Thymidine as a chemotherapeutic agent: pharmacologic, cytokinetic, and biochemical studies in a patient with T-cell acute lymphocytic leukemia

Biochemical studies suggested that leukemia T-cells have low levels of TTP-catabolizing enzyme activity and are uniquely sensitive to thymidine (dThd). A child with T-cell acute lymphocytic leukemia (ALL), whose peripheral blood lymphoblasts manifested very low TTP catabolic capacity, was treated with 75 g dThd/m2/day by constant iv infusion for two courses of 5 and 8 days. The dThd caused an initial accumulation of peripheral blood blasts in S-phase at the expense of cells in G1, followed by a rapid reversal of this pattern consistent with a block in late G1 and/or early S. Concurrently, a prompt reduction of blasts was found in the peripheral blood. However, dThd treatment neither decreased the number of lymphoblasts in the cerebrospinal fluid (CSF) nor cleared the marrow. No major toxicity was observed, but the effect of dThd on normal marrow elements could not be evaluated in this patient. Blood concentrations of dThd were 1.4-3.0 mM, and concentrations of thymine were in the same range; beta half-life for dThd was 48 minutes. Steady-state CSF dThd was 9% of the simultaneous serum level. Clearance measurements demonstrated that catabolism of dThd was saturated and that renal clearance was a major determinant of total body clearance during high-dose dThd infusion. A good correlation was found between biochemical and cytokinetic parameters and response to dThd for the peripheral blood lymphoblasts. However, dThd did not produce a useful remission in this case of T-cell ALL.

Reversible regulation by magnesium of chick embryo fibroblast proliferation

The rate of 3H-thymidine incorporation and of cell proliferation in chick embryo fibroblast cultures are reduced coordinately when the [Mg2+] of the external medium is reduced below the physiological concentration of about 0.8 mM. These effects of moderately reduced [Mg2+] and the accompanying change in appearance of the cells, resemble the effects produced by lowering the [serum] of the medium. Cells subjected to severe Mg2+ deprivation, especially at low [Ca2+], die and detach from the culture dish. Cells kept at a reduced rate of proliferation for three days by moderate Mg2+ deprivation are quickly restored to rapid proliferation upon restoration of the normal [Mg2+] of the medium. The rate of proliferation of the chick embryo cells is reduced markedly by lowering [Ca2+] about 100-fold, but unlike the case of Mg2+-deprivation this can occur without significant effect on the rate of 3H-thymidine incorporation. More severe Ca2+ deprivation, which does lower the rate of 3H-thymidine incorporation, produces retraction of cells from one another and from the dish, and results in a distinctly abnormal, rounded appearance. The results lend weight to the thesis that free [Mg2+] plays a central role within the cell in the coordinate control of metabolism and growth. They also suggest that the effects produced by varying [Ca2+] in the medium are caused by changes at the external surface of the cell.