Osteoarthritis is anecdotally considered one of the most common causes of chronic pain in birds.1 The prevalence of osteoarthritis in nonpasserine birds was estimated to be as high as 20% in a study2 of cadaveric bird skeletons, but epidemiologic research into osteoarthritis in companion parrots is lacking, and therefore, the prevalence may be higher. Long-term pain management is needed for parrots with osteoarthritis because parrots have a long life expectancy.1 Medical management of osteoarthritis includes administration of opioids, NSAIDs, polysulfated glycosaminoglycans, glucosamine, chondroitin, derivatives of γ-aminobutyric acid (eg, gabapentin and pregabalin), and amantadine.3,4
Amantadine increases the release of dopamine in the CNS, and via weak antagonism of N-methyl-d-aspartic acid receptors, it decreases tolerance to opioid medications.3,4 Administration of this drug is expected to reduce nociception associated with chronic pain, in which sensory neurons are hypersensitive to noxious stimuli and responsive to non-noxious stimuli (ie, central sensitization).4 Amantadine can be used alone for analgesia, but its weak affinity for N-methyl-d-aspartic acid receptors may necessitate multimodal treatments to effectively manage pain in veterinary patients.4,5 Amantadine is synergistic with NSAIDs, opioids, and derivatives of γ-aminobutyric acid.4 In humans, amantadine has been shown to decrease opioid use by patients after surgery, decrease pain in those with postherpetic neuralgia, and aid in neuropathic analgesia.6,7 Oral administration of amantadine at 3 to 5 mg/kg every 24 hours in conjunction with meloxicam significantly improved owner-assessed activity as rated with a Likert scale of dogs with osteoarthritis, compared with meloxicam alone.8 Through amantadine's unique mechanisms of action, it may provide analgesia in parrots with pain that is refractory to treatment with other drugs, and thus may be considered an adjunct for ameliorating chronic pain.
Information on the pharmacokinetics and pharmacodynamics of amantadine in birds is lacking, and knowledge of both is important to ensure appropriate dosing. Furthermore, knowledge of the adverse effects of amantadine after administration of a single dose and multiple doses is necessary if amantadine is to be used to treat chronic pain in birds.
The purpose of the study reported here was to determine the pharmacokinetics of amantadine after oral administration of a single dose and of multiple doses to orange-winged Amazon parrots (Amazona amazonica). We hypothesized that amantadine administered orally at a dosage of 5 to 20 mg/kg every 24 hours would yield plasma concentrations considered to be therapeutic in dogs with minimal adverse effects in parrots.8,9
Materials and Methods
Animals
Twelve orange-winged Amazon parrots (6 adults > 25 years old and 6 young birds < 25 years old; 6 males and 6 females) maintained as a research flock at the University of California-Davis were used in the study. Body weight ranged from 358 to 497 g. Beginning 14 days prior to the start of the study and during the study, birds were housed in a single room in individual stainless steel cages. The birds were provided with 12 hours of light and 12 hours of dark, a commercially pelleted diet,a and water ad libitum via a water line. They were considered healthy on the basis of historical observations, physical examination, CBC, and plasma biochemical analyses. None of the birds received medications or anesthetics for at least 1 month prior to collection of the blood sample. The study protocol was approved by the Institutional Animal Care and Use Committee at the University of California-Davis (protocol No. 20922).
The birds were evaluated 24 hours prior to and then every 12 hours for up to 48 hours after oral administration of amantadine for the preliminary phase and single-dose phase of the study and for up to 48 hours after the last dose was administered for the multiple-dose phase of the study. Before birds were handled, they were observed in their cages for overall attitude and behavior, including stereotypies. An agitation-sedation scoring system previously described for Hispaniolan Amazon parrots (Amazona ventralis) was used.10 Birds were monitored for gastrointestinal effects (eg, regurgitation, nausea-like behavior, decreased appetite, and diarrhea), tremors, and polyuria. Auscultation was performed of the heart for rate and rhythm and of the lungs and air sacs for rate, rhythm, and effort. The body weight of each bird was recorded daily.
Food was not withheld before amantadine was administered; however, amantadine was administered in the morning shortly after the lights were turned on, and typically the birds did not eat overnight.
Preliminary phase
The objectives of the preliminary phase of the study were to determine the appropriate dose of amantadine to administer and the times for blood sample collection in the subsequent single- and multiple-dose phases of the study. Six birds (3 males and 3 females [3 adults and 3 young birds]) were randomlyb assigned to 3 groups. A curved 10-gauge metal gavage tube was used to administer amantadine into the crop at doses of 5 mg/kg (n = 2), 10 mg/kg (2), or 20 mg/kg (2). Blood samples were collected at 0.5, 1, 2, 3, 4, 6, 8, 12, and 24 hours after amantadine administration for measurement of plasma amantadine concentration.
The single oral dose of amantadine at 5 mg/kg, 10 mg/kg, and 20 mg/kg resulted in mean ± SD Cmax of 667 ± 6 ng/mL, 977 ± 387 ng/mL, and 2,458 ± 377 ng/mL, respectively. In humans, the risk of toxicosis is increased when plasma amantadine concentrations exceed 1,000 ng/mL.11–15 Because the plasma concentrations in the birds were > 1,000 ng/mL after the 20-mg/kg dose was administered, the potential existed for drug accumulation to toxic concentrations. Therefore, that dose was not considered for administration in the single-dose phase of the study. Rather, the 10-mg/kg dose was selected; 10 mg/kg was the lowest dose that maintained plasma concentrations of at least 275 ng/mL, which is considered to be therapeutic in dogs,8,9 for at least 6 to 8 hours.
Single-dose phase
Six birds (3 males and 3 females [3 adults and 3 young birds]) that were not associated with the preliminary phase of the study orally received a single dose of amantadine at 10 mg/kg. Blood samples were collected at 0.5, 1.5, 3, 6, 24, 48, 72, 96, and 120 hours after amantadine administration for measurement of plasma amantadine concentration.
Nonparametric superposition analysis of the data of this single-dose phase was performed with commercially available pharmacokinetic modeling software.c Results of this analysis suggested the potential for drug accumulation and, thus, possible toxicosis if amantadine were administered to the birds at 10 mg/kg. Therefore, the 5-mg/kg dose was selected for the multiple-dose phase of the study.
Multiple-dose phase
Eight of the 12 birds (4 males and 4 females [4 adults and 4 young birds]) from the previous phases of the study were randomlyb selected for inclusion in the multiple-dose phase of the study to begin after a 2-month washout period. Birds were considered healthy on the basis of physical examination, CBC, and plasma biochemical analyses performed 2 weeks before this phase began.
After the washout period, amantadine was administered orally at 5 mg/kg every 24 hours for 7 days. Blood samples were collected at 0.5, 1.5, 3, 6, 24, 48, 72, 96, and 120 hours after amantadine administration on day 7 (ie, after the final dose) for measurement of plasma amantadine concentration.
Blood sample collection
For all blood sample collection times, 0.25 to 0.3 mL of blood was collected from the right or left jugular vein of manually restrained birds with a 28-gauge needle attached to a 0.3-mL syringe. The blood samples were transferred to pediatric collection tubes containing lithium-heparin,d placed in a cooler with ice packs, and centrifuged (3,500 × g for 6 minutes) within 1 hour after collection. Plasma was extracted, placed into labeled 0.5-mL cryovials,e and stored at −80°C until analysis.
Sample analysis
Prior to the study, a blood sample of volume not exceeding 1% of the bird's body weight was collected from each of the 12 birds to harvest blank plasma for preparation of the assay standards and quality control samples. Plasma concentrations of amantadine were quantified via tandem liquid chromatography–mass spectrometry with a previously published method.16 Prior to quantification, the method was validated with parrot plasma as a matrix. Standards were prepared in drug-free parrot plasma and ranged in amantadine concentration from 0.25 to 2,500 ng/mL. Standards and negative control samples were freshly prepared for each new sample run of the quantitative assay. The instrument response versus concentration of amantadine was linear, and the correlation coefficient was 0.99. The precision and accuracy of the assay were determined with analysis of quality control samples in replicates (n = 6). The intra- and interday precision and accuracy of the assay were within 15% of the nominal (theoretical) concentrations. The assay was optimized for a limit of quantification of 0.25 ng/mL and a limit of detection of 0.1 ng/mL.
Data analysis
Pharmacokinetic parameter estimates were determined via noncompartmental analysis with a commercially available software program.c Maximum plasma drug concentrations and tmax were obtained directly from review of the concentration and time data. Mean absorption time, plasma clearance, and apparent volume of distribution could not be determined because amantadine was not administered IV.
The AUC was calculated by use of the log-linear trapezoidal rule. Pharmacokinetic parameter estimates were determined at steady state after multiple-dose administration. The percentage fluctuation and accumulation index value for the multiple-dose phase of the study were calculated with the following equations:
where Cmean is the mean plasma concentration, Cmin is the minimum plasma concentration, λz is the terminal rate constant, and τ is the dosing interval.
Results
In all phases of the study, all birds were bright and responsive, maintained their body weight, had no signs of appetite or behavior changes, and had no change in urofeces as subjectively assessed. Birds were quiet after handling but did not have other outward signs of stress.
Plasma amantadine concentration-versus-time curves were prepared for the single- and multiple-dose phases of the study (Figure 1). After oral administration of a single 10-mg/kg dose of amantadine, amantadine was detectable in all samples above the target minimum concentration of 275 ng/mL for up to 48 hours, and after multiple 5-mg/kg doses of amantadine, amantadine was detectable in samples above the target concentration for at least 24 hours. Maximum plasma concentrations and tmax were similar after administration of a single dose and multiple doses (Table 1). The accumulation index values (mean ± SD, 1.83 ± 0.27) indicated evidence of weak accumulation after administration of multiple doses (ie, accumulation index value ≥ 1.2 but < 2).17
Mean ± SD plasma amantadine concentrations in orange-winged Amazon parrots (Amazona amazonica) after oral administration of a single dose of amantadine (10 mg/kg) to 6 parrots (A) and after oral administration of multiple doses of amantadine (5 mg/kg, q 24 h for 7 days) to 8 parrots (B). Time 0 was the time of amantadine administration for the single-dose phase of the study and time of last dose administration on day 7 for the multiple-dose phase of the study.
Citation: American Journal of Veterinary Research 81, 8; 10.2460/ajvr.81.8.651
Mean ± SD values of pharmacokinetic parameter estimates after oral administration of a single dose (10 mg/kg) of amantadine to 6 orange-winged Amazon parrots (Amazona amazonica) and multiple doses (5 mg/kg, q 24 h for 7 days) of amantadine to 8 orange-winged Amazon parrots.
| Parameter | Single dose | Multiple doses |
|---|---|---|
| tmax (h) | 3.8 ± 1.8 | 3.0 ± 2.4 |
| Cmax (ng/mL) | 1,174 ± 186 | 1,185 ± 270 |
| Cmin (ng/mL) | — | 348.5 ± 76.9 |
| Cmean (ng/mL) | — | 684.8 ± 137.3 |
| t1/2λ (h) | 23.2 ± 2.9 | 21.5 ± 5.3 |
| AUClast–∞ (μg·h/mL) | 38.6 ± 7.4 | 26.3 ± 5.7 |
| Fluctuation (%) | — | 114.8 ± 30.6 |
| Accumulation index | — | 1.83 ± 0.27 |
Pharmacokinetic parameter estimates associated with multiple doses are for steady state.
— = Not determined. AUClast–∞ = The AUC extrapolated from the time of the last dose to infinity. Cmean = Mean plasma concentration. Cmin = Minimum plasma concentration.
Discussion
In the present study, the pharmacokinetics of amantadine was evaluated in a psittacine species after oral administration of single and multiple doses. Oral administration of amantadine at 10 mg/kg once and at 5 mg/kg every 24 hours for 7 days yielded plasma concentrations above those considered therapeutic in dogs for at least 24 hours, without any observed adverse effects. The reported mean Cmax for dogs receiving a mean single therapeutic dose of 2.8 mg of amantadine/kg is 275 ng/mL (range, 225 to 351 ng/mL).9 Comparatively, the reported mean ± SD Cmax for cats receiving 5 mg of amantadine/kg is 1,142 ± 133 ng/mL.18 The orange-winged Amazon parrots appeared to have plasma amantadine concentrations that were comparable to those of cats.
The target plasma concentration of amantadine for the birds of the present study was that considered to be therapeutic in dogs. Although target plasma concentrations were achieved, we acknowledge that no pharmacodynamic data are available to support plasma concentrations of > 275 ng/mL as therapeutic for parrots. Further study is necessary to determine the efficacy of amantadine alone and combined with other analgesics in parrots with osteoarthritis. However, the study of amantadine for use in other avian species—chickens, ducks, and turkeys—cannot be pursued because the US FDA prohibits the administration of amantadine to those birds.19
The pharmacokinetic parameter estimates were comparable between single and multiple doses of orally administered amantadine, and the mean accumulation index value was low for the multiple doses, suggesting minimal drug accumulation. However, toxicosis may still develop depending on the metabolism and elimination of amantadine and the health of the parrot. The pharmacokinetic parameter estimates derived after administration of multiple doses of amantadine in the present study may not apply to unhealthy orange-winged Amazon parrots.
The organs responsible for metabolism and elimination of orally administered amantadine have not been identified in birds. For mammalian species that have been studied,20 amantadine is not or only minimally metabolized (humans, monkeys, and mice) or partially metabolized (dogs) and is primarily eliminated by the kidneys. If renal excretion is the primary route of elimination for amantadine in birds, then birds with kidney disease may be at increased risk for toxicosis because of high plasma amantadine concentrations. Studies are warranted to determine the mechanisms of metabolism and elimination for orally administered amantadine in orange-winged Amazon parrots.
We acknowledge several limitations of our study. Because amantadine was not administered IV, comparative bioavailability, plasma clearance, absorption time, and volume of distribution could not be determined. Because of the birds' relatively small circulating blood volume, compared with that for other species, the number of blood samples collected was limited; therefore, peak and trough plasma concentrations of amantadine also could not be determined. Additionally, amantadine was assumed to be minimally metabolized and eliminated via the kidneys, and these assumptions may not have been correct. If amantadine is eliminated via the kidneys, the pharmacokinetics in patients with kidney disease may be different from those reported for the healthy birds of the present study. Yet, the pharmacokinetics of amantadine administered daily for 1 week may not accurately reflect those associated with amantadine administered over longer periods (ie, months or years). Furthermore, the pharmacokinetics of amantadine derived from this single species of parrots may differ from those for other species of birds.
The results of the present study suggested that oral administration of amantadine to orange-winged Amazon parrots at 5 mg/kg every 24 hours maintained plasma concentrations above the target levels considered therapeutic in dogs and without obvious adverse effects. On the basis of these data alone, amantadine at this dosage may be useful as an adjunctive analgesic for otherwise healthy orange-winged Amazon parrots with pain that is attributable to osteoarthritis and refractory to other drugs. Although plasma concentrations considered to be therapeutic in dogs were attained, pharmacodynamics studies are warranted to investigate whether these plasma concentrations are therapeutic for this species.
Acknowledgments
Funded by the Richard M. Schubot Parrot Wellness and Welfare Program at the Center for Companion Animal Health, School of Veterinary Medicine, University of California-Davis.
The authors declare that there were no conflicts of interest.
Presented in abstract form at the Annual Conference of the Association of Avian Veterinarians, St Louis, September 2019.
ABBREVIATIONS
| AUC | Area under the concentration-versus-time curve |
| Cmax | Maximum plasma concentration |
| tmax | Time to maximum plasma concentration |
Footnotes
Roudybush Daily Maintenance, Roudybush Inc, Woodland, Calif.
Random.org, Randomness and Integrity Services Ltd, Dublin, Ireland. Available at: www.random.org. Accessed Feb 2, 2019.
Phoenix Winnonlin, version 8.2, Certara, Princeton, NJ.
Microtainer, Becton, Dickinson and Co, Franklin Lakes, NJ.
Bio Plas Inc, San Rafael, Calif.
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