In this study, the ability of deoxythymidine (dThd) to enhance selectively the metabolism of 1-β-d-arabinofuranosylcytosine (ara-C) in rats bearing transplantable colon carcinoma was investigated. A steady-state plasma level of 375 µm dThd was achieved within 3 h after initiation of a 24-h infusion of dThd (7 g/kg/day) with a concomitant 80% reduction in circulating 2′-deoxycytidine levels. Complete recovery to control values occurred within 6 to 8 h after termination of the infusion. Under the conditions of dThd infusion, the intracellular levels of 2′-deoxycytidine 5′-triphosphate rose from 0.15 to 60 pmol/mg tumor tissue, from 2.5 to 15 pmol/mg intestinal tissue, and from 0.07 to 0.25 pmol/106 bone marrow cells. During the steady-state plasma concentration of dThd, the intracellular concentration of 2′-deoxycytidine 5′-triphosphate in tumor tissue was reduced by 50% at 6 h after the initiation of dThd treatment with a complete recovery 9 h thereafter.

Differences in the apacity of tumor and host normal tissues to recover from the effects of dThd pretreatment were evaluated by measuring decreasing 1-β-d-arabinofuranosylcytosine 5′-triphosphate formation with time following dThd infusion. The ability to accumulate 1-β-d-arabinofuranosylcytosine 5′-triphosphate was reduced by 60 to 80% in normal tissues by 3 h after cessation of the dThd infusion but was decreased by only 15% in the tumor. These results suggested that delaying ara-C administration following dThd might result in less host toxicity while maintaining the antitumor effect. Sequential infusion of dThd (7 g/kg/day) for 24 h followed 3 h later by a 48-h infusion of ara-C (175 mg/kg/day), was as effective in reducing tumor mass as was dThd infusion immediately prior to ara-C and resulted in reduced host toxicity (less weight loss). The best schedule for the dThd-ara-C combination was two courses of alternating 24-h sequential infusions of dThd and ara-C with a 3-h delay in ara-C administration following dThd.

These data show that under the conditions used, reductions in intracellular 2′-deoxythymidine 5′-triphosphate pools by dThd in vivo do not appear to correlate with the antitumor activity of the dThd-ara-C combination. Intracellular 1-β-d-arabinofurano-sylcytosine 5′-triphosphate accumulation, however, was prolonged in rat colon tumor compared to normal tissues, and selectivity of the dThd-ara-C combination in favor of the tumor could be achieved by schedule modification.

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Supported in part by USPHS Grant CA-18240 from the National Cancer Institute, Department of Health, Education, and Welfare.

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