Modulation of the effect of 1-beta-D-arabinofuranosylcytosine by 6-mercaptopurine in L1210 cells.

In L1210 cells incubated with 1-beta-D-arabinofuranosylcytosine (ara-C), 6-mercaptopurine (6-MP) significantly potentiated 1-beta-D-arabinofuranosylcytosine 5'-triphosphate (ara-CTP) accumulation and ara-C incorporation into DNA (ara-C/DNA). The cytotoxicity of these two drugs was assessed to be at least additive by clonogenic assay. 1-beta-D-Arabinofuranosyluracil (ara-U) level in a cell suspension was suppressed by 6-MP in a concentration-dependent fashion, though intracellular cytidine deaminase (CDD) activity was not affected by 6-MP. In addition, extracted CDD activity was not directly inhibited by 6-MP or by its intracellular metabolites in vitro. After preincubation in the presence or absence of 6-MP, the cell suspension was fractionated to obtain the spent medium and cell pellet. Then, each fraction was incubated with ara-C. Ara-U formation in the spent medium was found to increase conspicuously in relation to the time of preincubation in the control and it was suppressed by 6-MP pretreatment. Ara-U formation in the cell compartment increased slightly in relation to the time of preincubation in the control and substantially no suppression of ara-U formation was observed in spite of 6-MP pretreatment. In conclusion, intracellularly synthesized CDD was thought to be rapidly shed into the medium and the released CDD could play an important role in ara-C inactivation. 6-MP interrupted some step between synthesis and shedding of CDD, resulting in a decrease of the ara-C deamination in the medium and enhancement of its antileukemic effect.