1. Department of Pharmacology and ‡Cancer Therapy & Research Center, The University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229-3900, United States
2. Natural Product Discovery Group, Institute for Natural Products Applications and Research Technologies, and ⊥Department of Chemistry & Biochemistry, Stephenson Life Science Research Center, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019-0390, United States