Transdermal macromolecular delivery: Real-time visualization of iontophoretic and chemically enhanced transport using two-photon excitation microscopy

Purpose. To investigate the transdermal delivery of a model macromolecule by passive and iontophoretic means following pretreatment with C-12-penetration enhancers and to visualise transport across human stratum corneum (SC) in real time. Methods. Transport studies of dextran, labelled with fluorescent Cascade Blue(R) (D-CB; M-R = 3 kDa) across human stratum corneum, were conducted during passive and iontophoretic modes of delivery following pretreatment with either dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulphate (SDS) or Atone((R)). Size-exclusion chromatography was used to assess maintenance of dextran structural integrity throughout experimental lifetime. Two-photon excitation microscopy was employed to visualise real-time dextran transport during current application. Results. The positively charged C-12-enhancer DTAB elevated passive D-CB steady-state flux (J(ss)) and was the only enhancer to do so above control during iontophoresis. The negatively charged SDS had the least effect during both stages. On-line macromolecular transport was visualised, indicating both inter- and intra-cellular pathways across SC during current application. No transport was visible across untreated SC during passive transport. Conclusions. Use of a positively charged enhancer may improve J(ss) of anionic macromolecular penetrants during passive and iontophoretic delivery. On-line visualisation of iontophoresis across SC was possible and can provide mechanistic insight into SC transport pathways.