1. Laboratory of Drug Discovery Research and Development, Developmental Therapeutics Program, Division of Cancer Treatment, Diagnosis, and Centers, National Cancer Institute−Frederick Cancer Research and Development Center, P.O. Box B, Building 1052, Room 121, Frederick, Maryland 21702-1201, and Laboratory of Analytical Chemistry, National Institute of Diabetes and Digestive Kidney Disease, 9000 Rockville Pike, Building 8, Room B2A23, Bethesda, Maryland 20892