Comparative and Veterinary Pharmacology

2.1.4 Cytochrome P450 1A2 in Dogs

A pre-clinical pharmacokinetic study of a novel cognitive enhancer (AC-3933) in
Beagle dogs revealed highly variable drug clearance with a bimodal distribution,
such that dogs could be divided into EM and PM groups. In vitro studies confirmed
50- to 100-fold lower intrinsic clearance (Vmax/Km) values in liver microsomes from
PM compared with EM dogs (Mise et al.2004b). Furthermore, CYP1A protein
could not be detected by immunoblotting in the PM dogs (Mise et al.2004a; Mise
et al.2004b) (Fig. 2 ). Two groups independently identified a nonsense mutation in
canine CYP1A2 cDNA that introduced a premature stop codon instead of an
arginine at position 373 (c.1117 C>T; R373X), thereby eliminating the predicted
heme-binding region and resulting in a non-functional enzyme (Mise et al.2004a;
Tenmizu et al. 2004 ). The genotypic frequency of the homozygous mutant geno-
type in CYP1A2 was 0.11 (in 149 Beagles) and 0.17 (in 65 Beagles) implying that
11–17% of Beagles would not express CYP1A2 protein (Mise et al.2004a;
Tenmizu et al. 2004 ). Pre-clinical studies of a novel phosphodiesterase type 4
inhibitor, YM-64227, also metabolised by canine CYP1A2, reported 17 and 27
times higher maximum plasma drug concentrations and bioavailability, respec-
tively for PM dogs compared with EM dogs following oral administration, thus
indicating a large effect of CYP1A2 deficiency on first-pass drug metabolism
(Tenmizu et al.2006a,2006b). More recently, the substrate specificity of canine
CYP1A2 was examined and, while there was overlap between human and canine
substrates, some typical human CYP1A2 substrates, including caffeine and oestra-
diol, were not good substrates for dog CYP1A2 (Mise et al. 2008 ). Nevertheless, as
the canine CYP1A2 R373X polymorphism represents a true “null” allele, this
polymorphism should provide a useful tool for in vitro and in vivo studies seeking
to identify the contribution of canine CYP1A2 to metabolism of drugs in dogs. To

PM1

m/mm/mm/mm/mwt/mwt/wtwt/wtwt/m

2 PM2PM3PM4EM1EM2EM3EM4

1

0

–1

–2

Log(SX5745/AC3933)

150 bp

CYP1A

EM1 EM2 PM1 PM2

CYP2B

CYP2C

CYP3A

100 bp

86 bp 50 bp

124 bp

38 bp

Fig. 2Canine CYP1A2 protein and activities in dog liver genotyped for theCYP1A2c.1117c>t
(R373X) polymorphism.Left panel(from Mise et al. (2004a)) shows the log ratios of metabolite
(SX5745) to parent (AC3933) concentrations in the plasma of four PMs and four EMs Beagle dogs
after being administered the putative CYP1A2 substrate, AC3933. Below are the CYP1A2
c.1117c>t genotypes for each animal determined by PCR-RFLP analysis (wt¼wild-type C
allele; m¼mutant T allele). Theright panel(from Mise et al. (2004b)) shows immunoblots of
liver microsomes from EM and PM dogs probed with antibodies that recognise CYP1A, CYP2B,
CYP2C, and CYP3A enzymes. PM dogs do not express any detectable CYP1A protein

54 C.M. Mosher and M.H. Court