Suppression of thymidine uptake of human lymphocytes by co-trimoxazole

Co-trimoxazole (trimethoprim-sulphamethoxazole) causes a decrease in the uptake of labelled thymidine in lymphocytes cultured in the presence of phytohaemagglutinin. This phenomenon was observed in 60% of 25 subjects. In cultures affected by the drug the mean suppression was 84%. A small decrease in thymidine uptake was noted with trimethoprim and sulphamethoxazole separately, but the effect was much more pronounced with the combination of the two drugs. The mechanism responsible for this phenomenon is discussed. The action is probably not due to the ability of these drugs to interfere with folic acid metabolism and it is likely that there is no direct effect on DNA synthesis.The suppression of thymidine uptake by lymphocytes in vitro in the presence of co-trimoxazole may not have any obvious clinical significance. However, in view of a report of an immunosuppressive action of trimethoprim in mice, it is possible that the leucopenia observed in some patients treated with this drug may have been caused by a similar mechanism.These experiments show that lymphocytes in vitro are suppressed by co-trimoxazole in concentrations comparable to, or smaller than, those found in vivo under normal therapeutic conditions. They are therefore likely to be clinically relevant.