Tamoxifen: 28-day oral toxicity study in the rat based on the Enhanced OECD Test Guideline 407 to detect endocrine effects

The main objective of this 28-day oral gavage toxicity study in the rat was to investigate which of the current and/or additional parameters of the OECD Test Guideline 407 would reliably and sensitively detect the endocrine-mediated effects of the nonsteroidal antiestrogen tamoxifen. In addition, as this study was performed using two subgroups of five animals of each sex run concurrently, it enabled an assessment of the intralaboratory reproducibility while also assessing the potential value of using ten animals of each sex per group instead of using the standard five animals of each sex per group stipulated by the current guideline. Tamoxifen was administered daily by gavage to groups of 7-week-old Wistar rats for at least 28 days at dose levels of 5, 30, or 200 microg/kg body weight. Additional parameters specified in the enhanced OECD Test Guideline 407 were spermatozoa enumeration and morphology of the cauda epididymis, hormonal analysis of the thyroid-stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4) levels, monitoring of the estrous cycle during week 4 of treatment to ensure females were in diestrus on the day of terminal sacrifice, organ weight of ovary, uterus, thyroid gland, prostate gland (ventral and dorsolateral parts), seminal vesicles with coagulation glands and pituitary gland, and microscopic investigation of the pituitary gland, vagina, mammary gland, seminal vesicles with coagulating glands, epididymis, and prostate gland (ventral and dorsolateral parts). Overall, 200 microg/kg per day was considered to be the Maximum Tolerated Dose (MTD) in both sexes, resulting in a marked reduction of body weight gain, together with slight effects on clinical signs, hematology, plasma chemistry, and microscopic changes in some endocrine tissues. Five micrograms per kilogram per day represented the No Observed Adverse Effect Level (NOAEL) in males and the No Observed Effect Level (NOEL) in females. At the intermediate dose level (30 microg/kg per day), the current OECD Test Guideline 407 was appropriate to detect the specific endocrine-related changes induced by tamoxifen in females, based on the histopathology findings observed in the ovary and the uterus. The additional parameters which were found to be changed in females (thyroid hormone levels, ovary and uterus weights, and histopathology of vagina) provided supplementary information further confirming tamoxifen-mediated endocrine effects. In males, when data from the current Test Guideline 407 were considered at the intermediate dose level, specific endocrine effects were only indicated on the basis of the histopathology findings observed in the prostate gland. The additional parameters examined which were found to be changed (prostate gland and seminal vesicle weights, and histopathology of seminal vesicle and mammary gland) were necessary to confirm the specific tamoxifen-mediated endocrine effects. Hence, amongst the additional parameters contained in the enhanced OECD Test Guideline 407, organ weights and histopathological examination of endocrine-related organs were the most helpful in confirming the detection of tamoxifen-mediated endocrine effects. The reproducibility evaluation showed that a group size of five animals of each sex consistently allowed the detection of endocrine effects with the current Test Guideline in both sexes at the high dose level and in females at the intermediate dose level. Doubling the animal number from five to ten of each sex per dose level did not notably increase the sensitivity of detection of endocrine-mediated effects.