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Figure 9.28 The effect of hyoscine and pH on its flux across a membrane; increasing the pH increases the flux as ionisation is decreased.

Reproduced from A. S. Michaels etal., A. I. Chem. J., 21, 985 (1975).

Figure 9.29 The major components of an iontophoretic drug-delivery system. To optimise iontophoretic transdermal drug delivery, the drug should be charged. It should be applied under an electrode of the same polarity, and a counterelectrode opposite in charge to the drug must be placed at an indifferent site on the body. Electric current must be allowed to flow for an appropriate duration at a level minimising skin irritation. Iontophoresis can deliver neutral drugs by inducing solvent flow (electro-osmosis) which results from the migration of ionic species in the applied field. Drug candidates for iontophoretic transdermal delivery must usually be in a salt form with adequate aqueous solubility, be stable in an aqueous environment, and be sufficiently potent to allow therapeutic doses to be delivered at an acceptable current density. Reproduced from G. A. Lattin etal., Ann. N.Y. Acad. Sci., 618, 450 (1991).

Figure 9.29 The major components of an iontophoretic drug-delivery system. To optimise iontophoretic transdermal drug delivery, the drug should be charged. It should be applied under an electrode of the same polarity, and a counterelectrode opposite in charge to the drug must be placed at an indifferent site on the body. Electric current must be allowed to flow for an appropriate duration at a level minimising skin irritation. Iontophoresis can deliver neutral drugs by inducing solvent flow (electro-osmosis) which results from the migration of ionic species in the applied field. Drug candidates for iontophoretic transdermal delivery must usually be in a salt form with adequate aqueous solubility, be stable in an aqueous environment, and be sufficiently potent to allow therapeutic doses to be delivered at an acceptable current density. Reproduced from G. A. Lattin etal., Ann. N.Y. Acad. Sci., 618, 450 (1991).

Current can be applied in a continuous manner using either direct current (e.g. 0.1 mA cm or pulsed (0.1 mA cm 2000 Hz) to either solutions or gels of a drug. Enhancement of migration results from several possible sources.

• Ion-electric field interaction (electro-repulsion)

• Convective flow (electro-osmosis)

• Current-induced increases in skin permeability

The flux of drug can be increased by increasing the current density, movement of small (mol. wt. < 500) neutral and charged compounds increasing linearly with current density; with larger drugs (mol. wt.« 1000), however, the fraction of total current flowing which is carried by the drugs is small because of competition for smaller ions. Electro-osmosis probably plays a greater role here. Figure 9.30 shows the effect of pH, osmolality and current density on the transport of buserelin, a nona-peptide, through stratum corneum.

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