Scientific evaluation of the data-derived safety factors for the acceptable daily intake. Case study: diethylhexylphthalate.

Diethylhexylphthalate causes peroxisome proliferation and is hepatocarcinogenic in rodents; it also displays reproductive and developmental toxicity in a variety of mammalian and non-mammalian species. These manifestations of toxicity have each been separately evaluated for the development of a data-derived safety factor and Tolerable Daily Intake (TDI). Using hepatocarcinogenicity as the pivotal study, the nature of toxicity factor of 10 is applicable and there are no adequate studies demonstrating a No-Observed-Adverse-Effect Level (NOAEL). If studies of less statistical sensitivity are used to derive the NOAEL and a factor of 0.1 is used for the relative sensitivity to humans of peroxisome proliferation (assuming this is linked mechanistically to carcinogenesis), a TDI of 1 mg/kg bw is obtained. The data-derived safety factor using peroxisomal proliferation as the pivotal end-point is 6.25, since the factor from trans-species toxicodynamics is 0.01, and the TDI derived from the NOAEL for peroxisome proliferation is thus 8 mg/kg bw. If teratogenicity is used as the pivotal study, the nature of toxicity attracts a factor of 10 and all the other aspects take default values because of the limited availability of relevant toxicodynamic and toxicokinetic data. The TDI derived from the NOAEL for teratogenicity is then 0.04 mg/kg bw and this confirms teratogenicity as the limiting aspect of toxicity defining the TDI. It also identifies the fact that appropriate toxicokinetic and toxicodynamic data related to the pregnant animal and fetus would facilitate a re-evaluation of the safety factor and TDI by replacing the current default values by data-derived values.