Phase 2 Enzyme Induction by the Major Metabolite of Oltipraz

Treatment for 48 h of murine Hepa 1c1c7 cells in culture with the cancer chemopreventive oltipraz (1) followed by addition of CD₃I and immediate cell lysis yields, by LC/MS analysis, three isotopomers of the methylated pyrrolopyrazine (2), a known human metabolite of oltipraz. The major isotopomer (58%) is the one containing two CD₃- groups attached to the pendant sulfur atoms of the pyrrolopyrazine ring, the others containing one CD₃- and one CH₃- group or two CH₃- groups. It is concluded from this that the unmethylated pyrrolopyrazine (4) is the major metabolite of oltipraz. Prodrugs 5 and 6, which have been shown to rapidly generate 4 in the presence of GSH at physiological pH, induce the phase 2 enzyme NQO1 in Hepa 1c1c7 cells with potencies on par with oltipraz itself: CD_NQO1 = 14.4 ± 1.3, 20.1 ± 4.6, and 23.6 ± 1.6 μM for oltipraz, 5, and 6, respectively. Pretreatment of oltipraz, 5, and 6 in cell culture media with 1 mM GSH, which is shown to immediately convert 5 and 6 to 4, followed by incubation with Hepa 1c1c7 cells shows similar potencies for oltipraz and the (decomposed) produrgs, with CD_NQ01 18.0 ± 4.4, μM for 5, 17.8 ± 0.2 μM for 6, and 13.5 ± 1.4, μM for oltipraz. Treatment with compound 6 of murine hepatoma cells containing a luciferase gene under the control of the antioxidant response element (ARE) from the mouse heme oxygenase (ho-1) gene elicits induction of luciferase activity, CD = 35.8 ± 2.8 μM, somewhat greater than the potency than oltipraz itself. Western blots of nuclear proteins isolated from Hepa 1c1c7 cells and probed with anti-Nrf2 indicate that as compared to vehicle DMSO, compound 6 stimulates nuclear translocation of Nrf2 from the cytosol. From this study, it is concluded that the major metabolite of the cancer chemopreventive oltipraz is a phase 2 enzyme inducer of comparable potency that activates the ARE and initiates nuclear translocation of transcription factor Nrf 2.