Reversibility of the toxicological changes induced by photomirex and mirex.

Photomirex (8-monohydromirex) is a known environmental contaminant. In a previous 28-day and 90-day study, photomirex was shown to cause a number of biochemical and histological alterations in male rats. The present study was undertaken to determine the reversibility of those changes and to compare both effects and reversibility with the present compound mirex. Male rats were fed diets containing 0, 0.05, 0.5, 5.0 and 50 ppm photomirex and 5 ppm or 50 ppm mirex for 28 days. Ten animals/group were killed at this time and the remainder placed on clean feed. Ten animals/group were killed after another 12, 24, and 48 weeks. The lower body weight gain in animals fed 50 ppm photomirex and 50 ppm mirex observed after 28 days on the test was not evidence after 12 weeks on the clean diet or thereafter. Reduced food consumption produced by these groups persisted for 12 weeks but not 24 weeks. Liver hypertrophy was observed in the 5.0-ppm and 50-ppm photomirex groups after 28 days and persisted in the highest dose group for 24 weeks on clean diet. Elevated serum sorbitol dehydrogenase activity was observed in the 50-ppm photomirex group and persisted for 12 weeks on clean diet. Histological changes in liver and thyroid were present up to 48 weeks on clean diet, whereas treatment-related testicular changes observed after 28 days were not evident after 12 weeks. Significant residues of photomirex were still evident in all tissues examined up to 48 weeks postexposure in the 50-ppm photomirex group. The data indicate that photomirex and mirex are extremely persistent compounds and can result in a variety of biochemical and histological changes long after exposure has ceased.