Objective—To describe the pharmacokinetics of
phenylbutazone and oxyphenbutazone after IV administration
in miniature donkeys.
Animals—6 clinically normal miniature donkeys.
Procedure—Blood samples were collected before
and 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300, 360,
and 480 minutes after IV administration of phenylbutazone
(4.4 mg/kg of body weight). Serum was analyzed
in triplicate by use of high-performance liquid
chromatography for determination of phenylbutazone
and oxyphenbutazone concentrations. The serum
concentration-time curve for each donkey was analyzed
separately to estimate model-independent pharmacokinetic
Results—Serum concentrations decreased rapidly
after IV administration of phenylbutazone, and they
reached undetectable concentrations within 4 hours.
Values for mean residence time ranged from 0.5 to
3.0 hours (median, 1.1 hour), whereas total body
clearance ranged from 4.2 to 7.5 ml/kg/ min (mean,
5.8 ml/kg/ min). Oxyphenbutazone appeared rapidly in
the serum; time to peak concentration ranged from
13 to 41 minutes (mean, 26.4 minutes), and peak concentration
in serum ranged from 2.8 to 4.0 mg/ml
(mean, 3.5 μg/ml).
Conclusion and Clinical Relevance—Clearance of
phenylbutazone in miniature donkeys after injection of
a single dose (4.4 mg/kg, IV) is rapid. Compared with
horses, miniature donkeys may require more frequent
administration of phenylbutazone to achieve therapeutic
efficacy. (Am J Vet Res 2001;62:673–675)