Novel Applications of Reverse Iontophoresis

: (Alternative Format Thesis)

Abstract

The ability to monitor the levels of analytes of interest within biological tissues is critical to many areas of scientific and medical research. However, the conventional methods employed to this end are often invasive and either painful to some extent in certain cases or at least partially destructive in others. Reverse iontophoresis (RI) is a technique which has been used, primarily, to extract analytes from human interstitial fluid to monitor their systemic concentrations. By virtue of its simplicity and non-invasive nature, RI is a powerful tool but the range of its potential applications has not been fully explored. This thesis identifies several technical challenges in the areas of topical drug delivery, disease diagnosis, and plant science, and examines whether RI may offer innovation in these situations.

Chapters 2 and 3 describes a body of research, the objective of which was to appraise the feasibility of RI to assess the skin disposition of topically applied drugs, using salicylic acid and nicotine as models. First, an in vitro permeation test was performed to facilitate measurement of each drug in the stratum corneum (SC) and viable tissue (VT) layers of the skin following a period of topical application. The capability of RI to extract the delivered drug from the skin was subsequently assessed. A simple kinetic model was proposed and applied to the RI cumulative extraction vs time profiles, enabling the delineation of drug sampled from the SC and VT compartments. The experimentally determined values and those predicted were in good agreement, meriting investigation in vivo in human volunteers. Encouragingly, consistency between measured and predicted SC values was observed, and VT concentrations estimated in vivo were in good agreement with those measured in vitro, providing a proof-of-concept demonstration of the approach. Overall, the results in Chapters 2 and 3 provide validation of RI to sample drug from the skin following its topical application, and of the modelling approach to adequately to derive key output metrics which can then be used to assess topical bioavailability.

Chapter 4 explored the application of RI as a tool to support the diagnosis of cutaneous disease; specifically, the method was used to examine whether the extraction of the skin cancer biomarkers tryptophan (Trp) and kynurenine (Kyn) was representative of their relative skin concentrations. The results show that rational selection of iontophoretic conditions – specifically, pH – may enable timely sampling of a Trp/Kyn ratio which accurately reflects the corresponding (epi)dermal levels.

The broad objective of Chapter 5 was to expand the application of RI beyond mammalian biomembranes and to investigate its ability to sample plant phytochemicals of interest from a morphologically complex plant biomass. The model biomass employed in vitro was the isolated peel of the fruit of Punica granatum. Using ellagic acid as a model compound of interest, these experiments were used to optimise the iontophoretic extraction conditions prior to in fructo testing. The technique demonstrated that several phytochemicals were successfully extracted from pomegranates after only relatively short periods of current application, suggesting that RI may offer a convenient sampling technique for metabolic profiling.

Following this proof-of-concept demonstration, Chapter 6 then demonstrates the timely RI extraction and subsequent quantification of several phytochemicals of cosmetic relevance from their source biomass. Furthermore, the RI extraction of punicalagin by RI was shown to correlate strongly with conventional (solvent) extraction, the implication being that rapid in situ phytochemical measurements may ultimately inform the harvesting process in a much less invasive and significantly more sustainable way.

This thesis advances the scope of practical reverse iontophoresis applications, providing an elegant solution to several pressing technical challenges and, potentially, expediting research in the relevant scientific disciplines.

Date of Award	24 May 2023
Original language	English
Awarding Institution	University of Bath
Sponsors	L'Oréal Research and Innovation, Aulnay-Sous-Bois, France
Supervisor	Richard Guy (Supervisor), Begona Delgado-Charro (Supervisor) & Sébastien Grégoire (Supervisor)