1. Natural Products Discovery Group, Institute for Natural Products Applications and Research Technologies, Department of Chemistry & Biochemistry, Stephenson Life Science Research Center, University of Oklahoma, Norman, Oklahoma 73019, United States
2. Natural Products Support Group, Leidos Biomedical Research, Inc., Frederick National Laboratory for Cancer Research, Frederick, Maryland 21702, United States
3. Natural Products Branch, Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Frederick, Maryland 21702, United States
4. Molecular Targets Program, Center for Cancer Research, National Cancer Institute, Frederick, Maryland 21702, United States