Biochemical studies with a new cytotoxic immunosuppressive agent, 3-acetyl-5-(4-fluorobenzylidene)-2,5-dihydro-4-hydroxy-2-oxothiophen (I.C.I. 47776).

1. A new cytotoxic agent, 3-acetyl-5-(4-fluorobenzylidene)-2,5-dihydro-4-hydroxy-2-oxothiophen (I.C.I. 47776), strongly inhibits protein and nucleic acid synthesis and, to a smaller extent, respiration in lymph-node cells and Landschütz ascites-tumour cells in vitro. 2. The activity of I.C.I. 47776 in vitro declines as the pH of the medium is increased and is inversely proportional to the concentration of serum in the medium. 3. The compound has no effect on the incorporation of leucine by a cell-free preparation from Landschütz ascites cells containing ATP and phosphoenolpyruvate. 4. I.C.I. 47776 stimulates glycolysis in suspensions of Landschütz ascites cells in the presence of excess of glucose but has no effect on glycolysis in suspensions of rat lymph-node cells. 5. I.C.I. 47776 markedly depresses ATP concentration in ascites cells in the absence of glucose but has no effect on the ATP concentration in the presence of glucose. The inhibition of protein synthesis by I.C.I. 47776 in ascites cells is, however, only partially reversed by the addition of glucose. 6. The ATP concentration of rat lymph-node cells incubated with I.C.I. 47776 in the absence of glucose is also markedly depressed but the addition of glucose increases the ATP concentration only slightly. Further, glucose has no effect on the inhibition of protein synthesis in lymph-node cells by I.C.I. 47776. 7. It is suggested that I.C.I. 47776 inhibits protein and nucleic acid synthesis in cell suspensions indirectly by acting as a mitochondrial poison. 8. The relevance of studies on the activity of I.C.I. 47776 in vitro to its cytotoxic and immunosuppressive action in vivo is discussed.