Results and Discussions
3.3 Passive and iontophoretic delivery using the coplanar set-up
The final part of the project focused on the development of a new experimental set-up for short duration iontophoresis that would better approximate clinical use. For buccal mucosa, the counter electrode can
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be placed on either the epithelial side or outside the oral cavity on the cheek. A mathematical simulation using the COMSOL Multiphysics software to compare the effect of electrode placement on drug delivery suggested that placing the counter electrode on cheek skin outside the oral cavity was more efficient for transbuccal drug delivery [45-47] and a three-compartment vertical diffusion cell, where both electrodes were on the epithelial side of the mucosa and which better mimicked in vivo conditions was used to investigate the buccal iontophoretic delivery of atenolol [28]. The coplanar iontophoretic system proposed in the present study enabled “same-side” current application since both electrode compartments were placed on the epithelial surface – there was no receiver compartment since the experiments were of short duration and the tissue was hydrated using a surgical absorbent pad moistened with PBS (Figure 2).
A bridging study was performed to compare passive and iontophoretic delivery of BUF from the hydrogel (containing 20 µmol of BUF) using conventional two-compartment vertical Franz cells and the coplanar set-up to confirm that changing the electrode configuration did not influence the iontophoretic delivery of BUF to the mucosa (Figure 5). After application of the hydrogel for 10 min, passive delivery from the Franz cell and the coplanar set-up was 4.2 ± 0.4 and 4.1 ± 1.8 µg/cm2 (p = 0.9065), the corresponding amounts after iontophoresis at 1 mA/cm2, were 304.2 ± 28.9 and 278.2 ± 40.3 µg/cm2, respectively – they were also statistically equivalent (p = 0.1164). The amounts of BUF deposited in the different layers of the epithelium after passive or iontophoretic application using the vertical diffusion cells or coplanar set-up were also statistically equivalent (Figure 5a and 5b). This confirmed that the electrode configuration did not affect iontophoretic transport of BUF.
The set-up was then used to explore iontophoretic delivery of BUF from a fast disintegrating oral film.
These dosage forms have gained increasing attention for their ease of application, discreetness and ability to control dosing. They dissolve in a few minutes upon hydration, increase the local bioavailability at the diseased sites in the mucosa and improve therapeutic response. An example of a film used for intra-oral disease management is Oramoist®, which is indicated for xerostamia [48]; the Rivelin® patch containing clobetasol is in Phase II clinical trial. Rapid transmucosal delivery of systemically-acting drugs is also possible: Onsolis® and Breakyl® contain fentanyl and are approved for management of acute cancer pain [49].
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(b)
Figure 5. Amounts of BUF deposited in successive layers of the mucosal epithelium by passive (10 min) and iontophoretic (10 min at 1 current density of mA/cm2) transport when using (a) two compartmental Franz diffusion cells and (b) the coplanar set-up.
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The fast dissolving film developed in this study contained the same molar amount of BUF (20 µmol) as the gel discussed above; it was moistened by the addition of 70 µL of water before being placing on the mucosa. A circular silver anode covered the film entirely and delivery studies were performed at the optimized parameters mentioned above. There was a statistically significant difference in the total passive delivery from the two formulations (4.1 ± 1.8 and 24.8 ± 5.9 µg/cm2 for the gel and film respectively; p < 0.0001) despite the same amount of BUF being applied to the mucosal surface from the two formulations. This was attributed to the lower water content and therefore higher concentration of BUF at the surface of the film facing the mucosa. Iontophoresis for 10 min at 1 mA/cm2 enabled 13-fold more BUF to be delivered to the epithelium as compared to passive administration (p < 0.0001) (Figure 6a). The total iontophoretic delivery of BUF from the hydrogel and the film was statistically equivalent (p > 0.05); thus, the difference in BUF concentration did not influence the amounts delivered by iontophoresis.
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Deposited amounts (µg/cm2)
Mucosal depth (µm)
Film-P Film-I
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Figure 6. (a) Passive and iontophoretic delivery of BUF to the mucosal epithelium from the film (b) Amounts and corresponding concentrations of BUF deposited by passive and iontophoretic transport in LP by the film. Suffix –P means passive (10 min) and –I means iontophoresis (10 min at 1 current density of mA/cm2).
It has been previously suggested that in a system where there are no competing ions present in the donor compartment, the flux of the drug becomes independent of the donor concentration and is governed by the ratio between the drug diffusivity and the counter-ion present in the receptor compartment [50-53]) and this has also been observed experimentally [50-54]. This could explain why iontophoretic delivery from the hydrogel and the hydrated film is statistically equivalent. Having said that, it is reasonable to assume that there is a threshold concentration for each molecule above which the independence of the flux from donor concentration is observed [52, 54]. This is consistent with the observation that iontophoretic delivery of BUF from the 5, 10, 20 mM solutions was statistically different (section 3.1).
The results are in agreement with those seen with huperzine A where a 4-fold increase flux was seen upon increasing the drug concentration from 1 to 4 mM [52]. In the experiments conducted using aqueous BUF solutions at 5, 10 and 20 mM, the ratio of BUF with the counter ion (Cl-) is in the range of 0.03-0.14 suggesting that the BUF concentration at 5 and 10 mM was still below the “threshold concentration” required so that the drug:counter-ion ratio is sufficient for flux independence to be observed [52]. More investigation is required to determine how this threshold concentration differs for individual cations.
Amount of BUF in lamina propria (µg/cm2)
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It has been reported that a BUF concentration of 1.8 µg/mL produces a significant vasodilation [55]. The amount of BUF deposited in the LP by iontophoresis from the BUF mucoadhesive film and the corresponding estimated concentrations (Figure 6b) suggest that iontophoresis resulted in BUF concentrations in the LP – the therapeutic target area – that were >700-fold higher than the concentration reported to inhibit collagen production. This demonstrates that further optimization of iontophoretic conditions and a reduction to an even shorter current application period is possible – this would obviously facilitate treatment and compliance.
Conclusions
Advances in drug delivery technologies and the advent of efficient drug delivery strategies mean that pharmacokinetic limitations of existing dosage forms can be overcome. Here, short duration constant current buccal iontophoresis was successfully used to deliver potentially supra-therapeutic amounts of BUF to the mucosa. Electrotransport of BUF could be modulated using iontophoretic parameters and this could prove to be a useful tool to attenuate BUF delivery as a function of disease severity.
Biodistribution studies provided precise information about the amounts of BUF delivered to the different tissues and showed that it was possible to reach the LP, which is the therapeutic target for treatment of OSF. A novel experimental set-up was also developed and tested in order to mimic in vivo conditions more closely. Nevertheless, it is worth noting that xerostomia and the diseased state of epithelium associated with OSF were not taken into account in this in vitro study and this could influence iontophoretic transport. However, it is considered that these would have a greater impact on passive delivery rather than iontophoretic transport but further preclinical studies are required.