Determination of the di-(2-ethylhexyl) phthalate NOAEL for reproductive development in the rat: importance of the retention of extra animals to adulthood.

Deriving No Observed Adverse Effect Level (NOAEL) or benchmark dose is important for risk assessment and can be influenced by study design considerations. In order to define the di-(2-ethylhexyl) phthalate (DEHP) dose-response curve for reproductive malformations, we retained more offspring to adulthood to improve detection of these malformations in the reproductive assessment by continuous breeding study design. Sprague-Dawley rats were given a dietary administration of 1.5 (control), 10, 30, 100, 300, 1000, 7500, and 10,000 ppm DEHP. Male pups were evaluated for gross reproductive tract malformations (RTMs) associated with the "phthalate syndrome." DEHP treatment had minimal effects on P0 males. There was a statistically significant increase in F1 and F2 total RTMs (testis, epididymides, seminal vesicle, and prostate) in the 7500-ppm dose group and F1 10,000-ppm dose group. The 10,000-ppm exposed F1 males did not produce an F2 generation. The NOAEL for F1 and F2 RTM combined data, because in utero exposures were similar, were 100 ppm (4.8 mg/kg/day), which was close to the 5% response benchmark dose lower confidence limit of 142 ppm. The utility of evaluating more pups per litter was examined by generating power curves from a Monte Carlo simulation. These curves indicate a substantial increase in detection rate when three males are evaluated per litter rather than one. A 10% effect across male pups would be detected 5% of the time if one pup per litter was evaluated, but these effects would be detected 66% of the time if three pups per litter were evaluated. Taken together, this study provides a well-defined dose response of DEHP-induced RTMs and demonstrates that retention of more adult F1 and F2 males per litter, animals that were already produced, increases the ability to detect RTMs and presumably other low-incidence phenomena.