title: Hair follicles contribute significantly to penetration through human skin only at times soon after application as a solvent deposited solid in man.
abstract: OBJECTIVE
  The aim of this study was to define the underlying relative penetration of caffeine through hair follicles and through intact stratum corneum with time in vivo through pharmacokinetic modelling.
  
  
  METHODS
  Caffeine plasma concentration-time profiles after topical application into skin with or without hair follicle blocking were modelled using the Wagner-Nelson method or a compartmental model with first order absorption and elimination. Pharmacokinetic parameters describing absorption rate and extent of absorption through hair follicles or the stratum corneum were determined separately and compared with each other.
  
  
  RESULTS
  The obtained pharmacokinetic parameters from the two methods were similar. The absorption rate constant of caffeine for hair follicles was nearly 10 times higher than that for the stratum corneum and the percentage of absorption from hair follicles was more than half of that of the stratum corneum. In addition, the absorption from the stratum corneum showed an approximately 10 min delay while there was no delay for absorption from hair follicles. All caffeine absorbed by hair follicles occurs within 30 min of application and accounts for 10.5 to 33.8% of the total amount absorbed across the skin for all subjects, whereas absorption of caffeine through the stratum corneum can occur over several hours.
  
  
  CONCLUSIONS
  Hair follicles contribute significantly to percutaneous absorption of caffeine after topical application in man in vivo only at times soon after application.
authors: Therapeutics Research Centre, School of Medicine, University of Queensland, Princess Alexandra Hospital, Woolloongabba, Brisbane, Australia. m.roberts@uq.edu.au
authors: Xin
authors: X
authors: Liu
authors: Jeff E
authors: JE
authors: Grice
authors: Juergen
authors: J
authors: Lademann
authors: Nina
authors: N
authors: Otberg
authors: Sindy
authors: S
authors: Trauer
authors: Alexa
authors: A
authors: Patzelt
authors: Michael S
authors: MS
authors: Roberts
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