53 C HAPTER 3 CHAPTER0S TEREOSELECTIVE TRANSPORT OF HYDROPHILIC QUATERNARY DRUGS BY HUMAN MDR1 AND RAT M DR 1 B P- GLYCOPROTEINS Guido JEJ Hooiveld *‡ Janette Heegsma ‡ Jessica E van Montfoort ‡§ Peter LM Jansen * Dirk KF Meijer ‡ Michael Müller * Groningen University Institute for Drug Exploration * Division of Gastroenterology and Hepatology ‡ Department of Pharmacokinetics and Drug Delivery University of Groningen, Groningen, The Netherlands § Division of Clinical Pharmacology and Toxicology University Hospital Zürich, Zürich, Switzerland Submitted