Induction of apoptotic cell death in chronic lymphocytic leukemia by 2-chloro-2′-deoxyadenosine and 9-β-D-arabinosyl-2-fluoroadenine

2-Chloro-2′-deoxyadenosine (CldAdo) and 9-β-D-arabinosyl-2-fluoroadenine (F-ara-A) have shown marked activity in the treatment of indolent lymphoid malignancies. Based on the susceptibility of various lymphocyte populations to apoptosis, we investigated whether CldAdo or F-ara-A would induce this process in lymphocytes from patients with chronic lymphocytic leukemia (CLL). In vitro exposure of leukemic lymphocytes to CldAdo or F-ara-A for 24 to 72 hours elicited features of apoptosis visible by light and electron microscopy. Analysis of DNA integrity showed DNA cleavage into nucleosomal-sized multimers. Using a quantitative assay, drug-induced DNA fragmentation was both time and dose dependent. Inhibition of active macromolecular synthesis did not prevent drug-induced fragmentation; however, both drug-induced and spontaneous DNA fragmentation were prevented by intracellular calcium chelation. In vitro culture with phorbol ester generally decreased drug-induced DNA cleavage. After prolonged incubation, CLL cells exhibited spontaneous cleavage; albeit, at significantly lower rates than drug-treated cells. Heterogeneity was observed for spontaneous and drug-induced DNA fragmentation and was significantly lower in B-leukemic cells obtained from patients with high-risk and refractory disease. We conclude that CldAdo and F-ara-A are potent inducers of apoptotic death in CLL and that this feature correlates with the disease status.