Immunosuppressive agent; blocks phagocytosis, cytokine production and proliferation of T and B cells. Non-competitively inhibits chymotrypsin-like activity of 20S proteasome; prevents degradation of IκBα, an endogenous blocker of NF-κB. Also inhibits farnesyltransferase and geranylgeranyltransferase I (IC50 values are 80 and 17 μM respectively) and displays antitumor activity against breast cancer in vivo.