of in vitro celecoxib hydroxylase activity in liver microsomal preparations from
representative EM and PM phenotyped dogs showed a similar pattern of bimodal
phenotypes and concordance between low in vitro celecoxib oxidation and low
in vivo celecoxib clearance (Fig. 1 ). Screening of recombinant canine CYP2B11,
CYP2C21, CYP2D15, and CYP3A12 identified CYP2D15 as the main isoform
mediating celecoxib hydroxylation, as well as bufuralol oxidation, which is a
selective marker for human CYP2D6 activity. Celecoxib metabolism was also
potently inhibited by quinidine, a selective inhibitor of human CYP2D6. Taken
together, these results indicate that there may be a genetic polymorphism associated
with canine CYP2D15 that results in decreased expression or activity of this
enzyme. Two amino acid variants of CYP2D15 (CYP2D152 – I250F and
I307V; CYP2D153 – G186S), as well as a putative splice variant that deleted 51
amino acids in exon 3 (CYP2B15del), were identified by sequencing Beagle dog
genomic DNA (Paulson et al. 1999 ) or cDNA derived from dog liver mRNA
(Roussel et al. 1998 ). Recombinant expression of these variants indicated only
minimal, if any, effect of the 2 and 3 amino acid variants, and almost complete
abolition of activity with the CYP2D15del variant. However, as yet it is not clear
whether the presence or amount of the CYP2D15del mRNA variant (relative to full
length UGT2B15 mRNA) is associated with decreased CYP2D15 activity in dog
liver. Also, there has been no attempt to correlate the presence of any CYP2D15
genetic polymorphism with celecoxib metabolism phenotype. This polymorphism
may affect metabolism of other drugs, including propranolol, bufuralol and dextro-
methorphan, which are known CYP2D15 substrates (Tasaki et al. 1998 ; Shou et al.
2003 ).
Fig. 1Polymorphism of celecoxib pharmacokinetics in Beagle dogs (from (Paulson et al. 1999 )).
Shown in theleft panelare plasma concentrations of celecoxib following intravenous administra-
tion of 5 mg/kg bodyweight of celecoxib to 19 male Beagle dogs. Results with female dogs were
similar.Solid linesindicate dogs with the PM phenotype, whiledotted linesindicate dogs with the
EM phenotype. Theright panelshows celecoxib and bufuralol hydroxylase activities for recom-
binant canine CYP isoforms indicating that both drugs are oxidised by CYP2D15 and that the exon
3 deletion (rCYP2D15delta) greatly decreases activity
52 C.M. Mosher and M.H. Court