1. Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Sorbonnelaan 16, P.O. Box 80082, 3508 TB Utrecht, The Netherlands, Philips Research Laboratories, Department of Biomolecular Engineering, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands, and Department of Drug Delivery Technology, Leiden/Amsterdam Center for Drug Research, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands