Phenylguanidines as selective nonpeptide melanocortin-5 receptor antagonists

Guanidine Derivatives Containing the Chalcone Skeleton Are Potent Antiproliferative Compounds against Human Leukemia Cells

In this study, we investigated the effects of eleven synthetic guanidines containing the 1,3-diphenylpropenone core on the viabilities of six human cancer cells. The most cytotoxic compound against human cancer cells of this series contains a N-tosyl group and a N-methylpiperazine moiety 6f. It was cytotoxic against leukemia cells (U-937, HL-60, MOLT-3, and NALM-6) with significant effects against Bcl-2-overexpressing U-937/Bcl-2 cells as well as the human melanoma SK-MEL-1 cell line. It exhibited low cytotoxicity against quiescent or proliferating human peripheral blood mononuclear cells. The IC₅₀ value for the leukemia U-937 cells was 1.6 ± 0.6 µM, a similar value to that in the antineoplastic agent etoposide. The guanidine containing a N-phenyl substituent 6i was also as cytotoxic as the guanidine containing the N-tosyl substituent and the N-methylpiperazine group 6f against human U-937 leukemia cells and both synthetic guanidines were potent apoptotic inducers. Cell death was mediated by the activation of the initiator caspase-9 and the executioner caspase-3, and associated with the release of cytochrome c. These synthetic guanidines are potent cytotoxic compounds against several human leukemia cells and even the human melanoma cell line SK-MEL-1 and might be useful in the development of new strategies in the fight against cancer.

De novo design and synthesis of dipyridopurinone derivatives as visible-light photocatalysts in productive guanylation reactions

Described here is the de novo design and synthesis of a series of 6H-dipyrido[1,2-e:2',1'-i]purin-6-ones (DPs) as a new class of visible-light photoredox catalysts (PCs). The synthesized DP1-5 showed their λ _Abs(max) values in 433-477 nm, excited state redox potentials in 1.15-0.69 eV and -1.41 to -1.77 eV (vs. SCE), respectively. As a representative, DP4 enables the productive guanylation of various amines, including 1°, 2°, and 3°-alkyl primary amines, secondary amines, aryl and heteroaryl amines, amino-nitrile, amino acids and peptides as well as propynylamines and α-amino esters giving diversities in biologically important guanidines and cyclic guanidines. The photocatalytic efficacy of DP4 in the guanylation overmatched commonly used Ir and Ru polypyridyl complexes, and some organic PCs. Other salient merits of this method include broad substrate scope and functional group tolerance, gram-scale synthesis, and versatile late-stage derivatizations that led to a derivative 81 exhibiting 60-fold better anticancer activity against Ramos cells with the IC₅₀ of 0.086 μM than that of clinical drug ibrutinib (5.1 μM).

The Melanocortin Receptor System: A Target for Multiple Degenerative Diseases

The melanocortin receptor system consists of five closely related G-protein coupled receptors (MC1R, MC2R, MC3R, MC4R and MC5R). These receptors are involved in many of the key biological functions for multicellular animals, including human beings. The natural agonist ligands for these receptors are derived by processing of a primordial animal gene product, proopiomelanocortin (POMC). The ligand for the MC2R is ACTH (Adrenal Corticotropic Hormone), a larger processed peptide from POMC. The natural ligands for the other 4 melanocortin receptors are smaller peptides including α-melanocyte stimulating hormone (α-MSH) and related peptides from POMC (β-MSH and γ-MSH). They all contain the sequence His-Phe-Arg-Trp that is conserved throughout evolution. Thus, there has been considerable difficulty in developing highly selective ligands for the MC1R, MC3R, MC4R and MC5R. In this brief review, we discuss the various approaches that have been taken to design agonist and antagonist analogues and derivatives of the POMC peptides that are selective for the MC1R, MC3R, MC4R and MC5R receptors, via peptide, nonpeptide and peptidomimetic derivatives and analogues and their differential interactions with receptors that may help account for these selectivities.

Structure-activity relationships of cyclic lactam analogues of alpha-melanocyte-stimulating hormone (alpha-MSH) targeting the human melanocortin-3 receptor

A variety of dicarboxylic acid linkers introduced between the alpha-amino group of Pro(6) and the -amino group of Lys(10) of the cyclic lactam alpha-melanocyte-stimulating hormone (alpha-MSH)-derived Pro(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)-Lys(10)-NH2 pentapeptide template lead to nanomolar range and selective hMC3R agonists and antagonists. Replacement of the Pro(6) residue and the dicarboxylic acid linker with 2,3-pyrazine-dicarboxylic acid furnished a highly selective nanomolar range hMC3R partial agonist (analogue 12, c[CO-2,3-pyrazine-CO-D-Phe-Arg-Trp-Lys]-NH2, EC50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a beta-turn-like structure with the D-Phe/D-Nal(2') residues, which explains the high potency of the corresponding peptides. Placement of a Nle residue in position 6 produced a hMC3R/hMC5R antagonist (analogue 15, c[CO-(CH 2)2-CO-Nle-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 12 and 17 nM, respectively), similarly to the previously described cyclic gamma-melanocyte-stimulating hormone (gamma-MSH)-derived hMC3R/hMC5R antagonists. These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-3 receptor.

Functional characterization of the modified melanocortin peptides responsible for ligand selectivity at the human melanocortin receptors

The melanocortin system plays an important role in energy homeostasis as well as skin pigmentation, steroidogenesis and exocrine gland function. In this study, we examined eight Ac-His-Phe-Arg-Trp-NH(2) tetrapeptides that were modified at the Phe position and pharmacologically characterized their activities at the human MCR wild-types and their mutants. Our results indicate that at the hMC1R, all D stereochemical modified residues at the Phe position of peptides increase cAMP production in a dose-dependent manner. At the hMC3R, the DPhe peptide dose dependently increases cAMP production but all other three tetrapeptides were not. At the hMC4R, both the DPhe and DNal(1') peptides induce cAMP production. However, both DTyr and DNal(2') were not able to induce cAMP production. Further studies indicated that at the hMC1R M128L mutant receptor, the all D-configured tetrapeptides reduce their potencies as compared to that of hMC1R wild-type. However, at the hMC3R and hMC4R L165M and L133M mutant receptors, the DNal(2') and DTyr tetrapeptides possess agonist activity. These findings indicate that DPhe in tetrapeptide plays an important role in ligand selectivity and specific residue TM3 of the melanocortin receptors is crucial for ligand selectivity.

Design of cyclic peptides with agonist activity at melanocortin receptor-4

A series of cyclic pentapeptides, c(His-D-Phe-Arg-Trp-Z) (Z=omega-amino acid), were prepared and biologically evaluated. The effects of increasing alkyl chain length of omega-amino acid on the functional activities and the receptor binding affinities for human melanocortin receptors (hMC-Rs) were studied. Compound 2 was an agonist for hMC-4R with an EC50 value of 15.4 nM, which was 4.7 times more potent than that of alpha-MSH. Compound 2 also showed a 4.3-fold higher hMC-4R selectivity over hMC-1R, thus providing us with information concerning size and chemical structure of the lactam ring for the development of the agonist with hMC-4R selectivity.

A chemogenomic analysis of the transmembrane binding cavity of human G-protein-coupled receptors

The amino acid sequences of 369 human nonolfactory G-protein-coupled receptors (GPCRs) have been aligned at the seven transmembrane domain (TM) and used to extract the nature of 30 critical residues supposed--from the X-ray structure of bovine rhodopsin bound to retinal--to line the TM binding cavity of ground-state receptors. Interestingly, the clustering of human GPCRs from these 30 residues mirrors the recently described phylogenetic tree of full-sequence human GPCRs (Fredriksson et al., Mol Pharmacol 2003;63:1256-1272) with few exceptions. A TM cavity could be found for all investigated GPCRs with physicochemical properties matching that of their cognate ligands. The current approach allows a very fast comparison of most human GPCRs from the focused perspective of the predicted TM cavity and permits to easily detect key residues that drive ligand selectivity or promiscuity.

Potent and selective MC-4 receptor agonists based on a novel disulfide scaffold

Extensive structure-activity relationship studies utilizing a beta-MSH-derived cyclic nonapeptide, Ac-Tyr-Arg-[Cys-Glu-His-D-Phe-Arg-Trp-Cys]-NH(2) (3), led to identification of a series of novel MC-4R selective disulfide-constrained hexapeptide analogs including Ac-[hCys-His-D-Phe-Arg-Trp-Cys]-NH(2) (12). The structural modifications associated with profound influence on MC-4R potency and selectivity were ring size, ring conformation, and the aromatic substitution of the D-Phe7. These cyclic peptide analogs provide novel and enhanced reagents for use in the elucidation of melanocortin-4 receptor-related physiology, and may additionally find application in the treatment of obesity and related metabolic disorders.