Synthesis and Crystallographic Study of N'-(1-benzylpiperidin-4-yl)acetohydrazide

As part of a study into new Fentanyl-derived opioid compounds with potent analgesic activity and reduced side effects the starting material title compound, C₁₄H₂₁N₃O (1), was synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The crystal structure is monoclinic Cc with unit cell parameters a = 14.1480(3) Å, b = 14.1720(4) Å, c = 27.6701(7) Å, β = 96.956(1)°, α = γ = 90°. The compound has crystallized with four crystallographically unique molecules in the asymmetric unit; each molecule has a very similar conformation and an analysis of the structure shows that although all four unique molecules overlay very well there is no evidence of pseudo-symmetry which would relate the molecules in the higher symmetry space group C2/c. The crystal packing consists of two separate hydrogen bonded chains which are linked together to form a thick 2D structure in the ab plane.

Solid-phase peptide head-to-side chain cyclodimerization: discovery of C(2)-symmetric cyclic lactam hybrid α-melanocyte-stimulating hormone (MSH)/agouti-signaling protein (ASIP) analogues with potent activities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the pharmacophores of the agouti-signaling protein (ASIP), an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. The designed hybrid ASIP/MSH pharmacophore was explored in monomeric cyclic, and cyclodimeric templates. The monomeric cyclic disulfide series yielded peptides with hMC3R-selective non-competitive binding affinities. The direct on-resin peptide lactam cyclodimerization yielded nanomolar range (25-120 nM) hMC1R-selective full and partial agonists in the cyclodimeric lactam series which demonstrates an improvement over the previous attempts at hybridization of MSH and agouti protein sequences. The secondary structure-oriented pharmacophore hybridization strategy will prove useful in development of unique allosteric and orthosteric melanocortin receptor modulators. This report also illustrates the utility of peptide cyclodimerization for the development of novel GPCR peptide ligands.

Biological and conformational evaluation of bifunctional compounds for opioid receptor agonists and neurokinin 1 receptor antagonists possessing two penicillamines

Neuropathic pain states and tolerance to opioids can result from system changes in the CNS, such as up-regulation of the NK1 receptor and substance P, lead to antiopioid effects in ascending or descending pain-signaling pathways. Bifunctional compounds, possessing both the NK1 antagonist pharmacophore and the opioid agonist pharmacophore with delta-selectivity, could counteract these system changes to have significant analgesic efficacy without undesirable side effects. As a result of the introduction of cyclic and topological constraints with penicillamines, 2 (Tyr-cyclo[d-Pen-Gly-Phe-Pen]-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)-Bzl]) was found as the best bifunctional compound with effective NK1 antagonist and potent opioid agonist activities, and 1400-fold delta-selectivity over the mu-receptor. The NMR structural analysis of 2 revealed that the relative positioning of the two connected pharmacophores as well as its cyclic and topological constraints might be responsible for its excellent bifunctional activities as well as its significant delta-opioid selectivity. Together with the observed high metabolic stability, 2 could be considered as a valuable research tool and possibly a promising candidate for a novel analgesic drug.

Synthesis and evaluation of new endomorphin-2 analogues containing (Z)-alpha,beta-didehydrophenylalanine (Delta(Z)Phe) residues

New endomorphin-2 (EM-2) analogues incorporating (Z)-alpha,beta-didehydrophenylalanine (Delta(Z)Phe) in place of the native phenylalanine in EM-2 are reported. Tyr-Pro-Delta(Z)Phe-Phe-NH(2) {[Delta(Z)Phe(3)]EM-2} (1), Tyr-Pro-Phe-Delta(Z)Phe-NH(2) {[Delta(Z)Phe(4)]EM-2} (2), and Tyr-Pro-Delta(Z)Phe-Delta(Z)Phe-NH(2) {[Delta(Z)Phe(3,4)]EM-2}(3) have been synthesized, their opioid receptor binding affinities and tissue bioassay activities were determined, and their conformational properties were examined. Compound 2 shows high mu opioid receptor selectivity and mu agonist activity comparable to those of the native peptide. The conformation adopted in solution and in the crystal by N-Boc-Tyr-Pro-Delta(Z)Phe-Phe-NH(2) (8) is reported.

Improving metabolic stability by glycosylation: bifunctional peptide derivatives that are opioid receptor agonists and neurokinin 1 receptor antagonists

In order to obtain a metabolically more stable analgesic peptide derivative, O-beta-glycosylated serine (Ser(Glc)) was introduced into TY027 (Tyr-d-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-3',5'-Bzl(CF(3))(2)) which was a previously reported bifunctional compound with delta/micro opioid agonist and neurokinin-1 receptor antagonist activities and with a half-life of 4.8 h in rat plasma. Incorporation of Ser(Glc) into various positions of TY027 gave analogues with variable bioactivities. Analogue 6 (Tyr-d-Ala-Gly-Phe-Nle-Pro-Leu-Ser(Glc)-Trp-NH-3',5'-Bzl(CF(3))(2)) was found to have effective bifunctional activities with a well-defined conformation with two beta-turns based on the NMR conformational analysis in the presence of DPC micelles. In addition, 6 showed significant improvement in its metabolic stability (70 + or - 9% of 6 was intact after 24 h incubation in rat plasma). This improved metabolic stability, along with its effective and delta selective bifunctional activities, suggests that 6 could be an interesting research tool and possibly a promising candidate as a novel analgesic drug.

Multiple N-methylation of MT-II backbone amide bonds leads to melanocortin receptor subtype hMC1R selectivity: pharmacological and conformational studies

Multiple N-methylation is a novel technology to improve bioavailability of peptides and increase receptor subtype selectivity. This technique has been applied here to the superpotent but nonselective cyclic peptide MT-II. A library of all possible 31 backbone N-methylated derivatives has been synthesized and tested for binding and activation at melanocortin receptor subtypes 1, 3, 4, and 5. It turned out that selectivity is improved with every introduced N-methyl group, resulting in several N-methylated selective and potent agonists for the hMC1R. The most potent of these derivatives is N-methylated on four out of five amide bonds in the cyclic structure. Its solution structure indicates a strongly preferred backbone conformation that resembles other alpha-MSH analogs but possesses much less flexibility and in addition distinct differences in the spatial arrangement of individual amino acid side chains.

Identification of novel pancreatic adenocarcinoma cell-surface targets by gene expression profiling and tissue microarray

Pancreatic cancer has a high mortality rate, which is generally related to the initial diagnosis coming at late stage disease combined with a lack of effective treatment options. Novel agents that selectively detect pancreatic cancer have potential for use in the molecular imaging of cancer, allowing for non-invasive determination of tumor therapeutic response and molecular characterization of the disease. Such agents may also be used for the targeted delivery of therapy to tumor cells while decreasing systemic effects. Using complementary assays of mRNA expression profiling to determine elevated expression in pancreatic cancer tissues relative to normal pancreas tissues, and validation of protein expression by immunohistochemistry on tissue microarray, we have identified cell-surface targets with potential for imaging and therapeutic agent development. Expression profiles of 2177 cell-surface genes for 28 pancreatic tumor specimens and 4 normal pancreas tissue samples were evaluated. Expression in normal tissues was evaluated using array data from 103 samples representing 28 organ sites as well as mining published data. One-hundred seventy unique targets were highly expressed in 2 or more of the pancreatic tumor specimens and were not expressed in the normal pancreas samples. Two targets (TLR2 and ABCC3) were further validated for protein expression by tissue microarray (TMA) based immunohistochemistry. These validated targets have potential for the development of diagnostic imaging and therapeutic agents for pancreatic cancer.

Design and synthesis of trivalent ligands targeting opioid, cholecystokinin, and melanocortin receptors for the treatment of pain

It has been known that co-administration of morphine with either cholecystokinin (CCK) receptor or melanocortin (MC) receptor antagonists enhance morphine's analgesic efficacy by reducing serious side effects such as tolerance and addiction. Considering these synergistic effects, we have designed trivalent ligands in which all three different pharmacophores for opioid, CCK, and MC receptors are combined in such a way as to conserve their own topographical pharmacophore structures. These ligands, excluding the cyclic compound, were synthesized by solid phase synthesis using Rink-amide resin under microwave assistance in very high yields. These trivalent ligands bind to their respective receptors well demonstrating that the topographical pharmacophore structures for the three receptors were retained for receptor binding. Ligand 10 was a lead compound to show the best biological activities at all three receptors.

Proton sharing and transfer in some zwitterionic compounds based on 4-oxo-4-((1-phenethylpiperidin-4-yl)(phenyl)amino)alcanoic acids

Three compounds, each derived from Fentanyl and differing essentially only in the length of a carboxylic acid chain, were synthesized and yielded four crystal structures three of which share several structural similarities, including the length of the chain, while the fourth, with a shorter chain, is quite different. The chain length has a significant influence on the crystal structures formed. The 'three atom' chain compounds are all solvated zwitterions which feature a hydrogen-bonded 'dimer' between adjacent zwitterions. The formation of this large dimer leaves available a second carboxylate O atom to take part in hydrogen bonding interactions with solvent molecules. The shorter 'two atom' chain compound was difficult to crystallize and required the use of synchrotron radiation to measure X-ray diffraction data. It does not form the same dimer motif observed in the 'three atom' chain compounds and has not formally formed a zwitterion; although there is evidence of proton sharing or disorder X-ray data are insufficient to create a disordered model, and the compound was modeled as formally neutral based on O-H and N-H distances. Room temperature analyses showed the proton transfer behavior to be independent of crystal temperature, and nuclear magnetic resonance studies show proton transfer behavior in solution. The formation of a zwitterionic hydrogen-bonded dimer is implicated in providing some stability during crystal growth of the easily crystallized 'three atom' chain compounds.

Organic chemistry and biology: chemical biology through the eyes of collaboration

From a scientific perspective, efforts to understand biology including what constitutes health and disease has become a chemical problem. However, chemists and biologists "see" the problems of understanding biology from different perspectives, and this has retarded progress in solving the problems especially as they relate to health and disease. This suggests that close collaboration between chemists and biologists is not only necessary but essential for progress in both the biology and chemistry that will provide solutions to the global questions of biology. This perspective has directed my scientific efforts for the past 45 years, and in this overview I provide my perspective of how the applications of synthetic chemistry, structural design, and numerous other chemical principles have intersected in my collaborations with biologists to provide new tools, new science, and new insights that were only made possible and fruitful by these collaborations.

Novel binding motif of ACTH analogues at the melanocortin receptors

The melanocortin receptor (MCR) subtype family is a member of the GPCR superfamily, and each of them has a different pharmacological profile with regard to the relative potency of the endogenous and synthetic melanocortin peptides. Alpha-MSH and ACTH are endogenous nonselective agonists for MC1R, MC3R, MC4R, and MC5R. In this study, we examined the role of Phe(7) in ACTH on human (h) MC1R, MC3R, and MC4R binding and signaling. Our results indicate that substitution of Phe(7) with d-Nal(2')(7) in ACTH1-24 yields a pharmacological profile different from that for substitution of Phe(7) with d-Nal(2')(7) in MSH in hMC1R, hMC3R, and hMC4R. N-d-Nal(2')(7)-ACTH1-24 is an agonist at hMC3R and hMC4R which did not change the peptide from an agonist to an antagonist at hMC3R and hMC4R. Further experiments indicate that N-d-Nal(2')(7)-ACTH1-17 is the minimal peptide required for hMC3R and hMC4R activation. Single-amino acid substitution studies of d-Nal(2')(7)-ACTH1-17 indicate that amino acid residues 15-17 in N-d-Nal(2')(7)-ACTH1-17 are crucial for hMC3R and hMC4R activation. Substitutions of these amino acid residues reduced or abolished agonist activity at hMC3R and hMC4R. Conformational studies revealed a new beta-turn (Arg(8)-Trp(9)-Gly(10)-Lys(11)) in N-d-Nal(2')(7)-ACTH1-17, compared to the beta-turn-like structure at NDP-alpha-MSH (His(6)-d-Phe(7)-Arg(8)-Trp(9)). Our results suggest that NDP-alpha-MSH and N-d-Nal(2')(7)-ACTH1-17 do not share the same binding site; the highly basic C-terminal fragment (Lys(15)-Lys(16)-Arg(17)) of N-d-Nal(2')(7)-ACTH1-17 induced a new beta-turn, and this shift contributed the selective agonist activity at hMC3R and hMC4R.

The biological activity and metabolic stability of peptidic bifunctional compounds that are opioid receptor agonists and neurokinin-1 receptor antagonists with a cystine moiety

In order to improve metabolic stability, a ring structure with a cystine moiety was introduced into TY027 (Tyr-D-Ala-Gly-Phe-Met-Pro-Leu-Trp-NH-[3',5'-(CF(3))(2)Bzl]), which is a lead compound of our developing bifunctional peptide possessing opioid agonist and NK1 antagonist activities. TY038 (Tyr-cyclo[D-Cys-Gly-Phe-Met-Pro-D-Cys]-Trp-NH-[3',5'-(CF(3))(2)Bzl]) was found as a highly selective delta opioid agonist over mu receptor in conventional tissue-based assays, together with an effective NK1 antagonist activity and good metabolic stability with more than 24h half life in rat plasma.

Solid-phase synthetic strategy and bioevaluation of a labeled delta-opioid receptor ligand Dmt-Tic-Lys for in vivo imaging

A general solid-phase synthetic strategy is developed to prepare fluorescent and/or lanthanide-labeled derivatives of the delta-opioid receptor (deltaOR) ligand H-Dmt-Tic-Lys(R)-OH. The high delta-OR affinity (K(i) = 3 nM) and desirable in vivo characteristics of the Cy5 derivative 1 suggest its usefulness for structure-function studies and receptor localization and as a high-contrast noninvasive molecular marker for live imaging ex vivo or in vivo.

Enhanced targeting with heterobivalent ligands

A novel approach to specifically target tumor cells for detection and treatment is the proposed use of heteromultivalent ligands, which are designed to interact with, and noncovalently crosslink, multiple different cell surface receptors. Although enhanced binding has been shown for synthetic homomultivalent ligands, proof of cross-linking requires the use of ligands with two or more different binding moieties. As proof-of-concept, we have examined the binding of synthetic heterobivalent ligands to cell lines that were engineered to coexpress two different G-protein-coupled human receptors, i.e., the human melanocortin 4 receptor (MC4R) expressed in combination with either the human delta-opioid receptor (deltaOR) or the human cholecystokinin-2 receptor (CCK2R). Expression levels of these receptors were characterized by time-resolved fluorescence saturation binding assays using Europium-labeled ligands; Eu-DPLCE, Eu-NDP-alpha-MSH, and Eu-CCK8 for the deltaOR, MC4R, and CCK2R, respectively. Heterobivalent ligands were synthesized to contain a MC4R agonist connected via chemical linkers to either a deltaOR or a CCK2R agonist. In both cell systems, the heterobivalent constructs bound with much higher affinity to cells expressing both receptors, compared with cells with single receptors or to cells where one of the receptors was competitively blocked. These results indicate that synthetic heterobivalent ligands can noncovalently crosslink two unrelated cell surface receptors, making feasible the targeting of receptor combinations. The in vitro cell models described herein will lead to the development of multivalent ligands for target combinations identified in human cancers.

Synthesis and evaluation of new endomorphin analogues modified at the Pro(2) residue

Six new endomorphin analogues, incorporating constrained amino acids in place of native proline have been synthesized. Residues of (S)-azetidine-2-carboxylic acid (Aze), 3,4-dehydro-(S)-proline (Delta(3)Pro), azetidine-3-carboxylic acid (3Aze) and dehydro-alanine (DeltaAla) have been used to prepare [Delta(3)Pro(2)]EM-2 (1), [Aze(2)]EM-1 (2), [Aze(2)]EM-2 (3), [3Aze(2)]EM-1 (4), [3Aze(2)]EM-2 (5) and [DeltaAla(2)]EM-2 (6). Binding assays and functional bioactivities for mu- and delta-receptors are reported. The highest affinity, bioactivity and selectivity are shown by peptides 2 and 3 containing the Aze residue.

Synthesis and investigations of double-pharmacophore ligands for treatment of chronic and neuropathic pain

Acids 9a-f as possible bivalent ligands designed as a structural combination of opioid mu-agonist (Fentanyl) and NSAID (Indomethacin) activities and produced compounds which were tested as analgesics. The obtained series of compounds exhibits low affinity and activity both at opioid receptors and as cyclooxygenase (COX) inhibitors. One explanation of the weak opioid activity could be stereochemical peculiarities of these bivalent compounds which differ significantly from the fentanyl skeleton. The absence of significant COX inhibitory properties could be explained by the required substitution of an acyl fragment in the indomethacin structure for 4-piperidyl.

Substitution of arginine with proline and proline derivatives in melanocyte-stimulating hormones leads to selectivity for human melanocortin 4 receptor

A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119, and Ac-NDP-gamma-MSH-NH(2) replaced by Pro or trans-/cis-4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a beta-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R and the development of therapeutic ligands selectively targeting the hMC4R.

3-[1-(4-Sulfamoylphenyl)-5-p-tolyl-1H-pyrazol-3-yl]propanoic acid and 3-[5-(4-bromophenyl)-1-(4-sulfamoylphenyl)-1H-pyrazol-3-yl]propanoic acid-dichloromethane-diethyl ether-water (2/0.72/1/1)

The syntheses of 3-[1-(4-sulfamoylphenyl)-5-p-tolyl-1H-pyrazol-3-yl]propanoic acid, C(19)H(19)N(3)O(4)S, (I), and 3-[5-(4-bromophenyl)-1-(4-sulfamoylphenyl)-1H-pyrazol-3-yl]propanoic acid-dichloromethane-diethyl ether-water (2/0.72/1/1), 2C(18)H(16)BrN(3)O(4)S.0.72CH(2)Cl(2).C(4)H(10)O.H(2)O, (II), are regiospecific. However, correct identification by spectroscopic techniques of the regioisomer formed is not trivial and single-crystal X-ray analysis provided the only means of unambiguous structure determination. Both structures make extensive use of hydrogen bonding and while compound (I) forms a straightforward unsolvated Z' = 1 structure, compound (II) crystallizes as an unusual mixed solvate, with two crystallographically unique molecules of the pyrazole derivative present in the asymmetric unit. The structure of (II) also features Br...Br interactions.

1-(4-Methoxy-phen-yl)-3-phenyl-1H-pyrazol-5-amine

The synthesis of the title compound, C₁₆H₁₅N₃O, is regiospecific and single-crystal X-ray diffraction provides the only means of unambiguous structural analysis, with the benzene ring bonded to the imine C atom. The phenyl ring and the essentially planar (r.m.s. deviation 0.0354 Å) methoxy­benzene group are rotated by 29.41 (5) and 37.01 (5)°, respectively, from the central pyrazole ring. An inter­molecular N—H⋯N hydrogen bond links symmetry-related mol­ecules into a C(5) chain, which runs parallel to the b axis.

3-[5-(4-Chlorophenyl)-1-(4-methoxyphenyl)-1H-pyrazol-3-yl]propionic acid and the corresponding methyl ester

The synthesis of 3-[5-(4-chlorophenyl)-1-(4-methoxyphenyl)-1H-pyrazol-3-yl]propionic acid, C(19)H(17)ClN(2)O(3), (I), and its corresponding methyl ester, methyl 3-[5-(4-chlorophenyl)-1-(4-methoxyphenyl)-1H-pyrazol-3-yl]propionate, C(20)H(19)ClN(2)O(3), (II), is regiospecific. However, correct identification of the regioisomer formed by spectroscopic techniques is not trivial and single-crystal X-ray analysis provided the only means of unambiguous structure determination. Compound (I) crystallizes with Z' = 2. The propionic acid groups of the two crystallographically unique molecules form a hydrogen-bonded dimer, as is typical of carboxylic acid groups in the solid state. Conformational differences between the methoxybenzene and pyrazole rings give rise to two unique molecules. The structure of (II) features just one molecule in the asymmetric unit and the crystal packing makes greater use than (I) of weak C-H...A interactions, despite the lack of any functional groups for classical hydrogen bonding.

Use of plasmon waveguide resonance (PWR) spectroscopy for examining binding, signaling and lipid domain partitioning of membrane proteins

AIMS

Due to their anisotropic properties and other factors, it has been difficult to determine the conformational and dynamic properties of integral membrane proteins such as G-protein coupled receptors (GPCRs), growth factor receptors, ion channels, etc. in response to ligands and subsequent signaling. Herein a novel methodology is presented that allows such studies to be performed while maintaining the receptors in a membrane environment.

MAIN METHOD

Plasmon waveguide resonance (PWR) spectroscopy is a relatively new biophysical method which allows one to directly observe structural and dynamic changes which occur on interaction of GPCRs (and other integral membrane proteins) with ligands and signaling molecules. The delta opioid receptor (DOR) and its ligands serve as an excellent model system to illustrate the new insights into GPCR signaling that can be obtained by this method.

KEY FINDINGS

Among our key findings are: 1) it is possible to obtain the following information directly and without any need for labels (radioactive, fluorescent, etc.): binding affinities, and the ability to distinguish between agonists, antagonists, inverse agonist, and partial agonists without a need for second messenger analysis; 2) it is possible to determine directly, again without a need for labels, G-protein binding to variously occupied or unoccupied DORs, and to determine which alpha-subtype is involved in allowing structurally different agonist ligands to have differential effects; 3) GTPgammaS binding can be examined directly; and 4) binding of the DOR with different ligands leads to differential segregation of the ligand-receptor complex into lipid rafts.

SIGNIFICANCE

The implications of these discoveries suggest a need to modify our current views of GPCR-ligand interactions and signaling.

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.

Melanotropins as drugs for the treatment of obesity and other feeding disorders: potential and problems

Current biological and pharmacological evidence suggests that the melanocortin 4 and melanocortin 3 receptors which are seven transmembrane G-protein coupled receptors (GPCRs) are involved in various aspects of energy balance and feeding behaviors in animals including humans. The natural endogenous ligands for these receptors are products of the gene pro-opiomelanocortin (POMC), and include alpha-melanocyte stimulating hormone, gamma-melanocyte stimulating hormone and perhaps other modified products of POMC. Thus well designed agonists and antagonists of these ligands might serve as drugs for the treatment of feeding disorders. However, these melanotropin peptides also can have other biological activities that involve the MC3R and MC4R, and these other biological properties will need to be modulated in ligands that are likely to be useful drugs for feeding disorders. Current progress in these areas with special emphasis on the MC3R will be discussed along with possible new directions that might be fruitful in these important aspects of contemporary biology and medicine.