A 3-(4-nitronaphthen-1-yl) amino-benzoate analog as a bifunctional AKR1C3 inhibitor and AR antagonist: Head to head comparison with other advanced AKR1C3 targeted therapeutics.

Drugs used for the treatment of castration resistant prostate cancer (CRPC) include Abiraterone acetate (Zytiga®) and Enzalutamide (XTANDI®). However, these drugs provide clinical benefit in metastatic disease for only a brief period before drug resistance emerges. One mechanism of drug resistance involves the overexpression of type 5 17-ß-hydroxysteroid dehydrogenase (aldo-keto reductase 1C3 or AKR1C3), a major enzyme responsible for the formation of intratumoral androgens that activate the androgen receptor (AR). 3-((4-Nitronaphthalen-1-yl)amino)benzoic acid 1 is a "first-in-class" AKR1C3 competitive inhibitor and AR antagonist. Compound 1 was compared in a battery of in vitro studies with structurally related N-naphthyl-aminobenzoates, and AKR1C3 targeted therapeutics e.g. GTx-560 and ASP9521, as well as with R-bicalutamide, enzalutamide and abiraterone acetate. Compound 1 was the only naphthyl derivative that was a selective AKR1C3 inhibitor and AR antagonist in direct competitive binding assays and in AR driven reporter gene assays. GTx-560 displayed weak activity as a direct AR antagonist but had high potency in the AR reporter gene assay consistent with its ability to inhibit the co-activator function of AKR1C3. By contrast ASP9521 did not act as either an AR antagonist or block AR reporter gene activity. Compound 1 was the only compound that showed comparable potency to inhibit AKR1C3 and act as a direct AR antagonist. Compound 1 blocked the formation of testosterone in LNCaP-AKR1C3 cells, and the expression of PSA driven by the AKR1C3 substrate (4-androstene-3,17-dione) and by an AR agonist, 5α-dihydrotestosterone consistent with its bifunctional role. Compound 1 blocked the nuclear translocation of the AR at similar concentrations to enzalutamide and caused disappearance of the AR from cell lysates. R-biaclutamide and enzalutamide inhibited AKR1C3 at concentrations 200x greater than compound 1, suggesting that its bifunctionality can be explained by a shared pharmacophore that can be optimized.

AKR1C3 as a target in castrate resistant prostate cancer.

Aberrant androgen receptor (AR) activation is the major driver of castrate resistant prostate cancer (CRPC). CRPC is ultimately fatal and more therapeutic agents are needed to treat this disease. Compounds that target the androgen axis by inhibiting androgen biosynthesis and or AR signaling are potential candidates for use in CRPC treatment and are currently being pursued aggressively. Aldo-keto reductase 1C3 (AKR1C3) plays a pivotal role in androgen biosynthesis within the prostate. It catalyzes the 17-ketoreduction of weak androgen precursors to give testosterone and 5α-dihydrotestosterone. AKR1C3 expression and activity has been implicated in the development of CRPC, making it a rational target. Selective inhibition of AKR1C3 will be important, however, due to the presence of closely related isoforms, AKR1C1 and AKR1C2 that are also involved in androgen inactivation. We examine the evidence that supports the vital role of AKR1C3 in CRPC and recent developments in the discovery of potent and selective AKR1C3 inhibitors. This article is part of a Special Issue entitled 'CSR 2013'.

Development of potent and selective indomethacin analogues for the inhibition of AKR1C3 (Type 5 17ß-hydroxysteroid dehydrogenase/prostaglandin F synthase) in castrate-resistant prostate cancer.

Castrate-resistant prostate cancer (CRPC) is a fatal, metastatic form of prostate cancer. CRPC is characterized by reactivation of the androgen axis due to changes in androgen receptor signaling and/or adaptive intratumoral androgen biosynthesis. AKR1C3 is upregulated in CRPC where it catalyzes the formation of potent androgens. This makes AKR1C3 a target for the treatment of CRPC. AKR1C3 inhibitors should not inhibit AKR1C1/AKR1C2, which inactivate 5α-dihydrotestosterone. Indomethacin, used to inhibit cyclooxygenase, also inhibits AKR1C3 and displays selectivity over AKR1C1/AKR1C2. Parallel synthetic strategies were used to generate libraries of indomethacin analogues, which exhibit reduced cyclooxygenase inhibitory activity but retain AKR1C3 inhibitory potency and selectivity. The lead compounds inhibited AKR1C3 with nanomolar potency, displayed >100-fold selectivity over AKR1C1/AKR1C2, and blocked testosterone formation in LNCaP-AKR1C3 cells. The AKR1C3·NADP(+)·2'-des-methyl-indomethacin crystal structure was determined, and it revealed a unique inhibitor binding mode. The compounds reported are promising agents for the development of therapeutics for CRPC.

N-Benzoyl anthranilic acid derivatives as selective inhibitors of aldo-keto reductase AKR1C3.

Human aldo-keto reductases AKR1C1-AKR1C3 are involved in the biosynthesis and inactivation of steroid hormones and prostaglandins and thus represent attractive targets for the development of new drugs. We synthesized a series of N-benzoyl anthranilic acid derivatives and tested their inhibitory activity on AKR1C enzymes. Our data show that these derivatives inhibit AKR1C1-AKR1C3 isoforms with low micromolar potency. In addition, five selective inhibitors of AKR1C3 were identified. The most promising inhibitors were compounds 10 and 13, with IC(50) values of 0.31 µM and 0.35 µM for AKR1C3, respectively.

Selective inhibitors of aldo-keto reductases AKR1C1 and AKR1C3 discovered by virtual screening of a fragment library.

Human aldo-keto reductases 1C1-1C4 (AKR1C1-AKR1C4) function in vivo as 3-keto-, 17-keto-, and 20-ketosteroid reductases and regulate the activity of androgens, estrogens, and progesterone and the occupancy and transactivation of their corresponding receptors. Aberrant expression and action of AKR1C enzymes can lead to different pathophysiological conditions. AKR1C enzymes thus represent important targets for development of new drugs. We performed a virtual high-throughput screen of a fragment library that was followed by biochemical evaluation on AKR1C1-AKR1C4 enzymes. Twenty-four structurally diverse compounds were discovered with low µM K(i) values for AKR1C1, AKR1C3, or both. Two structural series included the salicylates and the N-phenylanthranilic acids, and additionally a series of inhibitors with completely novel scaffolds was discovered. Two of the best selective AKR1C3 inhibitors had K(i) values of 0.1 and 2.7 µM, exceeding expected activity for fragments. The compounds identified represent an excellent starting point for further hit-to-lead development.

Crystal structures of AKR1C3 containing an N-(aryl)amino-benzoate inhibitor and a bifunctional AKR1C3 inhibitor and androgen receptor antagonist. Therapeutic leads for castrate resistant prostate cancer.

Castrate resistant prostate cancer (CRPC) is associated with increased androgen receptor (AR) signaling often brought about by elevated intratumoral androgen biosynthesis and AR amplification. Inhibition of androgen biosynthesis and/or AR antagonism should be efficacious in the treatment of CRPC. AKR1C3 catalyzes the formation of potent AR ligands from inactive precursors and is one of the most upregulated genes in CRPC. AKR1C3 inhibitors should not inhibit the related isoforms, AKR1C1 and AKR1C2 that are involved in 5α-dihydrotestosterone inactivation in the prostate. We have previously developed a series of flufenamic acid analogs as potent and selective AKR1C3 inhibitors [Adeniji, A. O. et al., J. Med. Chem.2012, 55, 2311]. Here we report the X-ray crystal structure of one lead compound 3-((4-(trifluoromethyl)phenyl) amino)benzoic acid (1) in complex with AKR1C3. Compound 1 adopts a similar binding orientation as flufenamic acid, however, its phenylamino ring projects deeper into a subpocket and confers selectivity over the other AKR1C isoforms. We exploited the observation that some flufenamic acid analogs also act as AR antagonists and synthesized a second generation inhibitor, 3-((4-nitronaphthalen-1-yl)amino)benzoic acid (2). Compound 2 retained nanomolar potency and selective inhibition of AKR1C3 but also acted as an AR antagonist. It inhibited 5α-dihydrotestosterone stimulated AR reporter gene activity with an IC(50)=4.7 µM and produced a concentration dependent reduction in androgen receptor levels in prostate cancer cells. The in vitro and cell-based effects of compound 2 make it a promising lead for development of dual acting agent for CRPC. To illuminate the structural basis of AKR1C3 inhibition, we also report the crystal structure of the AKR1C3·NADP(+)·2 complex, which shows that compound 2 forms a unique double-decker structure with AKR1C3.

Development of potent and selective inhibitors of aldo-keto reductase 1C3 (type 5 17ß-hydroxysteroid dehydrogenase) based on N-phenyl-aminobenzoates and their structure-activity relationships.

Aldo-keto reductase 1C3 (AKR1C3; type 5 17ß-hydroxysteroid dehydrogenase) is overexpressed in castration resistant prostate cancer (CRPC) and is implicated in the intratumoral biosynthesis of testosterone and 5α-dihydrotestosterone. Selective AKR1C3 inhibitors are required because compounds should not inhibit the highly related AKR1C1 and AKR1C2 isoforms which are involved in the inactivation of 5α-dihydrotestosterone. NSAIDs, N-phenylanthranilates in particular, are potent but nonselective AKR1C3 inhibitors. Using flufenamic acid, 2-{[3-(trifluoromethyl)phenyl]amino}benzoic acid, as lead compound, five classes of structural analogues were synthesized and evaluated for AKR1C3 inhibitory potency and selectivity. Structure-activity relationship (SAR) studies revealed that a meta-carboxylic acid group relative to the amine conferred pronounced AKR1C3 selectivity without loss of potency, while electron withdrawing groups on the phenylamino B-ring were optimal for AKR1C3 inhibition. Lead compounds did not inhibit COX-1 or COX-2 but blocked the AKR1C3 mediated production of testosterone in LNCaP-AKR1C3 cells. These compounds offer promising leads toward new therapeutics for CRPC.

Discovery of substituted 3-(phenylamino)benzoic acids as potent and selective inhibitors of type 5 17ß-hydroxysteroid dehydrogenase (AKR1C3).

Aldo-keto reductase 1C3 (AKR1C3) also known as type 5 17ß-hydroxysteroid dehydrogenase has been implicated as one of the key enzymes driving the elevated intratumoral androgen levels observed in castrate resistant prostate cancer (CRPC). AKR1C3 inhibition therefore presents a rational approach to managing CRPC. Inhibitors should be selective for AKR1C3 over other AKR1C enzymes involved in androgen metabolism. We have synthesized 2-, 3-, and 4-(phenylamino)benzoic acids and identified 3-(phenylamino)benzoic acids that have nanomolar affinity and exhibit over 200-fold selectivity for AKR1C3 versus other AKR1C isoforms. The AKR1C3 inhibitory potency of the 4'-substituted 3-(phenylamino)benzoic acids shows a linear correlation with both electronic effects of substituents and the pK(a) of the carboxylic acid and secondary amine groups, which are interdependent. These compounds may be useful in treatment and/or prevention of CRPC as well as understanding the role of AKR1C3 in endocrinology.