Observations on a proposed measure of genotoxicity in rat gastric mucosa

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Current methods for the study of the toxicological effects of antisecretory medications on the gastric mucosa possess disadvantages or limitations. A novel assay has been proposed to assess gastric mucosal genotoxicity in which the proton-pump inhibitor omeprazole has been reported to induce direct damage to cellular DNA, raising questions about the safety of this drug. To define the applicability of this proposed measure of genotoxicity and to examine the effects of omeprazole in this assay, control agents, known carcinogens, and omeprazole in various doses and formulations were administered to rats by gavage, followed by [3H]thymidine labeling of DNA in vivo approximately 14 hours later. The incorporation of the [3H]thymidine label into DNA of gastric mucosal cells liberated by limited pronase digestion was in close agreement with published results for negative and positive controls. Omeprazole, administered in doses ranging from 10 mg/kg to 300 mg/kg, did not increase [3H]thymidine incorporation into cellular DNA in this assay. The gastric carcinogen 1-methyl-2-nitro-1-nitrosoguanidine at 20 and 50 mg/kg increased [3H]thymidine incorporation. Pretreatment in vivo with hydroxyurea before [3H]thymidine labeling to inhibit replicative DNA synthesis suppressed [3H]thymidine incorporation more than 97% in negative controls and MNNG and more than 93% in omeprazole treatments. This indicates that replicative DNA synthesis was almost totally responsible for the [3H]thymidine incorporation and the contribution of unscheduled DNA synthesis to the total [3H]thymidine incorporation is minor. Flow cytometric analysis of the cell cycle of the gastric mucosal cells liberated by the limited pronase digestion indicated significant contamination of the preparation with dividing cells (4% in negative controls and 14% in MNNG-treated positive controls). These findings indicate that the proposed screening assay for genotoxicity in rat gastric mucosa is not a reliable measure of unscheduled DNA synthesis in its present form, and conclusions about genotoxic effects of any drug using this assay as initially proposed appear questionable.