Identification of Michael Acceptor-Centric Pharmacophores with Substituents That Yield Strong Thioredoxin Reductase Inhibitory Character Correlated to Antiproliferative Activity

Abstract

The role of thioredoxin reductase (TrxR) in tumorigenesis has made it an attractive anticancer target. A systematic approach for development of novel compounds as TrxR inhibitors is currently lacking. Structurally diversified TrxR inhibitors share in common electrophilic propensities for the sulfhydryl groups, among which include the Michael reaction acceptors containing an α,β-unsaturated carbonyl moiety. We aimed to identify features among structurally diversified Michael acceptor-based compounds that would yield a strong TrxR inhibitory character. Structurally dissimilar Michael acceptor-based natural compounds such as isobutylamides, zerumbone, and shogaols (SGs) were found to possess a poor TrxR inhibitory activity, indicating that a sole Michael acceptor moiety was insufficient to produce TrxR inhibition. The 1,7-diphenyl-hept-3-en-5-one pharmacophore in 3-phenyl-3-SG, a novel SG analog that possessed comparable TrxR inhibitory and antiproliferative potencies as 6-SG, was modified to yield 1,5-diphenyl-pent-1-en-3-one (DPPen) and 1,3-diphenyl-pro-1-en-3-one (DPPro, also known as chalcone) pharmacophores. These Michael acceptor-centric pharmacophores, when substituted with the hydroxyl and fluorine groups, gave rise to analogs displaying a TrxR inhibitory character positively correlated to their antiproliferative potencies. Lead analogs 2,2'-diOH-5,5'-diF-DPPen and 2-OH-5-F-DPPro yielded a half-maximal TrxR inhibitory concentration of 9.1 and 10.5 μM, respectively, after 1-h incubation with recombinant rat TrxR, with the C-terminal selenocysteine residue found to be targeted. Identification of Michael acceptor-centric pharmacophores among diversified compounds demonstrates that a systematic approach to discover and develop Michael acceptor-based TrxR inhibitors is feasible. A strong TrxR inhibitory character correlated to the antiproliferative potency is attributed to structural features that include an α,β-unsaturated carbonyl moiety centered in a DPPen or DPPro pharmacophore bearing hydroxyl and fluorine substitutions.