Breast cancer is one of the most common malignant tumors in women, aromatase inhibitors (AIs) are an important adjuvant therapy of breast cancer. Formestane (4-OHA), one of steroidal AIs, is used to treat advanced breast cancer and inhibits irreversibly aromatase activity. Several previous studies have found that aldo-keto reductases AKR1Cs are involved in the metabolism of many steroids and their derivatives. In the future they may be targets of therapy of hormonal diseases. We presumed that AKR1Cs may participate in site-specific metabolism of 4-OHA, affecting its therapeutic effect. In this paper, four active AKR1C isoforms were obtained by prokaryotic expression in vitro. The catalytic efficiency of AKR1Cs was detected by spectroscopic methods, and the effect of inhibitors on the catalytic reduction of 4-OHA by AKR1Cs was verified. It was found that four AKR1C isoforms can reduce keto-groups and double bonds of 4-OHA. AKR1C4 can rapidly catalyze the structural changes in 4-OHA with a conversion rate of almost 100%, followed by AKR1C3 and AKR1C1 with a conversion efficiency of about 30%. AKR1C2 has the lowest activity towards of 4-OHA, and the conversion efficiency is only about 20%. At the same time, the inhibitor showed a significant dose-effect relationship with AKR1Cs. Non-linear regression analysis showed that the inhibitors had a strong affinity for AKR1C3 and AKR1C4 with IC₅₀ values of 47.4 μmol/L and 54.68 μmol/L, respectively. The inhibition of AKR1C1 and AKR1C2 is relatively weak, and the IC₅₀ values are 77.37 μmol/L and 82.24 μmol/L, respectively. The above results indicate that 4-OHA can be rapidly metabolized by AKR1C4 which is expressed in the liver only which may contribute to the fact that 4-OHA and its many conjugated metabolites is not very effective after oral administration. Our data support the advantage of parenteral administration of the drug as a depot-formulation or as a preparation for transdermal delivery and also provides a new idea for further research on nano-drug carriers and derivatives of the drug in the future.