Design, synthesis, and evaluation of proline and pyrrolidine based melanocortin receptor agonists. A conformationally restricted dipeptide mimic approach.

The design, synthesis, and structure-activity relationships (SAR) of a series of novel proline and pyrrolidine based melanocortin receptor (MCR) agonists are described. To validate a conformationally constrained Arg-Nal dipeptide analogue strategy, we first synthesized and evaluated a test set of cis-(2R,4R)-proline analogues (21a-g). All of these compounds showed significant binding and agonist potency at the hMC1R, hMC3R, and hMC4R. Potent cis-(2S,4R)-pyrrolidine based MCR agonists (35a-g) were subsequently developed by means of this design approach. A SAR study directed toward probing the effect of the two chiral centers in the pyrrolidine ring on biological activity revealed the importance of the (S) absolute configuration at the 2-position for binding affinity, agonist potency, and receptor selectivity. Among the four sets of the pyrrolidine diastereomers investigated, analogues with the (2S,4R) configuration were the most potent agonists across the three receptors, followed by those possessing the (2S,4S) configuration.

Design and synthesis of potent and selective 1,3,4-trisubstituted-2-oxopiperazine based melanocortin-4 receptor agonists.

The design and synthesis of a series of potent 1,3,4-trisubstituted-2-oxopiperazine based MC4 agonists are described. The tripeptidomimetic analogs (12a,b and 23) and the dipeptidomimetic 27 displayed single-nanomolar binding affinity and agonist potency for MC4R and excellent selectivity for MC4R relative to MC1R.

Modifications of the human urocortin 2 peptide that improve pharmacological properties.

Recently, we demonstrated that the corticotropin releasing factor 2 receptor agonist, urocortin 2, demonstrated anti-atrophy effects in rodent skeletal muscle atrophy models. Compared to other CRF2R agonists however, the in vivo pharmacological potency of urocortin 2 is poor when it is administered by continuous subcutaneous infusion. Therefore, we attempted to modify the structure of urocortin 2 to improve in vivo efficacy when administered by subcutaneous infusion. By substituting amino acid residues in the linker region of urocortin 2 (residues 22-32), we have demonstrated improved in vivo potency without altering selectivity, probably through reduced CRFBP binding. In addition, attempts to shorten urocortin 2 generally resulted in inactive peptides, demonstrating that the 38 amino acid urocortin 2 peptide is the minimal pharmacophore.

Design, synthesis, and evaluation of proline based melanocortin receptor ligands.

A series of proline based melanocortin ligands has been developed on the basis of initial piperazine leads by using a more conformationally rigid scaffold. A number of these novel ligands showed significant binding affinity for MC3 and MC4 receptors.

Sauvagine analogs selective for corticotropin releasing factor 2 receptor: effect of substitutions at positions 35 and 39 on CRF2R selectivity.

Corticotropin releasing factor 2 receptor selective analogs of the amphibian peptide sauvagine, a member of the corticotropin releasing factor (CRF) peptide family, have therapeutic potential for the treatment of skeletal muscle atrophy. Previously, we demonstrated that [P11X12X13]Svg peptides have improved CRF2R selectivity, although not to the level of CRF2R selective hormones such as urocortin 2 and urocortin 3. Since we also demonstrated a potential for improvement in selectivity of sauvagine by modifications of residues 35 and 39, we investigated substitutions of these amino acids in selected [P11X12X13]Svg peptides. We have observed that substitution of Arg35 in sauvagine to Ala35 (the amino acid found in all CRF2R selective agonists), increased the selectivity of [P11, X12, X13]Svg analogs. In contrast, substitution of Asp39 in sauvagine to Ala39 (also the amino acid found in all CRF2R selective agonists) did not further increase the selectivity of [P11, X12, X13, A35]Svg analogs. Thus, the residues 35 along with 11, 12, and 13 in sauvagine represent important sites for improving CRF2R selectivity.

Discovery of corticotropin releasing factor 2 receptor selective sauvagine analogues for treatment of skeletal muscle atrophy.

The corticotropin release factor 2 receptor (CRF2R) has many biological activities including modulation of the stress response. Recently, we have demonstrated that CRF2R activation functions to prevent skeletal muscle wasting resulting from a variety of physiological stimuli. Thus we are interested in identifying CRF2R selective agonists with optimal pharmacological properties for use in treating muscle wasting diseases. Several CRF2R agonists are known including the frog peptide sauvagine (Svg), which display superior pharmacological properties compared to other CRF2R agonists. Unfortunately sauvagine is a nonselective CRFR agonist, thus making it of less utility due to side effects resulting from corticotropin release factor 1 receptor (CRF1R) activation. Because our initial modifications of Svg at position 11 improved CRF2R selectivity, we investigated the role of amino acids at positions 12 and 13 in Svg. We observed that phenylalanine, leucine, isoleucine, threonine, glutamine, histidine, and tyrosine at the 12th position were the strongest promoters of CRF2R selectivity whereas phenylalanine, glutamine, trytophane, tyrosine, valine, isoleucine, leucine, and 2-naphthylalanine were the preferred residues at the 13th position. Selective sauvagine peptides demonstrated improved antiatrophy effects in a mouse-casting model when compared to sauvagine itself. Thus, we demonstrate that the CRF2R selectivity can be improved by optimizing amino acids at positions 12 and 13 (all with proline at position 11) and that the selective sauvagine analogues demonstrate better in vivo efficacy than sauvagine itself.

Determinants of corticotropin releasing factor. Receptor selectivity of corticotropin releasing factor related peptides.

The corticotropin-releasing factor (CRF) peptide family is an important target in pharmaceutical research. The CRF system consists of two receptors, corticotropin releasing factor receptor 1 (CRF1R) and corticotropin releasing factor receptor 2 (CRF2R), a nonreceptor binding protein, and the peptide agonists of these receptors. The recent discovery of the CRF2R selective peptide agonists, UCN2, UCN3 and URP, prompted investigations into the structural source of CRF1R versus CRF2R selectivity of CRF peptide family members. Data from chimeric peptides demonstrated that amino acids in the N-terminus and C-terminus of CRF, UCN1, UCN2 and Sauvagine peptide families influence CRFR selectivity. Analysis of specific amino acid residues in the N-terminus and C-terminus demonstrated that the presence of a proline at position 11 and alanine at positions 35 and 39 (hCRF numbering) decreases CRF1R activity and increases CRF2R selectivity in CRF, UCN1 and sauvagine peptides. The availability of a large group of selective and nonselective CRF receptor peptide agonists will facilitate the development of CRF receptor selective drugs.

Novel tetrahydropyran-based peptidomimetics from a bioisosteric transformation of a tripeptide. Evidence of their activity at melanocortin receptors.

We have prepared novel peptidomimetics based on a 2,4,6-trisubstituted tetrahydropyran. This scaffold was constructed in an isosteric transformation using conceptual constraints imposed on a tripeptide moiety involving O(i)'-C(i+1)(gamma) and O(i)'-N(i+2) formal cyclization modes. A series of regioselective transformations commencing with a substituted dihydropyran-4-one readily provided the required analogues. Specific tetrahydropyrane analogues modeled on PheArgTrp as a truncated version of the melanocortin receptor message sequence, showed activity at the melanocortin receptors MC4R and MC1R. Thus, the 2,4,6-trisubstituted tetrahydropyran scaffold has provided a potentially useful peptidomimetic lead, and conceptual cyclization of peptide moieties can offer a valuable design strategy in peptidomimetic research.

Synthesis of 2,4,5-trisubstituted tetrahydropyrans as peptidomimetic scaffolds for melanocortin receptor ligands.

[reaction: see text] We have synthesized a series of 2,4,5-trisubstituted tetrahydropyran derivatives to determine the utility of this scaffold as a peptidomimetic platform. The key synthetic steps involved a palladium-mediated cross-coupling reaction of a dihydropyran-4-one moiety to introduce R(2) followed by a sequential regio- and diastereoselective reduction of sp(2) carbon centers. Selected compounds have shown biological activity at melanocortin receptors, indicating that this scaffold may be useful in the design of peptidomimetics relating to a tripeptide structure.

Chemoenzymic Approaches to the Preparation of 5-C-(Hydroxymethyl)hexoses.

Synthesis of 5-C-(hydroxymethyl)hexoses, carbohydrates resistant to metabolism, is described. These compounds are obtained in the reaction of hexose 6-aldehydes with formaldehyde. 5-C-(Hydroxymethyl)-L-arabino-hexopyranoses can be efficiently obtained from D-galactosides by a two-step chemoenzymic synthesis using galactose oxidase for the preparation of required hexose 6-aldehydes. This method is an example of carbohydrate synthesis without use of protecting groups. Other 5-C-(hydroxymethyl)hexoses are prepared by a typical chemical methodology requiring specific protection of the hexose hydroxyl groups.