Azapeptide Synthesis Methods for Expanding Side-Chain Diversity for Biomedical Applications

Mimicry of bioactive conformations is critical for peptide-based, medicinal chemistry because such peptidomimetics may augment stability, enhance affinity, and increase specificity. Azapeptides are peptidomimetics in which the alpha-carbon(s) of one or more amino acid residues are substituted by nitrogen. The resulting semicarbazide analogues have been shown to reinforce beta-turn conformation through the combination of lone pair lone pair repulsion of the adjacent hydrazine nitrogen and urea planarity. Substitution of a semicarbazide for an amino amide residue in a peptide may retain biological activity and add benefits such as improved metabolic stability. The applications of azapeptides include receptor ligands, enzyme inhibitors, prodrugs, probes, and imaging agents. Moreover, azapeptides have proven therapeutic utility. For example, the aza-glycinamide analogue of the luteinizing hormone-releasing hormone analogue Zoladex is a potent long-acting agonist currently used in the clinic for the treatment of prostate and breast cancer. However, the use of azapeptides was hampered by tedious solution-phase synthetic routes for selective hydrazine functionalization. A remarkable stride to overcome this bottleneck was made in 2009 through the introduction of the submonomer procedure for azapeptide synthesis, which enabled addition of diverse side chains onto a common semicarbazone intermediate, providing a means to construct azapeptide libraries by solution and solid-phase chemistry. In brief, aza residues are introduced into the peptide chain using the submonomer strategy by semicarbazone incorporation, deprotonation, N-alkylation, and orthogonal deprotection. Amino acylation of the resulting semicarbazide and elongation gives the desired azapeptide. Since the initial report, a number of chemical transformations have taken advantage of the orthogonal chemistry of semicarbazone residues (e.g., Michael additions and N-arylations). In addition, libraries have been synthesized from libraries by diversification of aza-propargylglycine (e.g., A(3) coupling reactions, [1,3]-dipolar cycloadditions, and S-exo-dig cyclizations) and aza-chloroalkylglycine residues. In addition, oxidation of aza-glycine residues has afforded azopeptides that react in pericyclic reactions (e.g., Diels-Alder and Alder-ene chemistry). The bulk of these transformations of aza-glycine residues have been developed by the Lubell laboratory, which has applied such chemistry in the synthesis of ligands with promising biological activity for treating diseases such as cancer and age-related macular degeneration. Azapeptide analogues of growth hormone-releasing peptide-6 (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, GHRP-6) have for example been pursued as ligands of the cluster of differentiation 36 receptor (CD36) and show promising activity for the development of treatments for angiogenesis-related diseases, such as age-related macular degeneration, as well as for atherosclerosis. Azapeptides have also been employed to make a series of conformationally constrained second mitochondria-derived activator of caspase (Smac) mirnetics that exhibit promising apoptosis-inducing activity in cancer cells. The synthesis of cyclic azapeptide derivatives was used to make an aza scan to study the conformation activity relationships of the anticancer agent cilengitide, cyclo(RGDf-N(Me)V), and its parent counterpart cyclo(RGDfV), which exhibit potency against human tumor metastasis and tumor-induced angiogenesis. Innovations in the synthesis and application of azapeptides will be presented in this Account, focusing on the creation and use of side-chain diversity in medicinal chemistry.