Suplatast-d4 Tosylate, is a labeled analogue of Suplatast (S841000), which is an antiallergic drug, that inhibits antigen-induced histamine release from mast cells as well as IgE antibody formation. Suplatast Tosylate suppresses interleukin (IL)-4 release from T cells, however in human peripheral basophils, Suplatast Tosylate inhibited the antigen-induced release of IL-13 but not IL-4.