Aldo-keto reductase (AKR) 1C3 catalyzes the NADPH-dependent reduction of Delta(4)-androstene-3,17-dione to yield testosterone, reduction of estrone to yield 17beta-estradiol and reduction of progesterone to yield 20alpha-hydroxyprogesterone. In addition, it functions as a prostaglandin (PG) F synthase and reduces PGH(2) to PGF(2)alpha and PGD(2) to 11beta-PGF(2). Immunohistochemistry showed that AKR1C3 is over-expressed in invasive ductal carcinoma of the breast. Retroviral expression of AKR1C3 in MCF-7 breast carcinoma cells shows that each of the assigned reactions occur in a breast cell microenvironment. Steroid and prostaglandin conversions were monitored by radiochromatography. Prostaglandin conversion was validated by a second method using HPLC coupled to APCI-MRM/MS. The combined effect of the AKR1C3 catalyzed 17- and 20-ketosteroid reductions will be to increase the 17beta-estradiol:progesterone ratio in the breast. In addition, formation of PGF(2) epimers would activate F prostanoid receptors and deprive PPARgamma of its putative anti-proliferative PGJ(2) ligands. Thus, AKR1C3 is a source of proliferative signals and a potential therapeutic target for hormone-dependent and -independent breast cancer. Two strategies for AKR1C3 inhibition based on non-steroidal anti-inflammatory drugs were developed. The first strategy uses the Ullmann coupling reaction to generate N-phenylanthranilate derivatives that inhibit AKR1C enzymes without affecting PGH(2) synthase (PGHS) 1 or PGHS-2. The second strategy exploits the selective inhibition of AKR1C3 by indomethacin, which did not inhibit highly related AKR1C1 or AKR1C2. Using known structure-activity relationships for the inhibition of PGHS-1 and PGHS-2 by indole acetic acids we obtained N-(4-chlorobenzoyl)-melatonin as a specific AKR1C3 inhibitor (K(I)=6.0muM) that does not inhibit PGHS-1, PGHS-2, AKR1C1, or AKR1C2. Both strategies are informed by crystal structures of ternary AKR1C3.NADP(+).NSAID complexes. The identification of NSAID analogs as specific inhibitors of AKR1C3 will help validate its role in the proliferation of breast cancer cells.