• View in gallery
    Figure 1—

    Plasma concentrations of trazodone (black symbols) and the trazodone metabolite m-CPP (white symbols) over time following IV or oral administration of trazodone to healthy adult Thoroughbred horses in an escalating manner during a pilot (dose finding) investigation (n = 2 horses/group). A—Concentrations at predetermined time points before, during, and after IV drug administration (0.5 mg/kg followed by 1.0 mg/kg and finally 2.0 mg/kg, with 15 minutes in between administrations). B—The same data in panel A from time 0 to 10 hours after the first drug administration are provided in greater detail. C—Concentrations at predetermined time points before, during, and after oral drug administration (2 mg/kg followed 1 hour later by 4 mg/kg). Time 0 data were obtained immediately prior to the first drug administration. Each symbol (circle or square) in a given panel represents an individual horse. All study horses participated in a fitness training program.

  • View in gallery
    Figure 2—

    Mean ± SD plasma concentrations of trazodone (black symbols) and m-CPP (white symbols) over time following IV (1.5 mg/kg; panel A) or oral administration (4 mg/kg; panel B) of the drug to healthy adult Thoroughbred horses (n = 6) in a pharmacokinetic and pharmacodynamic study. All horses received the IV formulation first and received the oral formulation after a 5-week washout period. Time 0 data were obtained immediately prior to drug administration.

  • View in gallery
    Figure 3—

    Mean ± SD chin-to-ground distance over time following IV (black circles) or oral administration (white circles) of trazodone to the same 6 horses as in Figure 2. See Figure 2 for remainder of key.

  • View in gallery
    Figure 4—

    Mean ± SD heart rate (A) and percentage atrioventricular block (B) over time following IV (black circles) or oral administration (white circles) of trazodone to the same 6 horses as in Figure 2. The percentage atrioventricular block was calculated as ([atrial beats – ventricular beats]/atrial beats) × 100. *Value differs significantly (P < 0.05) from that at time 0. See Figure 2 for remainder of key.

  • 1. Frazer A, Hensler JG. Serotonin. In: Siegel GJ, Agranoff BW, Albers RW, et al, eds. Basic neurochemistry. 6th ed. Philadelphia: Lippincott-Raven, 1999; 263292.

    • Search Google Scholar
    • Export Citation
  • 2. Stahl SM. Mechanism of action of trazodone: a multifunctional drug. CNS Spectr 2009; 14: 536546.

  • 3. Haria M, Fitton A, McTavish D. Trazodone. A review of its pharmacology, therapeutic use in depression and therapeutic potential in other disorders. Drugs Aging 1994; 4: 331355.

    • Search Google Scholar
    • Export Citation
  • 4. Mittur A. Trazodone: properties and utility in multiple disorders. Expert Rev Clin Pharmacol 2011; 4: 181196.

  • 5. Bossini L, Casolaro I, Koukouna D, et al. Off label uses of trazodone: a review. Expert Rev Clin Pharmacol 2012; 13: 17071717.

  • 6. Rotzinger S, Fang J, Baker GB. Trazodone is metabolized to m-chlorophenylpiperazine by CYP3A4 from human sources. Drug Metab Dispos 1998; 26: 572575.

    • Search Google Scholar
    • Export Citation
  • 7. Wen B, Ma L, Rodrigues D, et al. Detection of novel reactive metabolites of trazodone: evidence for CYP2D6-mediated bioactivation of m-chlorophenylpiperazine. Drug Metab Dispos 2008; 36: 841850.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8. Gruen ME, Sherman BL. Use of trazodone as an adjunctive agent in the treatment of canine anxiety disorders: 56 cases (1997–2007). J Am Vet Med Assoc 2008; 233: 19021907.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9. Jay AR, Krotscheck U, Parsley E, et al. Pharmacokinetics, bioavailability, and hemodynamic effects of trazodone after intravenous and oral administration of a single dose to dogs. Am J Vet Res 2013; 74: 14501456.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Knych HK, Arthur RM, Mitchell MM, et al. Pharmacokinetics and selected pharmacodynamics of cobalt following a single intravenous administration to horses. Drug Test Anal 2015; 7: 619625.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Yamaoka K, Nakagawa T, Uno T. Application of Akaike's information criterion (AIC) in the evaluation of linear pharmacokinetic equations. J Pharmacokinet Biopharm 1978; 6: 165175.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12. Mama KR, Grimsrud K, Snell T, et al. Plasma concentrations, behavioural and physiological effects following intravenous and intramuscular detomidine in horses. Equine Vet J 2009; 41: 772777.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Gruen ME, Roe SC, Griffith E, et al. The use of trazodone to facilitate postsurgical confinement in dogs. J Am Vet Med Assoc 2014; 245: 296301.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Orlando JM, Case BC, Thomson AE, et al. Use of oral trazodone for sedation in cats: a pilot study. J Feline Med Surg 2016; 18: 476482.

  • 15. Kale P, Agrawal YK. Pharmacokinetics of single oral dose trazodone: a randomized, two-period, cross-over trial in healthy, adult, human volunteers under fed condition. Front Pharmacol 2015; 6: 224.

    • Search Google Scholar
    • Export Citation
  • 16. Nilsen OG, Dale O. Single dose pharmacokinetics of trazodone in healthy subjects. Pharmacol Toxicol 1992; 71: 150153.

  • 17. Vatassery GT, Holden LA, Hazel DK, et al. Determination of trazodone and its metabolite, 1-m-chlorophenyl-piperzine, in human plasma and red blood cell samples by HPLC. Clin Biochem 1997; 30: 149153.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Mercolini L, Colliva C, Amore M, et al. HPLC analysis of the antidepressant trazodone and its main metabolite mCPP in human plasma. J Pharm Biomed Anal 2008; 47: 882887.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 19. Zanger UM, Schwab M. Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities and impact of genetic variation. Pharmacol Ther 2013; 138: 103141.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20. Corado CR. McKemie, Young A, Knych HK. Evidence for polymorphism in the cytochrome P450 2D6 gene in horses. J Vet Pharmacol Ther 2016; 39: 245254.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21. Corado CR. McKemie, Knych HK. Dextromethorphan and debrisoquine metabolism and polymorphism of the gene for cytochrome P450 isozyme 2D50 in Thoroughbreds. Am J Vet Res 2016; 77: 10291035.

    • Crossref
    • Search Google Scholar
    • Export Citation

Advertisement

Pharmacokinetics and selected pharmacodynamics of trazodone following intravenous and oral administration to horses undergoing fitness training

Heather K. KnychK. L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616
Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616

Search for other papers by Heather K. Knych in
Current site
Google Scholar
PubMed
Close
 DVM, PhD
,
Khursheed R. MamaDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

Search for other papers by Khursheed R. Mama in
Current site
Google Scholar
PubMed
Close
 DVM
,
Eugene P. SteffeyVeterinary Surgery and Radiology, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616

Search for other papers by Eugene P. Steffey in
Current site
Google Scholar
PubMed
Close
 VMD, PhD
,
Scott D. StanleyK. L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616
Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616

Search for other papers by Scott D. Stanley in
Current site
Google Scholar
PubMed
Close
 PhD
, and
Philip H. KassPopulation Health and Reproduction, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616

Search for other papers by Philip H. Kass in
Current site
Google Scholar
PubMed
Close
 DVM, PhD

Abstract

OBJECTIVE To measure concentrations of trazodone and its major metabolite in plasma and urine after administration to healthy horses and concurrently assess selected physiologic and behavioral effects of the drug.

ANIMALS 11 Thoroughbred horses enrolled in a fitness training program.

PROCEDURES In a pilot investigation, 4 horses received trazodone IV (n = 2) or orally (2) to select a dose for the full study; 1 horse received a vehicle control treatment IV. For the full study, trazodone was initially administered IV (1.5 mg/kg) to 6 horses and subsequently given orally (4 mg/kg), with a 5-week washout period between treatments. Blood and urine samples were collected prior to drug administration and at multiple time points up to 48 hours afterward. Samples were analyzed for trazodone and metabolite concentrations, and pharmacokinetic parameters were determined; plasma drug concentrations following IV administration best fit a 3-compartment model. Behavioral and physiologic effects were assessed.

RESULTS After IV administration, total clearance of trazodone was 6.85 ± 2.80 mL/min/kg, volume of distribution at steady state was 1.06 ± 0.07 L/kg, and elimination half-life was 8.58 ± 1.88 hours. Terminal phase half-life was 7.11 ± 1.70 hours after oral administration. Horses had signs of aggression and excitation, tremors, and ataxia at the highest IV dose (2 mg/kg) in the pilot investigation. After IV drug administration in the full study (1.5 mg/kg), horses were ataxic and had tremors; sedation was evident after oral administration.

CONCLUSIONS AND CLINICAL RELEVANCE Administration of trazodone to horses elicited a wide range of effects. Additional study is warranted before clinical use of trazodone in horses can be recommended.

Abstract

OBJECTIVE To measure concentrations of trazodone and its major metabolite in plasma and urine after administration to healthy horses and concurrently assess selected physiologic and behavioral effects of the drug.

ANIMALS 11 Thoroughbred horses enrolled in a fitness training program.

PROCEDURES In a pilot investigation, 4 horses received trazodone IV (n = 2) or orally (2) to select a dose for the full study; 1 horse received a vehicle control treatment IV. For the full study, trazodone was initially administered IV (1.5 mg/kg) to 6 horses and subsequently given orally (4 mg/kg), with a 5-week washout period between treatments. Blood and urine samples were collected prior to drug administration and at multiple time points up to 48 hours afterward. Samples were analyzed for trazodone and metabolite concentrations, and pharmacokinetic parameters were determined; plasma drug concentrations following IV administration best fit a 3-compartment model. Behavioral and physiologic effects were assessed.

RESULTS After IV administration, total clearance of trazodone was 6.85 ± 2.80 mL/min/kg, volume of distribution at steady state was 1.06 ± 0.07 L/kg, and elimination half-life was 8.58 ± 1.88 hours. Terminal phase half-life was 7.11 ± 1.70 hours after oral administration. Horses had signs of aggression and excitation, tremors, and ataxia at the highest IV dose (2 mg/kg) in the pilot investigation. After IV drug administration in the full study (1.5 mg/kg), horses were ataxic and had tremors; sedation was evident after oral administration.

CONCLUSIONS AND CLINICAL RELEVANCE Administration of trazodone to horses elicited a wide range of effects. Additional study is warranted before clinical use of trazodone in horses can be recommended.

Serotonin is a broadly distributed central and peripheral nervous system neurotransmitter, and modulation has various clinical effects ranging from facilitating sleep to combating nausea.1 Trazodone, a medication registered for use in human patients in the United States, modifies serotonin concentrations. It is considered unique among the SSRI drugs because it has dose-dependent serotonin antagonist and agonist (as an SSRI) properties.2,3 At lower doses, the antagonist properties predominate and result in hypnosis, thereby enhancing sleep in people. As the dose is increased, the SSRI effects predominate, and the drug has anxiolytic, antiobsessive, and antidepressant actions.2–5 In people, the effects of trazodone are attributed to both the parent drug and the active metabolite m-CPP.6,7 Although, to our knowledge, this metabolite has not been studied in other species, the parent compound trazodone has gained popularity for the treatment of anxiety disorders in dogs as well as for calming dogs requiring confinement after surgical procedures in both hospital and home environments.8,9

To the best of the authors’ knowledge, there are no reports describing the pharmacology of trazodone or m-CPP in horses. Given its pharmacological profile in people and in dogs, the drug is likely to have use as a behavioral modifier in horses. If behavioral effects similar to those observed in other species occur in horses, trazodone has the potential to be used as a calming medication in a variety of circumstances. Knowledge of the pharmacodynamic profile of the drug is essential to further identifying potential beneficial and adverse effects. Characterization of the pharmacokinetic profile of trazodone and m-CPP (if the metabolite is produced in horses) would additionally facilitate detection in the event of its inappropriate use to modulate performance in sport horses. The objective of the study reported here was to measure concentrations of trazodone and m-CPP in plasma and urine after administration of trazodone to healthy exercised horses and to concurrently assess selected physiologic and behavioral effects of the drug in this species.

Materials and Methods

Horses

Eleven 4- to 7-year-old healthy, exercise-fit Thoroughbred research horses were included in the prospective pharmacokinetic and pharmacodynamic study. The pilot investigation included 5 horses; 2 mares and 2 geldings (mean ± SD body weight, 412 ± 72 kg and 405 ± 57 kg for horses that received the drug IV or orally, respectively) were administered trazodone, and one 365-kg gelding received the vehicle (DMSO) only. Six horses (3 mares and 3 geldings) were used in the full pharmacokinetic-pharmacodynamic study. Each of these 6 horses received both treatments, with a slight weight difference between experiments (mean ± SD, 568 ± 39 kg and 562 ± 39 kg at the time of IV and oral drug administration, respectively). All horses were part of a research herd at the University of California-Davis and were deemed healthy on the basis of results of physical examination, a CBC, and serum biochemical analysis. All horses were exercised 5 d/wk according to standard protocols established by our laboratory.10 For study purposes, the exercise regimen was discontinued for the day of drug administration for each treatment and resumed after the 24-hour blood sample was collected. Horses were acclimatized to the study environment for ≥ 24 hours prior to data collection with no food or water restrictions.

Study horses received no medications for ≥ 3 weeks prior to commencement of the study. Hay was withheld for 8 hours prior to drug administration and for 2 hours afterward. Water was available ad libitum throughout the study. The study was approved by the Institutional Animal Care and Use Committee of the University of California-Davis.

IV catheter placement

On the day of IV trazodone administration, a 14-gauge, 5.25-inch IV cathetera was placed in the left jugular vein, and an identical catheter was placed in the right jugular vein after collection of baseline pharmacodynamic data and body weight determination. One catheter was used only for trazodone administration and the other only for sample collection. On the day of oral drug administration, only 1 IV catheter was placed. Prior to catheter placement, the hair was clipped and the skin area was prepared by completing a series of betadine and alcohol scrubs. The over-the-needle style catheters were inserted percutaneously with aseptic techniques and sutured into place once positioning in the vein was confirmed. The catheter used for trazodone administration was removed after IV drug delivery.

Test compound

Owing to a lack of commercially available injectable formulations of trazodone, the drug was compounded for IV administration. Trazodone hydrochloride powderb was purchased from a commercial source and dissolved in 17% DMSO in a sterile hood. Sterile saline (0.9% NaCl) solution was added to bring the test formulation to the appropriate concentration, and the solution was filter-sterilized. The drug was administered ≤ 10 minutes after mixing. The same powderb was used for oral administration.

Drug administration

Pilot investigation—No information pertaining to trazodone dosing in horses was available prior to the study. Therefore, a limited investigation was conducted to evaluate potential adverse effects and select a drug dose for the full pharmacokinetic-pharmacodynamic study. Two horses received trazodone IV, and another 2 horses received the drug orally. An additional horse was administered sterile saline solution with 17% DMSO (total volume, 11 mL) to ensure that the vehicle did not have any pharmacodynamic effects that would interfere with the assessment of the effects elicited by trazodone administration. The IV administration of trazodone was conducted in a dose escalation manner, whereby an initial dose of 0.5 mg/kg was followed by a dose of 1.0 mg/kg and finally a dose of 2.0 mg/kg, with 15 minutes in between administrations. All IV injections were administered for over 1 minute. For oral administration, a dose of 2 mg/kg was administered initially, followed by a dose of 4 mg/kg 1 hour later. Each oral dose was mixed into commercially available feedc (0.23 kg) moistened with 20 mL of saline solution prior to administration. Horses were monitored to ensure complete consumption of the grain and drug.

Full pharmacokinetic-pharmacodynamic study—The full study was conducted with a block design, whereby the 6 horses first received the IV formulation. Following a 5-week washout period, the same 6 horses were administered the drug orally. On the basis of the pharmacodynamic results of the pilot investigation, an IV dose of 1.5 mg/kg, administered over a 1-minute period, was chosen for the full study. For oral administration, 4 mg of trazodone hydrochloride powder/kg was added to grain (0.23 kg) as described for the pilot investigation, and horses were monitored to ensure complete consumption of the grain and drug.

Sample collection

Pilot investigation—A blood sample (10 mL) was collected at time 0 (immediately prior to drug administration), with additional samples collected 3, 5, 10, and 15 minutes after administration of dose 1; 3, 5, 10, and 15 minutes after administration of dose 2; and 5, 10, 15, 30, and 45 minutes and 1, 1.5, 2, 3, 4, 6, 8, 10, 24, 36, and 48 hours after administration of dose 3 for measurement of plasma drug concentrations. Prior to obtaining each sample for analysis of drug concentrations, 10 mL of blood was aspirated and discarded from the catheter and T-Port extension set (combined internal volume < 2 mL). The catheter was flushed with 10 mL of a dilute heparinized saline solution (10 IU/mL) after each sample collection. Blood samples were collected into EDTA-containing plasma tubesd and immediately placed on ice prior to centrifugation at 1,620 × g for 10 minutes at 4°C. Plasma was immediately transferred into storage cryovialse and stored at −20°C until analysis by LC-MS-MS. Samples were collected from the dedicated venous catheter up to the first 18 hours after drug administration (when the catheter was removed) and via direct venipuncture thereafter.

Full pharmacokinetic-pharmacodynamic study—Blood samples (10 mL) were collected at time 0 (immediately prior to drug administration) and at 2, 5, 10, 15, 30, and 45 minutes as well as 1, 1.5, 2, 3, 4, 6, 8, 10, 12, 18, 24, 36, and 48 hours after drug administration by the designated route for measurement of trazodone and m-CPP concentrations. Samples were collected as described for the pilot investigation.

Urine samples were collected from all horses by free catch at approximately 4, 24, and 48 hours after trazodone administration for measurement of trazodone and m-CPP concentrations. Samples were collected as close to the predetermined time points as possible, and the actual time of collection was recorded. Urine samples were stored at −20°C until analysis by LC-MS-MS.

Determination of drug concentrations

Plasma sample analysis—Trazodone and m-CPP concentrations were quantitated in equine plasma by LC-MS-MS analysis of protein-precipitated samples fortified with trazodonef or m-CPPg internal standards. Plasma calibrators were prepared by dilution of the working standard solutions with drug-free equine plasma to concentrations ranging from 0.1 to 5,000 ng/mL for trazodone and 0.1 to 40 ng/mL for m-CPP. Calibration curves and negative control samples were freshly prepared for each quantitative assay. In addition, quality control samples (equine plasma fortified with analyte [trazodone and m-CPP] at 3 concentrations within the standard curve) were included with each sample set as an additional means of checking accuracy.

Quantitative analysis of plasma and urine was performed on a triple-quadrupole mass spectrometerh coupled with a turbulent flow chromatography systemi and liquid chromatography systemj and operated in laminar flow mode. Chromatography was performed with a 10-cm × 2.1-mm C18 column.k

Detection and quantification were conducted by selective reaction monitoring of the initial precursor ion for trazodone (m/z, 372.1) and m-CPP (m/z, 197.1) and the labeled internal standards for trazodone (m/z, 378.1) and m-CPP (m/z, 205.1). The response for the product ions for trazodone (m/z, 78.1, 96.1, and 120.2) and m-CPP (m/z, 118.1 and 154.1) and the internal standards for trazodone (m/z, 78.1, 150.1, and 182.2) and m-CPP (m/z, 122.1, 123.2, and 158.1) were plotted, and peaks at the proper retention time were integrated with quantitative analysis software.l The software was used to generate calibration curves and quantitate trazodone and m-CPP in all samples by linear regression analysis. A weighting factor of 1/X (where X represents the area) was used for all calibration curves.

The responses for trazodone and m-CPP were linear and gave coefficients of determination (R2) of 0.99 or better. Accuracy was reported as percentage nominal concentration, and precision was reported as percentage relative SD. The technique was optimized to provide an LOQ of 0.1 ng/mL and a limit of detection of approximately 0.05 ng/mL for both trazodone and m-CPP.

Urine sample analysis—Urine calibrators were prepared by dilution of the working standard solutions in drug-free equine urine to concentrations of 0.1 to 300 ng/mL for trazodone and 0.1 to 2,500 ng/mL for m-CPP. Calibration curves and negative control samples were freshly prepared for each quantitative assay. In addition, quality control samples (equine urine fortified with each analyte [trazodone and m-CPP] at 3 concentrations within the standard curve) were included with each sample set as an additional means to verify accuracy.

Prior to analysis, 1 mL of urine was diluted with 0.1 mL of water containing 2.5 ng of trazodone internal standardf/mL or 25 ng of m-CPP internal standardg/mL and 0.4 mL of β-glucuronidase enzymem at 10,000 U/mL in 1.6M acetate buffer (pH, 5). The pH of samples was adjusted to 5 ± 0.5 with 2 N NaOH or 2 N HCl as necessary and heated in a sonicating water bath at 65°C for 2 hours with 99 minutes of sonication. After cooling to room temperature (approx 22°C), the pH was adjusted to 6 ± 0.5 with 1.6 mL of 0.6M phosphate buffer (pH, 6.5) and 2 N NaOH or 2 N HCl as needed. Samples were mixed gently and centrifuged at 3,310 × g for 5 minutes at 4°C. The samples were subjected to solid-phase extraction with commercially prepared 130-mg, 3-mL extraction columns.n Detection and quantification were performed as described for plasma samples, except that the trazodone product ions had an m/z of 78.1, 96.1, 120.2, and 148.1, and the m-CPP product ion had an m/z of 118.1.

The responses for trazodone and m-CPP were linear and gave coefficients of determination (R2) of 0.99. Accuracy and precision were reported as described for plasma samples. The technique was optimized to provide an LOQ of 0.1 ng/mL and a limit of detection of approximately 0.05 ng/mL for both trazodone and m-CPP. The extraction recovery for m-CPP in equine urine was 80%.

Pharmacokinetic analysis

Pharmacokinetic analysis was conducted by use of commercially available software.o Compartmental analysis was used for determination of pharmacokinetic parameters for trazodone following IV administration. The t1/2 λz, AUC from time 0 to infinity, and percentage of the AUC extrapolated to infinity following oral administration were determined by noncompartmental analysis. The t1/2 λz was calculated as 0.693/λz (where λz represents the slope of the terminal portion of the concentration-time curve), and AUC and area under the first moment of the concentration-time curve were calculated with the log up-linear down trapezoidal method and extrapolated to infinity by use of the last measured serum concentration divided by the terminal slope λz. Pharmacokinetics of m-CPP were calculated with noncompartmental methods as described.

The model that best fit the data was determined on the basis of analysis of the coefficient of variation, the Akaike Information Criterion,11 and visual inspection of the residual plots. A 3-compartment model (Cp = Ae−αt + Be−βt + C−γt, where Cp is the plasma drug concentration; A, B, and C are intercepts for the α, β, and γ phases, respectively, at time 0; e is the base of natural logarithms; t is time; and α, β, and γ represent slopes for the modeled equation for the α, β, and γ phases, respectively) yielded the best fit to plasma concentration data following IV administration. Additionally, a multiplicative weighting factor gave the best fit to trazodone concentration data on the basis of visual inspection of observed versus predicted and residual plots.

Behavioral and physiologic observations

All horses were observed for any adverse effects, including signs of excitation or sedation prior to and for ≥ 4 hours after trazodone administration in both the pilot investigation and full study. All behavioral assessments were made from outside the stall prior to collection of blood samples and other measurements. Additional assessments, for the full study only, were made in the following order at each time point by the same observers (KM and a non-author): ear position, head position, response to sound, response to placement of a foreign object in the mouth, body temperature, GI auscultation, and ability to ambulate. The observers were not blinded to treatment. Head position was assessed by measuring the distance from the most ventral aspect of the bony portion of the chin (rostroventral point of the mandibles) to the ground immediately prior to drug administration (time 0), at 5 minutes after drug administration (IV treatment only), and at 15 and 30 minutes and 1, 2, 4, 6, and 24 hours after drug administration (IV and oral treatments).

For assessment of drug effects on heart rate and rhythm, each horse was instrumented with a Holter monitorp for continuous long-term recording. Heart rate and rhythm were recorded from ≥ 30 minutes before drug administration until 4 hours after the medication was given. Heart rate was assessed at predetermined time points by manually counting P-QRS-T complexes over a 1-minute period. The percentage of atrial signals blocked at the atrioventricular node before and after trazodone administration was calculated as (atrial rate – ventricular rate)/atrial rate. The atrial and ventricular rates were determined by manually counting P waves and P-QRS-T complexes, respectively, over a 1-minute period at the predetermined time points.

Body temperature (measured rectally with a digital thermometer), respiratory rate (determined by visual observation), gastrointestinal sounds (evaluated by auscultation), ambulation, responses to sound and to a foreign object placed in the mouth, and ear position were all assessed immediately prior to and at 5, 15, and 30 minutes and 1, 2, 4, 6, and 24 hours after drug administration. Auscultation was performed for 15 seconds at each abdominal quadrant, and borborygmi were recorded as normal, increased, decreased, or absent; a numerical score equal to the number of borborygmi/30 s in each quadrant was assigned. A score of 0.5 was added for uncoordinated rumbling or gaseous sounds for each quadrant (maximum score of 5). Response to sound was assessed by partially filling a plastic gallon jug with pebbles and shaking it at a distance approximately 1.5 m from the horse as described previously.12 Ability to ambulate in and out of the stall was assessed by walking the horse over a wooden pole (10 cm high × 10 cm wide and approx 1.2 m long) placed on the ground at the entrance of the stall. A plastic 60-cm3 syringe was used to assess the reaction to placement of a foreign object in the mouth. Numerical scores for ambulation, response to sound, response to placement of a foreign object in the mouth, behavior, and ear position were assigned according to the observations (Appendix).

Statistical analysis

A log transformation was used to normalize pharmacokinetic data. Statistical analyses were performed with commercially available softwareq to evaluate potential differences in pharmacodynamic variables between time 0 (immediately prior to drug administration) and each posttreatment time point. Data were analyzed by a mixed-effects ANOVA, with horse as the random effect and time as the fixed effect. Post hoc comparisons of measurements between time 0 and posttreatment time points were performed with a Bonferroni multiple comparisons adjustment to preserve a nominal significance level of P < 0.05.

Results

Plasma drug concentrations and pharmacokinetics

The intraday accuracy and precision for LC-MS-MS analysis of trazodone and m-CPP were summarized (Table 1). Trazodone and m-CPP plasma concentrations for horses included in the pilot investigation and for horses used in the full pharmacokinetic-pharmacodynamic study are reported (Figures 1 and 2). In the full pharmacokinetic study, 2 of 6 horses studied had high plasma concentrations (> 130 ng/mL) following both IV and oral drug administration.

Figure 1—
Figure 1—

Plasma concentrations of trazodone (black symbols) and the trazodone metabolite m-CPP (white symbols) over time following IV or oral administration of trazodone to healthy adult Thoroughbred horses in an escalating manner during a pilot (dose finding) investigation (n = 2 horses/group). A—Concentrations at predetermined time points before, during, and after IV drug administration (0.5 mg/kg followed by 1.0 mg/kg and finally 2.0 mg/kg, with 15 minutes in between administrations). B—The same data in panel A from time 0 to 10 hours after the first drug administration are provided in greater detail. C—Concentrations at predetermined time points before, during, and after oral drug administration (2 mg/kg followed 1 hour later by 4 mg/kg). Time 0 data were obtained immediately prior to the first drug administration. Each symbol (circle or square) in a given panel represents an individual horse. All study horses participated in a fitness training program.

Citation: American Journal of Veterinary Research 78, 10; 10.2460/ajvr.78.10.1182

Figure 2—
Figure 2—

Mean ± SD plasma concentrations of trazodone (black symbols) and m-CPP (white symbols) over time following IV (1.5 mg/kg; panel A) or oral administration (4 mg/kg; panel B) of the drug to healthy adult Thoroughbred horses (n = 6) in a pharmacokinetic and pharmacodynamic study. All horses received the IV formulation first and received the oral formulation after a 5-week washout period. Time 0 data were obtained immediately prior to drug administration.

Citation: American Journal of Veterinary Research 78, 10; 10.2460/ajvr.78.10.1182

Table 1—

Accuracy and precision values for LC-MS-MS analysis of trazodone and the trazodone metabolite m-CPP in equine plasma and urine samples.

 PlasmaUrine
Product and concentrationIntraday accuracy (% nominal concentration)Intraday precision (% relative SD)Intraday accuracy (% nominal concentration)Intraday precision (% relative SD)
Trazodone (ng/mL)
 0.301025.01056.0
 40.01045.01035.0
 2001044.01095.0
m-CPP (ng/mL)
 0.3099.05.010014.0
 2.589.02.0NDND
 10.01015.0NDND
 100NDND98.04.0
 1,000NDND1052.0

ND = Not determined.

Total trazodone clearance was 6.85 ± 2.80 mL/min/kg, and the γ half-life was 8.58 ± 1.88 hours following IV administration of the drug. Following oral administration, bioavailability was 63.0% and the t1/2 λz was 7.11 ± 1.70 hours. Additional pharmacokinetic parameters are listed (Tables 2 and 3). The metabolite m-CPP appeared rapidly in equine plasma following both IV and oral administration. Mean maximum m-CPP concentrations were observed at 30 minutes following IV administration and 1.5 to 3 hours after oral administration of trazodone. Pharmacokinetic parameters for m-CPP are listed (Table 4). Urine concentrations of trazodone and m-CPP are also provided (Table 5).

Table 2—

Pharmacokinetic parameters of trazodone following IV administration of a single dose (1.5 mg/kg) to 6 healthy adult Thoroughbred horses enrolled in a training fitness program.

ParameterMean ± SD
A (ng/mL)2,982 ± 454
B (ng/mL)897 ± 336
C (ng/mL)55.7 ± 67.1
α (1/h)2.86 ± 1.32
β (1/h)0.41 ± 0.14
γ (1/h)0.08 ± 0.02
AUC0–∞ (ng•h/mL)4,420 ± 2,304
Vd1 (L/kg)0.38 ± 0.10
Vd2 (L/kg)0.36 ± 0.08
Vd3 (L/kg)0.33 ± 0.05
Vdss (L/kg)1.06 ± 0.07
Cl (mL/min/kg)6.85 ± 2.80
t1/2α (h)0.30 ± 0.16
t1/2β (h)1.87 ± 0.73
t1/2γ (h)8.58 ± 1.88

Pharmacokinetic parameters were determined by compartmental analysis. A, B, and C represent intercepts at time 0 (immediately before drug administration) for the α, β, and γ phases, respectively, and α, β, and γ represent slopes for the modeled equation for the α, β, and γ phases, respectively.

AUC0–∞ = Area under the concentration-versus-time curve from time 0 to infinity. Cl = Total body clearance. t1/2α = First distribution half-life. t1/2β = Second distribution half-life. t1/2γ = Elimination half-life. V1 = Volume of distribution of the central compartment. V2 = Volume of distribution of the first peripheral compartment. V3 = Volume of distribution of the second peripheral compartment. Vdss = Volume of distribution at steady state.

Table 3—

Pharmacokinetic parameters of trazodone following oral administration of a single dose (4 mg/kg) to the same 6 horses as in Table 2 after a 5-week washout period.

ParameterMean ± SD
Cmax (ng/mL)1,392 ± 1,978
tmax (h)1.71 ± 0.42
t1/2 λz (h)7.11 ± 1.70
AUC0–∞ (ng•h/mL)8,090 ± 2,495
AUC extrapolated (%)0.31 ± 0.11
F (%)63.0 ± 17.3

Pharmacokinetic parameters were determined by noncompartmental analysis.

AUC extrapolated = Percentage of area under the curve extrapolated to infinity. Cmax = Maximum plasma concentration. F = Bioavailability.

See Table 2 for remainder of key.

Table 4—

Pharmacokinetic parameters (mean ± SEM) of m-CPP following IV (1.5 mg/kg) or oral (4 mg/kg) administration of a single dose of trazodone to the same 6 horses as in Table 2.

ParameterIVPO
Cmax (ng/mL)8.44 ± 3.9223.6 ± 3.44
tmax (h)0.708 ± 0.2461.83 ± 0.38
t1/2 λz (h)5.97 ± 0.585.93 ± 0.60
AUC0–∞ (ng•h/mL)48.8 ± 14.5151 ± 44.7
AUC extrapolated (%)2.89 ± 0.610.98 ± 0.16

See Tables 2 and 3 for key.

Table 5—

Mean ± SD urine concentrations of trazodone and m-CPP following IV (1.5 mg/kg) or oral (4 mg/kg) administration of a single dose of trazodone to the same 6 horses as in Table 2.

 Trazodone (ng/mL)m-CPP (ng/mL)
Time (h)IVOralIVOral
4101.7 ± 16.8108.4 ± 64.71,110.2 ± 81.01,523.6 ± 1,003.8
241.8 ± 3.99.5 ± 13.555.7 ± 87.6220.6 ± 359.6
480.29 ± 0.240.57 ± 0.44.50 ± 6.7312.3 ± 16.7

Time 0 was immediately before drug administration.

Behavioral and physiologic changes

Pilot investigation—Following IV administration of trazodone at 0.5 mg/kg, 1 horse remained bright and alert and the second horse appeared quiet. At 1 mg/kg, the horse that was alert at the lower dose was quiet and calm and the second horse appeared hypersensitive to external stimuli. At the 2-mg/kg dose, both horses had signs of aggression including pinning the ears back, kicking, attempting to bite, and rearing. Both horses kicked at their flanks, had whole-body muscle tremors, and appeared ataxic, ultimately spending a large amount of time leaning on the stall wall (one commencing at 7 minutes after administration of the 2 mg/kg dose and the other at 15 minutes after receiving this dose). These signs persisted for approximately 35 to 45 minutes. Both horses that received trazodone orally appeared alert when administered 2-mg/kg and quiet and sedate (notable decrease in head height and movement) when the dose was increased to 4 mg/kg. Administration of DMSO alone did not appear to have any pharmacodynamic effects.

Full pharmacokinetic-pharmacodynamic study—Following IV administration of trazodone (1.5 mg/kg), 5 of 6 horses were ataxic, 1 of 6 had signs of excitation including circling in the stall, 3 of 6 had whole-body tremors, 6 of 6 had sweating (over the entire body), and 1 of 6 had head shaking behavior. All adverse effects were apparently resolved by 2 hours (range, 14 minutes to 2 hours) after IV drug administration in all horses. Following oral administration of trazodone (4 mg/kg), 4 of 6 horses had sweating (over the entire body), and 5 of 6 appeared sedate (quiet with drooping eyelids and not easily aroused by outside stimuli). The onset of signs of sedation varied from 45 minutes to 2 hours after the medication was consumed. One of 6 horses had signs of sedation for 2 hours and another for 4.5 hours after trazodone administration. No adverse behavioral effects were noted following oral administration. No significant changes in chin-to-ground distance were noted following oral or IV drug administration (Figure 3). Additional pharmacodynamic data are summarized (Table 6).

Figure 3—
Figure 3—

Mean ± SD chin-to-ground distance over time following IV (black circles) or oral administration (white circles) of trazodone to the same 6 horses as in Figure 2. See Figure 2 for remainder of key.

Citation: American Journal of Veterinary Research 78, 10; 10.2460/ajvr.78.10.1182

Table 6—

Results of assessment of selected pharmacodynamic variables following IV (1.5 mg/kg) or oral (4 mg/kg) administration of a single dose of trazodone to the same 6 horses as in Table 2.

 Time (h)
Variable00.080.250.51.02.04.06.024.0
Rectal temperature (°C)
 IV37.7 ± 0.637.3 ± 0.737.0 ± 0.636.6 ± 0.9*36.2 ± 1.2*36.2 ± 1.4*36.6 ± 1.2*37.0 ± 0.937.4 ± 0.6
 PO37.2 ± 0.637.1 ± 0.636.9 ± 0.536.4 ± 0.6*35.7 ± 1.0*35.8 ± 1.4*36.6 ± 1.2*37.4 ± 0.6
Respiratory rate (breaths/min)
 IV24.0 ± 5.014.0 ± 5.0*17.0 ± 5.0*21.0 ± 5.015.0 ± 4.015.0 ± 2.013.0 ± 4.015.0 ± 3.021.0 ± 8.0
 PO18.0 ± 4.014.0 ± 4.011.0 ± 2.0*11.0 ± 4.0*13.0 ± 5.0*11.0 ± 2.0*12.0 ± 3.0*14.0 ± 3.0
Borborygmi
 IV1.0 ± 0.91.0 ± 1.02.0 ± 1.01.2 ± 0.41.0 ± 0.91.3 ± 0.81.2 ± 1.21.3 ± 0.81.5 ± 0.8
 PO2.3 ± 0.81.3 ± 0.51.8 ± 1.01.0 ± 0.9*1.2 ± 0.81.8 ± 0.81.8 ± 1.21.3 ± 0.8
Behavior
 IV1.0 ± 0.02.2 ± 1.82.8 ± 2.0*2.5 ± 1.52.3 ± 1.41.8 ± 1.61.3 ± 0.51.3 ± 0.51.0 ± 0.0
 PO1.0 ± 0.01.0 ± 0.00.9 ± 0.22.2 ± 0.8*1.7 ± 0.8*1.3 ± 0.51.2 ± 0.41.0 ± 0.0
Ear position
 IV1.0 ± 0.01.2 ± 0.41.0 ± 0.01.2 ± 0.41.5 ± 0.5*1.2 ± 0.41.3 ± 0.51.2 ± 0.41.0 ± 0.0
 PO1.0 ± 0.01.0 ± 0.00.9 ± 0.21.7 ± 0.5*1.5 ± 0.6*1.5 ± 0.6*1.0 ± 0.01.0 ± 0.0
Response to foreign object placed in mouth
 IV1.0 ± 0.01.0 ± 0.01.0 ± 0.01.0 ± 0.01.0 ± 0.01.0 ± 0.01.0 ± 0.01.0 ± 0.01.0 ± 0.0
 PO1.2 ± 0.41.3 ± 0.51.2 ± 0.41.3 ± 0.51.2 ± 0.41.3 ± 0.51.2 ± 0.41.0 ± 0.0
Response to sound
 IV4.3 ± 1.23.6 ± 1.93.2 ± 2.03.4 ± 2.02.5 ± 2.02.0 ± 1.5*2.8 ± 1.72.3 ± 1.0*3.7 ± 1.8
 PO2.7 ± 1.42.5 ± 1.62.2 ± 1.82.7 ± 1.41.9 ± 142.3 ± 1.21.8 ± 1.43.0 ± 1.3
Ambulate into stall
 IV3.2 ± 0.83.0 ± 0.02.8 ± 0.53.2 ± 0.63.0 ± 0.62.5 ± 0.6*3.0 ± 0.43.0 ± 1.02.8 ± 0.5
 PO3.3 ± 0.53.2 ± 0.43.0 ± 0.63.3 ± 0.53.2 ± 0.43.0 ± 0.63.3 ± 0.53.0 ± 0.0
Ambulate out of stall
 IV2.7 ± 0.53.0 ± 0.03.0 ± 0.02.0 ± 0.82.7 ± 0.52.5 ± 0.83.0 ± 0.02.3 ± 0.82.8 ± 0.4
 PO3.0 ± 0.03.0 ± 0.03.0 ± 0.03.0 ± 0.03.0 ± 0.02.8 ± 0.42.5 ± 1.6*2.8 ± 0.4

All values are reported as mean ± SD. Hay was withheld from horses for 8 hours prior to drug administration and for 2 hours afterward (until collection of data for the 2-hour time point was complete). Borborygmi were assessed by auscultation of each abdominal quadrant for 15 seconds and scored as normal, increased, decreased, or absent; a numeric score equal to the number of borborygmi/30 s in each quadrant was assigned. A score of 0.5 was added for uncoordinated rumbling or gaseous sounds for each quadrant. Behavioral data were scored from 1 (least affected) to 4 or 5 (most affected). See Appendix for details.

Value is significantly (P < 0.05) different from that at time 0 (immediately prior to drug administration).

— = Not assessed.

Mean body temperature was decreased relative to time 0 from 30 minutes until 4 hours after IV administration and from 1 to 6 hours after oral administration of trazodone (Table 6). Mean respiratory rate was significantly decreased, compared with that at time 0, at 5 and 15 minutes after IV administration and from 30 minutes through 6 hours after oral administration of trazodone. Mean heart rate significantly increased, compared with that at time 0, from 3 minutes to 12 minutes after IV drug administration (Figure 4). No significant changes in heart rate were observed following oral drug administration, and no changes in percentage atrioventricular block were noted at any time after IV or oral drug administration.

Figure 4—
Figure 4—

Mean ± SD heart rate (A) and percentage atrioventricular block (B) over time following IV (black circles) or oral administration (white circles) of trazodone to the same 6 horses as in Figure 2. The percentage atrioventricular block was calculated as ([atrial beats – ventricular beats]/atrial beats) × 100. *Value differs significantly (P < 0.05) from that at time 0. See Figure 2 for remainder of key.

Citation: American Journal of Veterinary Research 78, 10; 10.2460/ajvr.78.10.1182

Discussion

The use of trazodone as an antianxiety medication in dogs8,13 and as a sedative in cats14 has been described. The present study was performed to evaluate the pharmacokinetics and selected pharmacodynamic effects of trazodone following IV or oral administration of a single dose to horses. We anticipated that this information would provide insight into the potential use of trazodone for clinical benefit to equine health and would provide initial baseline information necessary for regulatory affairs related to administration of the drug to equine athletes.

Trazodone absorption following oral administration to horses (tmax, 1.71 ± 0.42 hours [102 ± 25.2 minutes]) was more rapid than that previously reported for dogs (445 ± 271 minutes)9 but similar to that found for people (120 minutes).15 Although the reason for the difference in the rate of absorption between horses and dogs is not known, Jay et al9 suggested that differences in tmax between species is likely attributable to differences in gastric emptying times. In the present study, trazodone was administered as a powder with food, whereas in the aforementioned study9 of dogs, it was administered in tablet form. We consider it unlikely that the formulation was the sole explanation for the difference in tmax between studies, although disintegration of the tablets may have contributed somewhat to the prolonged absorption in dogs relative to horses. Although our results suggested that the rate of trazodone absorption is more rapid in horses relative to that reported for other species, the extent of absorption appears to be less, as the bioavailability of trazodone in the present study was 63.0 ± 17.3%, compared with 84.6 ± 13.2% in dogs9 and a range of 65% to 80% in people.16,17 Another potential reason for the apparently lower bioavailability was the delivery method of the drug. In the present study, trazodone was administered in grain. Although horses were observed while consuming the mixture, it is possible that some drug was lost from the oral cavity. Admittedly, administration via a stomach tube would have ensured that all drug reached the gastrointestinal tract. However, this route of administration was not chosen owing to concerns that sedation used for drug administration via this route would interfere with assessment of the pharmacodynamic effects of trazodone. Additionally, we sought to mimic practical administration of this compound in a manner that might be used by horse owners to deliver a prescribed treatment.

To the best of the authors’ knowledge, circulating concentrations of trazodone necessary for sedation or anxiolysis have not been established for horses or any other nonhuman animals. In people, a concentration range between 130 ng/mL and 2 μg/mL provides antidepressive effects,18 but plasma trazodone concentrations necessary for anxiolysis in human patients have not been reported. In the present study, mean plasma trazodone concentrations were within the therapeutic range necessary for antidepressive effects in humans for ≥ 4 hours after IV drug administration, with 2 of 6 horses having concentrations > 130 ng/mL for > 8 hours. Following oral administration, mean plasma trazodone concentrations exceeded 130 ng/mL for 6 hours after administration, with 2 horses having concentrations > 130 ng/mL for > 18 hours. These horses were the same horses that had circulating trazodone concentrations persistently > 130 ng/mL following IV administration.

The total body clearance of trazodone in horses (6.85 ± 2.80 mL/min/kg) in the present study was considerably slower than that reported for dogs (11.1 ± 3.56 mL/min/kg).9 Similarly, the elimination half-life for horses appeared prolonged relative to that described for dogs. This could potentially be attributable to species differences in the rate of metabolism; however, the apparent differences in elimination half-life might also be attributable to differences in sampling protocols and sensitivity of the analytical assay used to measure drug concentrations. In the present study, samples were collected for 48 hours, whereas in dogs, samples were collected for only 24 hours.9 Furthermore, the LOQ in the present study was 0.1 ng/mL, whereas in the study by Jay et al,9 the LOQ was 500 ng/mL. The longer sampling time and greater sensitivity in the present study likely required less extrapolation of the terminal portion of the plasma concentration-versus-time curve for calculation of the elimination half-life, compared with that in the study by Jay et al.9

In people, trazodone is extensively metabolized by CYP450 enzymes. One of the major metabolites of trazodone in people is the pharmacologically active compound m-CPP, which is produced following N-dealkylation by CYP450 3A4.6 This metabolite is either excreted or further metabolized. The most abundant m-CPP metabolite is designated M4 and is produced by CYP450 2D6.7 This metabolite is eliminated as a glutathione conjugate.7 Little is known about the metabolic pathway for trazodone in horses; however, m-CPP was rapidly detected in plasma samples collected in the present study following either IV or oral administration, albeit at much lower concentrations (< 10% of trazodone concentrations) relative to those reported for human patients (10% to 30% of trazodone concentrations).4 However, urine trazodone concentrations in horses of the present study were low, suggesting that metabolism is largely responsible for elimination in horses as has been reported for human patients, in which < 1% of the parent drug is excreted in urine.16 In the present study, only the m-CPP metabolite was evaluated; however, it is possible that, as for humans, m-CPP is either further metabolized to other metabolites or trazodone is converted to metabolites other than m-CPP. Interestingly, 2 horses appeared to eliminate trazodone much more slowly than the other 4 horses in our study following IV or oral administration. Both trazodone and m-CPP concentrations were subjectively higher and were detected in blood and urine for a prolonged period of time relative to results for the other horses. Additionally, although only mean pharmacokinetic parameters are reported here, the elimination half-life for trazodone was subjectively longer in these 2 horses, compared with those of the remaining 4. It is not possible to elucidate the reason for this apparent difference in trazodone clearance, and much remains to be determined regarding the metabolism of trazodone in horses; however, 1 possible explanation is that some horses may have genetic polymorphisms in CYP450 3A, CYP2D, or both. Polymorphisms have been identified in CYP450 3A4 and CYP450 2D6 in people,19 and preliminary evidence suggests the same may be true for CYP450 2D50 in horses.20,21

Aggressive behavior, including growling and attempting to bite, has been reported following IV administration of 8 mg of trazodone/kg in dogs.9 Although the dose in the present study was much lower than that administered to dogs, IV administration of 2 mg/kg in the pilot investigation elicited signs consistent with aggression in both horses that received it, including pinning the ears back, kicking, attempting to bite, and rearing. In human patients, it has been suggested that anxiogenic properties of trazodone are the result of high concentrations of the major metabolite m-CPP.4 Although concentrations of the metabolite were not measured in the study9 in dogs, it was suggested that this metabolite might have been responsible for the aggressive behavior observed at high doses. Although, to our knowledge, concentrations of m-CPP eliciting aggressive behavior have not been described in any species, it should be noted that concentrations of m-CPP were very low at all time points in the pilot investigation and in the pharmacokinetic-pharmacodynamic study.

For the full pharmacokinetic-pharmacodynamic study, a lower IV dose of trazodone (1.5 mg/kg) was selected to avoid eliciting the aggressive behavior observed in the pilot investigation. Signs of aggression were not noted at the lower dose, but signs of excitation including circling in the stall and whole body tremors were similar to those observed at the 2 mg/kg dose given in the pilot investigation. Ataxia was also observed at both doses, and ataxia has also been reported in dogs following IV administration of 8 mg of trazodone/kg. Consistent with the signs of excitation, a significant increase in heart rate was noted immediately after IV administration of 1.5 mg of trazodone/kg to horses, and this lasted until 15 minutes after drug administration. Similarly, transient tachycardia was reported in dogs immediately upon IV administration of the 8 mg/kg dose.9

Many of the adverse effects noted following IV administration of 1.5 or 2 mg of trazodone/kg in the present study were absent following oral administration of a 4-mg/kg dose. No significant differences were observed in chin-to-ground distance or in percentage atrioventricular block after either treatment; however, on the basis of subjective observations and behavioral scores, 5 of 6 horses were deemed sedate following both treatments. There were marked differences in responses between IV and oral administration, including sweating, ataxia, whole body tremors, and head shaking, and these differences were likely attributable to the marked differences in plasma concentrations between the 2 routes of administration.

This study had a few limitations. As there is no commercially available injectable formulation, trazadone hydrochloride powder was purchased from a chemical company and compounded for IV administration to these horses, which were part of a research herd. For consistency, we chose to use the same powder formulation for oral administration, even though trazodone tablets are commercially available for human use. The purity of the purchased powder was high; however, the composition may differ somewhat from the commercially available formulation. Therefore, the absorption kinetics reported here may vary from those of the commercially available formulation. For IV administration, trazodone powder was dissolved in DMSO. The present study included only 1 horse administered DMSO alone in the pilot investigation, and lack of inclusion of a nontreated group for behavioral assessment is another notable limitation. Furthermore, the absence of a blinded investigator in the present study for behavioral observations was less than optimal. Finally, the present study was conducted in a block design. While this design was chosen for logistical reasons, a block design does not allow for assessment of period effects and ideally the study would have been conducted in a balanced crossover manner.

Maximum plasma trazodone concentrations were achieved rapidly after oral administration, and the elimination half-life was prolonged relative to that reported for dogs and people. Administration of trazodone to horses elicited a range of effects, with aggression and excitation noted at high plasma concentrations and sedation evident at lower plasma concentrations. Trazodone is currently a prohibited substance in performance horses, and the study findings suggested that an extended withdrawal time would be needed; however, additional study is warranted before the clinical use of trazodone for treatment of horses.

Acknowledgments

Supported by the Center for Equine Health at the University of California-Davis.

The authors declare that there were no conflicts of interest.

The authors thank Dan McKemie, Kelsey Seminoff, Alexandria White, Stacy Steinmetz, and Sandy Yim for technical Assistance.

ABBREVIATIONS

AUC

Area under the curve

CYP450

Cytochrome P450

DMSO

Dimethyl sulfoxide

LC-MS-MS

Liquid chromatography–tandem mass spectrometry

LOQ

Limit of quantitation

m-CPP

m-chlorophenylpiperazine

m/z

Mass-to-charge ratio

t1/2 λz

Terminal-phase half-life

tmax

Time to maximum plasma concentration

SSRI

Selective serotonin reuptake inhibitor

Footnotes

a.

Angiocatheter, Becton Dickinson, Franklin Lakes, NJ.

b.

Trazodone HCl, Sigma-Aldrich Corp, St Louis, Mo.

c.

Equine Senior, Purina Animal Nutrition LLC, Shoreview, Minn.

d.

Becton Dickinson, Franklin Lakes, NJ.

e.

Phenix Research Products, Chandler, NC.

f.

d6-trazodone, Cerilliant, Round Rock, Tex.

g.

d8-mCPP, Cerilliant, Round Rock, Tex.

h.

TSQ Vantage, Thermo Scientific, San Jose, Calif.

i.

Thermo Scientific, San Jose, Calif.

j.

1100 series, Agilent Technologies, Palo Alto, Calif.

k.

ACE 3 C18 column, Mac-Mod Analytical, Chadds Ford, Penn.

l.

Xcaliber Quan Browser, version 3.1, Thermo Scientific, San Jose, Calif.

m.

β-glucuronidase, Sigma-Aldrich Corp, St Louis, Mo.

n.

CleanScreen DAU, United Chemical Technologies, Bristol, Penn.

o.

Phoenix WinNonlin, version 6.3, Pharsight Corp, Cary, NC.

p.

Forrest Medical, East Syracuse, NY.

q.

Stata/IC, version 13.1, StataCorp LP, Tex.

References

  • 1. Frazer A, Hensler JG. Serotonin. In: Siegel GJ, Agranoff BW, Albers RW, et al, eds. Basic neurochemistry. 6th ed. Philadelphia: Lippincott-Raven, 1999; 263292.

    • Search Google Scholar
    • Export Citation
  • 2. Stahl SM. Mechanism of action of trazodone: a multifunctional drug. CNS Spectr 2009; 14: 536546.

  • 3. Haria M, Fitton A, McTavish D. Trazodone. A review of its pharmacology, therapeutic use in depression and therapeutic potential in other disorders. Drugs Aging 1994; 4: 331355.

    • Search Google Scholar
    • Export Citation
  • 4. Mittur A. Trazodone: properties and utility in multiple disorders. Expert Rev Clin Pharmacol 2011; 4: 181196.

  • 5. Bossini L, Casolaro I, Koukouna D, et al. Off label uses of trazodone: a review. Expert Rev Clin Pharmacol 2012; 13: 17071717.

  • 6. Rotzinger S, Fang J, Baker GB. Trazodone is metabolized to m-chlorophenylpiperazine by CYP3A4 from human sources. Drug Metab Dispos 1998; 26: 572575.

    • Search Google Scholar
    • Export Citation
  • 7. Wen B, Ma L, Rodrigues D, et al. Detection of novel reactive metabolites of trazodone: evidence for CYP2D6-mediated bioactivation of m-chlorophenylpiperazine. Drug Metab Dispos 2008; 36: 841850.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8. Gruen ME, Sherman BL. Use of trazodone as an adjunctive agent in the treatment of canine anxiety disorders: 56 cases (1997–2007). J Am Vet Med Assoc 2008; 233: 19021907.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9. Jay AR, Krotscheck U, Parsley E, et al. Pharmacokinetics, bioavailability, and hemodynamic effects of trazodone after intravenous and oral administration of a single dose to dogs. Am J Vet Res 2013; 74: 14501456.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Knych HK, Arthur RM, Mitchell MM, et al. Pharmacokinetics and selected pharmacodynamics of cobalt following a single intravenous administration to horses. Drug Test Anal 2015; 7: 619625.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Yamaoka K, Nakagawa T, Uno T. Application of Akaike's information criterion (AIC) in the evaluation of linear pharmacokinetic equations. J Pharmacokinet Biopharm 1978; 6: 165175.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12. Mama KR, Grimsrud K, Snell T, et al. Plasma concentrations, behavioural and physiological effects following intravenous and intramuscular detomidine in horses. Equine Vet J 2009; 41: 772777.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Gruen ME, Roe SC, Griffith E, et al. The use of trazodone to facilitate postsurgical confinement in dogs. J Am Vet Med Assoc 2014; 245: 296301.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Orlando JM, Case BC, Thomson AE, et al. Use of oral trazodone for sedation in cats: a pilot study. J Feline Med Surg 2016; 18: 476482.

  • 15. Kale P, Agrawal YK. Pharmacokinetics of single oral dose trazodone: a randomized, two-period, cross-over trial in healthy, adult, human volunteers under fed condition. Front Pharmacol 2015; 6: 224.

    • Search Google Scholar
    • Export Citation
  • 16. Nilsen OG, Dale O. Single dose pharmacokinetics of trazodone in healthy subjects. Pharmacol Toxicol 1992; 71: 150153.

  • 17. Vatassery GT, Holden LA, Hazel DK, et al. Determination of trazodone and its metabolite, 1-m-chlorophenyl-piperzine, in human plasma and red blood cell samples by HPLC. Clin Biochem 1997; 30: 149153.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Mercolini L, Colliva C, Amore M, et al. HPLC analysis of the antidepressant trazodone and its main metabolite mCPP in human plasma. J Pharm Biomed Anal 2008; 47: 882887.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 19. Zanger UM, Schwab M. Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities and impact of genetic variation. Pharmacol Ther 2013; 138: 103141.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20. Corado CR. McKemie, Young A, Knych HK. Evidence for polymorphism in the cytochrome P450 2D6 gene in horses. J Vet Pharmacol Ther 2016; 39: 245254.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21. Corado CR. McKemie, Knych HK. Dextromethorphan and debrisoquine metabolism and polymorphism of the gene for cytochrome P450 isozyme 2D50 in Thoroughbreds. Am J Vet Res 2016; 77: 10291035.

    • Crossref
    • Search Google Scholar
    • Export Citation

Appendix 1

Scoring system for assessment of behavior following IV (1.5 mg/kg) or oral (4 mg/kg) administration of a single dose of trazodone to 6 healthy adult Thoroughbred horses enrolled in a training fitness program.

VariableScoreObservation
Behavior1Appears bright and alert, behavior and responsiveness deemed normal
 2Appears aware of observer presence (either sees or reacts to voice) and is alert and responsive
 3Appears quiet in the stall (eg, standing, recumbent, or sleeping) and minimally responsive to observer presence or voice call but other normal behaviors are apparent
 4Appears sedated (either standing or recumbent) and does not respond to observer presence or voice call
 5Appears agitated, hyperexcitable, hypersensitive, or aggressive (alone or in combination)
Ear position1Forward and erect; horse may move ears around as if listening
 2Relaxed and in a lateral position
 3Relaxed and back
 4Pinned back; not relaxed
Response to foreign object1Easy to place object in the horse's mouth; 1 person can complete
 2Placed but with minor restraint; 1 person can complete
 3More restraint required or needs 2 people to place object
 4Unable to place object
Ambulation in and out of stall1Jumps the pole
 2Clears the pole with all 4 feet
 3Hits the pole with 1 or 2 feet
 4Hits the pole with 3 or 4 feet
 5Unable to walk over the pole
Response to sound1No response
 2Slight or barely perceptible response (eg, ear movement)
 3Mild response (eg, head raises calmly)
 4Moderate response (eg, lifts head up briskly, ears pricked)
 5Dramatic response (eg, horse alert and in motion or whinnying)

Response to sound was assessed by partially filling a plastic gallon jug with pebbles and shaking it approximately 1.5 m away from the horse as described previously.12 Ability to ambulate in an out of the stall was assessed by walking the horse over a wooden pole (10 cm high × 10 cm wide and approx 1.2 m long) placed on the ground at the entrance of the stall. A plastic 60-cm3 syringe was used to assess the reaction to placement of a foreign object in the mouth.

Contributor Notes

Address correspondence to Dr. Knych (hkknych@ucdavis.edu).