Targeting thioredoxin reductase with organometallics

Abstract

The thioredoxin (Trx) system is a key antioxidant system to the regulation of oxidative stress in cells. Thioredoxin reductase (TrxR) is an enzyme which is central to the Trx system and is upregulated in cancer cells, making it a potential target in anti-cancer therapy. Its active site contains an S-Se bond (Cys497-Sec498). Since both sulphur and selenium are soft bases, we hypothesise that they may be targeted by organometallic compounds. In this work, two aspects on targeting TrxR with organometallics have been examined: (a) Developing organometallic complexes and derivatives of known drugs that act on TrxR, and (b) mechanistic and kinetic studies on the reaction of organometallic complexes with the S-Se bond. For the first aspect, two groups of organometallic complexes have been studied. The first group are the metallocenyl derivatives of ebselen, a known substrate of TrxR. They are found to inhibit TrxR with an IC50 of 2.5 ± 0.3 µM in a pure protein assay; they have a higher affinity towards TrxR (Km = 0.4 µM) than the natural substrate, Trx. The second is the carbonyl cluster Os3(CO)10(NCMe)2 which has already been shown to induce apoptosis. It is found to interact with selenyl-sulphide and selenides, besides its known reactivity with disulphides and sulfhydryl groups. It inhibits TrxR with an IC50 of 5.3 ± 0.9 mM as a non-substrate inhibitor, is selective even in the presence of high concentrations of GSH, and it has also been shown to inhibit TrxR in cell lysate. These results suggest that the inhibition of TrxR is a potential apoptotic pathway. The second aspect concerns the hypothesis that heterodinuclear organometallic complexes may react selectively with the hetero-dichalcogenide S-Se bond over the homo-dichalcogenide bonds S-S and Se-Se. Mechanistic and kinetic studies on the reactivity of homo/hetero bimetallic Ru and/or Mo complexes (M-M’, where M/M’=Ru/Mo), with two small-molecule mimics for the dichalcogenides, viz., PhEEPh and 1,8-naphthalenedichalcogenides, have been carried out. The reactions are initiated through photocleavage, by visible light, of the metal-metal bonds of the bimetallic complexes. While the Mo-Mo compound reacts faster with Se-containing molecules, the Ru-Ru compound preferred S-containing molecules; the Ru-Mo compound exhibits comparable kinetics for all three E-E’ molecules. A similar trend is observed for the naphthalene derivatives although the rates are significantly slower, postulated to be due to a chelate effect.