Drug Metabolism Letters, 2007, 1, 7-11 7 1872-3128/07 $50.00+.00 ©2007 Bentham Science Publishers Ltd. Verapamil, but not Probenecid, Co-Administration Can Convert Desloratadine to a Sedating Antihistamine in Mice Anand Katta 1 , Mugunthu Dhananjeyan 2 , Crystal Bykowski 2 , Paul Erhardt 2 Miles Hacker 1 , Donald B. White 1,3 and Kenneth Bachmann 1,* 1 Department of Pharmacology, the University of Toledo College of Pharmacy, Toledo, Ohio, USA; 2 Center for Drug Design and Development, the University of Toledo College of Pharmacy, Toledo, Ohio, USA; 3 Department of Mathe- matics, the University of Toledo, Toledo, Ohio, USA Abstract: The possibility that non-sedating antihistamines could elicit sedation in mice due to drug-induced inhibition of brain PgP was evaluated by measuring the ability of desloratadine alone or in combination with verapamil to cause ataxia in mice. Also, the concentrations of desloratadine in plasma and in brain homogenates were measured by liquid chroma- tography-mass spectrometry. Relative to methylcellulose (control) treatment, verapamil plus desloratadine decreased ro- tarod performance of mice. Plasma concentrations of desloratadine appeared comparable in the mice treated with either desloratadine or verapamil plus desloratadine, however the rate of decline of desloratadine from brain tissue was slower in mice treated with verapamil plus desloratadine compared to mice treated with desloratadine only. These data suggest that inhibition of brain PgP can convert desloratadine to a sedating antihistamine in mice. Key Words: PgP, desloratadine, antihistamines, pharmacokinetics, pharmacodynamics. INTRODUCTION Symptoms of allergic disorders often benefit from treat- ment with H1 histamine receptor antagonists (H1-antagonists). The so-called first generation H1-antagonists such as diphen- hydramine, an ethanolamine, are known to reach sufficient concentrations in the CNS to cause CNS side effects, most notably sedation [1]. The second generation H1-antagonists such as cetirizine, loratadine, fexofenadine, and deslorata- dine cause significantly less sedation owing to less accumu- lation in the CNS. Second generation H1-antagonists are therefore referred to as non-sedating antihistamines. A clear distinction between the CNS effects of loratadine and diphenhydramine in humans relative to their differences in CNS penetrability has been described [2]. The ability of P-glycoprotein (PgP) encoded by human MDR1 (ABCB1) and by rodent mdr1a and mdr1b to limit CNS exposure to numerous drugs by effluxing them from the CNS is well known [3]. PgP was shown to efflux second generation H1 antagonists much more effectively than first generation H1 antagonists in MDCK cells transfected with with MDR1 [4]. Brain-to-plasma AUC ratios of second gen- eration antihistamines were higher in mdr1a/b knockout mice than wild type mice, whereas for first generation anti- histamines the ratios were comparable for both strains [4]. Likewise, brain:plasma ratios of another second generation antihistamine, bepotastine, were higher in mdr1 knockout mice compared to wild-type mice [5]. In view of the well-characterized requirement of PgP activity to limit brain accumulation of second generation antihistamines, this study was designed to determine whether drug-mediated inhibition of PgP could elicit sedative effects *Address correspondence to this author at the Department of Pharmacology, the University of Toledo College of Pharmacy, Toledo, Ohio, USA; Tel:419-530-1912; Fax:419-530-1909; E-mail: kbachma@utnet.utoledo.edu from a non-sedating antihistamine, thereby giving rise to an otherwise unexpected drug-drug interaction. EXPERIMENTAL Chemicals Desloratadine (>97% purity) was purchased from Se- quoia Research Products (Oxford, UK). Diphenyhydramine, methylcellulose (MC), probenecid, and verapamil were pur- chased from Sigma-Aldrich (St. Louis, MO). Methanol, ace- tonitrile and formic acid (FA) were of HPLC grade and were purchased from Fisher Scientific (New Jersey, USA). HPLC grade water was obtained using a Milli Q system. Mouse plasma was purchased from Pel-Freeze ® Biologicals (Rogers, Arkansas, USA). Liquid nitrogen and argon gas were pur- chased in high purity (99.998%) from Linde Gas (Toledo, OH, USA). Animals Male CBL57 mice weighing ~ 25 g (19.9-36.7) were ac- quired from Harlan Sprague-Dawley (Indianapolis, IN), and maintained in the main campus vivarium. The study protocol was approved by the Institutional Animal Care and Use Committee of the main campus. Treatments All treatments were given by gavage. Drugs were pre- pared as suspensions in 1% methylcellulose (w/v in distilled water). Doses were prepared fresh daily, and administered in a volume of 10 l/g after an overnight fast. Animals were treated with either 100 mg/kg probenecid , 100 mg/kg desloratadine, 80 mg/kg diphenhydramine, 30 mg/kg verapamil, desloratatine + verapamil (100 mg/kg + 30 mg/kg, resp), probenecid+desloratadine (100 mg/kg + 100 mg/Kg, resp), or methylcellulose (MC). Verapamil was al- ways administered as two successive 15 mg/kg doses 10 min apart, since bolus 30 mg/Kg doses produced signs of toxicity