Does PBO Cause Cancer?
PBO caused liver tumors in mice following daily administration in the diet, but the data suggest that the mode by which these tumors were formed is not plausible in humans. Moreover, large quantitative differences exist between the doses of PBO that produce liver tumors in mice and human exposure to PBO. Many non-genotoxic (non-DNA-damaging) compounds like PBO have been shown to induce tumors in rodents especially if the compounds themselves are not particularly toxic to the animals and, thus, high doses are needed to meet requirement for a “maximum tolerated dose” in carcinogenicity studies.
Consideration of the relevance of such tumors to humans has been focus of an intense effort over the past two decades (Ames et al., 1993; Butler, 1996; Cohen and Ellwein, 1990; Cohen, et al., 2003, 2004; Grasso and Hinton, 1991; Grasso et al., 1991; Holsapple, et al., 2006; Loury et al., 1987; Wilson et al., 1992). Generally speaking, the means by which PBO causes mouse liver tumors is likely not relevant to humans. Consequently, the USEPA classified PBO only in the Group C-Possible Human Carcinogen (U.S. EPA, 1995) and no restrictions apply to any of the registered products.
Under the auspices of the United Nations, the Food and Agriculture Organization/World Health Organization (FAO/WHO) Joint Meeting on Pesticide Residues (JMPR) evaluated the entire body of toxicology of PBO several times since 1965. They concluded that, at doses up to internationally accepted standards for a Maximum Tolerated Dose, PBO is not considered to be carcinogenic in the mouse or rat, thus leading to the conclusion that PBO is not carcinogenic to humans (WHO, 1995).