Abstract B16: Development of potent and selective indomethacin analogs for the inhibition of AKR1C3 (type 5 17β-hydroxysteroid dehydrogenase) in CRPC

Abstract Castrate-resistant prostate cancer (CRPC) is a fatal metastatic form of prostate cancer that accounts for 30,000 deaths in the U.S. annually. Patients with advanced prostate cancer that initially respond to hormonal ablative therapy (orchiectomy or chemical castration) invariably develop CRPC. CRPC is characterized by reactivation of the androgen axis due to changes in androgen receptor (AR) signaling and/or adaptive intratumoral androgen biosynthesis. The latter is targeted with the new agent abiraterone acetate (a P450 17α-hydroxylase/17,20-lyase inhibitor) which prevents the production of the adrenal androgen precursor, dehydroepiandrosterone but has the unintended consequence of building up the potent mineralocortcoid, desoxycorticosterone. To circumvent this side effect, abiraterone acetate is co-administered with prednisone. AKR1C3 is among the most upregulated genes in CRPC and in soft tissue metastasis where its 17β-hydroxysteroid dehydrogenase activity converts the weak androgens, 4-androstene-3,17-dione and 5α-androstane-3,17-dione to the potent androgens, testosterone and 5α-dihydrotestosterone, respectively. The role of AKR1C3 in the pre-receptor regulation of ligands for the AR makes it a more selective target than abiraterone for the treatment of CRPC. Inhibitors of AKR1C3 should not inhibit the related isoforms, AKR1C1 and AKR1C2 since these enzymes inactivate 5α-dihydrotestosterone. Indomethacin, a therapeutic agent used to inhibit cyclooxygenase (COX) also inhibits AKR1C3 potently and displays selectivity over AKR1C1 and AKR1C2. Parallel synthetic strategies were used to generate libraries of indomethacin analogs which based on known structure-activity relationships should no longer inhibit COX enzymes but may retain AKR1C3 inhibitory potency and selectivity. Three classes of AKR1C3 inhibitors were discovered, that had the desired properties: indomethacin analogs (in which the 3′-side chain was modified), 2′-desmethyl-indomethacin analogs (which had either an acid or trifluoromethylsulfonamide substituent at the 3′-position), and 3′-alkyl-indomethacin analogs (which had either an acid or trifluoromethylsulfonamide substituent at the 2′-position). The lead compounds inhibited AKR1C3 with nanomolar potency, displayed over 100-fold selectivity for AKR1C3 over AKR1C1/AKR1C2 and produced robust inhibition of testosterone formation in an LNCaP-AKR1C3 prostate cancer cell line. Several had minimal inhibitory affects on COX-1 and COX-2. In a separate study, indomethacin blocked PSA and ERG expression, and cell proliferation in a VCaP xenograft model of CRPC providing in vivo efficacy data (Cai at al., Cancer Res. 71: 6503, 2011). 2′-Desmethyl-indomethacin 1 was crystallized in complex with AKR1C3 and NADP+, and diffraction data were obtained at 1.8 A resolution. The structure showed that in the absence of the 2′-methyl group the compound assumes a different binding mode to indomethacin. Compound 1 binds perpendicular to indomethacin such that the carboxylic acid on its indole ring is tethered to the oxyanion site and protrudes into a SP1 pocket as seen with the other NSAIDs like flufenamic acid. This model provides a binding pose for the 2′-desmethyl indomethacin analogs and also predicts a possible binding pose for the 3′-alkyl indomethacin series. The compounds reported are promising agents for the preclinical development of therapeutics for CRPC, which by targeting AKR1C3 will be more selective than P450 17α-hydroxylase/17,20-lyase inhibitors, and will not have to be co-administered with prednisone. The agents disclosed in this abstract are protected by US Provisional Patent Application No. 61/548,004. [Supported by R01-CA90744 and a Prostate Cancer Foundation Challenge Award to TMP and R01-CA889450 to LJM] Citation Format: Adegoke Adeniji, Andrew Liedkte, Michael C. Byrns, M Chen, Yi Jin, David Christianson, Lawrence J. Marnett, Trevor M. Penning. Development of potent and selective indomethacin analogs for the inhibition of AKR1C3 (type 5 17β-hydroxysteroid dehydrogenase) in CRPC [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr B16.