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.

The major pharmacophore for the melanocortin 1,3,4 and 5 receptors is the sequence -His-Phe-Arg-Trp-. There is a need for potent, biologically stable, receptor selective ligands, both agonists and antagonists, for these receptors. In this report we briefly examine the structural and biophysical approaches we have taken to develop selective agonist and antagonist ligands that can cross (or not) the blood brain barrier. Remaining questions and unmet needs are also discussed.

A new bicyclic template has been developed for the synthesis of peptide mimetics. Straightforward synthetic steps, starting from amino acids, allow the facile construction of a wide range of analogs. This system was designed to target the melanocortin receptors (MCRs), with functional group selection based on a known pharmacophore and guidance from molecular modeling to rationally identify positional and stereochemical isomers likely to be active. The functions of hMCRs are critical to myriad biological activities, including pigmentation, steroidogenesis, energy homeostasis, erectile activity, and inflammation. These G-protein-coupled receptors (GPCRs) are targets for drug discovery in a number of areas, including cancer, pain, and obesity therapeutics. All compounds from this series tested to date are antagonists which bind with high affinity. Importantly, many are highly selective for a particular MCR subtype, including some of the first completely hMC5R-selective antagonists reported. Keywords Melanocortins; Peptide mimetics; GPCRsThe human melanocortin receptors (hMCRs) comprise a family of five Type I, or rhodopsinlike, G-protein-coupled receptors (GPCRs) to which a wide array of biological functions has been ascribed. 1 Some examples include nociception, inflammation, energy balance, and sexual function. From the early understanding of the role MCRs play in pigmentation to recent revelations concerning their relevance to pain, new studies have continually uncovered crucial but previously unknown actions of this receptor system. Beyond advancing our knowledge of basic biology, the understanding and modulation of MCR function also has clinical relevance, with potential therapeutic value for addressing obesity, 2 cachexia, 3 pain, 4 inflammatory diseases, 5 and sexual dysfunction, 6 as well as the diagnosis and treatment of certain cancers. 7Much research to date has relied on natural and synthetic peptide ligands for these receptors. The MCRs are unique in that both endogenous agonists (α-, β-, γ-MSH, ACTH) and antagonists (agouti, AGRP) for the system have been discovered. 8 Each of the agonists contains the HisPhe-Arg-Trp tetrad, the minimum sequence necessary for activation of all melanocortin receptors. 9 Both endogenous antagonists contain an Arg-Phe-Phe sequence. Extensive melanotropin peptide structure-activity relationship (SAR) studies by our group and others have identified modifications which enhance potency, stability, or selectivity. 10 The value of

A variety of dicarboxylic acid linkers introduced between the α-amino group of Pro 6 and the ε-amino group of Lys 10 of the cyclic lactam α-melanocyte-stimulating hormone (α-MSH)-derived Pro 6 -DPhe7/D-Nal(2′) 7 -Arg 8 -Trp 9 -Lys 10 -NH 2 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]-NH 2 , EC 50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2′)-Arg-Trp-Lys]-NH 2 , IC 50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a β-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]-NH 2 , IC 50 = 12 and 17 nM, respectively), similarly to the previously described cyclic γ-melanocyte-stimulating hormone (γ-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.

In our ongoing research on the synthesis of constrained analogues of CCK/opioid chimeric peptides, a bicyclic dipeptide mimetic for Nle-Asp was designed and synthesized. Starting from β-allyl substituted aspartic acids, the terminal double bond was oxidized resulting in spontaneous cyclization to form racemic hemiaminals. Allylation of the hemiaminals afforded 5-allyl substituted proline analogues, which on oxidation, Horner-Emmons olefination, asymmetric hydrogenation, and bicyclization afforded bicyclic dipeptide mimetics for Nle-Asp. Constrained CCK/opioid peptide analogues containing bicyclic dipeptide mimetics for Nle-Gly, Nle-Asp, and homoPhe-Gly were then synthesized and analyzed at both the CCK and opioid receptors.Our group has recently been involved in the design and synthesis of chimeric peptides that interact with both the CCK and opioid receptors. 1 Constrained analogues of these peptides have been synthesized via disulfide and lactam cyclizations. 2 To further explore the topographical requirements for interaction of these peptides with receptors, bicyclic dipeptide mimetics for Nle-Gly (1a, 1b, 1c), and homoPhe-Gly 2 have been designed and synthesized (Fig. 1). 3 In this letter, the design and synthesis of indolizidinone bicyclic dipeptide mimetics for Nle-Asp 3 and the synthesis of peptides containing bicyclic dipeptide mimetics are discussed. The peptides were tested at both the CCK and opioid receptors.The synthesis of the indolizidinone type of bicyclic dipeptide mimetics has been reported by a number of authors. 4 In our lab, we have developed the synthesis of these mimetics from analogues of pyroglutamic acid. 5 The target compound can be obtained from lactam cyclization of dehydroamino acids derived from Horner-Emmons olefination of allyl substituted proline analogues.Alkylation of aspartic acid with different electrophiles has been reported by our group 3,6 and other authors. 7 Alkylation with allyl bromide in the presence of lithium bis(trimethylsilyl) amide (LHMDS) and HMPA resulted in the formation of two β-allyl substituted aspartic acids in a total yield of 57% and a ratio of 4:1 in favor of the (2 S, 3R)-5a isomer (Scheme 1).When 5a was subjected to ozonolysis, the resultant aldehyde spontaneously cyclized to the racemic hemiaminal 6 (Scheme 2). The hydroxyl group was then methylated and the resultant compound 7 reacted with BF 3 ·OEt 2 and allyl trimethyl silane at −78 °C→rt to give compounds 8a and 8b in 48% yield and a ratio of 1:1. When the minor isomer 5b was subjected to ozonolysis and allylation, only compound 8c was obtained in 51% yield. The structure of compound 8a was confirmed by X-ray crystallography (Fig. 2).Compound 8a was then subjected to ozonolysis and the resultant aldehyde subjected to a Horner-Emmons olefination 8 to give the dehydroamino acid 9a in 58% yield. When osmylation was used for the oxidation of 8a, the dehydroamino acid was obtained in 71% yield (Scheme 3). Oxidation of compound 8b by ozonolysis followed by Horner-Emmons olefination gave the de...