Assessment of Alpha-amino-beta-carboxymuconate-εpsilon-semialdehyde Decarboxylase (ACMSD) Inhibitors

The biodegradation of tryptophan in the kynurenine pathway provides quinolinic acid (QUIN), the precursor for the de novo synthesis of NAD⁺. This occurs through, α-amino-β-carboxymuconate-ε-semialdehyde (ACMS) which, nonenzymatically decays to form QUIN. ACMSD catalyzes the breakdown of ACMSD to 2-AMS. By inhibiting the α‑amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD), more ACMS cyclizes to form QUIN which increases NAD⁺ levels. In this study, diflunisal derivatives were used to assess the inhibition of ACMSD. Some of these derivatives were shown to have stronger inhibitions than diflunisal. The crystal structure of ACMSD in complex with the inhibition compounds showed similar binding modes with diflunisal but different second ring positioning. In this study, two diflunisal derivatives showed half-maximal inhibitory concentration (IC₅₀) values lower than KURA-ACMSD 17 (KA17), a previous diflunisal derivative with an IC₅₀ values of 1.32 ± 0.07 μM. The two compounds, PA-62 and PA-67 had IC50 values of 1.29 ± 0.17 μM and 0.66 ± 0.11 μM against the human ACMSD respectively. This was the first sub micromolar concentration from the diflunisal derivatives. This shows that the diflunisal derivatives can be improved upon to create a more potent inhibitor. The crystal structures of ACMSD in complex of with the diflunisal inhibitors were determined through X-ray crystallography using a bacterial version of the enzyme that had been previously demonstrated to share a similar active site architecture with the human ACMSD. This provided a foundation for understanding the inhibitory mechanism at the molecular level.