Synthesis of oligopeptide and peptidomimetic libraries

The elaboration of peptide libraries prepared by either chemical or biological methods in a format useful for discovery of peptides with specific biological activities was first introduced in the 1980s. A virtual explosion of activity in this area has occurred recently, and the basic approaches have been applied to a wide variety of chemistries and for all manner of biological, chemical and physical targets. Recent advances include new synthetic methodologies, new analytical methods, new design methods and new assay procedures.

Biological and conformational examination of stereochemical modifications using the template melanotropin peptide, Ac-Nle-c[Asp-His-Phe-Arg-Trp-Ala-Lys]-NH2, on human melanocortin receptors

Examination of conformationally constrained melanotropin peptide (Ac-Nle4-c[Asp5-His-Phe7-Arg-Trp9-Ala-Lys]-NH2) on four human melanotropin receptors (hMC1R, hMC3R, hMC4R, and hMC5R) resulted in identifying the importance of ligand stereochemistry at positions 5, 7, and 9 for agonist binding affinity and receptor selectivity. A trend in ligand structure-activity relationships emerged for these peptides, with the hMC1R and hMC4R possessing similar tendencies, as did the hMC3R and hMC5R. alpha-MSH (Ac-Ser-Tyr-Ser-Met4-Glu-His-Phe7-Arg-Trp-Gly-Lys-Pro-Val-NH2), NDP-MSH (Ac-Ser-Tyr-Ser-Nle4-Glu-His-D-Phe7-Arg-Trp-Gly-Lys-Pro-Val-NH2), and MTII (Ac-Nle4-c[Asp5,D-Phe7,Lys10]-alpha-MSH(4-10)-NH2) were also examined at each of these melanocortin receptors. Interestingly, the linear NDP-MSH possessed greater binding affinity for the hMC3R and hMC5R than did the cyclic analogue MTII. The peptide Ac-Nle-c[Asp-His-Phe-Arg-D-Trp9-Ala-Lys]-NH2 demonstrated the greatest differentiation in binding affinity between the hMC1R and hMC4R (78-fold). Analogue Ac-Nle-c[Asp-His-Phe7-Arg-Trp-Ala-Lys]-NH2 resulted in micromolar binding affinity (or greater) at the hMC3R and hMC5R, demonstrating the importance of D-Phe7 for ligand binding potency at these receptors. Ac-c[Asp-His-Phe-Arg-Trp-Ala-Lys]-NH2 resulted in loss of binding affinity at the hMC5R, implicating the importance of Nle4 (or a hydrophobic residue in this position) for binding to this receptor. Ac-Nle-c[D-Asp5-His-Phe-Arg-Trp-Ala-Lys]-NH2 was unable to competitively displace [125I]NDP-MSH binding at micromolar concentrations on the hMC3R and hMC5R, suggesting the importance of chirality of Asp5 either for ligand-receptor interactions or for orientation of the side chain lactam bridge and the structural integrity of the peptide conformation. Energy calculations performed for these peptides resulted in the identification of a low-energy ligand conformer family that is common to all the ligands. The differences in ligand binding affinities observed in this study are postulated to be a result of different ligand-receptor complexed interactions and not solely to the ligand structure.

Relative efficacies of delta-opioid receptor agonists at the cloned human delta-opioid receptor

The present study was conducted to determine the relative efficacies of the selective delta-opioid receptor agonists SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl )-3-methoxybenzyl]-N,N-diethylbenzamide), pCl-DPDPE (cyclic[D-Pen2,4'-ClPhe4,D-Pen5]enkephalin) and (-)-TAN67 ((-)-2-methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha-octahydro-quinolino-[2,3,3-g]isoquinoline). Experiments compared the abilities of the three drugs to competitively inhibit [3H]naltrindole binding and also stimulate [35S]GTPgammaS binding in membranes prepared from stably transfected Chinese hamster ovary (CHO) cells that express the cloned human delta-opioid receptor. Efficacy was determined according to the formula: efficacy = (E(max-A)/Emax)(A'/A + 1) X 0.5. Results show that SNC80 and pCl-DPDPE had efficacy values that were about 6-7 times greater than that of (-)-TAN67.

Structure-activity relationships of a series of [D-Ala2]deltorphin I and II analogues; in vitro blood-brain barrier permeability and stability

[D-Ala2]deltorphins are enzymatically stable, amphibian heptapeptides that have a higher affinity and selectivity for delta-opioid receptors than any endogenous mammalian compound known. This study investigated the in vitro blood-brain barrier permeability, using primary bovine brain microvessel endothelium culture, and the resistance to enzymatic degradation, in mouse 15% brain membrane homogenates and 100% plasma, of [D-Ala2]deltorphin I, [D-Ala2]deltorphin II and several analogues. Derivatives were designed with the addition of N-terminal neutral and basic amino acids or with alterations of the amino acids present within the deltorphin sequences. The results indicated that the N-terminal sequence and the amino acids in position 4 and 5 are critical to deltorphin analogue BBB permeability and biological stability, i.e., t 1/2 brain; 4.8 hr- [D-Ala2]deltorphin I; > 15 hr- [D-Ala2, Ser4, D-Ala5]deltorphin. Although, no analogue was found to increase the BBB permeability coefficient (PC; x10(-4) cm/min) of the parent compounds ([D-Ala2]deltorphin II, PC = 23.49 +/- 2.42) analogues were identified: [Arg0, D-Ala2]deltorphin II, PC = 19.06 +/- 3.73 and [Pro-1, Pro0, D-Ala2]deltorphin II, PC = 22.22 +/- 5.93; which had similar permeability coefficients, even though they had larger molecular weights and, in the case of the cationic prodrug, a significantly lower lipophilicity. These analogues provide directions in the development of future pro-drugs for the treatment of pain and this study further clarifies the structure-activity relationship of the deltorphins.

Peptide targeting and delivery across the blood-brain barrier utilizing synthetic triglyceride esters: design, synthesis, and bioactivity

As an approach to the development of therapeutically useful peptide pharmaceuticals that can penetrate the blood-brain barrier, we have designed and demonstrated the application of a carrier-targeting system. We have developed a prodrug design strategy that is designed to utilize membrane-bound enzymes whereby release of a bioactive peptide from a highly lipophilic triglyceride peptide-carrier is achieved in situ, thus attaining high localized concentrations of the bioactive peptide. Following localization of such a system, normal peptidase and lipase action is utilized to release the active peptide (deltorphin II) intact and in high concentration. At present, the exact mechanisms are unclear, but the observed results in which analgesia is observed following peripheral administration suggest that the active peptide is able to cross the blood-brain barrier and sustain prolonged periods of analgesia as determined by antinociception tests by release of the bioactive peptide. In vitro tests of binding and bioactivity by the peptide conjugate show essentially no potency in either target or control analogues, but potent antinociceptive effects are observed following peripheral administration.

Antipyretic role of endogenous melanocortins mediated by central melanocortin receptors during endotoxin-induced fever

Bacterial infection causes fever, an adaptive but potentially self-destructive response, in the host. Also activated are counterregulatory systems such as the pituitary-adrenal axis. Antipyretic roles have also been postulated for certain endogenous central neuropeptides, including the melanocortins (alpha-MSH-related peptides). To test the hypothesis that endogenous central melanocortins have antipyretic effects mediated by central melanocortin receptors (MCRs), we determined the effect of intracerebroventricular injection of a synthetic MCR antagonist, Ac-Nle4,c-[Asp5,DNal(2')7,Lys10]alpha-MSH(4-10)-NH2 (SHU-9119) in endotoxin-challenged rats. The efficacy and specificity of SHU-9119 as an MCR antagonist in the rat was first validated in vitro and in vivo. In vitro, in heterologous cells expressing either rat MC3-R or MC4-R, the major MCR subtypes expressed in brain, SHU-9119 showed no intrinsic agonism, but it inhibited alpha-MSH-induced cAMP accumulation (IC50 = 0.48 +/- 0.19 and 0.41 +/- 0.28 nM, respectively) and [125I]-[Nle4,DPhe7]-alpha-MSH binding (IC50 = 1.0 +/- 0.1 and 0.9 +/- 0.3 nM, respectively). In vivo, exogenous alpha-MSH (180 pmol) inhibited fever in rats when administered intracerebroventricularly 30 min after Escherichia coli lipopolysaccharide (LPS) (25 microg/kg, i.p.). When co-injected with alpha-MSH, SHU-9119 (168 pmol, i.c.v.) prevented the antipyretic action of exogenous alpha-MSH. In contrast, neither alpha-MSH nor SHU-9119, alone or in combination, affected body temperatures in afebrile rats. In LPS-treated rats, intracerebroventricular injection of SHU-9119 significantly increased fever, whereas intravenous injection of the same dose of SHU-9119 had no effect. Neither intracerebroventricular nor intravenous SHU-9119 significantly affected LPS-stimulated plasma ACTH or corticosterone levels. The results indicate that endogenous central melanocortins exert an antipyretic influence during fever by acting on MCRs located within the brain, independent of any modulation of the activity of the pituitary-adrenal axis.

Structure-activity studies of hydrophobic amino acid replacements at positions 9, 11 and 16 of glucagon

We have designed and synthesized eight compounds 2-9 which incorporate neutral, hydrophobic amino acid residues in positions 9, 11 and 16 of the glucagon molecule: (2) [desHis1, Val9. Ile11,16] glucagon amide, (3) [desHis1, Val9,11,16] glucagon amide, (4) [desHis1, Val9, Leu11,16]glucagon amide, (5) [desHis1, Nle9, Ile11,16]glucagon amide, (6) [desHis1, Nle9, Val11,16] glucagon amide, (7) [desHis1,-Nle9, Leu11,16] glucagon amide, (8) [desHis1, Val9, Leu11,16, Lys17,18, Glu21] glucagon amide and (9) [desHis1, Nle9, Leu11,16, Lys17,18, Glu21] glucagon amide. The effect of neutral, hydrophobic residues at positions 9, 11 and 16 led to good binding to the glucagon receptor. Compared to glucagon (IC50 = 1.5 nM), analogues 2-9 were found to have IC50 values of 6.0, 6.0, 11.0, 9.0, 2.5, 2.8, 6.5 and 7.0 nM, respectively. When these compounds were tested for their ability to block adenylate cyclase (AC) activity, they were found to be antagonists having no stimulation of adenyl cyclase, with pA2 values of 6.15, 6.20, 6.30, 7.25, 6.10, 7.30, 6.25 and 7.25, respectively.

Skin pigmentation and pharmacokinetics of melanotan-I in humans

A comparative pharmacokinetic trial was performed with a superpotent synthetic melanotropic peptide, [Nle4-D-Phe7]-alpha-MSHi-13 (melanotan-I or MT-I) given by three routes of administration. Plasma levels were measured by RIA and tanning was quantiated using serial reflectometry. Doses of 0.16 mgkg-1 were administered intravenously (IV) and orally (PO), and doses from 0.08 to 0.21 mg kg-1 subcutaneously (SC), in a randomized crossover fashion to three male volunteers over five consecutive days for 2 weeks (ten doses). The results indicate that the SC dose is completely bioavailable compared to the IV dose. No detectable drug levels were observed following PO dosing. The plasma half-lives following SC dosing ranged from 0.07 to 0.79 h for the absorption phase and from 0.8 to 1.7 h for the beta-phase. Clearance ranged from 0.12 to 0.19 L kg-1 h-1 and 3.9% or less of the dose was recovered in the urine. Side-effects were minimal, consisting of occasional gastrointestinal upset and facial flushing. Significant tanning of the forehead, arms, and neck was noted following IV or SC dosing. This effect peaked at 1 week following drug administration but was still present 3 weeks after completing the ten-dose regimen. It is concluded that SC administration is an efficacious method of delivering melanotan-I.

The entry of [D-penicillamine2,5]enkephalin into the central nervous system: saturation kinetics and specificity

The delta opioid receptor-selective, enzymatically stable peptide [D-Penicillamine2,5]enkephalin (DPDPE) has recently acquired special significance with the identification of a saturable uptake system for this analgesic into the CNS. The aim of the present study was to characterize further the entry of [3H]DPDPE into the brain and CSF by means of a bilateral in situ brain perfusion method. Initial experiments revealed a saturable [3H]DPDPE uptake into the brain that followed Michaelis-Menten type kinetics with a K(m) value of 45.5 +/- 27.6 microM, a V(max) value of 51.1 +/- 13.2 pmol x min(-1) x g(-1) and a K(d) value of 0.6 +/- 0.3 microl x min(-1) x g(-1). Uptake of [3H]DPDPE into the CSF could not be inhibited (K(d) = 0.9 +/- 0.1 microl x min(-1) x g(-1)). Entry of [3H]DPDPE into the CNS was not inhibited in the presence of 10 mM 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid (BCH) or 50 microM ICI 174,864, which suggests that the saturable mechanism does not involve the large neutral amino acid transporter or binding to opioid receptors. It would also appear that [3H]DPDPE is not in competition with either poly-L-lysine or insulin to enter the CNS. However, both of these substances significantly increased the CNS entry of [3H]DPDPE but not that of the vascular space marker [14C]sucrose, and this may have valuable clinical implications. It is not known at present which saturable uptake mechanism is responsible for the CNS entry of [3H]DPDPE, but overall the results suggest a carrier-mediated transport system.

Comparative biological activities of alpha-MSH antagonists in vertebrate pigment cells

We have previously reported that melatonin was an effective lightening agonist in the teleost Synbranchus marmoratus, the amphibians Rana pipiens and Bufo ictericus, and in the lizard Anolis carolinensis. The hormone, previously applied to the preparations, effectively inhibited alpha-MSH darkening activity in a dose-independent manner, and was also able to reverse MSH-induced darkening. We presently describe the inhibitory effect of the indoleamine on the murine melanoma cell proliferation. Interestingly, the hormone also stimulated tyrosinase activity, with a correlated increase in melanin content. We also demonstrate that in a diverse lizard species, Urosaurus ornatus, the indoleamine was totally ineffective. The competitive MSH antagonistic activity of H-His-D-Arg-Ala-Trp-D-Phe-Lys-NH2 has been demonstrated previously in R. pipiens and U. ornatus. Herein, its inhibitory activity is also reported in another lizard species, A. carolinensis. However, this MSH analogue was inactive in S. marmoratus, and in murine melanoma cells. On the other hand, the 7 thru 10 alpha-MSH fragment, Ac-Phe-Arg-Trp-Gly-NH2, although ineffective in S. marmoratus and R. pipiens, was an alpha-MSH antagonist in A. carolinensis. Surprisingly, in the melanoma cell line, the MSH fragment exhibited no agonist or antagonist activity, but dramatically potentiated the MSH-induced increase in tyrosinase activity. These data might suggest that the fragment is participating either in the process of facilitation or in positive cooperativity. The present results, taken together with our previously reported data, demonstrate a major interspecies diversity of the MC1 subtype of melanocortin receptor, and point out the relevance of the membrane microenvironment for the final receptor configuration.

Characterisation of D117A and H260A mutations in the melanocortin 1 receptor

Recent site directed mutagenesis studies on the melanocortin 1 (MC1) receptor have indicated the importance of D117 and H260 amino acid residues for the binding of alpha-MSH (melanocyte stimulating hormone). Here, we report the testing of 12 cyclic and linear MSH peptides on the D117A and H260A mutant receptors. Moreover, we constructed a double mutant which displayed a major loss in affinity for [Nle4, D-Phe7]alpha-MSH. Our new data of His6 and Phe7 substituted MSH peptides are compared with previous results and the hypothesis of putative interactions of D117 and H260 with single amino acids in the MSH peptide. Our conclusions are that the D117A and the H260A mutations may cause conformational changes in the receptor which can not be linked to any specific amino acid in the MSH-peptides.

Human epidermal melanocyte and keratinocyte melanotropin receptors: visualization by melanotropic peptide conjugated macrospheres (polyamide beads)

The objectives of this research were to determine whether melanotropin receptors are characteristic membrane markers of human epidermal melanocytes. Methodologies were developed to visualize these receptors by light microscopy. Multiple copies (up to a thousand) of [Nle4,D-Phe7] alpha-MSH, a superpotent analog of alpha-melanocyte stimulating hormone (alpha-MSH), were conjugated to a macromolecular carrier, large polyamide beads (macrospheres). Incubation in the presence of the I conjugated macrospheres resulted in binding of human epidermal melanocytes to the macrospheres. Specificity of the binding of melanocytes to the melanotropin-conjugated macrospheres was demonstrated by several studies: (i) Binding of melanocytes to the conjugate was specific since it could be blocked by prior incubation of the cells in the presence of the unconjugated hormone analog; (ii) The macrospheres after removal of the bound ligand did not bind to the melanocytes; (iii) Another peptide hormone ligand (e.g., a substance-P analog) attached to the macrospheres failed to bind to the melanocytes; (iv) B16/F10 mouse melanoma cells known to express melanotropin receptors bound to the macrospheres; (v) Cells of nonmelanocyte origin (e.g., mammary cancer cells, lung cancer cells, fibroblasts) did not bind to the macrospheres. One exception was that human epidermal keratinocytes also expressed melanotropin receptors as determined by all the criteria established for epidermal melanocytes. Thus, cell specific melanotropin receptors appear to be characteristic cell surface markers of epidermal melanocytes and keratinocytes.

Role of melanocortinergic neurons in feeding and the agouti obesity syndrome

Dominant alleles at the agouti locus (A) cause an obesity syndrome in the mouse, as a consequence of ectopic expression of the agouti peptide. This peptide, normally only found in the skin, is a high-affinity antagonist of the melanocyte-stimulating hormone receptor (MC1-R), thus explaining the inhibitory effect of agouti on eumelanin pigment synthesis. The agouti peptide is also an antagonist of the hypothalamic melanocortin-4 receptor (MC4-R). To test the hypothesis that agouti causes obesity by antagonism of hypothalamic melanocortin receptors, we identified cyclic melanocortin analogues that are potent agonists or antagonists of the neural MC3 (refs 11, 12) and MC4 receptors. Intracerebroventricular administration of the agonist, MTII, inhibited feeding in four models of hyperphagia: fasted C57BL/6J, ob/ob, and A(Y) mice, and mice injected with neuropeptide Y. Co-administration of the specific melanocortin antagonist and agouti-mimetic SHU9119 completely blocked this inhibition. Furthermore, administration of SHU9119 significantly enhanced nocturnal feeding, or feeding stimulated by a prior fast. Our data show that melanocortinergic neurons exert a tonic inhibition of feeding behaviour. Chronic disruption of this inhibitory signal is a likely explanation of the agouti obesity syndrome.

Blood-brain barrier permeability and bioavailability of a highly potent and mu-selective opioid receptor antagonist, CTAP: comparison with morphine

D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) is a cyclic, penicillamine-containing octapeptide that is structurally similar to somatostatin and displays greater antagonist potency and selectivity for mu-opioid receptors, compared with the classical mu-selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2. The aim of this study was to determine whether CTAP can enter the central nervous system (CNS) by crossing either the blood-brain barrier or the blood-cerebrospinal fluid barrier (CSF) and to characterize the mechanism of CNS entry. CNS entry of [3H]CTAP was compared with that of the vascular space marker [14C]inulin and the mu-agonist [3H]morphine. By using an in situ brain perfusion technique coupled to high-performance liquid chromatographic analysis, greater amounts of radioactivity were detected in the brain or CSF at most time points for [3H]CTAP, compared with [14C]inulin. [3H]CTAP was found to remain predominantly intact in the brain after a 20-min rat brain perfusion (62.8%). CTAP was also stable in the blood and serum of rats (T1/2 > 500 min), showing that the structure of this peptide offers enzymatic resistance. Additionally, [3H]CTAP was found to be extensively protein-bound to albumin in the perfusion medium (68.2%) and to proteins in rat serum (84.2%). Entry into the brain and CSF was not inhibited by the addition of unlabeled CTAP to the perfusion medium, suggesting that passage into the CNS is most likely through diffusion across the membranes that comprise the blood-brain barrier, rather than by saturable transport. Also, greater amounts of [3H]morphine entered both the brain and CSF after a 20-min brain perfusion, compared with [3H]CTAP. The increased CNS penetration observed for [3H]morphine, compared with [3H]CTAP, is likely due to the increased lipophilicity of morphine, as shown by its higher octanol/saline partition coefficient. Based on the pharmacokinetic profile, CTAP may be a promising mu-selective antagonist that can be used as a treatment for opiate overdose or addiction and also as a pharmacological tool to further understand opioid neurobiology.

Pure glucagon antagonists: biological activities and cAMP accumulation using phosphodiesterase inhibitors

Five new glucagon analogues have been designed, synthesized, characterized and their biological activities tested. The investigation was centered on modifications in the N-terminal region in particular, residues at Thr5, Phe6 and Tyr10 positions, with the goal of obtaining pure glucagon antagonists in our newly developed high sensitivity cAMP accumulation assay. The structures of the designed compounds are: [des-His1, des-Phe6, Glu9] glucagon-NH2 (1); [des-His1, des-Phe6, Glu9, Phe10]glucagon-NH2 (2); [des-His1, Tyr5, des-Phe6, Glu9]glucagon-NH2 (3); [des-His1, Phe5, des-Phe6, Glu9]glucagon-NH2 (4) and [des-His1, des-Phe6, Glu9, D-Arg18]glucagon-NH2 (5). The binding potencies IC50 values in (nM) were 48.0, 27.4, 26.0, 20.0 and 416.0, respectively. All of these analogues when tested in the classical adenylate cyclase assay demonstrate antagonist properties, and in competition experiments, all caused a rightward-shift of the glucagon stimulated adenylate cyclase dose-response curve. The pA2 values for these analogues were 8.20 (1); 6.25 (2); 6.10 (3); 6.25 (4); and 6.08 (5), respectively. A newly revised assay has been developed to determine the intracellular cAMP accumulation levels in hepatocytes at the highest possible sensitivity. Four of the five glucagon analogues in this report (analogues 1, 2, 4 and 5), did not activate the adenylate cyclase in the presence of Rolipram up to a maximal physiological concentration of 1 microM, and thus are pure antagonists.

Extraordinary potency of a novel delta opioid receptor agonist is due in part to increased efficacy

A new cyclic opioid peptide of sequence Tyr-D-Pen-Gly-Phe-Cys-Phe (HBP2) was examined in the mouse isolated vas deferens (MVD) bioassay. Studies with receptor-selective opioid antagonists showed the peptide to be highly selective for delta opioid receptors. HBP2 and the standard delta agonist DPDPE were simultaneously compared using the technique of partial irreversible receptor blockade; data were analyzed using the operational model of pharmacologic agonism. HBP2 was approximately 160 times as potent as DPDPE; estimation of the affinity and efficacy of the two peptides revealed that the potency increase was due to a 5.3-fold increase in efficacy, as well as a 37-fold increase affinity. This contrasts with our previous findings with other cyclic enkephalin analogs, in which increased affinity was achieved without a change in apparent efficacy. Analysis of concentration-response curve shape suggested in addition the possibility of heterogeneity in transduction mechanisms for MVD delta receptors.

Selectivity of cyclic [D-Nal7] and [D-Phe7] substituted MSH analogues for the melanocortin receptor subtypes

The binding of the 2 cyclic lactam MSH (4-10) analogues (MTII, SHU9119), and 5 cyclic [Cys4, Cys10] alpha-MSH analogues were tested on cells transiently expressing the human MC1, MC3, MC4 and MC5 receptors. The results indicate a differential importance of the C-terminal (Lys-Pro-Val) and N-terminal (Ser-Tyr-Ser) of cyclic [Cys4, Cys10] alpha-MSH analogues in binding to the MC receptor subtypes. Substitution of D-Phe7 by D-Nal(2')7 in both the cyclic lactam MSH (4-10) and the cyclic disulphide MSH (4-10) analogues resulted in a shift in favour of selectivity for the MC4 receptor; the disulphide analogue, [Cys4, D-Nal(2')7 Cys10] alpha-MSH (4-10) (HS9510), showing the highest selectivity for the MC4 receptor among all the substances tested. However, the cyclic lactams displayed an over all higher affinity for the MC receptors, than any of the cyclic disulphide MSH (4-10) analogues.

Structure-activity relationship of biphalin. The synthesis and biological activities of new analogues with modifications in positions 3 and 4

New analogues of biphalin [(Tyr-D-Ala-Gly-Phe-NH-)2] with modifications of amino acid residues in positions 3,3' and 4,4' have been synthesized. The potency and selectivity of these analogues were evaluated by competitive radioreceptor binding assay in the rat brain using [3H]CTOP (mu ligand) and [3H][p-Cl-Phe4]DPDPE (delta ligand) as ligands, and by bioassay in the mouse vas deferens (MVD, delta receptor assay) and guinea pig ileum (GPI, mu receptor assay). The symmetrical substitution of phenylalanine in positions 4 and 4' with p-fluorophenylalanine or p-nitrophenylalanine resulted in an enhancement of the affinity at both delta and mu receptors, with some increase of the selectivity for delta opioid receptors. The analogue containing p-chlorophenylalanine in positions 4 and 4' is the most selective to the delta receptors in this series, with a selectivity ratio about 5. The symmetrical substitution of the glycine-3 residue with phenylalanine resulted in a decrease of binding affinities and biological potencies at both mu & delta receptors.

Opioid peptides: simultaneous delta agonism and mu antagonism in somatostatin analogues

Four isomers of the Somatostatin analogue H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) were made with beta-MePhe in position 1 and assayed for opioid binding in rat brain, biological activity in MVD and GPI bioassays, and antinociception in mouse warm-water tail flick assays. The analogues displayed varying potencies and biological activities including: simultaneous delta receptor agonism/mu receptor antagonism, mu receptor antagonism, and delta receptor agonism. These analogues demonstrated that the N-terminal residue is important for receptor potency/selectivity and signal transduction. These analogues my represent leads to therapeutic agents that yield analgesia via delta agonist effects, yet lack side effects associated with mu activity.

Melanotropic peptide receptors: membrane markers of human melanoma cells

The objectives of this research were to determine whether melanotropin receptors are characteristic (constant) membrane markers of human melanoma cells. Methodologies were developed to visualize these receptors by fluorescence microscopy. Multiple copies (10-20) of both [Nle4,D-Phe7]alpha-MSH, a superpotent analog of alpha-melanocyte stimulating hormone (alpha-MSH), and a fluorophore, were conjugated to polyvinyl alcohol (PVA). Incubation in the presence of the multivalent macromolecular conjugate (FITC-PVA-MSH) resulted in binding of human epidermal melanocytes and keratinocytes and human melanoma cells (both melanotic and amelanotic) to the fluorescent conjugate. Binding of the conjugate to the cells exhibited a unique cluster pattern (capping) suggesting a receptor internalization related phenomenon. Most importantly, every cell of every melanoma cell line, melanotic or amelanotic, possessed receptors as visualized by fluorescence microscopy. Since the cells were not synchronized, some binding apparently took place during all phases of the cell cycle. Therefore, receptor expression appears not to be cell-cycle dependent. Specificity of binding of FITC-PVA-MSH was demonstrated by several studies. (i) Binding of the conjugate to melanoma cells could be blocked by prior incubation of the cells in the presence of the unconjugated hormone analog; [Nle4,D-Phe7]alpha-MSH. (ii) The macromolecular conjugate lacking bound ligand (FITC-PVA) did not bind to the melanoma cells. (iii) Another peptide, a substance-P analog, attached to the substrate (FITC-PVA-SP) failed to bind to the cells. (iv) With the exception of keratinocytes, other cells of nonmelanocyte origin (e.g., fibroblasts, spleen, liver, kidney cells, and mammary cancer cells, lung cancer cells) did not bind to the conjugate. Thus, cell-specific melanotropin receptors appear to be characteristic cell surface markers of epidermal melanocytes, keratinocytes, and melanoma cells. In several human melanoma cell lines these receptors appeared to be functional since [Nle4,D-Phe7]alpha-MSH stimulated tyrosinase activity. Fluorescent melanotropin conjugates might prove useful in determining whether all human melanoma (primary and metastatic) tumors possess such receptors. These receptors might then provide targets for melanotropic peptides for the identification, localization, and chemotherapy of melanoma.

Three-dimensional molecular models of the hMC1R melanocortin receptor: complexes with melanotropin peptide agonists

Three-dimensional molecular models of the human melanocortin receptor (hMC1R) have been developed based upon the electron cryo-microscopic structure of bacteriorhodopsin and the electron density footprint of bovine rhodopsin. alpha-Melanocyte-stimulating hormone, Ac-Ser-Tyr-Ser-Met4-Glu-His-Phe7-Arg-Trp-Gly-Lys-Pro-Val-NH2 (alpha-MSH, alpha-melanotropin), and the superpotent, prolonged acting agonists, Ac-Ser-Tyr-Ser-Nle4-Glu-His-DPhe7-Arg-Trp-Gly-Lys-Pro-Val-NH2 (NDP-MSH) and Ac-Nle4-c[Asp5-His6-DPhe7-Arg8-Trp9-Lys10]-NH2 (MTII), have been modeled into the proposed binding sites with specific ligand-receptor interactions identified. The melanotropin sidechain pharmacophores, DPhe7 and Trp9, are proposed to interact with a hydrophobic network of receptor aromatic residues in transmembrane regions 4, 5, 6, and 7. In addition, a hydrophilic network involving the ligand Arg8 and polar receptor residues located in transmembrane regions 2 and 3 were identified. Biological studies on alpha-MSH, NDP-MSH, MTII, and related peptides have been correlated with the proposed hMC1R model in terms of agonism, affinity, and prolongation. Finally, limited MC1R mutagenesis studies comparing alpha-MSH and NDP-MSH are interpreted within the context of the proposed hMC1R models.