Epilepsy is a common and serious neurological disorder to which a high proportion of patients continue to be considered “drug-resistant” despite the availability of a host of anti-seizure drugs. Investigation into new treatment strategies is therefore of great importance, one such strategy being the use of the nose to deliver drugs directly to the brain with the help of pharmaceutical formulation to overcome the physical challenges presented by this route. The overall aim of this thesis was to establish and apply a seizure model to the investigation of two types of particulate intranasal delivery systems; microparticles and cubosomes.
Chapter One introduces the topic of intranasal delivery of anti-seizure drugs, covering the link between the nose and seizures, pathways from the nose to the brain, current rudimentary formulations in clinical use, animal seizure models and their proposed application in studying intranasal treatments, and a critical discussion of relevant pre-clinical studies in the literature. Upon this, Chapter Two begins by validating a seizure model based on the Maximal Electroshock Seizure Threshold (MEST) test with the intention of using it to detect the effects of intranasally administered therapeutics. The design attempts to address previously scarcely acknowledged issues of sensitivity in the MEST model and confounding by anaesthetics which are currently necessary to reliably and ethically perform intranasal administration to the olfactory region in rats. The results show that the model was able to clearly detect a change in seizure threshold after administration of the positive control, intravenous phenytoin, which was supported by therapeutic brain and plasma concentrations of the drug as determined using an internally developed Liquid Chromatography Mass Spectrometry (LC-MS) assay. Importantly, this effect was able to be detected despite the use of the inhaled anaesthetic, isoflurane, to briefly sedate the animals 60 minutes prior to stimulation.
In Chapter Three, the seizure model is applied to the evaluation of tamarind seed polysaccharide (TSP) microparticles as a proposed intranasal delivery system for the pharmacokinetically troublesome anti-seizure drug phenytoin. In this first pharmacodynamic study, to the author’s knowledge, of a dry powder mucoadhesive microparticle formulation for seizure treatment, the model identified a peak anti-seizure effect time of 120 minutes after administration, which coincided with peak brain concentrations and supported its utilisation in intranasal delivery screening. Furthermore, the complementary demonstration of a histologically intact nasal epithelium and simultaneous measurement of phenytoin’s major metabolite, 5-(4-Hydroxyphenyl)-5-phenylhydantoin (4-HPPH) in brain tissue and plasma, supported the hypothesis of a direct intranasal delivery to the brain and the suitability of the microparticles for further trials.
In Chapter Four, the seizure model is applied to explore a potential new type of anti-seizure therapeutic, the endogenous endocannabinoid-like molecule, oleoylethanolamide (OEA), which has not yet had an effect on seizures documented. A cubosome dispersion was selected as the delivery vehicle, presenting one of the few pharmacodynamic in vivo studies conducted with this class of formulation to date. Given the unknown effects of oleoylethanolamide, it was firstly administered intravenously as a control, but no effect on seizure threshold was evident. Considering the complex nature of the hydrolysis-susceptible oleoylethanolamide and the self-assembling cubosome dispersion, complementary in vivo pharmacokinetic studies (utilising an internally developed LC-MS assay) and in vitro structural stability studies (utilising Small-angle X-ray Scattering (SAXS)) were conducted to further explore confounding factors. Despite presenting with complexities of their own, they overall supported the lack of pharmacodynamic effect seen after systemic administration. Intranasal studies were conducted in an attempt to bypass the challenges of systemic administration, but also demonstrated no measurable change in seizure threshold. Histological studies to determine a safe dose uncovered a toxicity of cubosomes to the nasal epithelium at the highest dose, independent of lipid type, which has not yet been described in any in vivo liquid crystalline nanoparticle studies to date and should be considered in future related work.
In summary, this thesis presents a tailored seizure model for screening intranasal delivery systems, a practical template for studying these systems in vivo, and a pre-clinical evaluation of two such systems. Notwithstanding the discussed limitations, it concludes that dry-powder mucoadhesive microparticles appear to be a promising platform for future study of intranasal anti-seizure drug delivery, while cubosomes and oleoylethanolamide may be better suited to other applications until a more thorough in vivo exploration of their respective fields exists
