Plasma phenytoin elimination rates were examined among twelve inbred
strains of mice. Two populations are identified the -the fast metabolizers' (BALB/cN,
C57BL/6N, C57BL/6J, AKR/N, AKR/J and C3H/HeN) having almost exactly twice as rapid
an elimination rate as the 'slow metabolizers' (CL/FR, CBA/J, DBA/2N, STAR/N, SJL/N,
DBA/2J and RF/N). The difference in elimination rate between C57BL/6J and DBA/2J
cannot be accountcd for by dissimilarities in volume of distribution. The phenytoin
elimination rate in the (C57BL/6J)(DBA/2J)F1 heterozygote is expressed as an additive
trait. A good correlation exists between phenytoin elimination rates in vivo and phenytoin
metabolism by liver microsornes in vitro, as determined by a newly described assay using
high-performance liquid chromatography. 3-Methylcholanthrene pretreatment does not enhance
phenytoin elimination or metabolism. The cytochrome P-450-mediated monooxygenase
metabolism of phenytoin is not associated with the Ah locus or with coat color
among progeny of the (C57BL/6N)(DBA/2N)F1 X DBA/2N backcross. Phenobarbital
pretreatment enhances phenytoin elimination and metabolism in both a fast metabolizer
(C5 7BL/6N) and a slow metabolizer (DBA/2N) strain. Phenobarbital pretreatment probably
also induces non-P-450 enzymes, such as those which form the phenytoin dihydrodiol and
the glucuronide and glutathione conjugates, in addition to inducing one or more forms of
P-450 that oxygenate phenytoin.
These data probably reflect allelic differences in a structural gene encoding for one
(or more) form(s) of control cytochrome P-450 that metabolizes phenytoin, rather than
allelic differences in a regulatory gene. The marked sensitivity of inbred mouse strains
CL/FR and A/J and the marked resistance of STAR/N, Swiss-Webster, and C57BL/6 to
phenytoin-induced cleft lip and/or palate cannot be explained by genetic differences in
phenytoin elimination rates or liver microsomal metabolism in vitro, as measured by the
methods described in this report.