title: Optimization of perfusate pH to improve microdialysis recovery of lipophilic compounds.
abstract: BACKGROUND
  Microdialysis (MD) allows sampling of compounds in-vivo from tissues' interstitial fluid. However, molecules insoluble at physiological pH have usually extremely low recovery. The addition of albumin to the perfusate or the use of isotonic lipoemulsion improves recovery of these molecules although it requires a cleaning step before HPLC analysis. This study investigates the possibility of improving the MD recovery of compounds insoluble at physiological pH but soluble at a different pH. The probe is perfused with an isotonic solution adjusted to pH values at which the compound has maximum solubility. Ketoconazole (KTC), clotrimazole (CLT) and tretinoin (TTN) were selected as model drugs because they are almost insoluble at pH 7.4 but soluble at pH 4 for KTC and CTL; and at pH 9 for TTN.
  
  
  METHODS
  Linear microdialysis probes were used to collect KTC, CLT or TTN from a standard solution of the compounds. Probes were perfused with 0.01 M pH 7.4 isotonic buffer solution (1) without or (2) with 5% Bovine Serum Albumin (BSA); or (3) with 20% isotonic lipoemulsion; or (4) with 0.01 M pH 4 isotonic buffer solution for KTC and CLT or 0.01 M pH 9 isotonic buffer solution for TTN. The method was then tested in-vivo, in rabbit skin, to assess the skin tolerance to the non-physiological perfusates and to monitor KTC and TTN delivery from commercial cream products.
  
  
  RESULTS
  In-vitro, the optimized-pH perfusate increased MD recovery significantly (P<0.001): 6.9 (KTC), 8.3 (CLT), and 2.0 (TTN) times compared to the physiological pH and 1.4 and 1.2 compared to the BSA and lipoemulsion respectively. No evidence of irritation or edema was observed in-vivo. However, KTC and TTN were not detected in-vivo with any of the modified perfusate tested.
  
  
  CONCLUSIONS
  These findings show that the optimized-pH perfusate effectively increases the in-vitro microdialysis recovery of KTC, CLT and TTN and that it is well tolerated in-vivo. However, the compounds tested (KTC and TTN) could not be detected in-vivo.
authors.affiliation: Division of Pharmaceutical Sciences, Arnold and Marie Schwartz College of Pharmacy and Health Sciences, Long Island University, Brooklyn, NY 11201, USA.
authors.forename: Emmanuelle S
authors.initials: ES
authors.lastname: Tre
authors.forename: Chanchal
authors.initials: C
authors.lastname: Patel
authors.forename: Shiv
authors.initials: S
authors.lastname: Aghara
authors.forename: Chirag
authors.initials: C
authors.lastname: Yadav
authors.forename: Grazia
authors.initials: G
authors.lastname: Stagni
mesh_terms: D000279:Administration, Cutaneous; D000818:Animals; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003022:Clotrimazole / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*; D045604:Extracellular Fluid / Q000378:metabolism; D005260:Female; D006863:Hydrogen-Ion Concentration; D007654:Ketoconazole / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*; D017551:Microdialysis / Q000379:methods*; D011817:Rabbits; D012710:Serum Albumin, Bovine / Q000737:chemistry; D012867:Skin / Q000378:metabolism; D012995:Solubility; D013997:Time Factors; D014212:Tretinoin / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*
chemical_list: D012710:Serum Albumin, Bovine; D014212:Tretinoin; D003022:Clotrimazole; D007654:Ketoconazole