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Pharmacokinetics and pharmacodynamics of acepromazine ... Pharmacokinetics and pharmacodynamics of acepromazine in horses P. J. Marroum, PhD; A. I. Webb, BVSc, PhD; Gina Aeschbacher,

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  • 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)

    Summary 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 conjugated 2-(1-hydroxyethyl)-7-hydroxy-promazine 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 sodium chloride.f

    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 output.

    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 data were

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