based inhibitors of xanthine oxidase is described. After a high-throughput screening campaign, an NMR based counterscreen was used to distinguish actives, which interact with XO in a reversible manner, from assay artefacts. This approach identified pyrimidone 1 as a reversible and competitiveinhibitor with good lead-like properties. A hit to lead campaign gave compound 41, a nanomolar inhibitor of hXO