Formation of the N-nitroso derivatives of six beta-adrenergic-blocking agents and their genotoxic effects in rat and human hepatocytes

Six beta-adrenergic-blocking drugs, atenolol, metoprolol, nadolol, oxprenolol, propranolol and sotalol, were found to react with sodium nitrite in HCl solution, yielding the corresponding N-nitrosamines. The genotoxic activity of the six nitrosamines was evaluated in primary cultures of both rat and human hepatocytes; DNA fragmentation was measured by the alkaline elution technique, and DNA repair synthesis by quantitative autoradiography. Positive dose-related responses were produced in cells of both species after 20 h of exposure to the following subtoxic concentrations: NO-propranolol, 0.01-0.1 mM; NO-oxprenolol, 0.03-1 mM; NO-atenolol and NO-metoprolol, 0.1-1 mM; and NO-nadolol and NO-sotalol, 0.3-3 mM. Modest but statistically significant differences between the DNA-damaging potencies for the two species were observed with NO-atenolol and NO-oxprenolol, which were both more active against rat hepatocytes, and with NO-propranolol, which was more active against human hepatocytes. At equal or higher concentrations, the six N-nitrosamines did not produce DNA fragmentation in Chinese hamster lung V79 cells; this indicates that they behave as indirectly acting compounds, which need to be transformed into reactive metabolites in order to exert a genotoxic effect.