1. CRC Drug Development Group and CRC Radiochemical Targeting and Imaging Group, Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester, M20 4BX, U.K., Department of Chemistry, UMIST, P.O. Box 88, Sackville Street, Manchester, M60 1QD, U.K., Chemistry Department, Cardiff University, P.O. Box 912, Cardiff, CF10 3TB, U.K., and Centre for Molecular Drug Design, Department of Chemistry, University of Salford, Manchester, M5 4WT, U.K.