Abstract
THE effectiveness of arabinosyl cytosine (cytarabine: 1-β-D-arabinofuranosylcytosine, ‘Cytosar’, abbreviated to ara-C) in the treatment of human leukaemia can be correlated with the rate of uptake of the molecule in vitro1 and its subsequent phosphorylation. In man, this nucleoside is also rapidly deaminated to produce arabinosyl uridine which has no therapeutic activity2: both liver and serum contain pyrimidine nucleoside deaminase3. Consequently the half-life of unchanged ara-C in human sera is brief (about 15 min−1) (ref. 2). These characteristics suggest a relationship between cytidine deaminase and the susceptibility of the leukaemic cell to ara-C, and we have found that initial therapeutic responses in human leukaemia are correlated with lower intracellular concentrations of deaminase than those present in non-responders., Furthermore, in sequential treatment schedules, increasing concentrations of deaminase are associated with markedly reduced susceptibility of the tumour cell to ara-C.
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