The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings were not evaluated in accordance with the levels of evidence for reproductive or developmental criteria established by NTP in March 2009. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
Diethylstilbestrol (DES), the least expensive and most widely used estrogen, was tested in Swiss mice using the RACB protocol. This study was one of the first RACB studies conducted. DES was chosen as a known positive, based on the extensive literature by MacLachlan and colleagues. There were two laboratories beginning to run RACB studies, and a DES study was performed at each of these laboratories to address the issue of interlaboratory variability. From the range finding study (Task 1), levels of 1.0, 10.0 and 50.0 ppb in feed were selected for the continuous breeding phase of the study. Based on body weights and measures of food consumption, the estimated average doses were approximately 0.15, 1.5, and 7.70 mgm/kg/d.
In Task 2, no adverse clinical signs were noted; one high dose female died of partner-inflicted wounds. Body weight was unaffected during Task 2. The mean number of litters/pair was reduced to nearly equal to 30% of control at the high dose, while the number of live pups/litter was reduced by 77% and 64% in the medium and high dose grups, respectively. Adjusted live pup weight was unaffected. The high dose group took longer to deliver each of the first 3 litters; no furth or fifth litters were delivered at 50 ppb.
Since adverse effects were noted in Task 2, Task 3 was conducted using the using the control and high dose mice. Compared to controls, only one-third as many exposed female mice delivered a litter, and these litters contained nearly equal to 60% fewer pups. Interestingly, for the exposed-female group, adjusted pup weight was reduced by nearly equal to 20%. The only adverse effect noted in the group containing exposed males was a 10% reduction in adjusted pup body weight. The larger adverse effects were clearly seen in exposed females.
After the Task 3 litters were evaluated, the F0 control and high dose mice were killed and necropsied. For females, the only effect noted was a nearly equal to 30% increase in pituitary weight. For females, 25% of controls had an unclear estrous cycle, while more than 75% of treated females did not have a clear estrous cycle. In males, a significant increase in pituitary weight (nearly equal to 15%) was also seen, along with 13-18% reductions in the weight of epididymis, cauda epididymis, and prostate. There were no demonstrable sperm effects.
Task 4, the F2 generation assessment, was not conducted.
This study replicated all of the salient features of the other DES study: female as the much-more-sensitive gender in the absence of significant weight effects. Differences between the results of the two studies include effects in the middle dose group, different responses in adjusted pup weight (increased at one lab, decreased at the other), and an effect on pup number in the middle dose in Task 2 (unchanged at one lab, reduced by 11% at the other). Nonetheless, in broad strokes, these studies confirmed the general replicability of the data from two different laboratories, and provided a sense of the degree of variation that might be expected across labs