IDO2 and COVID-19: ,8 IDO2 generates formylkynurenine, which can be metabolized further to kynurenine and downstream cyto- and neurotoxic metabolites, such as 3OH-kynurenine, 3OH-anthranilic acid and quinolinic acid, potentially leading to apoptosis.9, 10, 11 In tissue from fatal COVID-19 patients, IDO2 expression, kynurenine metabolites and markers of autophagy and apoptosis closely co-localized, suggestive of IDO2 contributing to the pathophysiology in fatal COVID-19.