Basis for the selective antibacterial activity in vitro of proton pump inhibitors against Helicobacter spp.

Abstract

Proton pump inhibitors of the benzimidazole type exert a specific antibacterial activity against Helicobacter pylori in vitro. In the present study, the basis for this selectivity was investigated, and in particular, various factors affecting the in vitro antibacterial activity of sulfide analogs of benzimidazoles were studied. Upon preincubation of omeprazole for a period of up to 72 h in a buffer at pH 7, a product was formed that was bactericidal for H. pylori but had no effect on urease activity. Sulfide constitutes the main end product of degradation. The sulfide analog of omeprazole (H 168/22) exerted a bactericidal activity specifically against both resting (in buffer) and growing (in broth) Helicobacter spp., and time-kill in buffer at pH 5 was enhanced compared to that at pH 7. There was no or very low covalent binding of 3H-labeled H 168/22 to Helicobacter spp. or to other gram-negative and gram-positive bacteria. In the presence of fetal calf serum (FCS) under the same conditions, binding was only slightly lowered while the killing activity was markedly reduced, indicating a probably nonspecific interaction with proteins and/or protection of bacterial target(s) by FCS. Addition of H 168/22 (four times the minimum bactericidal concentration [MBC]) to exponentially growing H. pylori immediately stopped growth, and after an incubation period of 20 h viable counts were reduced by >7 log10. One-hour exposure of H. pylori to the drug followed by repeated washing retarded growth by about 2 h, indicating that the effect is reversible after short-term exposure. MICs and MBCs of various sulfide structures were lower than those obtained in broth after the addition of the corresponding sulfoxide. Thus, the MBC of the sulfide structure of omeprazole against 140 clinical isolates of H. pylori ranged from 8 to 32 microg/ml, compared to an MBC of omeprazole of 32 to 128 microg/ml. A similar potency was also recorded against other helicobacters. In conclusion, formation of sulfides of benzimidazoles in culture media is the reason for the selective antibacterial effect against H. pylori. The sulfides rapidly exerted a reversible antibacterial activity, which was specific against both resting and growing Helicobacter spp. without any covalent protein binding.