Abstract||The common brushtail possum (Trichosurus vulpecula) is the most significant vertebrate pest in New Zealand as an ecological threat to the indigenous biodiversity and an economic threat as a vector for bovine tuberculosis. Biological control is considered to be the most accepted management strategy to reduce the population, specifically by impairing fertility. Successful development of a biocontrol agent (most likely a protein or peptide macromolecule) requires identification of a compound that is species-specific and potent. The challenge is also to deliver the bioactive to this free-ranging, widespread, feral animal and ensure sufficient bioavailability. Macromolecules have low oral bioavailability, thus new formulation strategies are required to enhance stability and absorption in the gastrointestinal tract (GIT) of T. vulpecula. Oral administration of the bioactive contained within a non-toxic bait is the most practical delivery strategy.
Essential to designing an oral delivery system is to quantify the transit time of different sized delivery systems. The gastrointestinal transit in T. vulpecula was investigated (n = 72) by gamma scintigraphy. Technetium-labelled (99mTc) anion exchange resin particles (75 - 125 (mu)m or 500 - 700 (mu)m) or solution (99mTc-DTPA) was administered orally. After 3, 6, 12, 24 or 32 h, distribution of radioactivity in excised GITs was determined. Transit profiles were similar for each formulation. For delivery to the hindgut, bioactives need protection for 12 h though the upper GIT. Particulate formulations may be retained in the caecum for up to 32 h. Transit time was not different between animals dosed in the evening or the morning. Furthermore, GIT morphology is different between specimens in this study from southern New Zealand and Australian specimens. This may reflect improved diet quality in New Zealand.
A model protein (insulin) was incorporated into poly(ethyl 2-cyanoacrylate) (PECA) nanoparticles prepared by interfacial polymerisation of water-in-oil microemulsions. The mean size of nanoparticles was 220 nm with a mean entrapment efficiency of 78%, determined using reverse phase HPLC. In vitro release of insulin from PECA nanoparticles in phosphate buffer (0.067 M, pH 7.4) at 37°C was triphasic and not all entrapped insulin was released.
Following in vitro incubation of nanoparticles with enzyme solutions prepared from the GIT of T. vulpecula, lumen enzymes were more aggressive towards insulin compared to mucosal enzymes and the hindgut lumen was the GIT region with the lowest degradation.
For the first time in a marsupial species, the in vivo pharmacokinetics of insulin-loaded, PECA nanoparticles were investigated following i.v. and intra-caecal administration and measured by radioimmunoassay. The low cross reactivity of human and endogenous brushtail possum insulin means that T. vulpecula is a suitable non-diabetic model to study pharmacokinetics of insulin. The i.v. pharmacokinetics of insulin solution and insulin-loaded nanoparticles were similar. On intracaecal dosing, co-administration of a permeation enhancer (EDTA) resulted in a small increase in plasma insulin concentration compared to insulin-loaded nanoparticles alone.
In conclusion, transit time to the caecum of T. vulpecula following oral delivery was 12 h for fluid and particulate formulations < 1 mm diameter and was independent of the time of day the dose was given. T. vulpecula is a potential non-diabetic model for the study of insulin pharmacokinetics. This thesis demonstrates the potential application of oral peptide and protein delivery technology in the area of wildlife management.