555. Targeting kynurenine aminotransferase III: discovery of a selective and brain-penetrant inhibitor for reducing synthesis of immune-induced kynurenic acid

Abstract

Abstract Background Kynurenic acid (KYNA) is a neuroactive metabolite produced via the kynurenine pathway. It acts as an endogenous N-methyl-D-aspartic acid (NMDA) receptor antagonist and is proposed to antagonize the α7-nicotinic acetylcholine receptor (α7nAChR). Elevated KYNA levels are observed in several conditions, including psychotic disorders and infectious diseases, where interference with glutamatergic and dopaminergic neurotransmission contributes to psychosis and cognitive decline. Under physiological conditions, kynurenine aminotransferase II (KAT II) is the main enzyme responsible for KYNA synthesis in the brain and has been the primary target of drug development. However, our recent findings demonstrate that immune activation induces KAT III expression, shifting KYNA synthesis predominantly to this isoform and establishing KAT III as the principal contributor to immune-induced KYNA production, for which no pharmaceutical inhibitors currently exist. Aims & Objectives To develop a potent, selective, and brain-penetrant inhibitor of human KAT III. Method To initiate this effort, we developed a medium-throughput screening assay for KAT III to identify candidate compounds based on biochemical-fluorometric assays. Biochemical enzymatic assay against rat KAT III to determine potency in animals. Affinity measurements by surface plasmon resonance (SPR) with both human and rat enzymes. X-ray crystallisation of hKAT III to guide structure-activity relationship (SAR) studies to identify key positions amendable to chemical modifications. In vitro ADME studies investigating properties such as metabolic stability, permeability, plasma protein binding, and thermodynamic solubility Pharmacokinetic evaluation and preliminary effect studies in naive and immune-stimulated rodents. Results Our lead compound, SLL-9183, exhibits nanomolar potency against human and rat KAT III, and nanomolar affinity for both species determined by SPR, with over 100-fold selectivity against the other KAT isoforms. The X-ray crystal structure of our inhibitor in complex with KAT III revealed no interaction with the enzyme’s cofactor, pyridoxal phosphate (PLP). Pharmacokinetic evaluation in rodents confirmed brain exposure, with free brain concentrations of SLL-9183 exceeding 10 times its Kd value measured by SPR. Preliminary data show that systemic administration of our inhibitor consistently and robustly suppresses immune-induced KYNA synthesis in mice, with reductions observed across several distinct brain regions, including the hippocampus (-51%), frontal cortex (-56%), and striatum (-78%). Discussion & Conclusions We have generated a portfolio of potent, selective, and brain-penetrant inhibitors of KAT III within our academic-led drug discovery and development (DDD) program and established a robust platform for the continued advancement of first-in-class KAT III inhibitors.