Inhibitors Targeting Insulin-Regulated Aminopeptidase (IRAP) : Identification, Synthesis and Evaluation

Abstract: Insulin-regulated aminopeptidase (IRAP) has emerged as a potential new therapeutic target for treatment of cognitive disorders. Inhibition of the enzymatic activity facilitates cognition in rodents. Potent and selective peptide and pseudopeptide based inhibitors have been developed, but most of them suffer from poor pharmacokinetics and blood-brain-barrier penetration. Hence, development of less-complex inhibitors with good pharmacokinetic properties are of great importance.The aim of this thesis was to identify and optimize new small-molecule based IRAP inhibitors for use as research tools to investigate the cognitive effects of IRAP inhibition. Adaptation of an existing enzymatic assay into a screening compatible procedure allowed the evaluation of 10,500 compounds as IRAP inhibitors. The screening campaign resulted in 23 compounds displaying more than 60% inhibition. Two of these compounds, a spiro-oxindole dihydroquinazolinone and an imidazo[1,5-α]pyridine, were further investigated in terms of structure-activity relationship, physicochemical properties, metabolic stability, and mechanism of inhibition.Spiro-oxindole dihydroquinazolinone based IRAP inhibitors were synthesized via fast and simple microwave-promoted reactions, either in batch or in a continuous flow approach. The most potent compounds displayed sub-µM affinity, and interestingly an uncompetitive mode of inhibition with the synthetic substrate used in the assay. Molecular modeling confirmed the possibility of simultaneous binding of the compounds and the substrate. Furthermore, the molecular modeling suggested that the S-enantiomer accounts for the inhibitory effect observed with this compound series. The compounds also proved inactive on the closely related enzyme aminopeptidase N. Unfortunately, the spiro-oxindole based inhibitors suffered from poor solubility and metabolic stability.Imidazo[1,5-α]pyridine based IRAP inhibitors were synthesized via a five step procedure, providing inhibitors in the low-µM range. The stereospecificity of a methyl group proved important for inhibition. The compound series displayed no inhibitory activity on aminopeptidase N. Intriguing, these compounds exhibit a noncompetitive inhibition mechanism with the model substrate. As observed for the spiro-compounds, the imidazopyridines suffered from both poor solubility and metabolic stability.  In summary, the work presented in this thesis provide synthetic procedures, initial structure-activity relationship, and pharmacological evaluation of two distinct inhibitors classes. The compounds are among the first non-peptidic IRAP inhibitors presented, serving as interesting starting points in the development of research tools for use in models of cognition.