Design, synthesis and biological evaluation of tetracyclic azafluorenone derivatives with topoisomerase I inhibitory properties as potential anticancer agents oleh Tsung-Chih Chen, Dah-Shyong Yu, Shiag-Jiun Chen, Chun-Liang Chen, Chia-Chung Lee, Ying-Yu Hsieh, Lien-Cheng Chang, Jih-Hwa Guh, Jing-Jer Lin, Hsu-Shan Huang