Investigation into the mechanism of action and chemical enhancement of transdermal iontophoreses of LHRH

Abstract

Recent developments in drug design and advances in mapping of the human genome have quickened the pace of development of peptide and protein drugs. Unfortunately, this class of compound often has specialised delivery requirements that preclude administration via traditional routes due to biological barriers, physicochemical drug properties, and issues of pharmacokinetics and pharmacodynamics. Transdermal drug delivery has advantages over other routes of administration. Also, the use of an external driving force, such as iontophoresis, is a technique that is well suited for delivery of these compounds. However, the large molecular size of peptides may limit the amounts of drug that can be delivered across the skin barrier. In this study the potential to enhance transdermal iontophoretic delivery of peptides was investigated. Two approaches to this were used: manipulating electrical field strengths to induce structural changes in the skin; and combining a chemical permeation enhancer with iontophoresis to determine if peptide delivery could be synergistically enhanced. In vitro permeability investigations were performed through human epidermal membrane (HEM) using the model peptide, luteinizing hormone releasing hormone (LHRH). Increasing the electrical field strength across the HEM resulted in marked porosity changes above 1 V. This was accompanied by dramatic increases in permeability enhancement of a small ionic solute, tetraethylammonium bromide (TEAB). However, the increases that were observed in LHRH permeability enhancement was several orders of magnitude less. The modified Nernst-Planck model, after correction for porosity changes, predicted results that matched the observed TEAB enhancement. However, LHRH enhancement predictions were markedly different from observed values indicating that the electrically-induced pathways were largely unavailable for LHRH transport, a finding that is consistent with recent reports characterising these pores. The influence of the voltage-induced pores on electroosmosis was also studied, since this transport mechanism is more important as molecular size increases. Sucrose, a small non-ionic polar solute, was used as an electroosmotic flow marker during these investigations. The enhancement of sucrose closely mirrored porosity increases in the membrane indicating considerable electroosmosis occurs within electrically-induced pores. These two parallel investigations showed moderate voltage iontophoresis (> 1 V) causes the induction of pores in the HEM, however, transport through these pores, despite significant electroosmosis, is limited to small solutes. The potential for synergism between iontophoresis and chemical enhancers to improve peptide delivery was investigated. It was hypothesised that oleic acid (OA) may cause synergistic enhancement of LHRH during iontophoresis by increasing the pore sizes of the HEM. Synergistic enhancement of LHRH was observed. Permeability and conductance changes, in addition to structural investigations (differential scanning calorimetry (DSC) and infrared spectroscopy (IR)) were also performed. OA treatment of the skin promoted the permeability of LHRH, sucrose, and TEAB during passive conditions. This suggests that pre-existing pores in the membrane increased in size. However when the enhancer was used in combination with iontophoresis, LHRH was the only solute to be co-enhanced. This finding was consistent with increases in pore size of the pre-existing pores in the stratum corneum as modelled using the hindered pore theory. Co-enhancement of LHRH may also be due to an effect of the enhancer on the accumulation of peptide in the stratum corneum. Interestingly, disruption of the fixed negative charges on the skin by the enhancer may have negatively influenced transport via decreases in electroosmosis. The disordered effect caused by the action of oleic acid on the stratum corneum was confirmed using DSC and IR investigations. In summary, the transport of peptides across the skin during iontophoresis may become less restricted following the pre-treatment of the barrier with suitable chemical permeation enhancers.