Synthesis and biological evaluation of conformationally restricted derivatives of tryptophan as NK1/NK2 ligands

The Neurokinins: Peptidomimetic Ligand Design and Therapeutic Applications

The neurokinins are indisputably essential neurotransmitters in numerous pathoand physiological events. Being widely distributed in the Central Nervous System (CNS) and peripheral tissues, their discovery rapidly promoted them to drugs targets. As a necessity for molecular tools to understand the biological role of this class, endogenous peptides and their receptors prompted the scientific community to design ligands displaying either agonist and antagonist activity at the three main neurokinin receptors, called NK1, NK2 and NK3. Several strategies were implemented for this purpose. With a preference to small non-peptidic ligands, many research groups invested efforts in synthesizing and evaluating a wide range of scaffolds, but only the NK1 antagonist Aprepitant (EMENDT) and its prodrug Fosaprepitant (IVEMENDT) have been approved by the Food Drug Administration (FDA) for the treatment of Chemotherapy-Induced and Post-Operative Nausea and Vomiting (CINV and PONV, respectively). While non-peptidic drugs showed limitations, especially in side effect control, peptidic and pseudopeptidic compounds progressively regained attention. Various strategies were implemented to modulate affinity, selectivity and activity of the newly designed ligands. Replacement of canonical amino acids, incorporation of conformational constraints, and fusion with non-peptidic moieties gave rise to families of ligands displaying individual or dual NK1, NK2 and NK3 antagonism, that ultimately were combined with non-neurokinin ligands (such as opioids) to target enhanced biological impact.

A flexible approach to the design of new potent substance P receptor ligands

The development of small-molecule antagonists of the substance-P-preferring tachykinin NK1 receptor offers an excellent opportunity to exploit these molecules as novel therapeutic agents in diverse pathologies such as depression, emesis or asthma. GR71251 has previously been identified as a potent and selective substance-P-receptor antagonist. We have therefore undertaken the synthesis of new pseudopeptidic analogues based on the C-terminal sequence of GR71251. The evaluation of binding affinities toward NK1 and NK2 receptors has enabled us to propose new selective NK1 ligands with high affinity. Structure-activity relationships showed that the Trp-OBzl(CF3)2 moiety is essential for NK1 affinity and that the introduction of building units such as spirolactam, lactam or proline, leading to a constrained peptide, increased selectivity for NK1 receptors. These compounds constitute a useful starting point for new substance P antagonists and represent an attractive lead series for further studies on the design of specific NK1 antagonists.

Conformation of the tripeptide Cbz-Pro-Leu-Trp-OBzl(CF3)2 deduced from two-dimensional 1H-NMR and conformational energy calculations is related to its affinity for NK1-receptor

Chemical modifications of dual NK1/NK2 ligand Cbz-Gly-Leu-Trp-OBzl(CF3)2 (1) enabled us to create a high NK1 selective ligand Cbz-Pro-Leu-Trp-OBzl(CF3)2 (2). A determination of the conformational behavior of tripeptide 2 in solution is described. The 1D and 2D 1H-NMR techniques (COSY and ROESY) were used to assign resonances. Observed interproton distance restraints were considered to characterize conformational behavior. Spectral data indicate that tripeptide 2 presents a rigidified structure in DMSO stabilized by H-bond in two gamma-turns. Agreement with experimental data was obtained by averaging the 1H-NMR parameters over several combinations of low-energy conformations.