Metabolism and cytotoxic effects of 2-chloroadenine, the major catabolite of 2-chloro-2 '-deoxyadenosine

EHEB cells, a continuous cell line derived from a patient with B cell chron ic lymphocytic leukemia (B-CLL), synthesized, when incubated with tritiated 2-chloro-2'-deoxyadenosine (CdA), labeled mono-, di-, and triphosphate rib onucleosides at a much higher rate than CdA deoxyribonucleotides. Further a nalysis revealed that these ribonucleotides were formed from labeled 2-chlo roadenine (CAde), which contaminated commercial tritiated CdA at a proporti on of 2-3%. Since CAde is the major catabolite of CdA measured in plasma af ter oral or intravenous administration of CdA to patients, its metabolism a nd in particular its potential cytotoxicity were investigated both in EHEB cells and in B-CLL lymphocytes. Phosphorylation of CAde was inhibited by ad enine, indicating that its initial metabolism most probably proceeds via ad enine phosyhoribosyltransferase (EC 2.4.2.7). In both cell types, chloro-AT P was the major metabolite formed from CAde and its concentration increased proportionally at least up to 50 mu M CAde. At high concentration, CAde me tabolism was accompanied by a decrease in intracellular ATP. Cytotoxicity o f CAde, evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetra-zolium bromide (MTT) assay, showed an IC50 of 16 mu M in EHEB cells and 5 mu M in B-CLL lymphocytes. At cytotoxic concentrations, apopain/caspase-3 activatio n and high molecular weight DNA fragmentation were observed, indicating cha t CAde cytotoxicity results from induction of apoptosis. However, since CAd e cytotoxicity requires higher concentrations than CdA, it probably does no t play a role in the therapeutic effect of CdA in the treatment of hematolo gic malignancies. (C) 2000 Elsevier Science Inc.