Current Medicinal Chemistry - Volume 11, Issue 21, 2004

This special issue reviews applications of various types of molecular constraint to bioactive molecules, especially those with therapeutic potential. Constraints are used not only for improving their pharmacological profiles, including activities and receptor specificities, of existing bioactive peptides but also to provide information on the way in which these peptides bind to their receptor and thus aid in the development of non-peptide, orally-deliverable pharmaceuticals. Leading off, Lowell et al overview the application of conformational constraints as a strategy for improved drug design and this provides a background for the more specialized reviews that follow. Backbone methylation of peptide hormones has proven to be a powerful method for changing their receptor specificity and Sagan et al review these effects and the chemistry of synthesizing backbone-methylated peptide analogues. Reissmann and Imhof describe the application of constrained amino ands and different cyclization techniques to the development of conformationally restricted bradykinin and somatostatin analogues. Calcitonin and parathyroid hormone are two peptide hormones currently used in the treatment of bone diseases, particularly osteoporosis. Kapurniotu reviews the use of conformational constraints for understanding the structural and conformational requirements for calcitonin bioactivity and for deriving more powerful agonists. Finally, Willick et al review the application of cyclization to understanding the receptor action of parathyroid hormone, parathyroid hormone-related peptide, and the osteogenic growth peptide and the development of analogues with improved activities and specificities. These reviews should be useful not only to workers with the peptides described here, but also to those working with others of the many important bioactive peptides. Such work promises to be a rich source of therapeutics for the future.

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.

Numerous backbone constraints can be used to develop pseudopeptides or pseudomimetics of biologically active peptides. Among those, N- and Cα-methyl amino acids that can be incorporated by solidphase peptide synthesis in a bioactive sequence represent important tools to restrictϕ and ψ angles of peptide backbone. This review will focus on the chemical syntheses of N- and Cα-methyl amino acids, their effects on peptide conformation and structure, and their role on the peptide stability towards enzymatic degradation and on the biological activities of the resulting analogues.

The structure-based design of peptide drugs requires the knowledge of the bioactive conformation. Studies on this receptor-bound 3D structure require linear or cyclic analogues with strongly reduced flexibility, but high biological activity, since only analogues with retained potency have preserved the bioactive conformation. Constrained amino acids containing double bonds or bulky substituents at the Nα-, Cα- and Cβ-atom as well as at the aromatic ring atom were successfully applied to obtain potent and stable analogues of bradykinin and somatostatin, which due to their restricted conformation were suitable objects for conformational studies. Besides the generation of constrained cyclic analogues with improved biological and pharmacological properties, cyclic peptides were used as convenient models for the study of turn formations. Cyclization of the linear peptide bradykinin was performed by linking the N-terminus and the C-terminus, and in both bradykinin and somatostatin by cyclization using the amino acid side chains and by backbone cyclization. The later requires the introduction of Nα-functionalised amino acids for ring closure which can be performed either through incorporation of Nβ-functionalised amino acids or dipeptide building units. Conformational analysis of a cyclic bradykinin analogue by means of NMR-studies together with molecular dynamics simulation led to a quasicyclic 3D structure with two turns and together with other 3D structures provided a pharmacophore model of bradykinin antagonists.

Restricting the conformational flexibility of medium-sized linear polypeptides is a valuable approach to identify and characterize the structural and conformational features that define their biological activities and to design analogs with enhanced agonistic or antagonistic properties and with potential therapeutic applications. The calcium-regulating and bone resorption-inhibiting hormone calcitonin (Ct) is a conformationally flexible polypeptide of 32 amino acid residues that has long been applied therapeutically for the treatment of osteoporosis and other bone disorder diseases. This review describes studies on the structural and conformational features of the Ct sequence that are relevant for Ct bioactivity and focuses on research work performed on rationally designed conformationally constrained Ct analogs as tools for the understanding of the molecular basis of Ct bioactivity and as potential candidates or lead structures for novel Ct-based bone disorder therapeutics.

Osteogenic peptides are, or have potential to be, therapies for the treatment of osteoporosis, fracture repair, and repair of loosened bone implants. Human parathyroid hormone has been approved for the treatment of post-menopausal osteoporosis. Constrained analogs of PTH and the parathyroid-hormone related peptide (PTHrP) have aided the understanding of how PTH and PTHrP bind to their common receptor and some of these analogs have improved properties that make them possible candidates for clinical trial. Cyclization by lactam formation has shown that a core region of human PTH (hPTH) from residues 16-26 binds as an α-helix to the receptor and that the biological effects are remarkably sensitive to ring size. Appropriate cyclization in this region of the molecule not only has yielded analogs with improved receptor activation but also ones less susceptible to protease degradation and thus more active in vivo. Cyclization has been less successful in the Nterminus region, residues 1-12, of hPTH(1-34) with only a cyclization between residues 6 and 10 showing some promise. The growing understanding of how this region binds to the receptor will lead to other productive constraints. This review also covers the potential of a different class of molecule, the osteogenic growth peptide (OPG), as an anabolic bone agent. These molecules have much weaker anabolic effects than PTH and cyclization does not result in improved activity. However, the information gained from these studies may yield analogs with better pharmacological profiles.

Protein kinase C (PKC) is a family of serine / threonine kinases that regulates a variety of cell functions including proliferation, gene expression, cell cycle, differentiation, cytoskeletal organization, cell migration, and apoptosis. The PKC signal transduction cascade coordinates complex physiological events including normal tissue function and repair. Disruption of the cellular environment through genetic mutation, disease, injury, or exposure to pro-oxidants, alcohol, or other insults can induce pathological PKC activation. Aberrant PKC activation can lead to diseases of cellular dysregulation such as cancer and diabetes. Can aberrant activation of PKC be reversedα Even 25 years after the identification of PKC, therapeutic regulation of PKC activity remains an emerging field. Because the function of each isoform remains to be elucidated, isoform specific control of gene expression is a current challenge. Natural compounds are important regulators of PKC activity, with both preventive and therapeutic efficacy. Antioxidants including vitamin A (retinoids), vitamin C (ascorbic acid) and vitamin E (tocopherols) show promise for reversal of PKC activation. ß-carotene and retinoids function as anticarcinogenic agents and antagonize the biological effects of pro-oxidants on PKC. Vitamin E reverses the deleterious effects of hyperglycemia and diabetes by down-regulating PKC activity. Antioxidants in red wine provide cardioprotective effects. However, alcohol consumption also induces oxidative stress and disrupts PKC and retinoid function in the fetus and the adult. This review examines modulation of PKC activity by natural compounds and pharmacologic analogues which can be used effectively to prevent or treat common diseases associated with aberrant activation of PKC.

Haemoglobinopathies differ in geographic prevalence but together are amongst the most common genetic disorders worldwide. Despite huge diagnostic progress, therapeutic options remain limited, with many treatments still at the experimental stage, no more so than in pregnancy: not only does the presence of a fetus subject treatments to greater limitations, but also any worsening of the anaemia as pregnancy progresses results in higher fetomaternal morbidity and mortality. Anaemia weakens the response to peripartum blood loss, with the risk of postpartum complications. Until recently the standard conventional therapy for severe anaemia was (repeated) blood transfusion, with its well-known risks. Recombinant human erythropoietin (rhEPO) can induce fetal haemoglobin and is a safer, if less immediately effective, alternative for the correction of anaemia in pregnant patients with haemoglobinopathy.