Interference with nucleoside transport in mouse lymphoma cells proliferating in culture.

The S-nitrobenzyl derivatives of 6-thio-9-/3-D-ribofuranosylpurine and 2-amino-6-thio-9-ß-D-ribofuranosylpurine (i.e.), and 6-[(4-nitrobenzyl)thio] -9-ß-D-ribofuranosylpurine (NBMPR) and 2-amino-6-[(4-nitrobenzyl)thiol] -9-0-D-ribofuranosylpurine, respectively), which have been previously recognized as potent inhibitors of nucleoside transport, protected cultured L5178Y cells against the antiproliferative effects of 6-(methylthio)-9-0-D-ribofuranosylpurine (MMPR), 8-azaadenosine, 5-bromodeoxyuridine, tubercidin, and formycin. For example, when cultures contained 0.01 ¿tMNBMPR, cells were able to proliferate at 90% of the control rate in the presence of 0.5 MM MMPR, a concentration of MMPR which by itself completely inhibits cell proliferation. NBMPR did not inhibit phosphorylation of MMPR in cell-free extracts of L5178Y cells but was a very potent inhibitor of the uptake of MMPR by those cells in culture. It is suggested that the reduced sensitivity of the cells to these cytotoxic nucleosides is due to a reduction in the rates at which the toxic nucleosides enter cells. NBMPR alone did not affect cell proliferation at concentrations up to 10 MM under usual culture conditions; however, 1.0 MMNBMPR was inhibitory when cells were made dependent upon exogenous thymidine by the inclusion of methotrexate in the culture medium. It is concluded that the ability of lymphoma cells to utilize exogenous thymidine was impaired in the presence of the transport inhibitors. NBMPR and several related compounds, which differed only in the S substituent, were compared with respect to their ability to protect cells against MMPR; compounds bearing the 4-nitrobenzyl group were the most effective.