Mechanism of selective inhibition of respiratory syncytial virus by a benzodithiin compound (RD3-0028)

RD3-0028, a compound with a benzodithiin structure, was found to be a poten t inhibitor of respiratory syncytial virus (RSV) replication. Its action is specific; no activity is seen against influenza A virus, measles virus, he rpes simplex virus type 1 or 2, or human cytomegalovirus, A time-dependent drug addition experiment indicated that the antiviral activity occurs in th e late stage of the RSV replication cycle, since this compound completely i nhibited syncytium formation even when added up to 16 hr after the infectio n of cell monolayers at an MOI of 3, RD3-0028 had no direct virucidal effec t on RSV, Western blotting analysis showed that RD3-0028 significantly decr eased the amount of RSV proteins released into the cell culture medium. Mor eover, five independent isolates of the RSV long strain were selected for g rowth in RD3-0028 (5-20 mug/ml). These resistant viruses were more than 80- fold less sensitive to RD3-0028 than the long strain. The F gene segment of each of these viruses was sequenced and in each case the mutant RNA segmen t contained at least one sequence alteration, converting asparagine 276 to tyrosine (F1 protein). These results suggest that RD3-0028 inhibits RSV rep lication by interfering with intracellular processing of the RSV fusion pro tein, or a step immediately thereafter, leading to loss of infectivity.