INTRACELLULAR PHARMACODYNAMIC STUDIES OF THE SYNERGISTIC COMBINATION OF 6-MERCAPTOPURINE AND CYTOSINE-ARABINOSIDE IN HUMAN LEUKEMIA-CELL LINES

Selective combinations of purine and pyrimidine analogs increase remis sion rates in pediatric patients with relapsed leukemias. The combinat ion of 6-mercaptopurine (6-MP) and cytosine arabinoside (ara-C) may ex hibit synergism similar to that observed for fludarabine and ara-C and may diminish the potential for development of resistance since the tw o drugs are activated by separate enzymatic pathways. To determine the efficacy of the combination against human leukemia cells, we investig ated the time-concentration relationships of the drugs given alone or in combination to the resultant cytotoxicity. To determine whether the combination leads to enhanced activity of deoxycytidine kinase (dCk), the rate-limiting enzyme in ara-C activation, we characterized the ce llular dCk in CCRF/CEM/0, CCRF/CEM/ara-C/7A, and CCRF/CEM/ara-C/3A mon oclonal cells before and after treatment with 6-MP. CCRF/CEM/0 (wild t ype), CCRF/CEM/ara-C/7A (approximate to 50% ara-C-resistant as determi ned by ara-C sensitivity assay and dCk characterization), and CCRF/CEM /ara-C/3A (approximate to 90% resistant to ara-C) human leukemia cells were incubated with various concentrations of 6-MP and ara-C given al one or in combination. Cell survival, inhibition of DNA synthetic capa city (DSC), ara-CTP anabolism, and dCk enzymatic characteristics were studied. Incubation of CEM/0 cells with 6-MP for 24 h, followed by ara -C for 48 h, increased cell-growth inhibition by approximately 0.5-1 l og(10), corresponding to 5- to 10-fold synergism, as compared with ara -C alone after identical drug incubation in al cell lines. Simultaneou s administration showed no synergism, whereas reversal of the sequence produced an antagonistic effect. The ara-CTP levels were 2- to 3.5-fo ld and 3- to 5-fold higher in CEM/0 and CEM/ara-C/7A cells, respective ly, in cells exposed to 6-MP followed by ara-C than in those exposed t o ara-C alone at the same concentrations. Furthermore, a progressive i ncrease in ara-CTP levels was noted in CEM/0 cells exposed to increasi ng concentrations of 6-MP followed by 10 or 20 mu M ara-C. A significa nt decrease in DSC was observed upon treatment of wild-type and ara-C- resistant cells with 6-MP and ara-C. The combination of 6-MP and ara-C exhibits significant sequence-specific synergism in both wild-type an d partially ara-C-resistant leukemia cell lines. The combination also exerts collateral sensitivity in the ara-C-resistant cell lines. 6-MP pretreatment may play a role in enhancing ara-C activation, thus produ cing drug synergism in sensitive and resistant leukemia cell lines.