the tail. In addition, a much larger dose of dynorphin A-(1-17) (1000 microg), when administered s. c. in the back or i.m. in the thigh, did not cause sedative or diuretic effects. These results suggest that in vivo opioid actions of dynorphin-related peptides can be differentiated locally in this procedure. They also indicate that local application of peptidic ligands may be a useful medication for localized pain.

Evaluation of new base-labile 2-(4-nitrophenylsulfonyl) ethoxycarbonyl (Nsc)-amino acids for solid-phase peptide synthesis

The 2-(4-nitrophenylsulfonyl)ethoxycarbonyl (Nsc) group is a new base-labile protecting group for solid-phase peptide synthesis, completely interchangeable with the fluorenylmethoxycarbonyl (Fmoc) protecting group, but with certain advantages. In this paper, we report a methodology with Nalpha-Nsc-protected amino acids for the synthesis of some melanotropins important to our research, namely, gamma-melanocyte-stimulating hormone (gamma-MSH), its [Nle3]-analogue, and a cyclic alpha-MSH/beta-MSH hybrid. We developed an efficient protocol for the synthesis of the cyclic MSH analogue that yielded this peptide in >98% purity. The gamma-MSH synthesis, which gave problems with both the Boc and Fmoc strategies, yielded the desired peptide by Nsc-chemistry but was accompanied by side products. Finally, the Nle3-gamma-MSH analogue was synthesized more efficiently using the Fmoc strategy, suggesting that Nsc-chemistry might not be the best methodology for certain sequences.