1. Department of Chemistry, Bedson Building, The University, Newcastle upon Tyne NE1 7RU, U.K.; Cancer Research Unit, Medical School, Framlington Place, Newcastle upon Tyne NE2 4HH, U.K.; AstraZeneca Pharmaceuticals, Alderley Park, Macclesfield, Cheshire SK10 4TG, U.K.; and Departments of Cellular and Molecular Physiology and Pharmacology, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, P.O. Box 850, Hershey, Pennsylvania 17033