Subchronic administration of phenobarbital alters the mutation spectrum oflacI in the livers of Big Blue (R) transgenic mice

Phenobarbital (PHE) is a liver carcinogen in B6C3F1 mice and a weak mutagen that does not appear to form DNA adducts. To investigate PHE mutagenicity in vivo, B6C3F1 Big Blue(R) male transgenic mice harboring the lambda LIZ s huttle vector containing the lad target gene were fed PHE at 2500 ppm for 1 80 days. A modest increase in the mutant frequency (MF) from 5.02 +/- 2.4 x 10(-5) in the control group to 6.88 +/- 0.754 x 10(-5) in the PHE-treated group, which was marginally different (p < 0.05), was obtained. To better a ssess the relevance of this increase in MF, a random collection of mutants from each PHE-exposed mouse was sequenced. After correcting for clonal expa nsion, which is the most conservative approach, the MF in the PHE-treated m ice decreased to 6.39 +/- 1.02 x 10(-5), an insignificant difference (p = 0 .10) from that in control group. Despite this modest increase in MF, the mu tation spectrum obtained from the PHE-exposed group was significantly diffe rent (p < 0.02) from the spontaneously derived spectrum in untreated mice. In the PHE-exposed mice, an increase in the mutation frequency of G:C > T:A and G:C > C:G transversions was recorded while the mutation frequency of G :C > A:T transitions remained the same in the two spectra, It is postulated that the increase in transversions at G:C base pairs found in the PHE-deri ved spectrum is likely due to oxidative damage as a result of induction of CYP2B isozymes by the chronic administration of PHE. Results from this stud y demonstrate that PHE alters the spectrum of mutations, rather than induci ng a significant global increase in the MF. The PHE-derived spectrum of lac I mutants from the liver of Big Blue(R) B6C3F1 male mice was remarkably sim ilar (p = 0.8) to that generated by oxazepam (OX), a compound which also in duces CYP2B isozymes following chronic administration of the drug. (C) 2000 Published by Elsevier Science B.V. All rights reserved.