MDL-899 is new phthalazine derivative which has been developed as a substitute for hydralazine, which has several undesirable effects, in the treatment of hypertension. The effects of MDL-899 on human lymphocyte functions are analyzed. This drug inhibited in a dose-related fashion the blastogenesis of lymphocytes upon PHA and Con A activation and down-regulated the activation of allogeneic mixed lymphocyte reactions (MLR). On the contrary the drug enhanced the activation of autologous MLR of non T/T type. This effect was five times higher on cells which carried the HLA DR 4 phenotype. The above reported observations suggest that MDL-899, as well as hydralazine, affects the in vitro responsiveness of human lymphocytes mostly in subjects with HLA-DR 4 phenotype. Whether the impact of MDL-899 on immune function gives rise to a lupus like syndrome is not known. For this reason further studies are warranted to assess its long-term in vivo effects.
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