In vitro and in vivo iontophoresis of a tripeptide across nude rat skin

The iontophoretic delivery across nude rat skin of a tripeptide (Threonme-Lysine-Proline), which is positively charged at pH 7.4, has been measured in vitro and in vivo. The peptide was delivered from a poloxamer gel which demonstrated thermoreversible properties: the gel is a liquid at room temperature, a feature which facilitates preparation, whereas, at 37°C, the gel solidifies and provides a welldefined delivery system. The in vitro studies showed that: (a) iontophoresis significantly enhanced peptide delivery compared to passive transport; (b) peptide delivery was directly proportional to the applied continuous direct current density over the range 0.18-0.36 mA/cm²; (c) following 6 h of iontophoresis, minimal degradation of the peptide in either the donor or receptor phases of the diffusion cell (by electrolysis and/or metabolism) was observed; (d) exposure of the skin to direct current prior to the application (without current) of the tripeptide lowered the barrier function to the passive transport of Thr-Lys-Pro. In vivo, a single intravenous injection of radiolabeled tripeptide was rapidly eliminated (primarily in the urine) with an apparent half-life of less than 30 min. When the peptide was delivered by iontophoresis, considerable enhancement over passive diffusion was again achieved; the cumulative delivery of tripeptide in vivo was predictable from the in vitro results. In addition, and once more in parallel to the in vitro findings, pre-exposure of the skin to the iontophoretic current lowered barrier function to the subsequent passive delivery of the peptide. Skin sectioning and radioassay demonstrated that, relative to passive diffusion, iontophoresis produced (i) greater localization of the peptide in the skin, and (ii) delivery of peptide to deeper layers of the skin. Finally, histological and electron microscopical examination or rat skin following 3 hours of iontophoresis (at 0.36 mA/ cm²) in vivo did not reveal any gross morphological changes.