Controlling moderate to severe pain in dogs on an outpatient basis is difficult, as there are few options that have demonstrated clinical efficacy for this use. Nonsteroidal anti-inflammatory drugs can be administered orally at home, but are effective for only mild to moderate pain and sometimes have adverse effects in the gastrointestinal tract, kidneys, and liver that can be severe.1 Transdermal fentanyl patches are used occasionally, but absorption of the drug by this route is variable in dogs and a used patch contains enough remaining fentanyl to be lethal for up to 8 human adults (ie, 8 times the lethal dose), which poses a human health hazard and may increase the risk of diversion and misuse or consequences of accidental exposure.2–4
Methadone has a short duration of action when administered parenterally and has a low bioavailability after oral administration in dogs.5,6 A strategy to increase the plasma drug concentrations and duration of methadone's effects would be to include a pharmacokinetic enhancer of methadone. A pharmacokinetic enhancer is a drug that affects the pharmacokinetics of another drug, such as methadone, when administered concurrently with that drug. A specific CYP inhibitor can be used as a pharmacokinetic enhancer by concurrent administration with a drug that is metabolized by the same CYP, with an expected result of higher oral bioavailability as a result of decreased first-pass metabolism and prolonged maintenance of drug concentrations owing to slower elimination. Results of previous studies7–9 have revealed clinical opioid effects after oral administration of methadone with CYP inhibitors used as pharmacokinetic enhancers. Significant central opioid effects (evidenced by decreased rectal temperature) were present for ≥ 8 hours after methadone was administered orally with fluconazole as a pharmacokinetic enhancer, compared with oral methadone administration alone in dogs.9 Rectal temperature decreases are predictable in dogs after opioid administration and are attributable to central opioid effects. Decreases in rectal temperature typically parallel analgesic effects in a dose-dependent manner in dogs, as both effects arise from activation of opioid receptors in the CNS.10–13 As such, monitoring rectal temperature in dogs is a fairly noninvasive method of assessing central opioid effects, and the results can be used to inform the design of other studies to assess the antinociceptive effects of opioids.
A concern with any opioid is the potential for misuse and abuse by people (including those who work with or administer drugs [eg, clients, veterinarians, and veterinary staff] and others who may gain access by theft for purposes of diversion or misuse) as well as the consequences of inadvertent exposure (eg, ingestion of a tablet, capsule, or patch by a child). A way to minimize the potential for misuse, abuse, and consequences of inadvertent exposure is to include an opioid receptor antagonist that is known to inhibit or decrease the effects of methadone in people but does not inhibit its opioid effects in dogs.
Naltrexone is primarily a μ opioid receptor competitive antagonist with a low oral bioavailability in dogs (approx 1% after oral administration), but it has higher bioavailability (5% to 40%) in people.14–18 People additionally produce an active naltrexone metabolite (β-naltrexol) that also primarily antagonizes μ opioid receptors (along with absorbed naltrexone), subsequently inhibiting or reversing the effects of opioids. In contrast, dogs apparently do not produce β-naltrexol as a substantial metabolite but form the inactive metabolite naltrexone glucuronide.15,16 Naltrexone has been incorporated into some FDA-approved human oral opioid medications formulated for abuse resistance; the naltrexone was sequestered so it was available for absorption only if the tablet was crushed, chewed, or dissolved. For example, morphine was combined with naltrexone at a ratio of 25:1, and oxycodone was combined with naltrexone at a ratio of 8.3:117,18 Oral administration of the intact morphine-naltrexone formulation resulted in very small amounts of β-naltrexol and naltrexone, but crushing the tablet increased the Cmax to 3.9 ng/mL. The terminal half-life of β-naltrexol in people is approximately 12 to 14 hours, and the drug is expected to produce prolonged antagonism of opioid receptors (up to 48 hours).14,17,18 Although these formulations are FDA approved and demonstrated to be effective analgesics with abuse-deterrent properties, the manufacturer made a business decision to discontinue the products because their exclusivity was expiring.19 Naltrexone could be incorporated into an oral formulation for dogs without sequestration or a need for advanced formulation chemistry because the drug has a low bioavailability after oral administration in this species and β-naltrexol is not a substantial metabolite in dogs; for these reasons, opioid receptor antagonist effects are not expected after oral naltrexone administration to dogs.
The purpose of the study reported here was to assess the pharmacokinetics and pharmacodynamic effects of naltrexone in a formulation with methadone and fluconazole (methadone-fluconazole-naltrexone) in healthy dogs. The specific aims were to determine the pharmacokinetics of methadone, fluconazole, naltrexone, β-naltrexol, and naltrexone glucuronide and to assess central opioid effects, measured as decreased rectal temperature and sedation, in dogs following oral administration of methadone-fluconazole-naltrexone. The hypotheses were that naltrexone would have low bioavailability in dogs after oral administration of methadone-fluconazolenaltrexone; naltrexone and its active metabolite, β-naltrexol, would not be measurable in dogs that received this combined drug treatment; the inactive metabolite, naltrexone glucuronide, would be measurable, indicating systemic absorption of naltrexone in dogs that received the treatment; and central opioid effects would be detected in dogs that received the methadone-fluconazole-naltrexone treatment.
Materials and Methods
Animals
The study was approved by the Institutional Animal Care and Use Committee at Kansas State University. Twelve healthy purpose-bred Beaglesa (8 neutered males and 4 spayed females; age, 3 to 4 years; body weight, 10.7 to 13.9 kg) were enrolled in 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 pairs or groups in indoor runs (dimensions, 10 × 10 feet) with free access to outdoor runs (10 × 30 feet) when the weather was considered appropriate. Supervised group interactions were also provided. Dogs interacted with people at least twice daily and were given access to toys with treats, chew toys, and balls. During the study period, the dogs were housed individually in runs (dimensions, 5 × 7 feet) that included elevated dog beds, and they were returned to their routine housing at the end of the study. The dogs were not used in other pharmacology studies for ≥ 8 weeks prior to the study reported here.
The study had a parallel design with 6 dogs/treatment group. To ensure equal distribution, dogs were initially blocked by sex and then randomly allocated to 1 of 2 treatments (methadone-fluconazole or methadone-fluconazole-naltrexone) by drawing names from a container. Groups of 4 dogs (2 dogs/treatment) were treated each day for 4 consecutive days, so all 12 dogs completed the study in a 4-day time frame. Access to food was withdrawn approximately 16 hours prior to administration of the methadone-containing formulation and replaced 10 hours after the treatment; water was provided at all times. Rectal temperature and sedation were monitored as described.
As previously described,9 every dog received 1 dose of fluconazole (100 mg, PO) 12 hours prior to administration of the randomly assigned study treatment. This treatment was intended to maximize potential interactions of fluconazole with methadonemetabolizing enzymes when the methadone-containing study treatments were administered.
The methadone-fluconazole treatment consisted of 10 mg of methadone hydrochloride and 50 mg of fluconazole (total doses) formulated for oral administration as follows: 10 (10-mg) methadone hydrochloride tabletsb and 5 (100-mg) fluconazole tabletsc (yielding a 1:5 ratio of methadone to fluconazole content) were weighed, then pulverized and homogenized with a mortar and pestle to a fine powder. The powder was aliquoted by weight and placed in size-0 gelatin capsulesd to provide the predetermined drug doses in each capsule. The methadone-fluconazole-naltrexone treatment consisted of 10 mg of methadone hydrochloride, 50 mg of fluconazole, and 2.5 mg of naltrexone hydrochloride. To prepare the oral formulation, 10 (10-mg) methadone hydrochloride tablets,b 5 (100-mg) fluconazole tablets,c and half of 1 scored (50-mg) naltrexone hydrochloride tablete (yielding a 4:1 ratio of methadone to naltrexone content) were weighed, then pulverized and homogenized with a mortar and pestle to a fine powder. The powder was aliquoted by weight and placed in size-0 gelatin capsules to provide the predetermined drug doses in each capsule. The only size of naltrexone hydrochloride tablet commercially available was 50 mg, and half of 1 scored tablet was used to minimize waste of methadone tablets while maintaining the desired ratios of methadone, fluconazole, and naltrexone. On the basis of FDA and United States Pharmacopeia guidelines, half of a scored naltrexone hydrochloride tablet contains 45% to 55% of the stated (labeled) content, just as a whole tablet contains 90% to 110% of the stated content.20,21 The contents of the capsules were known to only 1 investigator (BK) to avoid bias for subjective assessments such as sedation.
Rectal temperatures and sedation were evaluated at baseline (immediately prior to the assigned methadone-containing treatment [time 0]) and 1, 2, 4, 6, 8, 12, and 24 hours after the drug was administered. The same rectal thermometer was used to measure rectal temperatures throughout the study. Sedation was assessed by an experienced observer (KK or DCR) who was masked to the treatment. Sedation was scored with a categorical scale as follows: none (no apparent sedation), slight (almost normal; able to stand easily but appears somewhat fatigued, subdued, or somnolent), moderate (able to stand but prefers to be recumbent; appears sluggish, ataxic, or uncoordinated), profound (unable to rise but can show some awareness of environment, responds to stimuli through body movement, and may be laterally or sternally recumbent), or unresponsive (in a state of coma or semicoma in which little or no response can be elicited; remains in lateral recumbency).22
Blood was obtained through an aseptically placed jugular catheter at baseline and 20 and 40 minutes and 1, 2, 4, 6, 8, 12, and 24 hours after administration of the assigned methadone-containing treatment. Blood was placed in tubes that contained lithium heparin and immediately placed on ice. Within 4 hours after collection, the tubes were centrifuged at 3,000 × g for 15 minutes. Plasma was separated and stored at −70°C until analysis. Plasma was analyzed for methadone, fluconazole, naltrexone, β-naltrexol, and naltrexone glucuronide concentrations by ultrahigh-performance liquid chromatography with triple quadrupole mass spectrometry.g Plasma preparation was performed with pass-through platesh as previously described.9 Mobile phase A consisted of deionized water with 0.1% formic acid, and mobile phase B consisted of acetonitrile with 0.1% formic acid. The mobile phase gradient started at 85% mobile phase A followed by a linear gradient to 5% mobile phase A at 0.8 minutes, which was held until 1.2 minutes and followed by a linear gradient to 85% mobile phase A. The total run time was 2 minutes with a flow rate of 0.6 mL/min, and separation was achieved by use of a column designed for retention and separation of polar compoundsi maintained at 40°C.
Methadone d9j (m/z 319→268) was the internal standard for methadonej (m/z 310→265), and voriconazolek (m/z 350→281) was the internal standard for fluconazolel (m/z 307→220). Naltrexone d3j (m/z 345→270) was the internal standard for naltrexonej (m/z 342→324) and β-naltrexolj (m/z 344→326). Naltrexone glucuronide d3j (m/z 519→113) was the internal standard for naltrexone glucuronidej (m/z 516→113). The lower limits of quantification (signal-to-noise ratio ≥ 10:1) for methadone, fluconazole, naltrexone, β-naltrexol, and naltrexone glucuronide were 0.5 ng/mL, 0.5 μg/mL, 0.5 ng/mL, 1 ng/mL, and 10 ng/mL, respectively. The intraday accuracy for the methadone assay was 107%, 99%, and 102%, and the intraday CV was 8%, 2%, and 1% on replicates of 3 for methadone at concentrations of 0.5, 10, and 100 ng/mL, respectively. For the fluconazole assay, the intraday accuracy was 102%, 92%, and 96% and the intraday CV was 5%, 1%, and 5% on replicates of 3 for concentrations of 1, 10, and 100 μg/mL, respectively. The naltrexone assay had an intraday accuracy of 106%, 106%, and 104% and intraday CV of 14%, 1%, and 5% on replicates of 3 for 0.5, 10, and 100 ng/mL, respectively. The intraday accuracy for the β-naltrexol assay was 111%, 106%, and 105% and the intraday CV was 6%, 6%, and 3% on replicates of 3 for 1, 10, and 100 ng/mL, respectively. The intraday accuracy for the naltrexone glucuronide assay was 106%, 100%, and 101%, and the intraday CV was 3%, 1%, and 5% on replicates of 3 for 10, 100, and 500 ng/mL, respectively.
Pharmacokinetic data were analyzed by noncompartmental methods with computer software.m The summary pharmacokinetic data are presented as geometric mean and range, and the plasma concentration-versus-time profile is presented as arithmetic mean ± SD.23 The Cmax and time to maximum concentration were determined directly from the data. The terminal half-life and terminal rate constant were determined with log-linear regression and included ≥ 3 time points on the terminal portion of the curve. The AUC was calculated with the linear trapezoidal method, and AUC0–∞ was determined with the following formula: AUC + (plasma drug concentration at 24 h/terminal half-life). The relative fraction of the methadone dose absorbed was calculated by dividing the geometric mean AUC0–∞ of methadone for dogs that received the methadone-fluconazole-naltrexone treatment by the geometric mean AUC0–∞ of methadone for dogs that received the methadone-fluconazole treatment.
Statistical analysis
Statistical analyses of pharmacodynamic data were performed with computer software.n Normality of the data was confirmed with the Shapiro-Wilk test, and equal variance was confirmed with Spearman rank correlation. Baseline rectal temperatures, body weights, and drug doses were compared between the 2 treatment groups with t tests and are reported as arithmetic mean ± SD. Rectal temperature data were assessed for within-group differences by a repeated-measures ANOVA followed by the Holm-Sidak post hoc test for comparisons with baseline (time 0) when significant differences were detected. Values of P < 0.05 were considered significant.
Results
Body weight of the dogs ranged from 11.9 to 13.8 kg in the methadone-fluconazole treatment group and from 10.7 to 13.9 kg in the methadone-fluconazolenaltrexone treatment group. Body weight did not differ significantly (P = 0.197) between the methadone-fluconazole and methadone-fluconazole-naltrexone treatment groups (arithmetic mean ± SD, 12.9 ± 0.8 kg and 12.0 ± 1.3 kg, respectively). The arithmetic mean ± SD capsule weights were 101.0 ± 0.9% (range, 99.6% to 102.2%) and 100.0 ± 0.5% (range, 99.4% to 100.9%) of the targeted values for the methadone-fluconazole and the methadone-fluconazole-naltrexone treatments, respectively. The mean doses of fluconazole and methadone administered, as calculated on the basis of body weight, did not differ significantly between treatment groups (Table 1). Adverse effects were not observed other than the reported sedation and changes in rectal temperature.
Doses of methadone hydrochloride, fluconazole, and naltrexone hydrochloride calculated on the basis of body weight for 12 healthy Beagles that received methadone-fluconazole (n = 6) or methadone-fluconazole-naltrexone (6) in a study to determine the effects of coadministration of naltrexone on the pharmacokinetics and pharmacodynamics of a methadone-fluconazole combination administered orally to dogs.
| Methadone-fluconazole | Methadone-fluconazole-naltrexone | |||
|---|---|---|---|---|
| Treatment (mg/kg) | Mean ± SD | Range | Mean ± SD | Range |
| Fluconazole | ||||
| Initial dose | 7.76 ± 0.52 | 7.25–8.44 | 8.38 ± 0.87 | 7.19–9.35 |
| Coadministered* | 3.88 ± 0.26 | 3.62–4.22 | 4.19 ± 0.44 | 3.60–4.67 |
| Methadone | 0.78 ± 0.05 | 0.72–0.84 | 0.84 ± 0.09 | 0.72–0.93 |
| Naltrexone | — | — | 0.21 ± 0.02 | 0.18–0.23 |
All dogs received an initial dose of 100 mg of fluconazole, PO, and then received the assigned combined drug treatment (methadone [10 mg] plus fluconazole [50 mg], with or without naltrexone [2.5 mg] in the same capsule), PO, 12 hours later. For both combined drug treatments, FDA-approved tablets were crushed, homogenized, and placed in a gelatin capsule for administration.
Dose of drug given in the same capsule with methadone or methadone and naltrexone.
— = Not applicable.
The pharmacokinetics of methadone, fluconazole, and naltrexone glucuronide are summarized (Table 2; Figures 1–3). Variability in plasma methadone concentrations was large among dogs in the methadone-fluconazole group, partly owing to the variability in the time to maximum concentration (range, 1 to 8 hours). One of these dogs also had lower relative absorption of or exposure to methadone than the other dogs; parameters of the drug for this dog included a Cmax of 9.9 ng/mL (range for the remaining 5 dogs, 23.9 to 75.1 ng/mL) and an AUC0–∞ of 115 ng·h/mL (range for the remaining 5 dogs, 221.0 to 1,113.1 ng·h/mL).
Pharmacokinetics parameters for methadone, fluconazole, and naltrexone glucuronide (inactive metabolite of naltrexone) after oral administration of methadone-fluconazole or methadone-fluconazole-naltrexone to the dogs in Table 1.
| Product and parameter | Methadone-fluconazole | Methadone-fluconazole-naltrexone |
|---|---|---|
| Methadone | ||
| AUC (%) | 13.5 (5.9–44.1) | 10.5 (7.4–15.0) |
| AUC0–∞ (ng·h/mL) | 389.2 (115.7–1,113.1) | 349.2 (289.1–438.0) |
| Cmax (ng/mL) | 35.1 (9.9–75.1) | 33.5 (25.7–46.7) |
| tmax (h) | 2.52 (1.00–8.00) | 3.17 (2.00–4.00) |
| λz (h−1) | 0.0875 (0.0615–0.1164) | 0.0977 (0.0843–0.1116) |
| t1/2 (h) | 7.92 (5.95–11.28) | 7.09 (6.21–8.22) |
| Frel (%) | — | 90 (—) |
| Fluconazole | ||
| Cmax (μg/mL) | 16.3 (13.6–19.4) | 15.8 (14.3–16.5) |
| tmax (h) | 2.05 (0.33–6.00) | 1.83 (1.00–2.00) |
| t1/2 (h) | 20.21 (14.41–28.92) | 15.62 (14.29–17.72) |
| Naltrexone glucuronide | ||
| Cmax (ng/mL) | — | 104.1 (60.6–150.7) |
| tmax (h) | — | 1.39 (0.33–2.00) |
| t1/2 (h) | — | 8.66 (2.99–20.63) |
Data are represented as geometric mean (range). The time of administration for the methadone-containing treatment was considered time 0.
λz = Terminal rate constant. Frel = Relative fraction of the dose absorbed. t1/2 = Terminal half-life. tmax = Time to maximum concentration.
See Table 1 for drug dose information and remainder of key.
Arithmetic mean ± SD plasma methadone concentrations for 12 healthy Beagles that received methadone-fluconazole (black circles; n = 6) or methadone-fluconazolenaltrexone (white squares; 6) in a study to determine the effects of coadministration of naltrexone on the pharmacokinetics and pharmacodynamics of a methadone-fluconazole combination administered orally to dogs. All dogs received an initial dose of 100 mg of fluconazole, PO, and then received the assigned combined drug treatment (methadone [10 mg] plus fluconazole [50 mg], with or without naltrexone [2.5 mg] in the same capsule), PO, 12 hours later (time 0). See Table 1 for drug doses calculated on the basis of body weight.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma methadone concentrations for 12 healthy Beagles that received methadone-fluconazole (black circles; n = 6) or methadone-fluconazolenaltrexone (white squares; 6) in a study to determine the effects of coadministration of naltrexone on the pharmacokinetics and pharmacodynamics of a methadone-fluconazole combination administered orally to dogs. All dogs received an initial dose of 100 mg of fluconazole, PO, and then received the assigned combined drug treatment (methadone [10 mg] plus fluconazole [50 mg], with or without naltrexone [2.5 mg] in the same capsule), PO, 12 hours later (time 0). See Table 1 for drug doses calculated on the basis of body weight.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma methadone concentrations for 12 healthy Beagles that received methadone-fluconazole (black circles; n = 6) or methadone-fluconazolenaltrexone (white squares; 6) in a study to determine the effects of coadministration of naltrexone on the pharmacokinetics and pharmacodynamics of a methadone-fluconazole combination administered orally to dogs. All dogs received an initial dose of 100 mg of fluconazole, PO, and then received the assigned combined drug treatment (methadone [10 mg] plus fluconazole [50 mg], with or without naltrexone [2.5 mg] in the same capsule), PO, 12 hours later (time 0). See Table 1 for drug doses calculated on the basis of body weight.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma naltrexone glucuronide concentrations for the dogs in Figure 1. See Figure 1 for key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma naltrexone glucuronide concentrations for the dogs in Figure 1. See Figure 1 for key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma naltrexone glucuronide concentrations for the dogs in Figure 1. See Figure 1 for key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma fluconazole concentrations for the dogs in Figure 1. See Figure 1 for key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma fluconazole concentrations for the dogs in Figure 1. See Figure 1 for key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD plasma fluconazole concentrations for the dogs in Figure 1. See Figure 1 for key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Naltrexone glucuronide was detected only in the methadone-fluconazole-naltrexone group, and all dogs in this group had naltrexone glucuronide detected in their plasma after treatment. None of the dogs in the methadone-fluconazole-naltrexone group had the active metabolite β-naltrexol detected, and only 1 dog had naltrexone detected at 3 time points (40 minutes, 1 hour, and 2 hours after administration) with a peak concentration of 3.1 ng/mL
Comparison of rectal temperatures between groups after the administration of methadone-containing treatments is not reported because the post hoc power with an α of 0.05 was low (0.3) when the baseline rectal temperatures were compared. In contrast, post hoc power analysis for within-treatment comparison of rectal temperatures had a power of 1 with an α of 0.05. Mean rectal temperature was significantly decreased from baseline in the methadone-fluconazole group from 2 to ≥ 12 hours and in the methadone-fluconazole-naltrexone group from 2 to ≥ 8 hours (Figure 4). Mean rectal temperature for the methadone-fluconazole-naltrexone group was not significantly (P = 0.221) different from baseline at the 12-hour time point, and values for both groups were not significantly different from baseline at the 24-hour time point.
Arithmetic mean ± SD rectal temperatures for the dogs in Figure 1. Within-group differences in temperature are shown. *Measurement is significantly (P < 0.05) different from the baseline (immediate pretreatment) value for the methadone-fluconazole group. †Measurement is significantly (P < 0.05) different from the baseline value for the methadone-fluconazolenaltrexone group. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD rectal temperatures for the dogs in Figure 1. Within-group differences in temperature are shown. *Measurement is significantly (P < 0.05) different from the baseline (immediate pretreatment) value for the methadone-fluconazole group. †Measurement is significantly (P < 0.05) different from the baseline value for the methadone-fluconazolenaltrexone group. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Arithmetic mean ± SD rectal temperatures for the dogs in Figure 1. Within-group differences in temperature are shown. *Measurement is significantly (P < 0.05) different from the baseline (immediate pretreatment) value for the methadone-fluconazole group. †Measurement is significantly (P < 0.05) different from the baseline value for the methadone-fluconazolenaltrexone group. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.656
Sedation scores were variable in both groups (Table 3). No dogs had signs of profound sedation; all were capable of walking, and only 1 dog in each group had signs of moderate sedation (at 1 time point each). No dogs had signs of sedation 24 hours after administration of the assigned methadone-containing treatment.
Results of sedation scoring for the dogs in Table 1 after oral administration of methadone-fluconazole or methadone-fluconazole-naltrexone.
| Methadone-fluconazole | Methadone-fluconazole-naltrexone | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Time (h) | None | Slight | Moderate | Profound | Unresponsive | None | Slight | Moderate | Profound | Unresponsive |
| 0 | 6 | 0 | 0 | 0 | 0 | 6 | 0 | 0 | 0 | 0 |
| 1 | 5 | 1 | 0 | 0 | 0 | 5 | 1 | 0 | 0 | 0 |
| 2 | 3 | 2 | 1 | 0 | 0 | 5 | 1 | 0 | 0 | 0 |
| 4 | 2 | 4 | 0 | 0 | 0 | 2 | 3 | 1 | 0 | 0 |
| 6 | 3 | 3 | 0 | 0 | 0 | 1 | 5 | 0 | 0 | 0 |
| 8 | 5 | 1 | 0 | 0 | 0 | 4 | 2 | 0 | 0 | 0 |
| 12 | 6 | 0 | 0 | 0 | 0 | 5 | 1 | 0 | 0 | 0 |
| 24 | 6 | 0 | 0 | 0 | 0 | 6 | 0 | 0 | 0 | 0 |
Values represent the number of dogs in each group that had the described degree of sedation. Time 0 assessment was immediately prior to administration of the methadone-containing treatment.
Discussion
Results of a previous study9 revealed that orally administered fluconazole had significant effects on the pharmacokinetics and pharmacodynamics of orally administered methadone in healthy dogs. In that study,9 the bioavailability of methadone and duration of methadone exposure (detectable plasma concentrations) were significantly greater when the drug was administered with a dose of fluconazole; additionally, rectal temperature was significantly decreased (a measurable opioid effect) when dogs received fluconazole in addition to methadone, but not when methadone was given alone. Mean rectal temperatures were similarly decreased after administration of methadone-fluconazole or methadone-fluconazole-naltrexone in the present study. Previous studies10–12 have also identified a dose-response relationship with decreases in rectal temperature in dogs and a dose-response relationship with increases in analgesic effects of opioids. As such, rectal temperature is potentially useful as a marker for centrally mediated opioid effects in dogs.
The pharmacokinetics of methadone, naltrexone glucuronide, and fluconazole were investigated in the present study after oral administration of a methadone-fluconazole-naltrexone formulation to dogs. We found a lack of measurable plasma concentrations of the active metabolite β-naltrexol as well as the sporadic detection of a low concentration of naltrexone. Only a single dose of the assigned drug combination was administered to dogs in this study, and additional studies assessing the pharmacokinetics of methadone-fluconazole-naltrexone after administration of multiple doses are needed.
The plasma concentrations of fluconazole overlapped between dogs of the methadone-fluconazole and methadone-fluconazole-naltrexone groups in our study. Results of a previous study9 suggested there may be a dose- or concentration-dependent effect of fluconazole on methadone metabolism. The similar pharmacokinetics and plasma concentrations of fluconazole between the 2 treatment groups in the present study suggested that similar effects on methadone metabolism can be expected for dogs administered methadone and fluconazole with or without naltrexone if metabolism effects are indeed related to fluconazole concentrations.
Similar to a method used in the previous study,9 fluconazole was administered orally approximately 12 hours prior to the methadone-fluconazole or methadone-fluconazole-naltrexone treatments. However, to our knowledge, an optimal time to administer fluconazole prior to oral methadone treatment of dogs has not been established, and it is not known whether this prior treatment is needed when a combined oral treatment that includes methadone and fluconazole is administered. Since fluconazole is a competitive or non-competitive CYP inhibitor, it is likely that there is no need for its prior administration to inhibit methadone metabolism.24 Studies are needed to assess the effects of coadministration of fluconazole with methadone (without prior fluconazole administration) on methadone pharmacokinetics in dogs. It is possible fluconazole could have other effects on methadone pharmacokinetics, but fluconazole does not appear to affect drug transporters, including P-glycoprotein,25 which can contribute to renal and biliary elimination of drugs.25
Because of the potential for abuse, misuse, and accidental exposure to opioid medications, we were interested in evaluating the effects of incorporating an opioid abuse deterrent into the methadone-fluconazole formulation with the addition of naltrexone. Our results confirmed previous findings15,16 that plasma concentrations of naltrexone are low after oral administration to dogs and that dogs produce little or no β-naltrexol after naltrexone administration. Opioid effects, as measured by decreased rectal temperature, were detected in both the methadone-fluconazole and methadone-fluconazole-naltrexone groups in the present study. Fluconazole is a CYP inhibitor, but naltrexone is metabolized primarily to glucuronide conjugates in dogs (through UDP-glucuronosyltransferase), so we expected naltrexone glucuronide would remain the prominent metabolite in dogs.15 The lack of measurable concentrations of β-naltrexol and rare measurements of naltrexone in 1 dog were consistent with the results of previous studies15,16 of this drug after administration to dogs. Results of human medical studies17,18 have revealed that orally administered naltrexone antagonizes opioid receptors and thus the effects of opioid receptor agonists (eg, morphine or oxycodone); when ingested concurrently with these drugs, naltrexone significantly decreased feelings of drug liking and a so-called drug high, compared with morphine or oxycodone alone. Therefore, naltrexone functions as an opioid abuse deterrent in people when used in this manner.
A specific ratio of methadone to naltrexone to decrease the abuse potential of methadone has not been reported. The ratio of morphine to naltrexone is 25:1,17 and the ratio of oxycodone to naltrexone is 8.3:118 in the previously described formulations created to decrease abuse potential of morphine and oxycodone, respectively. In the present study, we used a methadone-to-naltrexone ratio of 4:1 (ie, a larger proportion of naltrexone). On the basis of morphine milligram–equivalent calculations (a method used to estimate total daily opioid doses in people by multiplying the dose of opioids other than morphine by a predetermined conversion factor and calculating the sum for all opioids administered), 1 mg of methadone represents 4 morphine milligram equivalents,26 suggesting that a methadone-to-naltrexone ratio of 6.25:1 could have the desired effects of limiting unintended exposure or abuse. However, the relatively higher potency of methadone is primarily attributable to its longer half-life and subsequent accumulation with multiple doses in people, and it is unclear whether the calculated ratio is appropriate to achieve these desired effects. Human medical studies would be needed to assess the effects on opioid liking and elicitation of drug withdrawal in opioid-dependent patients with the formulation containing the methadone-to-naltrexone ratio used in this study.
In people, fluconazole is considered a weak inhibitor of methadone metabolism (resulting in a > 1.25-fold but < 2-fold increase in the AUC for methadone), and as such, dose adjustments are not needed for methadone when it is administered concurrently with fluconazole.27 Therefore, fluconazole is not a pharmacokinetic enhancer of methadone in humans. In contrast, fluconazole increased the AUC for methadone 98- to 176-fold in dogs, which qualifies the drug as a strong inhibitor (defined as a product that results in a > 5-fold increase in the AUC of the target28) and a pharmacokinetic enhancer of methadone in this species. These differences in the effects of fluconazole are likely attributable to an affinity for specific enzymes that metabolize methadone in dogs, rather than in people. Since fluconazole has such a minor effect on the metabolism of methadone and subsequent drug exposure in humans, the inclusion of fluconazole in the formulation of methadone and naltrexone (ie, methadone-fluconazole-naltrexone) is unlikely to have a consequence on the amount of naltrexone needed to decrease the abuse potential of methadone. However, as previously mentioned, studies would be required to determine whether this specific combination has anti-abuse potential in people.
In the present study, significantly decreased mean rectal temperatures were detected at all evaluated time points from 2 to 12 hours after methadone-fluconazole administration and from 2 to 8 (but not 12) hours after methadone-fluconazole-naltrexone administration. These data supported the finding that including naltrexone in the formulation does not block centrally mediated opioid effects of orally administered methadone-fluconazole in dogs. However, studies specifically assessing other effects, including antinociceptive or analgesic, antitussive, and antidiarrheal effects, are still needed.
Sedation was observed in some of the dogs after administration of methadone-fluconazole or methadone-fluconazole-naltrexone. The highest degree of sedation was deemed moderate, and this was found at only 1 time point for 1 dog/treatment group (all dogs were able to stand and walk after treatment). Slight to moderate sedation may be a desirable effect in a perioperative situation. Since sedation is a central opioid effect, it is dose dependent, and when this effect is undesirable, it can likely be decreased by decreasing the methadone dose.
Long-term administration of fluconazole to human patients can select for resistant fungal organisms, with fluconazole-resistant Candida spp specifically described.29 Candida albicans is part of the normal gastrointestinal microbiota in dogs, and long-term administration of fluconazole may select for resistant organisms.30 However, we anticipate the clinical application of methadone-fluconazole-naltrexone would be for acute pain management (eg, perioperative use or treatment of patients with trauma). Fungal diseases caused by organisms that primarily reside in the environment (eg, histoplasmosis, blastomycosis, coccidiomycosis, or aspergillosis) are unlikely to have selection pressure from fluconazole administration to dogs, as environmental application of antifungal compounds for agriculture purposes is thought to be the primary factor driving resistance for those organisms.31–33 It is important to realize that other commonly used drugs can also affect microbial populations in dogs as a result of incidental antimicrobial activity. For example, carprofen, acepromazine, amitriptyline, clomipramine, and fluoxetine were all shown to inhibit Staphylococcus pseudintermedius growth in vitro, but to the authors' knowledge, their effects on antimicrobial resistance of organisms in dogs have not been reported.34
Overall, data from the study reported here indicated that incorporation of naltrexone does not antagonize the centrally mediated opioid effects of an orally administered methadone-fluconazole formulation in dogs. Mean methadone concentrations were > 10 ng/mL from at least 40 minutes to 12 hours after administration of either study treatment at the doses provided in this study. These data provide a basis and rationale for future investigation of antinociceptive or analgesic, antitussive, and antidiarrheal effects of orally administered methadone-fluconazole-naltrexone in dogs.
Acknowledgments
The study was performed at Kansas State University.
Funding was provided by the American Veterinary Medical Foundation Susan I. Maylahn Canine Pain Management Grant (awarded 2018). Software license for Phoenix was provided by Certara USA Incorporated (Princeton, NJ) as part of the Centers of Excellence program.
Kansas State University has applied for a patent covering the intellectual property reported in this manuscript.
ABBREVIATIONS
| AUC | Area under the concentration-versus-time curve |
| AUC0–∞ | Area under the concentration versus-time-curve from time 0 to infinity |
| Cmax | Maximum observed concentration |
| CV | Coefficient of variation |
| CYP | Cytochrome P450 |
Footnotes
Marshall Bioresources, North Rose, NY.
Elite Pharmaceuticals Inc, Northvale, NJ.
Manufactured for Blue Point Laboratories by Glenmark Pharmaceuticals, Colvale-Bardez, Goa, India.
Capsuline, Pompano Beach, Fla.
Mallinckrodt Inc, Hazelwood, Mo.
Acquity Prominence UPLC, Waters Corp, Milford, Mass.
TQD, Waters Corp, Milford, Mass.
Ostro Pass-through Sample Preparation Plate, Waters Corp, Milford, Mass.
Acquity UPLC column, HSS T3, 1.8 μm, 2.1 × 50 mm, Waters Corp, Milford, Mass.
Cerilliant Corp, Round Rock, Tex.
Acros Organics, Morris, NJ.
Sigma-Aldrich Corp, St Louis, Mo.
Phoenix 64, Certara Inc, Princeton, NJ.
SigmaPlot, version 12.5, Systat Software Inc, San Jose, Calif.
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