The effect of a series of structurally-diverse histamine type II (H2)-receptor antagonists on histamine-mediated immunosuppression of human peripheral blood lymphocytes (HPBL) has been examined. This analysis of structure-activity relationships was undertaken to examine the validity of the recent proposal arising from clinical studies that H2-receptor antagonists containing a furan ring were devoid of the effects on lymphocyte function reported previously in studies using cimetidine and other H2-receptor antagonists containing an imidazole nucleus. Cimetidine and two furan-containing antagonists, ranitidine and SKF 93479, were found to be devoid of any effect of PHA-induced proliferation of human peripheral blood lymphocytes over a wide concentration range (10-4 to 10-10M). At high drug concentrations (10-3M) significant suppression of mitogen stimulation was observed but this was accompanied by significant cytotoxicity. All three antagonists were effective in reversing the suppression of PHA-stimulation of HPBL induced by exogenous histamine. Reversal of histamine-induced immunosuppression was obtained at drug concentrations (2.0 × 10-4 to 1.0 × 10-6M) which were non-toxic and did not affect PHA-induced proliferation in the absence of histamine. Ranitidine was the most potent antagonist in reversing histamine-mediated immunosuppression. The ability of structurally-diverse H2-receptor antagonists to modify the action of histamine on lymphocyte function lends support to the view that histamine exerts its effects via classical H2-receptors. The ability of ranitidine to alter lymphocyte responsiveness in analogous fashion to cimetidine indicates that the possibility of alterations in immune function should be considered in clinical studies using ranitidine and other furan-containing H2-receptor antagonists.
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