Effect of 2′-deoxycoformycin on the inhibition of deoxyribonucleic acid synthesis by 9-β-d-arabinofuranosyladenine 5′-triphosphate

A biochemical basis for the increased cytotoxicity of 9-β-d-arabinofuranosyladenine (ara-A) in the presence of 2′-deoxycoformycin (dCF), a potent inhibitor of adenosine deaminase, has been investigated in CHO cells. Ten micromolar dCF was not toxic to this cell line, and it effected a 7-fold increase in the cytotoxicity of 50 μM ara-A during a 24-hr incubation. In the absence of dCF, CHO cells deaminated more than 85% of the initial amount of ara-A within 3 hr after drug addition. 9-β-d-Arabinofuranosyladenine 5′-triphosphate (ara-ATP) accumulated in these cells for only 6 hr to 150 μM, after which time the cellular ara-ATP concentration declined. At the conclusion of the incubation, no ara-ATP was detectable in the cells. In contrast, the presence of dCF maintained high levels of ara-A extracellularly and allowed a continuous intracellular accumulation of ara-ATP. It was demonstrated that the presence of dCF did not alter the relationship between the intracellular ara-ATP concentration and cellular DNA synthetic capacity (DSC). A concentration of 2.5 μM ara-ATP, accumulated during a 75-min drug incubation, was necessary to decrease DSC by 50%. The accumulation of more than 400 μM ara-ATP did not affect the intracellular concentrations of the four deoxyribonucleoside triphosphates. Furthermore, after a 3-hr incubation with ara-A alone or in the presence of dCF, DSC was inhibited more than 95% but increased substantially within 6 hr following drug washout. Recovery of DSC to more than 25% of the initial value corresponded to a decline in intracellular ara-ATP concentration to less than 35 μM. Thus, the presence of dCF can increase the duration of inhibition of DSC by allowing greater intracellular accumulation of ara-ATP, resulting in greater cell death.