Relationship between cytostatic activity of oxazolopyridocarbazoles and accessibility of DNA intercalation sites in living bacteria.

The ability of oxazolopyridocarbazole (OPC) derivatives to interact with DNA in living bacteria through reversible intercalation has been determined by using as probes their selective mutagenic effect on Salmonella typhimurium TA 1977 and TA 1537 as detected by frame-shift-1 reversion, the absence of intervention of the error-prone repair system on the mutagenic efficiency, the absence of induction of the SOS functions, and the absence of effect of recA and uvrB mutations on their bacteriostatic properties. Involvement of simple reversible intercalation as the event responsible for the bacteriostatic effect of the drugs has been further investigated by the establishment of a significant correlation between the maximum number of accessible intercalating sites in living bacteria and the bacteriostatic effect expressed in terms of the ED50. This correlation has been established by using bacteria spontaneously exhibiting different sensitivities toward the drugs as well as a resistant strain obtained by adaptation in the presence of increasing amounts of isopropyl-OPC. The number of intercalating sites in living bacteria was determined by using the change in the fluorescence properties of the drugs upon binding to intercalating sites. The results obtained clearly demonstrate that the number of intercalating sites is the parameter that controls the bacteriostatic effect of the drugs, indicating that DNA is the target for these drugs and that reversible intercalation is responsible for the cytostatic effect.