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. Author manuscript; available in PMC: 2024 Mar 19.
Published in final edited form as: J Vet Pharmacol Ther. 2022 Sep 26;45(6):516–529. doi: 10.1111/jvp.13095

Dosing protocols to increase the efficacy of butorphanol in dogs

Dariyan Springfield 1, Butch KuKanich 1, Mackenzie Gray 1, Kate KuKanich 2, Poyu Lai 2
PMCID: PMC10949855  NIHMSID: NIHMS1966587  PMID: 36164256

Abstract

The purpose of this study was to improve butorphanol dosing in dogs. Twelve Beagles (6 male, 6 female) were enrolled. Six were randomly allocated to each butorphanol treatment: IV (0.4 mg/kg), IV loading dose (0.2 mg/kg) with IV CRI (0.2 mg/kg/hr for 8 hours), SC (0.4 mg/kg), SC (0.8 mg/kg) with an equal volume sodium bicarbonate (SC-bicarbonate) and IV after CYP inhibitors. We hypothesized the CRI would produce longer durations than IV bolus, and SC-bicarbonate suspension would produce longer durations than SC. Hypothermia, an opioid effect paralleling antinociception in dogs, and sedation were evaluated. Pharmacokinetics and CYP inhibitor effects on butorphanol pharmacokinetics were determined. Rectal temperatures were significantly lower than baseline from 1.5–4 hours (IV), 1–5 hours (CRI) and 2–7 hours (SC-bicarbonate), but not after SC. Dogs in all treatments had sedation. Butorphanol’s half-life was ~1.5 hr. SC-bicarbonate had lower bioavailability (61%) relative to SC with no sustained release, and the CRI mean steady state plasma concentration was 43.1 ng/mL. CYP inhibitors had minor pharmacokinetic effects on butorphanol. Butorphanol 0.4 mg/kg IV and 0.2 mg/kg loading dose with 0.2 mg/kg/hr CRI decreased rectal temperature, but 0.4 mg/kg SC did not. Further studies are required to determine clinical analgesia of butorphanol.

Keywords: butorphanol, pharmacokinetics, pharmacodynamics, canine, cytochrome P450

Introduction

Butorphanol interacts with the mµ-opiate receptor as a partial agonist and the kappa-opiate receptor as an agonist and is commonly used in veterinary medicine (Garner et al., 1997). It produces typical opioid effects in dogs including analgesia and sedation, with minimal cardiopulmonary depression at clinically relevant dosages (Houghton, et. al., 1991; Sawyer, et. al. 1991; Trim, 1993). Butorphanol reportedly provides mild to moderate analgesic effects at specific dosages with minimal adverse effects and has lower abuse potential compared to pure mµ-agonist opioids such as morphine and hydromorphone (Houghton et al., 1991; Sawyer er al 1991; Wegner et al., 2008). Butorphanol is used in an extra-label manner for its opioid analgesic activity in dogs, since it is only FDA-approved for its antitussive effects in dogs.

Currently, canine pharmacokinetic data for butorphanol via different routes of administration and doses are limited to subcutaneous, intramuscular and epidural administration (Pfeffer et al., 1980; Troncy et al, 1996). The terminal half-life after SC and IM administration was 1.5–1.7 hr at 0.25 mg/kg with rapid absorption and CMAX range 25–33 ng/mL (Pfeffer et al., 1980). No pharmacokinetic data were found for IV, PO or constant rate infusion.

Available pharmacodynamic data reveal that butorphanol provides opioid effects such as sedation, miosis, hypothermia and variable antinociception activity based upon the dose in dogs. Butorphanol produced a dose related miosis after subcutaneous administration in dogs with a maximum effect at 0.25 mg/kg SC (Pircio et al., 1976). An objective model of thermal antinociception in dogs resulted in butorphanol (0.4 mg/kg IV) producing equianalgesic peak effects and duration compared to hydromorphone at 0.1 mg/kg IV with butorphanol’s peak effect at 1 hour and decreasing at 2 and 4 hours (Wegner et al., 2008). Butorphanol, at 0.4 mg/kg IV and 0.8 mg/kg SC, produced significant antinociception with a canine colonic distension model (Houghton, et. al., 1991; Sawyer, et. al. 1991). Other studies have demonstrated butorphanol has minimal effects on various cardiovascular and respiratory factors when administered intravenously alone or in combination with a commonly used sedative tranquilizer (Trim, 1983; Houghton, et. al., 1991; Sawyer, et. al. 1991; Gomes et al., 2018).

Rectal temperature is a parameter used as a non-invasive biomarker for central opioid effects that parallels analgesic/antinociceptive dose-response effects (Vaupel & Jasinski, 1997; KuKanich et al., 2019, 2020, 2021). Butorphanol decreases the rectal temperature in dogs, but the duration of effect was not determined (Gomes et al., 2018).

The objective of this study was to document the pharmacokinetics and pharmacodynamics of butorphanol after administration via different routes of administration and in combination with other commercially available drugs to healthy dogs. We hypothesized that administering butorphanol at 0.4 mg/kg intravenous (IV) or subcutaneous (SC) would last about 1–2 hours due to its short half-life, while butorphanol administered as a 0.2 mg/kg loading dose followed by 0.2 mg/kg/hr constant rate infusion (CRI) would provide a prolonged and consistent opioid effect throughout the 8-hour infusion due to consistent plasma concentrations. We also hypothesize that butorphanol injection when mixed with commercially available sodium bicarbonate injection, resulting in a butorphanol precipitate, then administered subcutaneously (SC-bicarbonate) would produce a prolonged opioid effect. We hypothesize this prolonged effect would be due to slow absorption because of the poorly water-soluble precipitate would produce a pharmacokinetic flip-flop (the slow absorption being the rate limiting step for elimination). Finally, we hypothesized prior administration of cytochrome P450 inhibitors would result in significantly decreased clearance, prolonged half-life and prolonged duration of opioid effects due to inhibition of butorphanol metabolism. The long-term goal is creating protocols with potential clinical application while using commercially available formulations to provide extended opioid effects including adequate analgesia.

Methods

Animals

All study protocols were approved by the Kansas State University Institutional Animal Care and Use Committee (protocol no. 4553). Twelve healthy purpose bred Beagles (6 males; 6 females) aged 1 year and weighing 5.4 – 9.8 kg were included in the study (Figure 1). The study was conducted in two phases in which 12 dogs were used for phase 1, and 6 of the same dogs from phase 1 were used for phase 2. Dogs were initially separated by sex (6 male and 6 female). Three females and three males were randomly chosen for each group to ensure equal proportion of males (n=3) and females (n=3). Each dog was randomly selected by choosing one piece of paper with the dog’s name, and a separate piece of paper with a number that determines the treatment it was to receive out of a different box. All dogs had received a health examination prior to treatment and inclusion into the study. Health status was determined on the basis of history and the results of physical examination, CBC, and serum biochemical analysis. Dogs were routinely housed in compatible pairs or groups in indoor runs (10 × 10 feet) with outdoor access when the weather was appropriate. During the study period, the dogs were housed individually in runs (5×7 feet) that included elevated dog beds, and they were returned to their routine housing at the end of the study. Dogs were housed in 2×2 to 3×5 feet stainless steel kennels during sample collection. Prior to any treatment, the dogs were fasted for at least 12 hours and water was offered after the 8-hour time point.

Figure 1.

Figure 1.

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Flow chart of the study design assessing the pharmacokinetics and pharmacodynamics of butorphanol in dogs (n=6 enrolled per treatment). IV bolus = 0.4 mg/kg IV; IV CRI = 0.2 mg/kg loading dose followed by 0.2 mg/kg/hr for 8 hours; SC = 0.4 mg/kg SC; SC-bicarbonate = 0.8 mg/kg butorphanol SC mixed with an equal volume of 8.4% sodium bicarbonate; IV inhibitors = 0.4 mg/kg IV 1.5–2 hours after the combination of chloramphenicol 50 mg/kg PO, cimetidine 50 mg/kg PO, citalopram 2 mg/kg PO and ketoconazole 20 mg/kg PO.

Drug Administration

The overall study was formatted in an incomplete block design with n=6 dogs per treatment with no more than 3 treatments per dog (Figure 1). There was at least 10 days between treatments based on the short half-life of butorphanol (<2 hours) and the total amount of blood obtained (maximum of 30 mL per dog per crossover). Phase 1 consisted of four different treatment groups (n = 6 dogs per group). Group 1 dogs were administered butorphanol at 0.4 mg/kg IV once (IV bolus) and by an IV loading dose of 0.2 mg/kg followed immediately by a constant rate infusion at 0.2 mg/kg/hr for 8 hours equaling a total dose of 1.8 mg/kg over 8 hours (IV CRI) in a randomized, complete block crossover. Prior to CRI administration a jugular catheter (19-gauge, 1”) was placed aseptically for drug infusion and was removed at the end of the infusion. The infusion was administered using a calibrated fluid pump (Practivet, Tempe AZ, USA) with a burette (JorVet, J-468B, Jorgenson Laboratories Inc, Loveland CO, USA) attached to an IV infusion set (Practivet). Group 2 dogs were administered butorphanol 0.4 mg/kg SC once (SC) and butorphanol at 0.8 mg/kg mixed with equal volume of 8.4% sodium bicarbonate (1:1) administered subcutaneously once (SC-bicarbonate) in a randomized, complete block crossover. Unpublished observations resulted in the 1:1 ratio producing better precipitation (visual interpretation) versus a 2:1 or 1:2 ratio. The alkaline bicarbonate solution caused precipitation of butorphanol within 30 minutes and resulted in a poorly water-soluble drug suspension.

The second phase of the study was conducted at least 4 weeks after Phase 1. Six dogs from Phase 1 were included who had received butorphanol as an IV bolus. In this phase, each dog received the same treatment. The following purported CYP inhibitors (targeted doses) were all administered concurrently 1.5–2 hours prior to butorphanol 0.4 mg/kg IV: chloramphenicol 50 mg/kg PO, cimetidine 50 mg/kg PO, citalopram 2 mg/kg PO and ketoconazole 20 mg/kg PO (IV inhibitors) (Matsui et al, 1995; KuKanich & KuKanich, 2015; KuKanich et al 2017; Perez Jimenez et al, 2019). Dosing the inhibitors at 1.5–2 hours prior to butorphanol administration was based on previous studies that documented the TMAX of the oral inhibitors in dogs: chloramphenicol (1.3 hours) cimetidine (1.8 hours), citalopram (1 hour), ketoconazole (2 hours) (KuKanich & KuKanich 2015; Matsui et al 1995).

Sample Collection and Processing

Approximately 24 hours prior to butorphanol administration, an aseptic jugular catheter was placed (19-gauge x 8–12”) after mild sedation using acepromazine 0.04 mg/kg SC and hydromorphone 0.05 mg/kg SC. The area over the jugular vein was clipped and cleaned with chlorhexidine scrub and isopropyl alcohol. A second jugular catheter was placed in the IV CRI group for drug administration. The catheters in the IV CRI group were marked “sample only” and “infusion only.” The jugular catheters were bandaged throughout the study and removed after the last blood sample was obtained for each treatment. If blood could not be collected from the jugular catheter, blood was obtained with phlebotomy using a 21 gauge x 1” needle from the cephalic vein.

Blood samples, 3 mL per time point, were collected for each treatment group at predetermined time points (Table 1). Blood was placed in heparin tubes then stored on ice until centrifugation. The blood was centrifuged at (3,000 x g for 15 minutes) with the plasma separated and stored at −80 Celsius until analysis for butorphanol plasma concentrations.

Table 1.

Sample collection schedule used to assess the pharmacokinetics and pharmacodynamics of butorphanol administered to dogs. IV bolus = 0.4 mg/kg IV; IV CRI = 0.2 mg/kg loading dose followed by 0.2 mg/kg/hr for 8 hours; SC = 0.4 mg/kg SC; SC-bicarbonate = 0.8 mg/kg butorphanol SC mixed with an equal volume of 8.4% sodium bicarbonate; IV inhibitors = 0.4 mg/kg IV 1.5–2 hours after the combination of chloramphenicol 50 mg/kg PO, cimetidine 50 mg/kg PO, citalopram 2 mg/kg PO and ketoconazole 20 mg/kg PO. RT = rectal temperature.

Samples collected
Time SC and SC-Bicarbonate IV bolus / IV inhibitors IV CRI
0 minutes Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
5 minutes N/A Blood Blood
10 minutes Blood Blood Blood
20 minutes Blood N/A N/A
30 minutes Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
45 minutes Blood N/A N/A
1 hour Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
1.5 hours RT, sedation RT, sedation RT, sedation
2 hours Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
3 hours RT, sedation RT, sedation RT, sedation
4 hours Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
5 hours RT, sedation RT, sedation RT, sedation
6 hours Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
7 hours RT, sedation RT, sedation RT, sedation
8 hours Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
8.5 hours N/A N/A Blood
9 hours RT, sedation RT, sedation Blood, RT, sedation
10 hours RT, sedation RT, sedation Blood, RT, sedation
11 hours RT, sedation RT, sedation RT, sedation
12 hours Blood, RT, sedation Blood, RT, sedation Blood, RT, sedation
24 hours RT, sedation RT, sedation RT, sedation
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Pharmacodynamic Assessment

Dogs were evaluated for rectal temperature (Table 1) using a single digital thermometer purchased new and calibrated by the manufacturer (AmerisourceBergen), and for sedation using a previously published sedation scale (Appendix 1) at predetermined time points (Kukanich et al., 2021; Martinez et al., 2014).

Plasma Butorphanol Concentrations

Plasma butorphanol concentrations were determined by liquid chromatography with triple quadrupole mass spectrometry (Acquity UPLC, TQD, Waters Corporation) based on a previous method (Paine, et. al., 2020) with some modifications. Canine plasma standards, canine plasma quality controls and incurred plasma samples were extracted and analyzed identically. Plasma, 200 mcL, was added to 200 mcL of 2% ammonium hydroxide with 100 ng/ml medetomidine (internal standard) and mixed by aspiration with the pipette. Microelution plates (Oasis HLB µElution plates, Waters Corporation, Milford, MA, USA) were conditioned with 200 mcL of methanol, followed by 200 mcL of 2% ammonium hydroxide; the plasma mixture was loaded, then washed with 200 mcL 5% methanol in 2% ammonium hydroxide and the sample eluted with 50 mcL of methanol with 0.1% formic acid into a 96 well plate. Deionized water, 50 mcL was then added to the 96 well plate and a cap mat placed. The liquid chromatography mobile phase consisted of A: 0.1% formic acid in water and B: acetonitrile with 0.1% formic acid with a flow rate of 0.6 mL/min. The mobile phase started at 95% A at time 0 with a linear gradient to 20% A at 2.8 minutes, back to 95% A at 2.81 minutes with a total run time of 3.5 minutes. A reversed phase C18 column achieved chromatographic separation (Acquity UPLC HSS T3, 2.1×50 mm, 1.8 µM, Waters Corporation). The retention times of butorphanol and the internal standard medetomidine were both 1.6 minutes. The qualifying and quantifying ions for butorphanol were m/z (mass to charge ratio) 328.10→310.11 and 201.03→94.94 for medetomidine. Mass spectrometer settings were optimized using the automated tuning feature included in the software which also was used for integrating the chromatograms (MassLynx 4.1, Waters Corporation). Standard curves in canine plasma were linear between 0.5 and 500 ng/mL and accepted if the coefficient of determination was at least 0.99 and predicted values were within 15% of the actual concentration, except at 0.5 ng/mL where within 20% of the actual concentration was considered acceptable. The lower limit of quantification was 0.5 ng/mL which had a signal to noise ratio >10. A daily run was accepted if at least 6/9 quality controls were within 15% of the actual concentration and no more than 1 failing QC could occur at the low, middle or high concentration. The interday accuracy and precision were determined on replicates of 5 at each of the following concentrations: low = 1 ng/mL, middle = 50 ng/mL and high = 500 ng/mL. The accuracy at 1, 50 and 500 ng/mL was 100%, 107% and 95% of the actual concentration, respectively. The precision (% coefficient of variation) at 1, 50 and 500 ng/mL was 5, 5 and 6%, respectively.

Pharmacokinetic Analysis

Pharmacokinetic analyses were conducted using computer software and noncompartmental methods (Phoenix, 64, Certara, Princeton, NJ, USA). The area under the curve extrapolated to infinity (AUCinf), percent of the AUCinf extrapolated (AUC extrapolated), maximum plasma concentration (CMAX), time to maximum plasma concentration (TMAX), concentration back extrapolated to time 0 (C0), terminal slope (λz), terminal half-life (T ½ λz), mean residence time extrapolated to infinity (MRT), volume of distribution at steady state (Vss), volume of distribution by area method (Vz) and plasma clearance (Cl) were calculated using the computer software. Terminal slopes were estimated using at least 3 time points on the log-linear terminal slope of the plasma concentration vs. time curve with a 1/y2 weighting. The areas under the curve were estimated using the linear trapezoidal method. For the constant rate infusion, the steady state plasma concentration (Css) was determined for each dog by averaging the plasma concentrations for each time point at steady state. The plasma clearance during the IV CRI was calculated by dividing the infusion rate by the Css. The fraction of the dose absorbed was determined by dividing the dose normalized AUCinf for each route.

Statistical Analysis

For IV bolus and SC rectal temperature data, an ANOVA test was used since​ the data were normally distributed with equal variance. The rectal temperature data for the IV CRI and sodium bicarbonate group were not normally distributed with uniform variance so a Kruskal-Wallis ANOVA on ranks was used. A significance level of P<0.05 was used for all evaluations. Analysis was performed using statistical software (Sigma-Plot, version 12.5, Systat Software Inc, San Jose, California). For the pharmacokinetic parameters of IV bolus, IV with inhibitors and IV CRI plasma samples, the post hoc power analyses were <0.8 (with a P<0.05) for the AUCinf, C0 and MRT, therefore statistical analyses were not reported for these pharmacokinetic parameters. For the SC and SC-bicarbonate groups, the post hoc power analyses were <0.8 for all of the pharmacokinetic parameters and therefore not reported.

Results

Rectal Temperature

One dog, a male, removed its infusion catheter during the CRI and it was uncertain when the removal occurred; therefore, it was excluded from all data analyses. The rectal temperature changes from baseline (°F) at predetermined time points (Figure 2) after butorphanol IV bolus (Appendix 2), IV CRI (Appendix 3), SC (Appendix 4), and SC-bicarbonate (Appendix 5) administration. There was a statistically significant decrease in rectal temperature, compared to baseline measurements, for IV bolus (1.5–4 hr), IV CRI (1–5 hr), SC-bicarbonate (2–7 hr) administration of butorphanol. A statistically significant decrease in rectal temperature did not occur after SC administration. Rectal temperature is not reported for the IV with inhibitors treatment group due to a malfunctions with the facility’s central air conditioning, subsequent use of supplemental portable air conditioning units and the resultant fluctuating room temperatures.

Figure 2.

Figure 2.

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Mean change in rectal temperature (mean) in Fahrenheit (°F) after the administration of butorphanol. Colored symbol indicates significant (P<0.05) difference compared to time 0 within each treatment. See Figure 1 for the treatment key.

Sedation

Following IV bolus administration, at least one dog had moderate sedation or higher (sedation score >1) from 1.5–2 hours (Figure 3, Appendix 6). During IV CRI administration, moderate sedation or greater in at least 1 dog was observed from 0.5–8 hours. After the SC administration, moderate sedation or greater was observed in at least 1 dog from 1.5–2 hours. For the SC-bicarbonate administration, moderate sedation or greater was observed from 1–2 hours. No dog had a sedation score of unresponsive (score of 4). No dog in any treatment group had moderate or greater sedation after 8 hours. Sedation scores were not recorded in 3 dogs (2 dogs at 24 hours in the IV bolus group and 1 dog at 12 hours in SC-bicarbonate group).

Figure 3.

Figure 3.

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The number of dogs at each time with moderate sedation or greater (i.e. number of dogs with sedation score >1). See Figure 1 for treatment key.

Pharmacokinetics

Plasma concentrations and pharmacokinetic parameters of butorphanol after IV bolus, IV CRI, and IV with inhibitors are summarized in Table 2 and Figure 4 and after SC and SC-bicarbonate administration can be found in Table 3 and Figure 5. The actual doses of the CYP inhibitors were mean (range): chloramphenicol 52.5 (41.7–61.7) mg/kg PO, cimetidine 49.3 (45.0–55.6) mg/kg PO, citalopram 2.4 (2.0–3.3) mg/kg PO, and ketoconazole 21.0 (16.7–24.7) mg/kg PO. The geometric mean (range) of the butorphanol plasma concentration extrapolated to time 0 (C0) after IV bolus and IV with inhibitors administration were 182.9 ng/mL (142.7–279.9 ng/mL), and 130.3 ng/mL (110.0–157.8 ng/mL), respectively. The terminal half-lives (T ½ λz) were not significantly different between the IV bolus (mean = 1.47, range 1.19–1.71 hr) and IV CRI groups (1.42 hr, range 0.99–1.83 hr), but both were significantly different compared to the IV inhibitors group (mean = 3.5 hr, range 2.1–10.0 hr, P=0.001). The plasma clearance (Cl) for the IV CRI group (mean=77.3 mL/min/kg, range 63.3–86.7 mL/min/kg) was significantly higher (P<0.001) compared to the IV bolus (mean=46.5 mL/min/kg, range 40.9–61.0 mL/min/kg) and IV with inhibitors (mean= 52.9 mL/min/kg, range 39.5–65.3 mL/min/kg). The clearance was not significantly different (P>0.05) between the IV bolus and IV inhibitor groups. The volumes of distribution (Vz, Vss) were significantly different between the IV bolus and IV with inhibitors. The IV CRI group steady state plasma concentration (Css) had a geometric mean of 43.1 ng/mL and range of (38.5–52.7 ng/mL).

Table 2.

Pharmacokinetic parameters for butorphanol following IV bolus administration, or administered in combination with cytochrome P450 inhibitors, or administered as a constant rate infusion with an IV loading dose. The post hoc power analyses was <0.8 with a P<0.05 for the AUCinf, C0 and MRT, therefore statistical analyses were not reported for these pharmacokinetic parameters. Superscript letters indicate significant differences within a row. See Table 1 for key.

  IV Bolus IV inhibitors IV CRI
Parameter Units Geometric Mean Minimum Maximum Geometric Mean Minimum Maximum Geometric Mean Minimum Maximum
AUC extrapolated % 1.0 0.5 2.2 2.9 1.6 9.1 N/A N/A N/A
AUCinf hr*ng/mL 143 109 163 126 102 169 N/A N/A N/A
C0 ng/mL 182.9 142.7 279.9 130.3 110.0 157.8 N/A N/A N/A
Cl mL/min/kg 46.5 a 40.9 61.0 52.9 b 39.5 65.3 77.3 a,b 63.3 86.7
λz /hr 0.472 a 0.406 0.584 0.196 a,b 0.069 0.327 0.488 b 0.378 0.698
T ½ λz hr 1.47 a 1.19 1.71 3.5 a,b 2.1 10.0 1.42 b 0.99 1.83
MRT hr 1.26 1.02 1.67 2.1 1.4 3.6 N/A N/A N/A
Vss L/kg 3.51 a 2.64 4.21 6.8 a 4.9 14.3 N/A N/A N/A
Vz L/kg 5.91 a 5.25 6.67 16.2 a 10.5 56.4 N/A N/A N/A
Css ng/mL N/A N/A N/A N/A N/A N/A 43.1 38.5 52.7
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Figure 4.

Figure 4.

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Plasma profile (mean and standard deviation) of butorphanol administered intravenously. See Figure 1 for key.

Table 3.

Pharmacokinetic parameters for butorphanol following subcutaneous administration, or combined with sodium bicarbonate then administered subcutaneously. The post hoc power analyses was <0.8 with a P<0.05 for all of the pharmacokinetic parameters and therefore not reported. See Table 1 for key.

  SC SC-bicarbonate
Parameter Units Geometric Mean Minimum Maximum Geometric Mean Minimum Maximum
AUC extrapolated % 2.5 0.3 22.5 4.6 0.5 63.3
AUCinf hr*ng/mL 123.3 101.8 152.4 151 58 301
CMAX ng/mL 74.2 48.6 90.0 52.0 8.7 156.2
TMAX hr 0.21 0.17 0.33 0.22 0.17 0.50
Λz /hr 0.378 0.127 0.696 0.163 0.031 0.421
T ½ λz hr 1.83 1.00 5.47 4.25 1.65 22.51
MRT hr 2.09 1.24 5.23 4.88 1.94 29.61
F relative % 61 22 108
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Figure 5.

Figure 5.

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Plasma profile (mean and standard deviation) of butorphanol administered subcutaneously. See Figure 1 for key.

The geometric mean (range) maximum plasma concentration (CMAX) following SC and SC-bicarbonate administration was 74.2 ng/mL (48.6–90.0 ng/mL) and 52.0 ng/mL (8.7–156.2 ng/mL), respectively. The relative bioavailability following SC-bicarbonate administration compared to SC administration was 61% and ranged from 22% to 108%. The geometric mean of the terminal half-life (T ½ λz) of SC and SC-bicarbonate was 1.83 (1.00–5.47) hr and 4.25 (1.65–22.51) hr, respectively. The time to reach maximum plasma concentration (TMAX) after SC and SC-bicarbonate administration was 0.21 (0.17–0.33) hr and 0.22 (0.17–0.50) hr, respectively.

Adverse Effects

Following IV bolus administration, vomiting occurred in 1/6 dogs, and hypersalivation was seen in 1/6 dogs. The IV inhibitors group had 4/6 dogs with vomiting and 4/6 dogs with hypersalivation. During IV CRI administration, vomiting occurred in 1/6 dogs, diarrhea occurred in 1/6 dogs and hypersalivation occurred in 1/6 dogs. Following SC administration vomiting occurred in 3/6 dogs, an observation of a painful injection occurred in 1/6 dogs, diarrhea occurred in 3/6 dogs and hypersalivation occurred in 2/6 dogs. After SC-bicarbonate administration vomiting occurred in 1/6 dogs, diarrhea occurred in 2/6 dogs and hypersalivation occurred in 1/6 dogs. Diarrhea in all but one dog was characterized by small fecal amounts with mucous. Diarrhea in 1 dog was a large amount of soft stool.

Discussion

Butorphanol is an opioid but it has lower potential for abuse or diversion than full mµ agonists. As such, establishing a safe and effective analgesic dose will not only provide an animal welfare benefit, but also a One Health benefit by decreasing the use of drugs with high abuse potential (mµ opioid agonists). This study was designed to enhance previously reported data (Houghton et al., 1991; Wegner et al., 2008) which demonstrated 0.4 mg/kg IV butorphanol produced significant antinociceptive effects. Wegner et al (2008) also demonstrated butorphanol produced near identical peak antinociceptive effects and duration compared to 0.1 mg/kg IV hydromorphone.

A previous study demonstrated dose dependent effects of morphine and methadone on antinociception and decreases in rectal temperature in dogs (Vaupel & Jasinski, 1997). We monitored central opioid effects using rectal temperature as a noninvasive marker in lieu of an induced pain model in order to minimize animal discomfort and stress. Butorphanol induced a significant opioid effect that was measured as a decrease in rectal temperature in the IV bolus, IV CRI, and SC-bicarbonate groups, but not in the SC group.

The time course of the IV bolus group’s opioid effect in this study, significantly decreased rectal temperature from 1.5–4 hours, was similar to the time course of the antinociceptive effect (0.5–4 hours) previously reported using thermal noiciception (Wegner et al., 2008) and both studies using 0.4 mg/kg IV. A colonic distension nociception model documented significant effects 30 minutes (the latest time point assessed) after 0.4 mg/kg IV, but not doses 0.2 mg/kg IV or lower (Houghton, et, al., 1991). These data, in addition to previously reported data (Vaupel & Jasinski, 1997) suggest that rectal temperature changes induced by butorphanol may be an easy noninvasive method of assessing central opioid effects in preclinical models in dogs. Further studies simultaneously assessing the effects of butorphanol on rectal temperature and antinociception are needed.

In contrast to the results of the preclinical trials demonstrating significant opioid effects of 0.4 mg/kg butorphanol IV in this study and others (Houghton et al., 1991; Wegner et al., 2008), a clinical trial of 0.4 mg/kg IV butorphanol provided insufficient analgesia in dogs undergoing soft tissue surgery (Gomes et. al., 2020). However, a major limitation of Gomes et. al., 2020 was that the intervention point for rescue analgesia in that study (Glasgow Composite Measure Pain Scale total score >3.5) was set below the validated intervention, >5 total score (Reid, et. al., 2007). The Glasgow Composite Measure Pain Scale uses six behavioral categories which are not specific for pain and using a low cutoff may not distinguish between postoperative sedation, anxiety and pain. For example, based on the Gomez study (2020) intervention point, a mildly sedated dog who is slow to stand and indifferent to its surroundings, would have been judged as needing rescue analgesia but may not have been painful (Reid, et. al., 2007). Therefore, Gomes et al (2020) had a bias for treatment failure and false positives for treatment failure which is not indicative of true treatment failure. Regardless, clinical trial data using validated methods are needed to confirm the opioid analgesia induced by butorphanol in dogs.

There were some unexpected results in this study. The significant effect of butorphanol on rectal temperature during the IV CRI was lost prior to discontinuing the infusion and despite consistent plasma drug concentrations. There are several explanations why this may have occurred. The most likely reason is the animal caretakers would come into the study room for “welfare checks” between the 6 and 8 hour time points. As seen in Appendix 3, the variability in the rectal temperature increased between 6 and 8 hours. The animals became excited when they saw the caretakers potentially causing an increase in temperature and is a factor that needs mitigated when using rectal temperature to measure central opioid effects. This affected the IV CRI group to a greater extent than the other groups as the opioid effect was lost in the other groups prior to this disturbance. It is also possible that true pharmacodynamic tolerance developed, but this has not been previously documented to occur within hours, but typically occurs in a time frame from days to weeks (Smith et al., 1999). The rectal temperature data were corrupted during the IV inhibitors treatment due to the air conditioning malfunctioning during the study, also a shortcoming of measuring this parameter.

Another unexpected result was that subcutaneous butorphanol had insignificant effects on rectal temperature. Although this was unexpected, it is similar to a study using a colonic distension nociceptive model in dogs in which doses up to 0.4 mg/kg SC failed to produce a significant response at 45 minutes, but 0.8 mg/kg SC did produce a significant effect (Sawyer et al., 1991). In order for a drug to produce an effect, it has to reach the target (receptor) in sufficient concentrations for a sufficient period of time (e.g. total exposure). We hypothesize that sufficient concentrations were able to reach opioid receptors in the CNS, but not for a sufficient period of time after SC administration. The CMAX of SC butorphanol was 74.2 ng/mL which exceeded the Css during the IV CRI suggesting sufficient concentrations were achieved. However, butorphanol plasma concentrations rapidly decreased after SC administration which failed to provide sufficient time exposure. Wegner et al 2008, demonstrated peak antinociceptive effect after IV administration occurred at 60 minutes suggesting a delay in the transfer of butorphanol into the central compartment. There is also going to be a delay from the time opioid receptors are activated in the hypothalamus until the dogs’ rectal temperature drops. We hypothesize the combined lag of drug penetration into the CNS, delay for subsequent drop in rectal temperature along with the rapid drug elimination obscured rectal temperature effect in these dogs (i.e. there was a drop in rectal temperature after SC administration, but it was not significant). In comparison, the IV bolus group had much higher drug concentrations after administration (the mean 5 minute plasma concertation was 143.9 ng/mL) which allowed more drug to diffuse into the CNS and increased total exposure to the opioid receptors compared to SC administration. Butorphanol is often administered as part of many preanesthetic protocols for dogs, but based on these data collectively (Sawyer et al., 1991), the effective SC butorphanol dose for analgesia is likely greater than 0.4 mg/kg and needs to be better defined.

The SC-bicarbonate group (0.8 mg/kg SC butorphanol) had significant effects on rectal temperature, whereas the SC group (0.4 mg/kg) did not. This is most likely due to the overall higher drug exposure (higher AUCinf) from the higher dose administered. We hypothesized a prolonged effect would occur with a poorly water-soluble suspension, due to drug precipitation, and resulting in a sustained release when administered SC. Based on the pharmacokinetic parameters, the SC-bicarbonate group did not produce a consistent sustained release profile and the greater magnitude and duration of effect was most likely a dose dependent effect. Dose escalation studies would confirm or refute the dose-response hypothesis.

Sedation is a well-known effect of opioids and can be desirable to calm a fractious or anxious animal or could be an adverse effect if normal mentation is desired. Sedation was observed in this study when rectal temperature decreases were not significant. Again, these are similar results to a previous study that reported that the degree of sedation does not correlate with the degree of antinociception and should not be used as an indicator for analgesia (Houghton, et. al., 1991; Sawyer, et. al., 1991). However, if sedation without significant analgesia is needed for procedures that produce little to no pain (e.g. radiographs, ultrasound, etc.), lower doses of butorphanol could be used for sedation than required for analgesia.

This is the first study to comprehensively report the pharmacokinetics of butorphanol when administered an IV bolus, IV CRI and SC in addition to the novel formulation SC bicarbonate and assessing the effects of purported CYP inhibitors in healthy dogs. The IV pharmacokinetic data are similar to other opioids and the SC data nearly proportional to previously reported values (Pfeffer et al., 1980). The addition of bicarbonate to the commercially available butorphanol solution for injection effectively decreased the relative bioavailability and seemed to provide marginal benefits for sustained absorption compared to the commercially available solution. Likewise, the IV inhibitors group had marginal overall effects indicating that, chloramphenicol, cimetidine, citalopram and ketoconazole administered as a single clinically relevant dose are unlikely to have a clinically meaningful pharmacokinetic interaction. Further studies with other CYP inhibitors and multiple doses of CYP inhibitors are needed.

The loading dose and IV CRI of butorphanol produced consistent effects, but the clearance was greater than expected. This may be due to a truly higher clearance, but could also be an indication of drug loss in the fluid administration set and an overall lower dose administered than expected. Further studies should assess the potential for drug binding/loss of butorphanol when administered as an IV CRI as described in this study. Regardless, effective concentrations were achieved and maintained over the 8-hour infusion.

Adverse effects, other than sedation, were noted within each treatment group such as vomiting, hypersalivation and diarrhea. Vomiting and hypersalivation are typical adverse effects of opioids in dogs. Diarrhea was an unexpected observation, but based on its appearance most seemed to be stress diarrhea and may be more associated with changes to the dogs’ daily routines, transport between long-term housing to the study areas and placement of jugular catheters versus a true drug adverse effect. Gastrointestinal adverse effects in phase 2 could also have been due to the inhibitors. No interventions were needed for the gastrointestinal adverse events. Painful administration of butorphanol was observed in the SC treatment group (1 dog) and could be due to its acidic formulation or could have been an individual animal response.

As with all studies, this study raises more potential research questions. A future study could assess the dose-response of SC butorphanol at equal doses to SC-bicarbonate and would help discern drug dose versus formulation as the factor for the better response observed in this study. A study directly correlating the response relationships of butorphanol for rectal temperature and antinociception will be helpful in better defining the potential for more widespread use of rectal temperature as a noninvasive surrogate marker for butorphanol antinociception in dogs. The use of doses effective in this study (i.e. 0.4 mg/kg IV and 0.2 mg/kg IV followed by 0.2 mg/kg/hr) should be assessed in clinical patients for analgesia.

All studies have limitations. Limitations of our study included difficulties with the research facility and its infrastructure which caused an inability to perform the study without outside interferences. For example, interferences from animal caretakers throughout the study and loss of air conditioning during one of the crossovers influenced data collection and results. We also had a larger magnitude of variability within our study for both pharmacokinetic and pharmacodynamics parameters than anticipated indicating a larger sample size is needed (i.e. >6 dogs per treatment) for future studies. Only single doses were assessed for each route/formulation (excluding the IV CRI) and the effects of multiple doses and dose response relationships for each formulation and route would help define the reasons for some of the results we observed.

In conclusion, IV bolus butorphanol (0.4 mg/kg) and a loading dose of butorphanol (0.2 mg/kg IV) followed by a constant rate infusion (0.2 mg/kg/hr) produced central opioid effects. However, 0.4 mg/kg SC butorphanol failed to produce significant opioid effects. The use of butorphanol (0.8 mg/kg) mixed with bicarbonate to form a poorly water soluble butorphanol suspension produced marginal additional effects, but that may have been due to a dose difference. There were minimal effects on the pharmacokinetics of IV butorphanol when administered after a cocktail of purported canine CYP inhibitors.

Acknowledgments

This study was funded by the KuKanich Clinical Pharmacology Research Program, and a National Institutes of Health (T35OD029981) grant that funded MG while a member of the Kansas State Veterinary Research Scholars Program. This study was presented as an oral abstract (DS) at the American Academy of Veterinary Pharmacology and Therapeutics Online Student Research Symposium May, 2022, as an oral abstract (MG) at the Kansas State Phi Zeta Research Day, March, 2022, and as a poster (MG) at the virtual National Veterinary Scholar Symposium, August, 2021. Phoenix 64 software was graciously provided by Certara as an academic license through their Centers of Excellence Program.

Funding

This study was funded by the KuKanich Clinical Pharmacology Research Program, and a National Institutes of Health (T35OD029981) grant that funded MG while a member of the Kansas State Veterinary Research Scholars Program.

Appendix

Appendix 1. Sedation Score Scale (Martinez et al., 2014)

0 - No Sedation Present.

1 - Slight Sedation – almost normal; able to stand easily, but appears somewhat fatigued, subdued or somnolent.

2 - Moderate Sedation – able to stand but prefers to be recumbent; sluggish; ataxic or uncoordinated.

3 - Profound Sedation – unable to rise, but can exhibit some awareness of environment; responds to stimuli through body movement; may be lateral or sternal recumbency.

4 - Unresponsive – in a state of coma or semi-coma from which little or no response can be elicited; remains in lateral recumbency.

Appendix 2.

Appendix 2.

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Change in rectal temperature (mean and standard deviation) in Fahrenheit (°F) after administration of butorphanol IV bolus (0.4 mg/kg IV) to healthy dogs (n=6). The colored symbols indicate statistically significant decreases in rectal temperature when compared to baseline between the 1.5–4 hour time points.

Appendix 3.

Appendix 3.

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Change in rectal temperature (mean and standard deviation) in Fahrenheit (°F) after the administration of butorphanol loading dose (0.2 mg/kg IV) and IV CRI (0.2 mg/kg/hr) to healthy dogs (n=5). The colored symbols indicate statistically significant decreases in rectal temperature when compared to baseline between the 1–5 hour time points.

Appendix 4.

Appendix 4.

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Change in rectal temperature (mean and standard deviation) in Fahrenheit (°F) after the administration of butorphanol SC (0.4 mg/kg) to healthy dogs (n=6). Statistically significant decreases in rectal temperature when compared to baseline did not occur with this group.

Appendix 5.

Appendix 5.

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Change in rectal temperature (mean and standard deviation) in Fahrenheit (°F) after the administration of SC-bicarbonate butorphanol (0.8 mg/kg butorphanol). The colored symbols indicate statistically significant decreases in rectal temperature when compared to baseline between the 2–7 hour time points.

Appendix 6.

The number of dogs within each sedation score category. 0=No Sedation Present; 1=slight sedation; 2=moderate sedation; 3=profound sedation; 4=unresponsive. No sedation scores of 4 were recorded. See Appendix 1 for detailed description of the sedation scoring rubric.

IV bolus IV inhibitors IV CRI SC SC-bicarbonate
Sedation Score Sedation Score Sedation Score Sedation Score Sedation Score
Time (hr) 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3
0 6 0 0 0 6 0 0 0 5 0 0 0 6 0 0 0 6 0 0 0
0.5 0 6 0 0 0 6 0 0 0 3 2 0 0 6 0 0 0 6 0 0
1 0 6 0 0 0 6 0 0 0 3 1 1 0 6 0 0 0 5 1 0
1.5 0 5 1 0 0 6 0 0 0 3 1 1 0 5 1 0 0 5 1 0
2 0 5 1 0 0 4 2 0 0 3 2 0 0 5 1 0 0 5 1 0
3 0 6 0 0 0 5 1 0 0 3 2 0 1 5 0 0 0 6 0 0
4 1 5 0 0 3 3 0 0 0 4 1 0 2 4 0 0 0 6 0 0
5 1 5 0 0 6 0 0 0 0 4 1 0 2 4 0 0 2 4 0 0
6 5 1 0 0 6 0 0 0 0 4 1 0 4 2 0 0 5 1 0 0
7 5 1 0 0 6 0 0 0 0 4 1 0 6 0 0 0 5 1 0 0
8 6 0 0 0 6 0 0 0 0 3 2 0 6 0 0 0 6 0 0 0
9 6 0 0 0 6 0 0 0 0 5 0 0 6 0 0 0 6 0 0 0
10 6 0 0 0 6 0 0 0 0 5 0 0 6 0 0 0 6 0 0 0
11 6 0 0 0 6 0 0 0 2 3 0 0 6 0 0 0 6 0 0 0
12 6 0 0 0 6 0 0 0 4 1 0 0 6 0 0 0 5 0 0 0
24 4 0 0 0 6 0 0 0 5 0 0 0 6 0 0 0 6 0 0 0
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Footnotes

Data Availability Statement

Data available on reasonable request from the authors.

Conflict of Interest Statement

The authors declare no conflict of interest to disclose.

Animal Welfare and Ethics Statement

The author confirms that they have adhered to the US standards for the protection of animals used for scientific purposes. This study was reviewed and approved by Kansas State University Institutional Animal Care and Use Committee (protocol no. 4553).

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