Abstract:
BACKGROUND : The plight of the white rhinoceros (Ceratotherium simum) and the increasing need of treatment options for injured poaching victims led to the
necessity to expand the knowledge on applicable drugs in this endangered species. With very little information available on drug pharmacokinetics in rhino,
veterinarians have to rely on information generated from other species. The horse
being a closely related species, has served as the model for dose extrapolations.
However, from recent research on enrofloxacin and carprofen, the white rhino
showed considerable differences in the pharmacokinetic properties of these drugs
in comparison to the horse. While the reason for the differences is unknown, a likely
cause may be a difference in present cytochrome P450 (CYP450), which may result
in the rhino being genetically deficient in certain enzyme families.
METHODS : For this paper we assess the degree of similarity of the CYP genome
sequences across the different species, using BLAT (BLAST-like alignment tool)
for the alignment of the nucleotide sequences of the equine CYP450 with
potential homologous nucleotide sequences of the published database from white
rhinos and other mammalian species (cow, pig, dog, sheep, elephant, mouse
and human).
RESULTS : The white rhino nucleotide sequences were 90.74% identical to the equine
sequences. This was higher than the degree of similarity between any of the
other evaluated species sequences. While no specific CYP family were found to be
deficient in the published rhino genome, the horse genome contained additional
genetic sequence for a larger number of iso-enzymes that were not present in
the rhino.
DISCUSSION : In pharmacokinetic study, it is well known that absence of a metabolic
enzyme will result in constraints in drug metabolism and drug elimination.
While this was our speculation, comparison to the horse and other mammalian
species indicate that all the described CYP genes required for metabolism are present
within the rhino genome. These results leave functional differences in enzyme
activity and a lack of isoenzymes as the likely reason for the constraint in drug metabolism. Despite a more than 90% similarity of the equine and rhino gene
sequences, seemingly small differences can have major effects on drug metabolism.
Thus, in spite of the close anatomical relationship, the rhino should not simply be
treated like a big horse.
