77 ARA-CTP METABOLISM FOLLOWING HYDROXYUREA OR METHOTREXATE TREATMENT IN HUMAN LEUKEMIA CELL LINES

Using human leukemia cell lines, we investigated the biochemical basis for the synergistic interaction between ara-C and hydroxyurea(HU) or methotrexate(MTX). In the cells of B cell phenotype, pretreatment with 1mM HU increased ara-CTP formation up to 2~10 fold, whereas it did not have significant effects on T cell lines. The phosphorylation of ara-A or deoxyadenosine to the corresponding triphosphates was also augmented by HU, indicating the activation of deoxycytidine(CdR) kinase in intact cells through HU treatment. As a possible mechanism, we propose the important role of de novo CdR production, since (i) the changes of pyrimidine dNTP pools by HU (the decrease in dCTP and increase in TTP) were equally seen in both cell lines, (ii) the excretion of CdR into the medium was much higher in B cell lines, which was significantly inhibited by HU. On the other hand, MTX(1μM) increased ara-CTP generation significantly in both cell lines, albeit it was less than 50%. These results suggest the cell type specific effectiveness of such combination chemotherapyin acute leukemia.