1. Radioimmune and Inorganic Chemistry Section, Radiation Oncology Branch, and Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, 10 Center Drive, Bethesda, Maryland 20892-1088, and Laboratory of Medicinal Chemistry, Center for Cancer Research, National Cancer Institute, National Institutes of Health, 376 Boyles Street, NCI-Frederick, Frederick, Maryland 21702