Abstract/Summary

This PhD project aims to develop a novel polymer-drug conjugate (PDC) via free radical polymerisation for extended release and decreased permeation of p-menthane 3,8-diol (PMD) when applied topically onto the skin. The rational behind this was the volatile nature of PMD (evaporates quickly) and reports of the side effects associated with the topical absorption of the PMD. For this purpose, firstly hyaluronic acid (HA) was chosen as a polymer to conjugate PMD but despite exploring various synthetic strategies, and changing reaction parameters including the molecular weight, drug, reaction time etc, a conjugate could not be produced. After these initial attempts, an alternative route was selected for the PDC synthesis based on synthesis of polymerisable PMD conjugate and its subsequent co- and homopolymerisation. PMD was conjugated with acryloyl chloride via an ester bond to form acryloyl-PMD (APMD), which was subsequently copolymerised with acrylic acid (AA) to form a series of copolymers poly(AA-co-APMD). The copolymers were characterised by 1 H NMR and FT-IR for their structural elucidation, which was then followed by molecular weight characterisation, thermal analysis by TGA and DSC, reactivity ratio studies, turbidimetric analysis and drug loading. The properties of these copolymers were affected by the molar ratios of AA and APMD, where the AA incorporation into the final copolymer was 3´ higher than the APMD. In order to assess the amount of drug (PMD) released from the copolymer, an in vitro experiment was performed by using porcine liver esterases (PLEs) to cleave the ester bond from the substrate (copolymer). It was found that ~45% of the drug was released over five days. To investigate the reason for the comparatively modest drug release, two experiments were performed to investigate the effect of copolymer molecular weight and enzyme activity on PMD release. It was found that molecular weight did impact on drug release whilst addition of fresh enzymes showed that ester bond cleavage was not limited by enzyme activity in our study. Penetration and permeation of the copolymer and free drug (PMD) through excised full thickness porcine ear skin was investigated. The Franz diffusion cell studies showed no permeation of the copolymer as compared to the PMD. Moreover, tape stripping revealed that almost ~90% of the copolymer was found on the outer surface of the skin as compared to PMD which was found in all the layers of the skin. A new model using planaria fluorescence assay was developed as a novel method to pre-screen potential irritants such as our copolymer or PMD. The model was developed using a range of known skin irritants to include non-, mild-, moderate- and strong-irritants. The results showed that this model was able to successfully differentiate strong irritants from the non-irritants, whilst the other irritant classes also showed good correlation between the fluorescence intensity (FI) of the vii planaria after irritant and then fluorescent dye exposure and the known literature primary irritation index (PII). This test demonstrated that that the copolymer is unlikely to be a significant irritant when applied topically. Overall, this project has demonstrated the feasibility of the copolymer approach as strategy to develop extended-release insect repellents whilst reducing transdermal permeation of the small molecular weight active ingredient and hence minimising any adverse systemic effects.