Investigation into gastrointestinal absorption of xenobiotics by the koala
A project undertaken at the Faculty of Veterinary Science, The University of Sydney, and supervised by Dr Merran Govendir
Koalas are regarded as specialist herbivores as they eat a highly restricted diet of eucalypts. Eucalypts are known to contain toxic plant secondary metabolites (PSMs) such as tannins and turpenes in high concentrations. It appears that koalas absorb less nutrients from their diet relative to other herbivorous species, perhaps as a protective mechanism to minimise absorption and the effects of PSMs. High concentrations of PSMs in foliage also appear to inhibit foraging by koalas and PSMs become elevated (and nutrient content decreases) in eucalypts when their normal environment is disturbed. Therefore the koala's ability to use certain habitats may be determined in part, by the degree to which they can cope with these toxins.
So it was not surprising, that when treating koalas with antibiotics for infectious diseases for a previous project, the investigators observed that normal dog and cat dose rates for oral medication failed to result in detectable blood concentrations in koalas. Using the treatment of koalas with appropriate oral medication as a model, the investigators hope to confirm that there is a reduced absorption of oral medication (xenobiotics) and explain why this it occurs. Is the reduced absorption of medication due to gut factors such as the presence of concentrated eucalypt ingesta, or other gut environment factors such as types and function of gut bacteria or gut pH conditions? Or is the reduced absorption due to gut cellular pumps that prevent absorption of medication, or once the medication is absorbed, does the liver rapidly break down the drug molecules so that they are no longer detectable in the blood in their original form, or is the reduced oral absorption a combination of all or some of these factors?
Another unknown in the existing knowledge is that current dose rates of both oral and injectable medications used to treat disease in koalas have never been scientifically determined, i.e. pharmacokinetic profiles for common agents have not been documented. Therefore this investigation will also seek to optimise the dose rate and dose frequency of various medications for koalas by both the oral and injectable routes.
Therefore the specific aims of this project include
- to investigate the rate of liver metabolism in koalas by comparing the half-lives of therapeutic agents. Particularly to compare the half-lives of agents which undergo liver metabolism with those that undergo primarily kidney elimination.
- to investigate whether P-glycoprotein pumps (cellular membrane pumps that prevent xenobiotics staying within cells) exist within the cells of the intestinal mucosa and if so, chart their distribution, density and population in various regions of the lower GIT.
- to investigate the characteristics of the koala gut lumen itself such as the pH of the different regions of the GIT and the effect pH on drug absorption, the extent and effect of stomach digestion on absorption and to identify the principal populations of bacterial flora in the different regions of the GIT with respect to their role in absorption and
- to determine the optimum dose rates for antimicrobial agents used to treat infectious diseases and analgesic / anti-inflammatory drugs for koalas by measuring the therapeutic concentrations obtained in the plasma and correlating these with the patient's clinical response.
Outcomes as at November 2011
A pharmacokinetic model was used to investigate how koalas detoxify their diet. Commonly used therapeutic drugs were administered to koalas by the oral and / or subcutaneous routes as well as the intravenous route.
Determination of the drug concentrations in koala plasma were conducted via liquid chromatography (LC). The LC assay was validated for each drug.
All drugs administered by mouth (i.e. meloxicam [an anti-inflammatory and analgesic drug], fluconazole [an antifungal drug] and enrofloxacin [an antibacterial drug]) were found to have poor oral absorption compared to most other species for which comparative data exists.
The reasons for the poor oral absorption have not yet been elucidated, however an in vitro study demonstrated that meloxicam is no longer detectable when mixed with the gut contents of the large intestine and caecum, inferring that meloxicam undergoes significant binding with an unknown substance/s at this location.
One of the project’s objectives was to investigate the distribution of P glycoprotein molecular efflux pumps in the gut mucosa. These pumps transport xenobiotics (foreign substances) out of cells and therefore may inhibit oral absorption. This investigation is still in progress.
All drugs except one were found to have a rapid clearance from koala plasma (blood). From the pharmacokinetic profiles of enrofloxacin, fluconazole, meloxicam, and chloramphenicol sodium succinate (an antibacterial drug), the current recommended dose rates and dosing frequencies (which have been extrapolated from other species) are unlikely to result in therapeutic plasma concentrations in koalas. The exception was aqueous chloramphenicol base which had a reasonable half-life of approximately 15 hours, although this formulation also failed to reach therapeutic plasma concentrations at the recommended dose.
An in vitro microsomal assay was developed to investigate the rate of metabolism of drugs in koalas. Microsomes are metabolism enzymes found primarily in the liver but do occur elsewhere such as the gut cells. From preliminary studies, meloxicam metabolism in the koala liver occurs approximately five times faster than in rats and mice, suggesting that the koalas’ hepatic metabolism is much faster than that of many other species. This assay is currently being used to investigate drug metabolism within the enterocytes of the gastrointestinal tract and whether metabolism also occurs within the kidney.
This project supports the idea that koalas deal with their naturally occurring toxic diet due to mechanisms in the gut to minimise toxin absorption, and that those toxins that are absorbed, are metabolised rapidly. Investigation into further understanding the physiological adaptations of the koala to detoxify it diet, using a pharmacological model is ongoing.
This project demonstrated that it is important that investigation into the pharmacokinetics of other therapeutic drugs commonly administered to koalas continues as for many drugs for therapeutic purposes in this species, it is not appropriate to extrapolate the dose rate and dosing frequency from other species.
The outcomes of this project will improve the therapeutic outcomes of drug administration to koalas, facilitate an understanding of the issues when administering drugs to other herbivores, and may suggest strategies to improve oral administration of those drugs which presently have poor oral absorption in many species.
These investigations assist in the understanding of the physiological adaptations of the koala to detoxify its diet in order to assist this species survival if the concentration of constituents within the eucalypt foliage is likely to be affected, as has been reported, by prolonged elevations in environmental temperatures and an increased incidence of droughts.