1. Cancer Research Unit and Department of Chemistry, University of Newcastle, Newcastle upon Tyne, NE2 4HH U.K., AstraZeneca Pharmaceuticals, Alderley Park, Cheshire, U.K., Laboratory of Molecular Biophysics and Department of Biochemistry, University of Oxford, Oxford, OX1 3QU U.K., and Developmental Therapeutics Program, National Cancer Institute, Bethesda, Maryland 20892-7458