In vitro, ASP9521 inhibited the conversion of androstenedione into testosterone by recombinant human and cynomolgus monkey AKR1C3 in a concentration-dependent manner, with IC50 values of 11 and 49 nmol/L, respectively. In contrast, ASP9521 did not inhibit the conversion by rat and mouse homologues (AKR1C1 and AKR1C6, respectively) up to a concentration of 10 μmol/L. ASP9521 showed moderately high selectivity (>100-fold) for human AKR1C3 (IC50: 120 nmol/L) over the human isoform AKR1C2 (IC50: >20,000 nmol/L).