Isolation of thymidine-resistant cells from a thymidine-sensitive acute lymphoblastic leukemia cell line

Malignant cells have enhanced sensitivity to inhibition of growth by thymidine. Cell growth of the permanent lymphoid cell line CCRF-CEM, originating from a patient with acute lymphoblastic leukemia, is inhibited by 3 x 10(-5) M thymidine, compared to 1 to 5 x 10(-3) M thymidine required to inhibit growth of normal lymphoid lines. Thymidine-resistant cells were isolated at a frequency of approximately 1/100,000 cells after cloning CCRF-CEM cells in medium containing 5 x 10(-4) M thymidine. The resistant cells lacked the enzyme thymidine kinase, had a 20-fold decrease of thymidine uptake, and were resistant to 1 x 10(-4) M 5-bromo-2-deoxyuridine. The cells were sensitive to 1 x 10(-5) M methotrexate, even in the presence of exogenously added thymidine and hypoxanthine. The data indicate that a small fraction of malignant cells may escape the toxic effect of high thymidine therapy and therefore, require additional chemotherapy for their control.