1. Departments of Pharmaceutical Biology and Medicinal Chemistry, University Centre for Pharmacy, Groningen Institute for Drug Studies, University of Groningen, A. Deusinglaan 1, NL-9713 AV Groningen, The Netherlands, Department of Radiobiology, Groningen Institute for Drug Studies, University of Groningen, Bloemsingel 1, NL-9713 BZ Groningen, The Netherlands, and Institut für Pharmazeutische Biologie der Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany