Design and Synthesis of Angiotensin IV Peptidomimetics Targeting the Insulin-Regulated Aminopeptidase (IRAP)

Abstract: Peptidomimetics derived from the bioactive hexapeptide angiotensin IV (Ang IV, Val1-Tyr2-Ile3-His4-Pro5-Phe6) have been designed and synthesized. These peptidomimetics are aimed at inhibiting the insulin-regulated amino peptidase (IRAP), also known as the AT4 receptor. This membrane-bound zinc-metallopeptidase is currently under investigation regarding its potential as a target for cognitive enhancers. The work presented herein was based on stepwise replacement of the amino acid residues in Ang IV by natural and unnatural amino acids, non-peptidic building blocks, and also on the introduction of conformational constraints. Initially, we focused on the introduction of secondary structure mimetics and backbone mimetics. The C-terminal tripeptide His-Pro-Phe was successfully replaced by a ?-turn mimetic scaffold, 2-(aminomethyl)phenylacetic acid (AMPA), which was coupled via an amide bond to the carboxyl terminus of Val-Tyr-Ile. Substitution of Val-Tyr-Ile, Val-Tyr, Tyr-Ile and Tyr, respectively, by 4-hydroxydiphenylmethane scaffolds comprising a 1,3,5-substituted benzene ring as a central moiety unfortunately rendered peptidomimetics that were less potent than Ang IV. The subsequent approach involved the introduction of conformational constraints into Val-Tyr-Ile-AMPA by replacing Val and Ile by amino acid residues appropriate for disulfide cyclization or ring-closing metathesis. Chemically diverse structures encompassing an N-terminal 13- or 14-membered macrocyclic tripeptide and a C-terminal non-peptidic moiety were developed. Tyr2 and AMPA were modified to acquire further knowledge about the structure-activity relationships and, in addition, to improve the metabolic stability and reduce the polarity. Several of the compounds displayed a high capacity to inhibit IRAP and exhibited Ki values in the low nanomolar range. Hence, the new compounds were more than ten times more potent than the parent peptide Ang IV. Enhanced selectivity over the closely related aminopeptidase N (AP-N) was achieved, as well as improved stability against proteolysis by metallopeptidases present in the assays. However, additional investigations are required to elucidate the bioactive conformation(s) of the relatively flexible N-terminal macrocycles. The compounds presented in this thesis have provided important information on structure-activity relationships regarding the interaction of Ang IV-related pseudopeptides and peptidomimetics with IRAP. The best compounds in the series constitute important starting points for further discovery of Ang IV peptidomimetics suitable as tools in the investigation of IRAP and other potential targets for Ang IV. The literature presents strong support for the hypothesis that drug-like IRAP inhibitors would serve as a new type of future cognitive enhancers with potential use in the treatment of cognitive disorders, e.g. Alzheimer’s disease.