In association with the international validation project to establish an OECD Enhanced Test Guideline 407, we performed a 28-day repeated-dose toxicity study of
methoxychlor, a
chlorinated hydrocarbon pesticide with pro-estrogenic and anti-androgenic activities. Attention was paid to the sensitivity of certain additional parameters for detecting endocrine related effects of
endocrine disrupting chemicals based on the existing TG 407. Seven-week-old Crj:CD(SD)IGS rats were allocated to one of four groups, each consisting often males and ten females, and
methoxychlor was administered once daily by gavage at doses of 0 (control), 20, 100 or 500 mg/kg
body weight per day. Male rats were killed on the day after the 28th administration. Female rats were killed on the day of the diestrus stage during 4 days after the 28th administration. Male rats receiving
methoxychlor showed mainly
atrophy of mammary acinus in the 20 mg/ kg and higher groups, together with decreases in prostate and seminal vesicle weights, and
atrophy of epididymis, prostate, seminal vesicle and coagulating gland in the 100 and 500 mg/kg groups. In addition, decrease in serum
testosterone level, increase in
follicle-stimulating hormone level, decrease in testis and epididymis weights,
atrophy of semiferous tubules and Leydig cells, decrease in the number of sperm in the caudal epididymis and their motility were observed in the 500 mg/kg group. Female rats receiving
methoxychlor showed mainly abnormal estrous cycles, decrease in serum
luteinizing hormone level, decrease in ovary weight, proliferation of mammary acinus,
atrophy of ovary due to decrease in follicles and corpus luteum in histopathology,
hypertrophy of endometrial epithelium of uterus and vagina epithelium in the 100 and 500 mg/kg groups. Among the parameters tested in the present experimental system, effects of
methoxychlor on endocrine-related organs were detected with regard to serum
hormone, organ weights, histopathological examination in both sexes, estrus cycle in females and sperm examination in males. Based on these results, a no-observed-adverse-effect level (NOAEL) in the present study was estimated to be below 20 mg/kg per day. In particular, the adverse effects were effectively detected in organ weights of accessory sex organs and histopathological examination.