Resistance to 2-chloro-2 '-deoxyadenosine of the human B-cell leukemia cell line EHEB

The effects of 2-chloro-2'-deoxyadenosine (CdA, cladribine), an adenosine d eaminase-resistant analogue toxic for both proliferating and resting lympho id cells, were investigated in the human leukemia cell line EHEB, which was derived from a patient with B-cell chronic lymphocytic leukemia. These cel ls were found to be less sensitive to CdA than B-cell chronic lymphocytic l eukemia lymphocytes (similar to 25-fold) and other human lymphoblastic cell lines (10-1000-fold). Phosphorylation of CdA by deoxycytidine kinase and i ntracellular accumulation of 2-chloro-2'-deoxyadenosine triphosphate (CdATP ) were similar in EHEB cells and in other CdA-sensitive cell lines. In cont rast, the inhibitory effect of CdA on ribonucleotide reductase activity, wh ich was investigated in situ by the conversion of cytidine into deoxyribonu cleotides and its incorporation into DNA, was much less pronounced in EHEB cells than in other human lymphoblastic cells. Accordingly, concentrations of deoxynucleoside triphosphates did not decrease and even tended to rise. Unexpectedly, incorporation of thymidine and deoxycytidine into DNA was inc reased severalfold after a 24-h incubation with CdA. CdA also increased the activities of deoxycytidine kinase and thymidine kinase approximately 4-fo ld. Analysis of the cell cycle by flow cytometry showed that after 24 h, Cd A provoked an increase in the proportion of cells in S phase, synthesizing DNA. We conclude that the EHEB cell line is resistant to the cytotoxic acti on of CdA not only because of a lack of inhibition of ribonucleotide reduct ion but also because CdA, in contrast with its known effects, provokes in t his cell line an increase in the proportion of cells replicating their DNA. Unraveling of the mechanism of this effect may shed light on clinical resi stance to CdA.