Clinically relevant deoxycytidine levels are high enough to profoundly alter 9-beta-D-arabinofuranosylguanine cytotoxicity for human T-cell acute leukemia cells in vitro.

Plasma deoxycytidine levels can vary markedly during chemotherapy, from < 0.05 microM to at least 10.3 microM in T-cell acute lymphoblastic leukemia (T-ALL). This study demonstrates that clinically relevant deoxycytidine levels can dramatically protect human T-ALL cells against 9-beta-D-arabinofuranosylguanine (araG), a promising drug in this leukemia. At 0.4, 1.2, 3.6, and 10.8 microM deoxycytidine, the dose of araG required to kill 50% of MOLT3 T-ALL cells increased 4.23 +/- 1.95-(mean +/- SEM), 23.1 +/- 5.42-, 39.3 +/- 19.3-, and 67.0 +/- 11.5-fold compared to araG without deoxycytidine. Such deoxycytidine concentrations sharply reduced intracellular araG levels and blocked inhibition of DNA synthesis even in the presence of 160 and 640 microM araG. These data offer the first evidence that clinically relevant deoxycytidine levels could profoundly modulate araG toxicity in T-ALL.

Deoxycytidine in human plasma: potential for protecting leukemic cells during chemotherapy.

Degradation of DNA produces deoxycytidine. Metabolism of deoxycytidine to dCTP inhibits phosphorylation of cytosine arabinoside (araC), fludarabine (FaraA) and 2-chlorodeoxyadenosine (CdA) by deoxycytidine kinase. This study measured plasma deoxycytidine in healthy adults and two leukemia patients and then determined how clinically relevant deoxycytidine levels would affect drug toxicity in human leukemia and lymphoma cells. Deoxycytidine was well below 0.05 microM in ten healthy persons. In the leukemia patients it was <0.05 and 0.44 microM before chemotherapy, rising to 10.3 and 5.5 microM during treatment. A broad range of clinically relevant deoxycytidine levels were high enough to profoundly decrease araC, FaraA and CdA toxicity in MOLT3, CA46 and HL60 leukemia/lymphoma cells and to change dCTP, DNA synthesis and drug incorporation into DNA in a manner consistent with prior mechanistic studies. Varying deoxycytidine levels could be an important factor influencing leukemia therapy.

Deoxyribonucleoside triphosphate pools and thymidine chemosensitization in human T-cell leukemia.

Thymidine kills cells by depleting dCTP stores. The present experiments tested whether deoxycytidine, by replenishing dCTP pools, could prevent thymidine cytotoxicity and thymidine's enhancement of carboplatin killing in two human T-cell acute leukemia cell lines. MOLT3 and JM cells were exposed to combinations of thymidine, deoxycytidine, and carboplatin and then assessed for survival, the magnitude of thymidine-carboplatin chemosensitization, and changes in deoxyribonucleoside triphosphate pools. For both cell lines, deoxycytidine (up to 144.5 micrograms/ml x 24 h) completely restored dCTP pools but only partially protected against thymidine cytotoxicity (100-1000 micrograms/ml x 24 h) and thymidine-carboplatin sensitization (up to 60 micrograms carboplatin/ml during the last hour of thymidine). This contrasts with complete protection in prior studies using other cell types. Thymidine alone markedly increased dTTP and dGTP pools and decreased dCTP; dATP pools underwent a sharp decline which has not been observed before in any cell line. In subsequent studies 0.0336-137.3 micrograms deoxyadenosine/ml partially prevented cytotoxicity and carboplatin sensitization by 300 micrograms thymidine/ml. Together, deoxycytidine and deoxyadenosine completely prevented thymidine-carboplatin sensitization even though dATP and dCTP pools were not entirely returned to normal. These findings are discussed in regard to the unusual sensitivity of T-cell malignancies to thymidine toxicity, mechanisms of cytotoxicity and chemosensitization by thymidine, and the possibility of thymidine selectively sensitizing T-cell malignancies to killing by alkylating agents.