The reasons why patients with leukemia fail to respond to chemotherapy, in which the nucleoside analogues play an important role, are largely unknown. Deoxyribonucleoside kinases catalyze the first step in the intracellular phosphorylation of nucleoside analogues leading to activation of these prodrugs. 5′-nucleotidases are the catabolic members of the substrate cycles assumed to be involved in the regulation of intracellular deoxyribonucleoside triphosphate pools and may play an important role in the outcome of the treatment of cancer and viral diseases. Thus, the efficacy of nucleoside analogues may be depending on a high ratio of deoxynucleoside kinase activity to 5′-nucleotidases level activity. The two primary nucleoside purine analogues are cladribine and fludarabine used in treatment of chronic lymphocytic leukemia (CLL). In the present study the activity of several 5‘-nucleotidases were measured in peripheral blood cells from 59 CLL patients by a radiochemical HPLC-based method. We found high inter-individual variation in cNI median 132, 95% CI (119-157), cNII median 478, 95% CI (427-616), dNt median 3265, 95% CI (2690-3573) and ecto median 1875, 95% CI (1901-4171) pmol/mg protein/min using AMP, IMP, dUMP and CMP as substrates, respectivelty. Using fludarabine monophosphate as substrates we found a clear correlation between the degrading activity of fludarabine monophosphate and the cytosolic 5′-nucleotidase, cNII activity (spearman rank test Rho=0.47 p=0.0002). Also, there was an excellent correlation between cladribine-monophosphate degrading activity and the cytosolic 5′-nucleotidase, cNI activity (spearman rank test Rho=0.63 p<0.0001). We conclude that the cytotoxic activity of these nucleoside analogues is associated with both the activity of 5‘-NT and dCK kinase. Different treatment effects on CLL patients of cladribine and fludarabine may be explained by different activity of cNI and cNII in patients. Clinical outcome of therapy in patients with CLL may be related to both anabolic and catabolic enzymes which should be considered in the development of new drugs.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]