SN34037 is a specific inhibitor of Aldo-keto reductase 1C3 (AKR1C3, EC 1.1.1.188) with the ability to inhibit the cytotoxic activity of PR-104A. SN34037-sensitive coumberone reduction provides a rapid and specific assay for AKR1C3 activity. SN34037 inhibits the aerobic cytotoxicity of PR-104A in TF1 erythroleukemia cells with high AKR1C3 expression, but not in Nalm6 pre-B acute lymphoblastic leukemia cells with low AKR1C3 expression.