title: Transdermal penetration of vasoconstrictors--present understanding and assessment of the human epidermal flux and retention of free bases and ion-pairs.
abstract: OBJECTIVE
  As reductions in dermal clearance increase the residence time of solutes in the skin and underlying tissues we compared the topical penetration of potentially useful vasoconstrictors (VCs) through human epidermis as both free bases and ion-pairs with salicylic acid (SA).
  
  
  METHODS
  We determined the in vitro epidermal flux of ephedrine, naphazoline, oxymetazoline, phenylephrine, and xylometazoline applied as saturated solutions in propylene glycol:water (1:1) and of ephedrine, naphazoline and tetrahydrozoline as 10% solutions of 1:1 molar ratio ion-pairs with SA in liquid paraffin.
  
  
  RESULTS
  As free bases, ephedrine had the highest maximal flux, Jmax = 77.4 +/- 11.7 microg/cm2/h, being 4-fold higher than tetrahydrozoline and xylometazoline, 6-fold higher than phenylephrine, 10-fold higher than naphazoline and 100-fold higher than oxymetazoline. Stepwise regression of solute physicochemical properties identified melting point as the most significant predictor of flux. As ion-pairs with SA, ephedrine and naphazoline had similar fluxes (11.5 +/- 2.3 and 12.0 +/- 1.6 microg/cm2/h respectively), whereas tetrahydrozoline was approximately 3-fold slower. Corresponding fluxes of SA from the ion-pairs were 18.6 +/- 0.6, 7.8+/- 0.8 and 1.1 +/- 0.1 respectively. Transdermal transport of VC's is discussed.
  
  
  CONCLUSIONS
  Epidermal retention of VCs and SA did not correspond to their molar ratio on application and confirmed that following partitioning into the stratum corneum, ion-pairs separate and further penetration is governed by individual solute characteristics.
authors.affiliation: Therapeutics Research Unit, Department of Medicine, University of Queensland, Princess Alexandra Hospital, Brisbane, Queensland 4102, Australia.
authors.forename: Sheree E
authors.initials: SE
authors.lastname: Cross
authors.forename: Melanie J
authors.initials: MJ
authors.lastname: Thompson
authors.forename: Michael S
authors.initials: MS
authors.lastname: Roberts
mesh_terms: D000279:Administration, Cutaneous; D000328:Adult; D004817:Epidermis / Q000378:metabolism*; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007477:Ions; D014662:Vasoconstrictor Agents / Q000493:pharmacokinetics*
chemical_list: D007477:Ions; D014662:Vasoconstrictor Agents