In vitro hepatic microsomal metabolism of meloxicam in koalas (Phascolarctos cinereus), brushtail possums (Trichosurus vulpecula), ringtail possums (Pseudocheirus peregrinus), rats (Rattus norvegicus) and dogs (Canis lupus familiaris)

Pharmacokinetic Profile of Fentanyl in the Koala (Phascolarctos cinereus) after Intravenous Administration, and Absorption via a Transdermal Patch

Simple Summary

Koalas can be injured by cars and bushfires, and be affected by painful infectious diseases. When koalas undergo surgery to repair broken bones, they require analgesia. Fentanyl is a potent opioid that can be administered during surgery to provide analgesia. This study describes the rate of elimination of fentanyl in koalas’ blood when administered as a single intravenous injection and consequently calculates the dose rate to administer a constant rate fentanyl infusion into the koalas’ veins to provide short-term pain control. Fentanyl can also be absorbed via the skin into the circulation when applied as a transdermal patch. Although the data for transdermal fentanyl patch absorption is from two koalas only, the results demonstrate that when a patch is applied, pain control is likely to occur 12 h after application to koalas’ skin. Fentanyl may provide effective pain control to koalas either as an intravenous infusion or as a transdermal patch.

Abstract

Fentanyl was administered as a single intravenous bolus injection at 5 µg/kg to five koalas and fentanyl plasma concentrations for a minimum of 2 h were quantified by an enzyme-linked immunosorbent assay (ELISA). The median (range) fentanyl elimination half-life and clearance were 0.53 (0.38–0.91) h, and 10.01 (7.03–11.69) L/kg/h, respectively. Assuming an analgesic therapeutic plasma concentration of 0.23 ng/mL (extrapolated from human studies), an intravenous constant infusion rate was estimated at approximately between 1.7 to 2.7 µg/kg/h (using the clearance 95% confidence intervals). A transdermal fentanyl patch was applied to the antebrachium of an additional two koalas for 72 h. Fentanyl plasma concentrations were determined during the patch application and after patch removal at 80 h. The fentanyl plasma concentration was greater than 0.23 ng/mL after 12 to 16 h. While the patch was applied, the maximum fentanyl concentration was approximately 0.7 ng/mL from 32 to 72 h. Fentanyl plasma concentrations increased to 0.89 ng/mL 1 h after the patch was removed, and then decreased to a mean of 0.47 ng/mL at 80 h. The transdermal fentanyl patch is likely to provide some level of analgesia but should be initially co-administered with another faster acting analgesic for the first 12 h.

Pharmacokinetic profile of injectable tramadol in the koala (Phascolarctos cinereus) and prediction of its analgesic efficacy

Tramadol is used as an analgesic in humans and some animal species. When tramadol is administered to most species it undergoes metabolism to its main metabolites M1 or O-desmethyltramadol, and M2 or N-desmethyltramadol, and many other metabolites. This study describes the pharmacokinetic profile of tramadol when a single subcutaneous bolus of 2 mg/kg was initially administered to two koalas. Based on the results of these two koalas, subsequently 4 mg/kg as a single subcutaneous injection, was administered to an additional four koalas. M1 is recognised as an active metabolite and has greater analgesic activity than tramadol, while M2 is considered inactive. A liquid chromatography assay to quantify tramadol, M1 and M2 in koala plasma was developed and validated. Liquid chromatography-mass spectrometry confirmed that M1 had been identified. Additionally, the metabolite didesmethyltramadol was identified in chromatograms of two of the male koalas. When 4 mg/kg tramadol was administered, the median half-life of tramadol and M1 were 2.89 h and 24.69 h, respectively. The M1 plasma concentration remained well above the minimally effective M1 plasma concentration in humans (approximately 36 ng/mL) over 12 hours. The M1 plasma concentration, when tramadol was administered at 2 mg/kg, did not exceed 36 ng/mL at any time-point. When tramadol was administered at 2 mg/kg and 4 mg/kg the area under the curve M1: tramadol ratios were 0.33 and 0.50, respectively. Tramadol and M1 binding to plasma protein were determined using thawed, frozen koala plasma and the mean binding was 20% and 75%, respectively. It is concluded that when tramadol is administered at 4 mg/kg as a subcutaneous injection to the koala, it is predicted to have some analgesic activity.

In vitro hepatic metabolism of mefloquine using microsomes from cats, dogs and the common brush-tailed possum (Trichosurus vulpecula)

Feline infectious peritonitis (FIP) is a systemic, fatal, viral-induced, immune-mediated disease of cats caused by feline infectious peritonitis virus (FIPV). Mefloquine, a human anti-malarial agent, has been shown to inhibit FIPV in vitro. As a first step to evaluate its efficacy and safety profile as a potential FIP treatment for cats, mefloquine underwent incubation in feline, canine and common brush-tailed possum microsomes and phase I metabolism cofactors to determine its rate of phase I depletion. Tramadol was used as a phase I positive control as it undergoes this reaction in both dogs and cats. Using the substrate depletion method, the in vitro intrinsic clearance (mean ± S.D.) of mefloquine by pooled feline and common brush-tailed possum microsomes was 4.5 ± 0.35 and 18.25 ± 3.18 μL/min/mg protein, respectively. However, phase I intrinsic clearance was too slow to determine with canine microsomes. Liquid chromatography-mass spectrometry (LC-MS) identified carboxymefloquine in samples generated by feline microsomes as well as negative controls, suggesting some mefloquine instability. Mefloquine also underwent incubation with feline, canine and common brush-tailed possum microsomes and phase II glucuronidative metabolism cofactors. O-desmethyltramadol (ODMT or M1) was used as a positive control as it undergoes a phase II glucuronidation reaction in these species. The rates of phase II mefloquine depletion by microsomes by all three species were too slow to estimate. Therefore mefloquine likely undergoes phase I hepatic metabolism catalysed by feline and common brush-tailed possum microsomes but not phase II glucuronidative metabolism in all three species and mefloquine is not likely to have delayed elimination in cats with clinically normal, hepatic function.

Intrinsic clearance rate of O-desmethyltramadol (M1) by glucuronide conjugation and phase I metabolism by feline, canine and common brush-tailed possum microsomes

Quantitative aspects of in vitro phase II glucuronidative metabolism of O-desmethyltramadol (O-DSMT or M1), the active metabolite of the analgesic drug tramadol, by feline, canine and common brush-tailed possum hepatic microsomes are described.Whilst previous studies have focused on the phase I conversion of tramadol to M1, this is the first report in which the phase II glucuronidative metabolic pathway of M1 has been isolated by an in vitro comparative species study.Using the substrate depletion method, microsomal phase II glucuronidative in vitro intrinsic clearance (Clint) of M1 was determined.The in vitro Clint (mean ± SD) by pooled common brush-tailed possum microsomes was 9.9 ± 1.7 μL/min/mg microsomal protein whereas the in vitro Clint by pooled canine microsomes was 1.9 ± 0.07 μL/min/mg microsomal protein. The rate of M1 depletion by feline microsomes, as measured solely by high pressure liquid chromatography, was too slow to determine. Liquid chromatography-mass spectrometry identified O-DSMT glucuronide in samples generated from all three species' microsomes, although the amount detected under the feline condition was minimal.This study indicates that M1 likely undergoes in vitro phase II glucuronidation by canine and common brush-tailed possum microsomes and, to a minor extent, by feline microsomes. The rate of depletion of M1 by phase I metabolism was also undertaken.When incubated with phase I co-factors and common brush-tailed possum microsomes or canine microsomes, M1 had an in vitro Clint of 47.6 and 22.8 μL/min/mg microsomal protein, respectively. However, due to a lack of CYP2B-like activity in the feline liver, unsurprisingly, M1 did not deplete when incubated with feline microsomes. Consequently, major M1 elimination pathways, using feline microsomes, were not determined."

Effects of Eucalypt Plant Monoterpenes on Koala (Phascolarctos Cinereus) Cytokine Expression In Vitro

Protective immunity is crucial for survival of any species, though the koala as a specialist feeder adapted to an exclusive diet of eucalypts that contain plant secondary metabolites of varying toxicity and of immunomodulatory property. Being constantly exposed to such dietary chemicals it raises the question of their immune effects in a specialist eucalypt feeder. This study demonstrates that natural levels of circulating eucalypt plant secondary metabolites have dose dependent in vitro effects on cytokine expression of koala peripheral blood mononuclear cells, suggesting a potential trade-off of reduced function in multiple arms of the immune system associated with koala's use of its specialized dietary niche.

Pharmacokinetic profile of amoxicillin and its glucuronide-like metabolite when administered subcutaneously to koalas (Phascolarctos cinereus)

Amoxicillin was administered as a single subcutaneous injection at 12.5 mg/kg to four koalas and changes in amoxicillin plasma concentrations over 24 hr were quantified. Amoxicillin had a relatively low average ± SD maximum plasma concentration (C_max ) of 1.72 ± 0.47 µg/ml; at an average ± SD time to reach C_max (T_max ) of 2.25 ± 1.26 hr, and an elimination half-life of 4.38 ± 2.40 hr. The pharmacokinetic profile indicated relatively poor subcutaneous absorption. A metabolite was also identified, likely associated with glucuronic acid conjugation. Bacterial growth inhibition assays demonstrated that all plasma samples other than t = 0 hr, inhibited the growth of Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 29213 to some extent. Calculated pharmacokinetic indices were used to predict whether this dose could attain a plasma concentration to inhibit some susceptible Gram-negative and Gram-positive pathogens. It was predicted that a twice daily dose of 12.5 mg/kg would be efficacious to inhibit susceptible bacteria with an amoxicillin minimum inhibitory concentration (MIC) ≤ 0.75 µg/ml such as susceptible Bordetella bronchiseptica, E. coli, Staphylococcus spp. and Streptococcus spp. pathogens.

Identification and quantification of domestic feline cytochrome P450 transcriptome across multiple tissues

Understanding of cytochrome P450 (CYP) isoform distribution and function in the domestic feline is limited. Only a few studies have defined individual CYP isoforms across metabolically relevant tissues, hampering the ability to predict drug metabolism and potential drug-drug interactions. Using RNA sequencing (RNA-seq), transcriptomes from the 99 Lives Cat Genome Sequencing Initiative databank combined with experimentally acquired whole transcriptome sequencing of healthy, adult male (n = 2) and female (n = 2) domestic felines, expression of 42 CYP isoforms were identified in 20 different tissues. Thirty-seven of these isoforms had not been previously reported in cats. Depending on the tissue, three to twenty-nine CYP isoform transcripts were expressed. The feline genome annotations did not differentiate CYP2E1 and 2E2 genes, demonstrating poor annotation for this gene using the reference genome. As the majority of the sequences are based on automated pipelines, complete cDNA sequences for translation into CYP protein sequences could not be determined. This study is the first to identify and characterize 37 additional CYP isoforms in feline tissues, increasing the number of identified CYP from the previously reported seven isoforms to 42 across 20 tissues.

Adaptation and conservation insights from the koala genome

The koala, the only extant species of the marsupial family Phascolarctidae, is classified as 'vulnerable' due to habitat loss and widespread disease. We sequenced the koala genome, producing a complete and contiguous marsupial reference genome, including centromeres. We reveal that the koala's ability to detoxify eucalypt foliage may be due to expansions within a cytochrome P450 gene family, and its ability to smell, taste and moderate ingestion of plant secondary metabolites may be due to expansions in the vomeronasal and taste receptors. We characterized novel lactation proteins that protect young in the pouch and annotated immune genes important for response to chlamydial disease. Historical demography showed a substantial population crash coincident with the decline of Australian megafauna, while contemporary populations had biogeographic boundaries and increased inbreeding in populations affected by historic translocations. We identified genetically diverse populations that require habitat corridors and instituting of translocation programs to aid the koala's survival in the wild.

Translating Pharmacokinetic and Pharmacodynamic Data into Practice

Pharmacokinetic (PK) and pharmacodynamic (PD) publications provide scientific evidence for incorporation in evidence-based veterinary medicine, aiding the clinician in selecting doses and dosing intervals. PK and PD studies have reported wide variations within exotic species, due to physiologic differences in absorption, distribution, metabolism, and excretion. PK studies offer species-specific data to help tailor doses and dosing routes to individual patients, minimize toxicity, and provide a cornerstone for PD studies to determine drug efficacy. This article reviews the application of PK parameters and the challenges in determining the PD activity of drugs, with a particular emphasis on exotic species.

The Educated Guess: Determining Drug Doses in Exotic Animals Using Evidence-Based Medicine

Lack of species-specific pharmacokinetic and pharmacodynamic data is a challenge for pharmaceutical and dose selection. If available, dose extrapolation can be accomplished via basic equations. If unavailable, several methods have been described. Linear scaling uses an established milligrams per kilograms dose based on weight. This does not allow for differences in species drug metabolism, sometimes resulting in toxicity. Allometric scaling correlates body weight and metabolic rate but fails for drugs with significant hepatic metabolism and cannot be extrapolated to avians or reptiles. Evidence-based veterinary medicine for dose design based on species similarity is discussed, considering physiologic differences between classes.

TREATMENT OF MYCOTIC RHINOSINUSITIS IN A BENNETT'S WALLABY ( MACROPUS RUFOGRISEUS) USING TOPICAL VORICONAZOLE SUSPENDED IN A REVERSE THERMODYNAMIC PLURONIC GEL

An approximately 4-yr-old female Bennett's wallaby ( Macropus rufogriseus) was evaluated for chronic left-sided facial swelling and nasal discharge. Computed tomography, endoscopy, biopsy, mycologic culture, and panfungal polymerase chain reaction were consistent with destructive mycotic rhinosinusitis. The patient's infection was treated with a long-term injectable antibiotic, oral antifungal therapy, and multiple intranasal infusions of voriconazole suspended in a reverse thermodynamic pluronic gel. This case represents the first documented case of mycotic rhinosinusitis in a macropod and underlines the importance of advanced cross-sectional imaging in the diagnosis, monitoring, and management of nasal cavity disease in zoo animals.

The bioavailability of medetomidine in eight sheep following oesophageal administration

There is sound evidence that medetomidine is an effective analgesic for acute pain in sheep. In this study, 15 μg kg(-1) of medetomidine was administered intravenously, and into the oesophagus, in a cross-over study, using eight sheep. Following intravenous administration, medetomidine could be detected in the plasma of these sheep for 120-180 min but following oesophageal administration, medetomidine could not be detected in the plasma of any sheep at any of 17 time points over four days. It is suspected that this is due to high first pass metabolism in the liver. Consequently, we conclude that future studies investigating the use of analgesics in orally-administered osmotic pumps in sheep should consider higher doses of medetomidine (e.g. >100 μg kg(-1)), further investigations into the barriers of medetomidine bioavailability from the sheep gut, liver-bypass drug delivery systems, or other α2-adrenergic agonists (e.g. clonidine or xylazine).

From Leaf Metabolome to In Vivo Testing: Identifying Antifeedant Compounds for Ecological Studies of Marsupial Diets

Identifying specific plant secondary metabolites that influence feeding behavior can be challenging, but a solid understanding of animal preferences can guide efforts. Common brushtail possums (Trichosurus vulpecula) predominantly eat Eucalyptus species belonging to the subgenus Symphyomyrtus, and avoid eating those belonging to the Monocalyptus subgenus (also called subgenus Eucalyptus). Using an unbiased (1)H NMR metabolomics approach, a previous study identified unsubstituted B ring flavanones in most species of monocalypts examined, whereas these compounds were absent from symphyomyrtles. We hypothesised that unsubstituted B ring flavanones act as feeding deterrents for common brushtail possums. In the current study, we tested this hypothesis by comparing how much possums ate of a basal diet, with diets containing one of four structurally related compounds; pinocembrin, flavanone (unsubstituted B ring flavanones), chrysin (the flavone analogue of pinocembrin), and naringenin (a flavanone with B ring substitution). We found that pinocembrin and flavanone deterred feeding relative to the basal diet, but that chrysin and naringenin did not at equivalent concentrations. Thus, unsubstituted B-ring flavanones may explain why brushtail possums avoid eating monocalypt species. Furthermore, small differences in the structure of secondary compounds can have a large impact on antifeedant properties. These results demonstrate that metabolomics can be a valuable tool for ecologists seeking to understand herbivore feeding preferences.

Pharmacokinetics of enrofloxacin following oral and subcutaneous administration in the common ringtail possum (Pseudocheirus peregrinus)

[Correction added on 23 March 2015, after first online publication: Terminal half-life values of enrofloxacin is corrected in the fourth sentence of the abstract] Clinically healthy common ringtail possums (n = 5) received single doses of 10 mg/kg enrofloxacin orally and then 2 weeks later subcutaneously. Serial plasma samples were collected over 24 h for each treatment phase, and enrofloxacin concentrations were determined using a validated HPLC assay. Pharmacokinetic parameters were determined by noncompartmental analysis. Following oral administration, plasma concentrations were of therapeutic relevance (Cmax median 5.45 μg/mL, range 2.98-6.9 μg/mL), with terminal-phase half-life (t½ ) shorter than in other species (median 3.09 h, range 1.79-5.30 h). In contrast, subcutaneous administration of enrofloxacin did not achieve effective plasma concentrations, with plasma concentrations too erratic to fit the noncompartmental model except in one animal. On the basis of the AUC:MIC, enrofloxacin administered at 10 mg/kg orally, but not subcutaneously, is likely to be effective against a range of bacterial species that have been reported in common ringtail possums.

Perceived efficacy of analgesic drug regimens used for koalas (Phascolarctos cinereus) in Australia

Recent publications report that some therapeutic drugs used in koalas (Phascolarctos cinereus) have poor oral absorption and are rapidly eliminated. Therefore, information on both the analgesic drug dosage regimens used to treat koalas in Australia and koala caretakers' perceptions of the efficacy of these drugs to control pain was collected for the purpose of identifying the most popular analgesics to prioritize future analgesic pharmacokinetic studies for this species. A one-page, double-sided questionnaire was distributed both electronically and by mail to Australian koala care facilities such as zoos and wildlife hospitals. Information was received from 13 respondents. Nonsteroidal anti-inflammatory drugs (NSAIDs) were the most frequently used analgesics, followed by full micro- and partial opioid receptor agonists and acetaminophen with or without codeine. The full micro-opioid receptor agonists and acetaminophen with or without codeine were most consistently considered efficacious, with wider variation in perceived efficacy of the NSAIDs. Analgesic drug combinations were generally thought efficacious.