Intermittent Dosing with Oltipraz: Relationship between Chemoprevention of Aflatoxin-induced Tumorigenesis and Induction of Glutathione S-Transferases

Oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione] protects against chemical carcinogenesis in several animal models and is currently under evaluation as a possible chemopreventive agent in humans. Ideally, clinical chemopreventive interventions use dosing regimens that maximize efficacy while minimizing toxicity. Toward this end, the chemopreventive efficacy achieved by administration of intermittent doses of oltipaz was evaluated in rats. F344 rats were treated with oltipraz (0.5 mmol/kg, p.o.) once weekly, twice weekly, or daily over a 5-week period. After the first week, all rats were gavaged with 20 µg/kg of aflatoxin B1 for 28 consecutive days. Livers were analyzed 2 months after the last aflatoxin B1 dose, and the volume of liver occupied by glutathione S -transferase (GST)-P positive foci, a presumptive marker of neoplasia, was observed to be decreased >95%, >97%, or >99% in livers of rats receiving once-, twice-weekly or daily oltipraz treatments, respectively. The chemopreventive actions of oltipraz have been associated with increases in the levels of phase 2 detoxifying enzymes, such as the glutathione S -transferase isozymes. Accordingly, GST conjugation activity measured with 1-chloro-2,4-dinitrobenzene as substrate increased 1.5-, 1.8-, or 2.4-fold for the once-weekly, twice-weekly or daily treatments, respectively, throughout a 7-day period. Quantitative HPLC analyses of GST subunits 24 h after 2 or 7 daily administrations of oltipraz showed that the levels of subunits Yb1, Yp, Yc2, and Ya2 were increased with maximum elevations of 5.6-, 11.1-, 6.4-, and 10.4-fold, respectively. In comparison, levels of subunits Yb2 and Yc1 were modestly elevated 1.8- to 2.6-fold, respectively, whereas subunit Ya1 was not induced. Remarkably, the levels of subunit Yp and Ya2 remained elevated ∼2.3-fold 7 days after a single dose of oltipraz. In contrast, the levels of subunits Yb1 and Yc2 diminished to approximate control levels within 7 days after a single dose of oltipraz. GST mRNA levels for Ya, Yb, and Yp were measured by Northern blot analysis and were found to be elevated maximally to 13.7-, 13.5-, and 3.9-fold, respectively, after two daily oltipraz doses. Interestingly, GST Ya and Yb mRNA diminished to constitutive levels after 7 daily doses of oltipraz, with no corresponding decreases in GST subunit or activity levels. The levels of GST Ya and Yb mRNA decreased to constitutive levels within 4 days after a single oltipraz administration, whereas GST Yp mRNA levels remained elevated throughout the 7-day follow-up period. These results suggest that the protracted pharmacodynamic actions of oltipraz on enzyme induction may account for the marked reduction in the hepatic burden of aflatoxin B1-induced putative preneoplastic tumors after intermittent dosing. Consequently, scheduling of intermittent dosing protocols may sustain efficacy while improving drug tolerance and patient compliance over long-term treatments. These properties of oltipraz increase its attractiveness for clinical chemopreventive interventions.