Abstract
Film forming systems offer a number of advantages for topical and transdermal drug delivery, in particular enabling production of a supersaturated state which can greatly improve drug absorption and bioavailability. However the suitability of individual film forming polymers to stabilise the supersaturated state and optimise delivery of drugs is not well understood. This study reports the use of differential scanning calorimetry (DSC) to measure the solubility of methylphenidate both as the free base and as the hydrochloride salt in two polymethacrylate copolymers, Eudragit RS (EuRS) and Eudragit E (EuE) and relates this to the ability of films formed using these polymers to deliver methylphenidate across a model membrane. EuRS provided greater methylphenidate delivery when the drug was formulated as the free base in comparison EuE because the lower solubility of the drug in EuRS provided a higher degree of drug saturation in the polymeric film. In contrast EuE provided greater delivery of methylphenidate hydrochloride as EuRS could not prevent its crystallisation from a supersaturated state. Methylphenidate flux across the membrane could be directly related to degree of saturation of the drug in the film formulation as estimated by the drug solubility in the individual polymers demonstrating the importance of drug solubility in the polymer included in film forming systems for topical/transdermal drug delivery. In addition DSC has been demonstrated to be a useful tool for determining the solubility of drugs in polymers used in film forming systems and the approaches outlined here are likely to be useful for predicting the suitability of polymers for particular drugs in film forming transdermal drug delivery systems.
Highlights
Film forming systems for topical or transdermal application contain drug and film forming excipients along with volatile solvent(s) in a formulation which typically presents as a solution or spray
It was noted in contrast to the other formulations, the film formed by the 1.2% Methylphenidate hydrochloride USP (MPH-HCl) EudragitÒ RS (EuRS) formulation rapidly became cloudy following application to the silicone membrane, suggesting that crystallisation of the drug may have occurred in the film which would be expected to reduce or prevent drug permeation [11]
The metered dose aerosol produced films used in this study exhibited drug transport data that was linear when plotted against the square root of time consistent with Fickian diffusion as described through the Higuchi model of drug release
Summary
Film forming systems for topical or transdermal application contain drug and film forming excipients along with volatile solvent(s) in a formulation which typically presents as a solution or spray. Film forming systems offer a number of advantages over more conventional formulation types; they can provide a unit dose, improve drug delivery, be applied to large application areas and their rapidly drying/absorbing nature can help to minimise transference losses of product onto clothes or other people. As such a number of topical, film forming pharmaceutical products have been successfully marketed [1]. Developing film forming systems with improved bioavailability for drugs that are delivered transdermally and may be abused, such as opioids (e.g. fentanyl) or stimulants (e.g. methylphenidate), may help to assist in the development of products that are less attractive for drug abuse, as a result of a lower abusable drug content [4]
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