A deoxycytidine kinase-deficient variant of a human promyelocytic leukemic cell line (HL-60/ara-C) has been isolated and characterized. These cells are capable of proliferating in the presence of 10-6 m 1-β-d-arabinofuranosylcytosine (ara-C), a level achieved in the plasma of leukemic patients undergoing conventional-dose ara-C therapy. The cells share numerous biological and biochemical features with the parent line, including: morphology; rate of growth; cloning characteristics; karyotype; rates of DNA, RNA, and protein synthesis; and ability to undergo terminal differentiation in the presence of agents such as 12-O-tetradecanoylphorbol acetate and dimethyl sulfoxide. In contrast, these cells display a great reduction in the total intracellular accumulation of ara-C following a 4-hr exposure to 10-6 m ara-C (2.4 versus 99.0 pmol ara-C/106 cells). Resistant cells exposed to 10-6 m ara-C for 1 hr also exhibited a reduction in the generation [1.2 versus 31.9 pmol 1-β-d-arabinofuranosylcytosine 5′-triphosphate (ara-CTP)/106 cells] and the 4-hr retention (0.30 versus 3.87 pmol ara-CTP/106 cells) of ara-CTP, the lethal ara-C metabolite, in comparison to parent cells. Incorporation of ara-C into resistant HL-60 cell DNA was also profoundly decreased. These biochemical alterations were associated with a 1000-fold decrease in the sensitivity of clonogenic cells to continuously administered ara-C (ara-C 50% inhibitory concentration: 1.8 × 10-6 m for HL-60/ara-C; 3.0 × 10-9 m for HL-60). A variety of antagonists of de novo pyrimidine synthesis inhibited the growth of ara-C-sensitive and -resistant cells to a similar extent. When HL-60 cells were exposed to a lethal concentration of thymidine (5 × 10-3 m), coadministration of 5 × 10-6 m deoxycytidine restored 90 ± 4% (S.D.) of colony-forming capacity. Normal human bone marrow progenitor cells were protected to a similar degree by 3 × 10-3 m deoxycytidine. In contrast, deoxycytidine concentrations as high as 5 × 10-3 m were unable to confer any protection to HL-60/ara-C cells under identical conditions. These studies suggest that an enzymatic perturbation rendering human leukemic cells highly resistant to ara-C may be exploited to achieve a selective in vitro chemotherapeutic effect.
Portions of this material have been presented in preliminary form at the American Society of Hematology, Washington, DC, meeting (1982) and have been published in abstract form (3). This work was supported by CA-13696-13 and CA-36501-01, both awarded by the National Cancer Institute, and The William J. Matheson Foundation.