Inhibition of Insulin-Regulated Aminopeptidase by Imidazo [1,5-α]pyridines—Synthesis and Evaluation

Abstract

Inhibition of insulin-regulated aminopeptidase (IRAP) has been shown to improve cognitive functions in several animal models. Recently, we performed a screening campaign of approximately 10,000 compounds, identifying novel small-molecule-based compounds acting as inhibitors of the enzymatic activity of IRAP. Here we report on the chemical synthesis, structure-activity relationships (SAR) and initial characterization of physicochemical properties of a series of 48 imidazo [1,5-α]pyridine-based inhibitors, including delineation of their mode of action as non-competitive inhibitors with a small L-leucine-based IRAP substrate. The best compound displays an IC50 value of 1.0 µM. We elucidate the importance of two chiral sites in these molecules and find they have little impact on the compound’s metabolic stability or physicochemical properties. The carbonyl group of a central urea moiety was initially believed to mimic substrate binding to a catalytically important Zn2+ ion in the active site, although the plausibility of this binding hypothesis is challenged by observation of excellent selectivity versus the closely related aminopeptidase N (APN). Taken together with the non-competitive inhibition pattern, we also consider an alternative model of allosteric binding.

The Discovery of New Inhibitors of Insulin-Regulated Aminopeptidase by a High-Throughput Screening of 400,000 Drug-like Compounds

“…More recently, our group screened a library of 10,500 small molecules and identified various lead compounds [44], including arylsulfonamides [45], that enhance dendritic spine density in primary hippocampal neuron cultures [46], and un-competitive IRAP inhibitors encompassing a spiro-oxindole dihydroquinazolinone scaffold [47] and an imidazo [1,5α]pyridine scaffold [48]. Additional selective inhibitor leads targeting an allosteric site in IRAP were identified after applying a virtual docking approach [49], and it was reported that modulators of the structurally related hERAP2 have also been identified from a highthroughput screen (HTS) [50].…”

Stabilization of the open conformation of Insulin-Regulated Aminopeptidase by a novel substrate-selective small molecule inhibitor

“…As a result, several efforts have been initiated to develop pharmacologically useful inhibitors of IRAP's aminopeptidase activity, in particular in the areas of cognition 7,8 and fibrosis 9,10 . IRAP inhibitors have been developed either by rational substrate-inspired design or after random chemical or virtual library screening and include pseudophosphinic peptide transition-state-analogues, aryl sulfonamides, benzopyran derivatives, diaminobenzoic acid derivatives, imidazopyridines and cyclic peptide analogues [11][12][13] . Several of these compounds have been shown to be active in cellular and in in vivo systems relating to IRAP biological activities, such as promoting the formation of functional dendritic spines in primary hippocampal neuron cultures 14 , regulation of acetylcholine-mediated vasoconstriction 15 and glucose tolerance in insulin-resistant Zucker fatty rats 16 .…”