Assessment of oral toxicity and safety of pentamethylchromanol (PMCol), a potential chemopreventative agent, in rats and dogs

Abstract

2,2,5,7,8-Pentamethyl-6-chromanol (PMCol) was administered by gavage in rats for 28 days at dose levels of 0, 100, 500, and 2000 mg/kg/day. PMCol administration induced decreases in body weight gains and food consumption, hepatotoxicity (increased TBILI, ALB, ALT, TP; increased relative liver weights; increased T4 and TSH), nephrotoxicity (increased BUN and BUN/CREAT, histopathology lesions), effect on lipid metabolism (increased CHOL), anemia, increase in WBC counts (total and differential), coagulation (FBGN↑and PT↓) and hyperkeratosis of the nonglandular stomach in the 2000 mg/kg/day dose group (in one or both sexes). In the 500 mg/kg/day dose group, toxicity was seen to a lesser extent. In the 100 mg/kg/day dose group, only increased CHOL (females) was observed. To assess the toxicity of PMCol in male dogs it was administered orally by capsule administration for 28 days at dose levels of 0, 50, 200 and 800 mg/kg/day (four male dogs/dose group). PMCol treatment at 800 mg/kg/day resulted in pronounced toxicity to the male dogs. Target organs of toxicity were liver and thymus. Treatment at 200 mg/kg/day resulted in toxicity consistent with slight adverse effect on the liver only. The results of the safety pharmacology study indicate that doses of 0, 50, 200 and 800 mg/kg administered orally did not have an effect on the QT interval, blood pressures and body temperatures following dosing over a 24-h recording period. Under the conditions of this study, the no-observed-adverse effect level (NOAEL) for daily oral administration of PMCol by gavage for 28 days to male rats was 100 mg/kg/day and 50 mg/kg in male dogs. In female rats, the NOAEL was not established due to statistically significant and biologically meaningful increases in CHOL level seen in the 100 mg/kg/day dose group. The results of these studies indicated that administration of PMCol at higher dose levels resulted in severe toxicity in dogs and moderate toxicity in rats, however, administration at lower levels is considered to be less likely to result in toxicity following 28 days of exposure. Sex-related differences were seen in rats. Male rats appeared to have greater sensitivity to nephrotoxicity, while female animals had a greater incidence of hepatoxicity and changes in hematological parameters evaluated, especially at a dose of 500 mg/kg/day, which correlated to the higher plasma drug levels in female rats. It appeared that dogs were generally more sensitive than rats to oral administration of PMCol. Further examination of the potential toxic effects of PMCol in longer term studies is required prior to understanding the full risks of PMCol administration as a chemopreventative agent.

Introduction

The prevention of cancer by chemopreventive agents is preferable to the use of chemotherapy against the established disease. Antioxidants, such as vitamin E, are being investigated for efficacy in cancer prevention. There are indications that it may be efficacious against bladder (Liang et al., 2008) and mammary cancer (Shen et al., 2008, Sharhar et al., 2008, Suh et al., 2007). Vitamin E consists of a head (chroman ring) which carries the active antioxidant group, and a phytyl tail. Chromanol-type compounds act as an antioxidant in biological systems by reduction of oxygen-centered radicals (Gregor et al., 2005, Tyurina et al., 1995). The chroman ring (Fig. 1), 2,2,5,7,8-Pentamethyl-6-chromanol (PMCol), has shown antiandrogen activity in prostate carcinoma cells and is thus considered a potent chemopreventive agent of androgen dependent cancers such as prostate cancer (Thompson and Wilding, 2003). An advantage of PMCol over vitamin E as a chemopreventative agent is its water soluble properties, as well as purported androgen antagonist activity. Absence of the phytyl chain confers a marked increase in water solubility of PMCol (Thompson and Wilding, 2003). This provides a major advantage in the evaluation of its chemopreventative effects, given that in vitro work in cellular suspensions and in growth plates requires the test agents to be in solution. In the presence of PMCol, the androgen-stimulated biphasic growth curve of LNCaP human prostate carcinoma cells was shifted to the right, indicating a reduction in cancer cell growth. Authors reported that the PMCol-induced growth shift was similar to that produced by treatment with the pure antiandrogen bicalutamide (i.e., Casodex), indicative of androgen receptor (AR) antagonist activity. The concentration of PMCol which provides AR antagonist activity is lower than that which shifts the LNCaP cell growth curves. Additional evidence of the antioxidant activity of PMCol is the protection of erythrocytes from hemolysis when exposed to the free radical generator (Koga et al., 1998). Results of further testing indicate that PMCol seems to be slightly anchored within membranes because of the lack of a hydrophobic side chain (Koga et al., 1998). The safety of vitamin E has been previously investigated and the toxicity to several organ systems identified at high doses in rats (Abdo et al., 1986). The safety of PMCol had not been previously established, which is a necessary step in the progression of this compound for use as a potential chemopreventative agent.