| Summary: | Vaiproic acid (VPA) is a commonly used anticonvulsant agent that is
often given in combination with carbamazepine (CBZ) to maximize seizure
control. The incidence of VPA associated hepatotoxicity is increased when
coadministered with other anticonvulsants. One or more of VPAs metabolites
may be responsible for the hepatotoxicity.
The consequences of induction by CBZ, carbamazepine-10,11-epoxide
(CBZE), phenobarbital (PB) and clofibrate (CFB) on the metabolism of VPA and
(E)-2-ene VPA by rat hepatic microsomes were examined. Total hepatic
cytochrome P-450 content and the isozyme(s) of cytochrome P-450 induced by
these agents were determined and compared. The metabolism of VPA was
monitored following induction by quantitating the microsomal formation of 4-
ene VPA, 3-OH VPA, 4-OH VPA, 4-keto VPA and 5-OH VPA. For (E)-2-ene VPA,
the formation of (E)-2,4-diene VPA and (E,E)-2,3’-diene VPA was monitored.
Adult male Long Evans rats (4/group) were treated intraperitoneally (i.p.)
with one of the following: CBZ 100 mg/kg or CBZE 50 mg/kg every 12 h for 3, 7,
10 or 14 days, CFB 350 mg/kg for 7 days, or PB 75 mg/kg for 4 days. The mean
hepatic cytochrome P-450 content for the CBZ treatment groups over the ten
day treatment period was significantly enhanced 1.5 to 1.8 fold compared to the
vehicle control group while CBZE treatment did not appear to affect total
hepatic cytochrome P-450 content. PB treatment resulted in a significant 1.9
fold increase compared to the vehicle control, but a significant increase was not
observed for the CFB treatment group.
Immunoblot analysis indicated that cytochrome P-450b was induced by
PB and also by CBZ and CBZE. Cytochrome P-450b constituted 65 % of the
total hepatic cytochrome P-450 content in the PB induced microsomes and
ranged from 31 to 66 % in the CBZ and CBZE groups over the 14 day treatment
period. Pentoxyresorufin, a substrate for cytochrome P-450b, was preferentially
metabolized compared to ethoxyresorufin by microsomal protein isolated from
PB, CBZ and CBZE treated rats. Mean pentoxyresorufin 0-dealkylation rates
for the CBZ, CBZE and PB treatment groups were enhanced 12 to 53 fold when
compared to their respective vehicle control groups.
The metabolism of VPA and (E)-2-ene VPA was enhanced by PB, CBZ
and to a lesser extent by CBZE treatment. CFB pretreatment did not have any
significant effects on the metabolism of VPA or (E)-2-ene VPA. An antibody
directed against rat cytochrome P-450b was effective in completely inhibiting
the metabolism of VPA to 4-ene VPA by microsomal protein isolated from PB or
CBZ 3 day treated rats. The formation of 4-OH VPA and 4-keto VPA was
inhibited greater than 75 % in the presence of the antibody. The metabolism of
(E)-2-ene VPA to (E)-2,4-diene VPA by microsomal protein from PB or CBZ 3
day treatment groups was inhibited 89 % and 85 % respectively, in the presence
of the anti-rat cytochrome P-450b antibody.
Three days of treatment with CBZ at a dose of 100 mg/kg i.p. every 12 h
was as effective as typical PB treatment for inducing total hepatic cytochrome
P-450 content, cytochrome P-450b, pentoxyresorufin 0-dealkylation, and the in
vitro metabolism of VPA and (E)-2-ene VPA. CBZE, used at an equivalent
molar dose as CBZ, also may be as effective an inducer as PB. Cytochrome P
450b plays an important role in the in vitro metabolism of VPA and (E)-2-ene
VPA to 4-ene VPA and (E)-2,4-diene VPA, respectively. These two metabolites
are known hepatotoxins and their enhanced formation in the presence of
enzyme inducing agents are likely responsible for VPA associated
hepatotoxicity. The isozyme of cytochrome P-450 induced by CBZ or CBZE has
not previously been identified.
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