Potentiation of methotrexate lymphocytotoxicity in vitro by inhibitors of nucleoside transport.

Modulation of nucleic acid antimetabolite cytotoxicity by preformed purines and pyrimidines may not only complicate the interpretation of drug sensitivity tests and other in vitro studies but also adversely affect treatment in vivo. Previously we reported that in a lymphocyte clonal assay, thymidine and hypoxanthine released from dead or damaged cells reduced methotrexate cytotoxicity. We now report that the nucleoside transport inhibitor dipyridamole (DP), at 1.0 microM, abolished 3H-thymidine uptake into PHA stimulated lymphocytes, potentiated methotrexate cytotoxicity and reversed modulation of methotrexate cytotoxicity by exogenous thymidine and hypoxanthine. Normal growth of lymphocytes at high density was unaffected by 1.0-5.0 microM dipyridamole, while growth at low densities was only slightly reduced. Hydroxy-nitrobenzylthioguanosine (555) was a less potent inhibitor of 3H-thymidine uptake and was toxic to normal lymphocytes at concentrations inhibiting 3H-thymidine uptake. Nucleoside transport inhibitors isolate the cellular effects of nucleic acid antimetabolites, and provide a tool to study mechanisms of antifolate cytotoxicity.