Transdermal Macromolecular Delivery: Real-Time Visualisation of Iontophoretic and Chemically Enhanced Transport Using Two-Photon Excitation Microscopy

Purpose. To investigate the transdermal delivery of a modelmacromolecule by passive and iontophoretic means following pretreatment withC₁₂-penetration enhancers and to visualise transport across humanstratum corneum (SC) in real time.Methods. Transport studies of dextran, labelled with fluorescentCascade Blue® (D-CB; M_R = 3 kDa) across human stratum corneum,were conducted during passive and iontophoretic modes of deliveryfollowing pretreatment with either dodecyltrimethylammoniumbromide (DTAB), sodium dodecyl sulphate (SDS) or Azone®.Size-exclusion chromatography was used to assess maintenance of dextranstructural integrity throughout experimental lifetime. Two-photonexcitation microscopy was employed to visualise real-time dextran transportduring current application.Results. The positively charged C₁₂-enhancer DTAB elevated passiveD-CB steady-state flux (J_ss) and was the only enhancer to do soabove control during iontophoresis. The negatively charged SDS had theleast effect during both stages. On-line macromolecular transport wasvisualised, indicating both inter- and intra-cellular pathways across SCduring current application. No transport was visible across untreatedSC during passive transport.Conclusions. Use of a positively charged enhancer may improve J_ssof anionic macromolecular penetrants during passive and iontophoreticdelivery. On-line visualisation of iontophoresis across SC was possibleand can provide mechanistic insight into SC transport pathways.