Covalent binding of styrene to DNA in rat and mouse

Covalent binding of [7-3H]styrene (S) to DNA in vivo was measured and evaluated in a quantitative manner in order to investigate whether DNA adduct formation could form a mechanistic basis for tumor induction in a carcinogenicity bioassay. [7-3H]S was administered by inhalation in a closed chamber to male and female CD rats and B6C3F1 mice. After 4.5-6 h (rats) and 6-9 h (pools of four mice), S doses of 23-39 and 85-110 mg/kg respectively had been metabolized. DNA was purified to constant specific radioactivity which was measurable in all samples. DNA was enzymatically degraded to the 3'-nucleotides which were separated by HPLC for the detection of radiolabeled nucleotide-S adducts. The fractions with the normal nucleotides contained most of the radioactivity. In mouse liver DNA, a minute but significant level of adduct radioactivity was also detected. In the units of the Covalent Binding Index CBI = (mumol adduct/mol DNA nucleotide)/(mmol chemical/kg body wt), values of 0.05-0.09 and 0.07-0.18 were calculated for males and females respectively. In the rat, no DNA adducts were detectable in the liver at a limit of detection of 0.1 CBI units. Two of the four lung samples of the female rats showed adduct-related radioactivity corresponding to 0.07 CBI units. The CBI values are compatible with styrene 7,8-oxide as the reactive intermediate. The data are compared with CBI values and carcinogenic potencies of established genotoxic carcinogens. It is concluded that the DNA-binding potency of S is so low that significant tumor induction in a standard bioassay for carcinogenicity is unlikely to be due to DNA adduct formation alone. Consequences for a human risk estimation are discussed.