The rifamycin derivative AF/013 is cytolytic.

When intact cells (HeLa S-3, mouse 3T3 fibroblasts, hen erythrocytes) are incubated with the rifamycin SV derivative AF/OI3, which is widely used as a potent inhibitor of mammalian DNA-dependent RNA polymerases, a rapid and extensive disintegration of the plasma membrane takes place. This cell disruption results, for example, in hemolysis of the erythrocytes at concentrations above 50 JLg/ml, in the release of cytoplasmic components, including ribosomes, and in dramatic ultrastructural changes in the HeLa and mouse 3T3 cells. In HeLa cells incubated for 30 min in 100 JLg/ml of the drug, about 70% of the RNA, 44% of the protein, and 26% of the phospholipids leak out into the medium. The permeability of the plasma membrane is markedly increased, even at relatively low AF/OI3 concentrations. In the presence of 20 JLg/ml of the drug, for example, 70% of the trichloracetic acid-soluble radioactivity and 10% of the trichloracetic acid-precipitable RNA are released from [3H]uridine-Iabeled He La cells. The membranolytic action of AF/013 is higher than that of the nonionic surfactant Triton X100. This high toxicity indicates that AF/013 cannot be used as a specific inhibitor of RNA polymerases in vivo.