Pharmacokinetics and pharmacodynamics
of acepromazine in horses
P. J. Marroum, PhD; A. I. Webb, BVSc, PhD; Gina Aeschbacher, Dr Med Vet; S. H. Curry, PhD, DSc (Med)
A specific, sensitive, reverse-phase high-per-
formance liquid chromatographic assay for aceprom-
azine, with analytic sensitivity as low as 5 ng/ml of
plasma, and electrochemical detection with an oxi-
dation potential of 0. 7 V, was used to study the phar-
macokinetics of acepromazine given at a dosage of
0.15 mg/kg of body weight in horses. The relation
between effect and pharmacokinetics of the drug was
examined. The effects studied included those on
blood pressure, pulse, PCV, measures of respiration
function, and sedation. Intravenously administered
doses led to a biphasic concentration decay pattern
with an a-phase distribution half-life of< 3 minutes.
The J3-phase half-life was in the range of 50 to 150
minutes. The CNS effects peaked at 20 minutes after
administration, and the hemodynamic effects peaked
at 100 minutes. In all horses, the most sensitive var-
iable was the PCV, which decreased by up to 20% (P
< 0.0001). Systolic, diastolic, and mean blood pres-
sures decreased (P < 0.0001); heart rate was unchan-
ged (P > 0.05). Neither blood gas tensions nor blood
pH changed noticeably (P > 0.05). In all horses stud-
ied, acepromazine had a significant (P < 0.0001) sed-
ative effect, as observed by posture and alertness.
None of the observed pharmacodynamic effects cor-
related well with plasma acepromazine concentra-
tion. These effects persisted beyond the time of
detectable acepromazine concentration, indicating
that they might be caused by active metabolites, or
that their timing could result from complex phar-
macokinetic compartment influences.
Acepromazine is a sedative/tranquilizer com-
monly used in horses, cats, and dogs. Its pharmaco-
Received for publication Mar 12, 1993.
From the Department of Pharmaceutics (College of Phar-
macy), and the Department of Medical Science (College of Veter-
inary Medicine), Health Science Center, University of Florida,
Gainesville, FL 31610.
Dr. Marroum's present address is Division of Biopharmaceu-
tics, US Food and Drug Administration, 5600 Fishers Lane, Rock-
ville, MD 20857. Dr. Curry's present address is Fison's Pharma-
ceuticals, Divisional Research and Development, Jefferson Road,
PO Box 1710, Rochester, NY 14603-1710.
Published as Florida Agricultural Experimental Station Journal .
Series No. R-034251.
Address reprint requests to Dr. Curry.
logic effects are similar to those of other pheno-
thiazines, but it is considered more potent than the
prototype drug, chlorpromazine.1 Despite its wide-
spread use, little is known of the metabolism and
pharmacokinetics of acepromazine. In fact, it appears
that only 3 relevant reports have been published.2-4
In 1 study, 2 acepromazine was detected in horses for
8 hours after IV administration of 0 .3 mg/kg of body
weight.2 The plasma decay was biexponential, with
mean a-phase half-life of 4.2 minutes and mean J3-
phase half-life of 184.8 minutes. The apparent vol-
ume of distribution was 6.6 L/kg. Protein binding
exceeded 99%. Drug distribution was approximately
even between plasma and RBC. Dewey et al3 reported
that the major urinary metabolite of acepromazine in
mares is unconjugated 2-(1-hydroxyethyl) promazine
sulfoxide. Conjugated 7-hydroxy-acepromazine and
also were isolated. Miller et al4 studied oral admin-
istration of a paste form of acepromazine and its tim-
ing in relation to feeding.
The objectives of the study reported here were
to use a high-peformance liquid chromatography
(HPLC) assay for acepromazine in biological fluids,
and to describe the pharmacokinetic profile after IV
administration of 0.15 mg/kg in horses. Parallel stud-
ies of the pharmacokinetic properties of the drug and
various pharmacodynamic measurements, such as
blood pressure, heart rate, degree of sedation, PCV,
and arterial blood gas tensions, were conducted.
Materials and Methods
The following analytic grade materials were
used: acepromazine maleate8; trimeprazine tartrateb;
acetonitrile, hexane, ammonium acetate, and sodium
acetatec; hexamethyldisilazined; sodium heparin e; and
Apparatus-The HPLC system consisted of a sol-
vent delivery system, an automatic injector, and a
data module,g a recorder, h and an electrochemical
detector.i The HPLC column was nitrile-bonded (13
cm X 5 µmi).
a Fort Dodge Laboratories, Fort Dodge, Iowa.
b Smith Klfue & French Laboratories, Philadelphia, Pa.
c Fisher Scientific, Pittsburgh, Pa.
d SCM Specialty Chemicals, Gainesville, Fla.
e Lyphomed Inc, Rosemont, ill.
f Kendall McGaw Laboratories, Inc, Irvine, Calif.
g Model 6000, WISP 710~ and model M730, Waters Asso-
ciates, Millford, Mass.
h Recordall 5000, Fisher Scientific, Pittsbur~ Pa.
l Model 5100A Colochem, ESA Inc, Bedford, Mass.
1 Zorbax CN, Mac-Mod Analytical Inc, Chadds Ford, Pa.
Am J Vet Res, Vol SS, No. 10, October 1994
Analysis of acepromazine in biological fluids-Sam-
ples (0.5 to 2 ml, depending on availability, and ac-
curately measured) of biological fluids, such as
plasma, were alkalinized by addition of 0.1 ml of lN
NaOH. An appropriate amount of the internal stan-
dard trimeprazine (to a concentration of 150 ng/ml)
was added. This alkaline sample was then extracted
with 5 ml of hexane for 1 hour, using a mechanical
shaker that caused inversion of the tube with a fre-
quency of at least 30 cycles/min. After centrifugation,
an aliquot of the hexane phase was removed and
evaporated to dryness at 25 C under a constant
stream of nitrogen. If any emulsion persisted after
centrifugation, the contents of the tube were gently
stirred with a glass rod, followed by further centri-
fugation. The residue was redissolved in an appro-
priate volume of mobile phase, usually 250 µl, and
an aliquot of this reconstituted residue was injected
into the chromatographic system. The mobile phase
consisted of 75:25 acetonitrile-0.lM acetate buffer,
pH 4.75. The flow rate was 1.2 ml/min. The oxidation
potential was set at 0.7 V for the analytic cell and
0.75 V for the guard cell.
Phannacokinetics of acepromazine after N admin-
istration of 0.15 mg/kg-The kinetic studies were con-
ducted, using 4 female and 2 male adult horses that
had been donated to the College of Veterinary Med-
icine at the University of Florida. Five horses were
used in the pharmacokinetic and the pharmacody-
namic studies. One horse (No. 6) was only used in
pharmacokinetic studies. Ages ranged from 2 to 20
years, with that that for horse 6 unknown. Four were
Thoroughbred, and 2 were Arabian. Body weight
ranged from 389 to 543 kg. All were thought to be
healthy at the time of the investigation, but during
the course of the investigation, horse 1 was found to
have a vegetative lesion on the aortic valve, on the
basis of echocardiography indicating regurgitation,
and later postmortem examination revealing marked
pulmonary congestion. Prior to the study, horses
were acclimatized to their surroundings. Aceproma-
zine maleate solution (concentration, 0.5 mg/ml) was
injected into the right jugular vein, and blood samples
from the left jugular vein were withdrawn through a
14-gauge, 5-inch Teflon over-the-needle catheter
into heparinized evacuated tubes. Catheters were
flushed with heparinized saline solution. Sample col-
lection times were 0 (predos~) and 1.5, 3, 4.5, 6, 7.5,
9, 12, 15, 18, 24, 30, 45, 60, 75, 90, 120, 150, 180, 210,
240, 300, and 360 minutes after injection or a selec-
tion of times thereof. Each venous blood sample,
consisting of 5 ml, was used to measure PCV, then
was centrifuged immediately afterward at 800 X g to
separate plasma and RBC. Samples from 4 of the
horses (2 male and 2 female) also were collected from
the arterial line installed for blood gas analysis.
Blood pressure measurements-Systemic systolic,
diastolic, and mean blood pressures were obtained by
transducing the pressures obtained by use of a per-
cutaneous catheter placed in the transfacial artery. The
transducer was powered by a multichannel oscillo-·
scope with digital pressure and heart rate displays.
Am J Vet Res, Vol 55, No. 10, October 1994
Heart rate-The heart rate was measured by the
rate counter on the multichannel oscilloscope that
determined the number of arterial pulse waves per
10-second period and gave a beats-per-minute count
Blood gas analysis-Systemic arterial blood gas
tensions and pH were measured from samples col-
lected anaerobically through the arterial catheter into
a heparinized plastic syringe. These samples were an-
alyzedk within 2 hours of collection.
Sedative effects-Such effects were evaluated, us-
ing a scale devised for the purpose of this work (Ap-
pendix). Degree of sedation, general behavior,
posture, and general alertness of each horse were
evaluated. All horses were rated by the same person
to avoid any observer-dependent subjective differ-
ences. Rating was on an open basis, without blinding.
Evaluation and fitting of phannacokinetic data-
Plasma concentrations of acepromazine were evalu-
ated, using commercial software packages.Lm After IV
administered doses, the equation for the linear sum
of exponentials was:
Cpt = Ae-a.t + Be-~t
where Cp is the plasma concentration, A, B, a, and
~are constants, and tis time in minutes. Goodness-
of-fit parameters were the sum of squared residuals,
sum of weighted squared residuals, and correlation.
Statistical review of phannacodynamic data-Raw