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Mechanism of Uptake of Nitrosoureas by L5178Y Lymphoblasts in Vitro¹

Department of Medicine, University of Manitoba, and The Manitoba Institute of Cell Biology, Winnipeg, Manitoba, R3E OV9, Canada

The mechanism of uptake of nitrosoureas by L5178Y cells in vitro was investigated. A time course of the uptake of radioactivity on incubation of L5178Y lymphoblasts with [¹⁴C]-1,3-bis(2-chloroethyl)-1-nitrosourea was linear for 30 min and then entered a plateau phase; it was markedly temperature dependent. A similar time course for cells incubated with [¹⁴C]ethylene-labeled 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea reached equilibrium rapidly, was temperature independent, and resulted in a relatively low level of uptake of radioactivity. However, cells treated with 3-[cyclohexyl-¹⁴C]-1-(2-chloroethyl)-1-nitrosourea had a time course that was linear for 30 min, resulted in much higher levels of uptake of radiactivity, and was strongly temperature dependent. These findings, at least for 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, suggest that some drug decomposition precedes uptake.

The percentage of radioactivity found in the cell sap fraction was at least 85% of total cell activity when cells were incubated with any of the three ¹⁴C-labeled nitrosoureas. Furthermore, thin-layer chromatography of the cell sap fraction revealed the presence of free intact drug. These findings indicate that intracellular uptake of intact nitrosoureas occurred. A time course of uptake of intact 1,3-bis(2-chloroethyl)-1-nitrosourea reached equilibrium rapidly with cell/medium distribution ratios of 0.2 to 0.6 and was temperature independent. The addition of excess unlabeled 1,3-bis(2-chloroethyl)-1-nitrosourea or 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea had no effect on uptake of [¹⁴C]-1,3-bis(2-chloroethyl)-1-nitrosourea. These findings suggest that uptake of intact 1,3-bis(2-chloroethyl)-1-nitrosourea was by passive diffusion. A time course of the uptake of intact 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea with either [¹⁴C]ethylene- or ring-labeled drug rapidly reached equilibrium, was temperature independent, and attained a cell/medium ratio greater than unity. Uptake of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea was sodium independent and was unaffected by the metabolic inhibitors (sodium fluoride, sodium cyanide, or 2,4-dinitrophenol) or by urea, a potential physiological competitor. Furthermore, addition of unlabeled 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea or 1,3-bis(2-chloroethyl)-1-nitrosourea had no effect on uptake of labeled 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea. These findings suggest that uptake of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea also occurs by passive diffusion.

¹ This work was supported by grants from the National Cancer Institute of Canada and the Medical Research Council of Canada.

² Postdoctoral Fellow of the Medical Research Council of Canada.

Received 9/ 1/76. Accepted 12/17/76.

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