Endemic Alkanna cappadocica was used to isolate novel antitumor molecules from Turkish landscapes in our previous studies. In this study, deoxyalkannin (ALCAP1), beta,beta-dimethylacrylalkannin (ALCAP2), acetylalkannin (ALCAP3), and alkannin (ALCAP4) as well as the novel isolated compounds 5-methoxydeoxyalkannin (ALCAP5), 8-methoxydeoxyalkannin (ALCAP6), 5-methoxyacetylalkannin (ALCAP7), 5-methoxy-beta,beta-dimethylacrylalkannin (ALCAP8) were characterized. The topoisomerase I (topo I) inhibitory activity of ALCAPs was investigated using in vitro plasmid relaxation assay and found that ALCAP2, 3, 4 and 7 were potent inhibitors at 2-6 mu M concentrations. Further, DNA damage response to ALCAP treatments was also studied by measuring the H2AX((S139)) and ATM((S1981)) phosphorylations. ALCAP2, 7 and 8 induced the DNA damage and apoptosis, consistently resulted in PARP cleavage at nanomolar concentrations in K562 leukemia cells. Moreover, when the free radical (ROS) generating capacity of the compounds was studied by 2',7'-dichlorofluorescein-diacetate assay using flow cytometry, we found that a known antioxidant N-acetyl-cysteine almost completely abrogated the H2AX((S139)) phosphorylations and the caspase 3 cleavage and activation. Thus, gamma H2AX((S139)) foci formation remained higher than the control, and an increase in CHK2((T68)) phosphorylation was observed by ALCAP2 and 7 treatments suggested that, these compounds can be potential therapeutics against tumor cell growth because of their unique DNA damaging abilities additional to enzyme inhibition similar to those of doxorubicin. (C) 2011 Elsevier Inc. All rights reserved.