1. Natural Product Discovery Group, Institute for Natural Products Applications and Research Technologies, Stephenson Life Science Research Center, University of Oklahoma, Norman, Oklahoma 73019, United States
2. Department of Chemistry & Biochemistry, Stephenson Life Science Research Center, University of Oklahoma, Norman, Oklahoma 73019, United States
3. Department of Pharmacology, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229, United States
4. Mays Cancer Center, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229, United States