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Pippione et al. Steroidogenic enzymes in prostate cancer

In an attempt to identify potential AKR1C3 inhibitors Since carboxylic acids are likely to be transported
based on known natural-based pharmacophores, into cells by carrier-mediated processes rather than
Tian et al. [54] studied the blocking mechanism of passive diffusion [57] , there are potential advantages in
berberine (2,3-methylenedioxy-9,10-dimenthoxyproto- finding non-carboxylate inhibitors [58,59] . Following this
berberine chloride; 12). This isoquinoline alkaloid rationale, we have applied a scaffold hopping strategy
screened from a traditional Chinese medicine replacing the benzoic acid moiety of flufenamic acid
monomer library, was shown to prevent AKR1C3- with an acidic hydroxyazolecarbonylic scaffold [60] .
mediated intratumoral steroidogenesis incastrated nu/ In particular, differently N-substituted hydroxylated
nu mice bearing subcutaneous LNCaP xenografts. triazoles were designed to simultaneously interact
The authors found that berberine inhibited AKR1C3- with both subpockets 1 and 2 in the active site of
expressing 22Rv1 PCa cell proliferation and AKR1C3, larger for AKR1C3 than other AKR1Cs
decreased cellular T formation in a dose-dependent isoforms. Through computational design and iterative
manner, provided the experimental basis for the use of rounds of synthesis and biological evaluation, novel
berberine as the lead compound for the further design, compounds were reported, sharing high selectivity (up
research, and development of AKR1C3 inhibitors. to 230-fold) for AKR1C3 over 1C2 isoform and minimal
COX1 and COX2 off-target inhibition. A docking study
Baccharin (3-prenyl-4-(dihydrocinnamoyloxy)cinnamic of compound 17, the most interesting compound
acid, 13) is a constituent in the ethanol extract of of the series, suggested that its methoxybenzyl
Brazilian propolis [55] , which is a natural resinous substitution has the ability to fit inside subpocket 2,
substance collected by honeybees and has been used being involved in π-π staking interaction with Trp227
in alternative medicine to treat inflammation, liver (partial overlapping) and in a T-shape π-π staking with
disorders, and stomach ulcers. Recently Endo et al. [56] Trp86. This compound was also shown to diminish
found that baccharin is a selective and potent inhibitor testosterone production in the AKR1C3-expressing
of AKR1C3, correlating with the antiproliferative effect 22RV1 prostate cancer cell line while synergistic
of baccharin against human PC3 PCa cells. Baccharin effect was observed when 17 was administered in
was shown to exhibita 900-fold selectivity for AKR1C3 combination with abiraterone or enzalutamide.
over the other three AKR1C isoforms. Due to its [61]
high inhibitory selectivity, baccharin represented a Heinrich et al. also reported on a non-carboxylate
inhibitor class of phenylpyrrolidin-2-one derivatives,
promising lead for the development of more potent obtained modulating 18, an inhibitor deriving from a
and specific agents targeting AKR1C3. The structure high-throughput screen [62] . This modulation afforded
activity relationship (SAR) of propolis-derived cinnamic compound 19, named later as SN33638, that inhibited
acids suggested that the 3-prenyl moiety of baccharin AKR1C3 without forming a direct interaction with the
is responsible for the selective binding to AKR1C3 [56] . oxyanion hole in the active site. Furthermore, in a
Endo et al. [46] also reported on the commercially cell-based assay, 19 was shown to be more potent
available 3,4-dihydroxybenzaldehyde, derivatives than the carboxylic acid analogue 18 (ratio IC 50 (enz)/
configured with 3-aliphatic and aryl ethers instead IC 50 (cell) was 0.48 for 18 vs. 8.5 for 19), suggesting
of the 3-prenyl moiety. Within the series of aliphatic a pharmacological disadvantage for the acids in
ethers, AKR1C3 inhibition was shown to decrease PCa cells [61] . The authors explored the role of the
proportionally with increase in the aliphatic chain sulphonamide substituent and probed its affinity within
lengths. Compound 14, possessing an n-butyl ether, the enzyme hydrophobic pocket bound by residues
showed the highest inhibitory potency. Within the Met120, Asn167, Tyr216, Phe306, Phe311, Tyr317,
series of aromatic ethers, two benzyl ether derivatives, Pro318 and Tyr319 [Figure 8] [61] . SAR studies of potent
15 and 16, showed an equivalent inhibitory potency to and selective non-carboxylate AKR1C3 inhibitor 19
baccharin. The molecular docking of 15 in the crystal showed that while the sulphonamide function was still
structure of AKR1C3 informed the design of a novel as critical as in 18 [62] , there was much more tolerance
baccharin-based inhibitor (16a) with improved potency for the sulphonamide substituent, with a range of
(Ki 6.4 nmol/L), which may be due to the introduction monocyclic six-membered ring analogues retaining
of a new interaction between the 3-hydroxyl group of activity and AKR1C selectivity. Crystal structure
the benzyl moiety of 16a and Tyr24 of the enzyme. studies show that the 2-pyrrolidinone was located in
The inhibitory selectivity of 16a for AKR1C3 over other the SP3 pocket but did not bind to the oxyanion site,
human AKR1C isoforms was comparable or superior and variations in the position, co-planarity or electronic
to that of baccharin. Additionally, 16a significantly nature of the pyrrolidinone ring abolished or severely
decreased the cellular metabolism by AKR1C3 at diminished activity. The effectiveness of compounds at
much lower concentrations than baccharin. inhibiting AKR1C3 activity in cells broadly correlated

Journal of Cancer Metastasis and Treatment ¦ Volume 3 ¦ December 12, 2017 339