Modulation of the Effect of l‐β‐D‐Arabinofuranosylcytosine by 6‐Mercaptopurine in L1210 Cells

In L1210 cells incubated with l‐β‐D‐arabinofuranosylcytosine (ara‐C), 6‐mercaptopurine (6‐MP) significantly potentiated 1‐β‐D‐arabinofuranosylcytosme 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. l‐β‐D‐Arabinofuranosylnracil (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.