Heteroaromatic
quinols 4-(benzothiazol-2-yl)-4-hydroxycyclohexa-2,5-dienone (1) and
4-(1-benzenesulfonyl-1H-indol-2-yl)-4-hydroxycyclohexa-2,5-dienone (2) exhibit potent and selective antitumor activity against colon, renal, and
breast carcinoma cell lines in vitro (GI50 < 500 nmol/L). In vivo growth inhibition of renal, colon, and breast xenografts has been observed. Profound G2-M cell cycle block accompanied down-regulation of cdk1 gene transcription was corroborated by decreased CDK1
protein expression following treatment of HCT 116 cells with growth inhibitory concentrations of 1 or 2. The chemical structure of the
quinol pharmacophore 4-(hydroxycyclohexa-2,5-dienone) suggested that these novel agents would readily react with nucleophiles in a double Michael (beta-
carbon) addition. Indeed, COMPARE analysis within the National Cancer Institute database revealed a number of chemically related
quinone derivatives that could potentially react with
sulfur nucleophiles in a similar manner and suggested that
thioredoxin/
thioredoxin reductase signal transduction could be a putative target. Molecular modeling predicted covalent irreversible binding between
quinol analogues and
cysteine residues 32 and 35 of
thioredoxin, thereby inhibiting
enzyme activity. Binding has been confirmed, via mass spectrometry, between reduced human
thioredoxin and 1. Microarray analyses of untreated HCT 116 cells and those exposed to either 1 (1 micromol/L) or 2 (500 nmol/L and 1 micromol/L) determined that of > or =10,000
cancer-related genes, expression of
thioredoxin reductase was up-regulated >3-fold. Furthermore,
quinols 1 and 2 inhibited
insulin reduction, catalyzed by
thioredoxin/
thioredoxin reductase signaling in a dose-dependent manner (IC50 < 6 micromol/L). Results are consistent with a mechanism of action of novel antitumor
quinols involving inhibition of the small redox
protein thioredoxin.