RD3-0028, a compound with a benzodithiin structure, was found to be a potent inhibitor of respiratory syncytial virus (RSV) replication. Its action is specific; no activity is seen against influenza A virus, measles virus, herpes simplex virus type 1 or 2, or human cytomegalovirus. A time-dependent
drug addition experiment indicated that the
antiviral activity occurs in the late stage of the RSV replication cycle, since this compound completely inhibited syncytium formation even when added up to 16 hr after the
infection of cell monolayers at an MOI of 3.
RD3-0028 had no direct virucidal effect on RSV. Western blotting analysis showed that
RD3-0028 significantly decreased the amount of RSV
proteins released into the cell culture medium. Moreover, five independent isolates of the RSV long strain were selected for growth in
RD3-0028 (5-20 microg/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 segment contained at least one sequence alteration, converting
asparagine 276 to
tyrosine (F1
protein). These results suggest that
RD3-0028 inhibits RSV replication by interfering with intracellular processing of the RSV fusion
protein, or a step immediately thereafter, leading to loss of infectivity.