Preclinical toxicology studies with azithromycin: genetic toxicology evaluation.

Azithromycin was subjected to a series of three in vitro and one in vivo genetic toxicology assays for the detection of drug-associated gene or chromosomal effects. In the Ames Salmonella typhimurium tester strains TA1535, TA1537, TA98 and TA100, the presence of azithromycin was not associated with any increase in the number of his- revertants. Urine from mice dosed with up to 200 mg/kg of azithromycin also had no effect on the number of revertants in these same strains suggesting the absence of mutagenic excretory products following oral exposure. When tested up to the cytotoxic level of 240 micrograms/ml, azithromycin caused no increase in the mutant frequency at the thymidine kinase locus of L5178Y/TK cells. Both the mammalian and microbial gene mutation assays included the presence of rat-liver postmitochondrial (S9) fraction for the detection of mutagenic biotransformation products. Mitogen-stimulated human lymphocytes cultured in the presence of 2.5-7.5 micrograms/ml azithromycin for 24 h or 30.0-40.0 micrograms/ml azithromycin for 3 h in the presence of rat S9 had chromosomal aberration frequencies that were no different than negative control cells even though slight to moderate mitotic suppression was associated with these concentrations. In vivo assessment of this compound was completed in male and female mice with a single oral dose of 200 mg/kg followed by sacrifice at 6, 24 or 48 h later and metaphase analysis of bone marrow for chromosomal aberrations. No statistically significant elevations of chromosomally aberrant cells were found. We conclude that azithromycin does not cause gene mutations in microbial or mammalian cells, or chromosomal aberrations in cultured human lymphocytes or in mouse bone marrow in vivo.

Genetic profile of a nalidixic acid analog: a model for the mechanism of sister chromatid exchange induction.

In recent years, evidence has accumulated that suggests that mammalian topoisomerase may play a role in the formation of spontaneous or chemically induced sister chromatid exchange (SCE). In microbial systems, nalidixic acid is known to disrupt the function of a topoisomerase-like enzyme, DNA gyrase. To explore the possible relationship to topoisomerase function and SCE formation in mammalian cells, an analog of nalidixic acid with potent topoisomerase II inhibitory activity was selected for examination in a variety of genetic toxicology assays. This analog, CP-67,015, proved to be a positive direct-acting mutagen in the L5178Y/TK+/-, CHO/HGPRT, and V79/HGPRT systems. However, no gene mutational activity was observed using the Ames test in direct plate, mouse and rat metabolic activation, and mouse urine tests. In vitro cytogenetic studies showed strong clastogenic activity in human lymphocytes and in CHO cells. Compound-induced chromosome damage was also observed in vivo in mouse bone marrow cells. Surprisingly, SCE studies in vitro in human lymphocytes or CHO cells showed only slight increases, even at levels producing severe chromosome breakage. Mouse bone marrow showed no significant elevation of SCE following parenteral treatment with CP-67,015. These results, taken together, demonstrate that CP-67,015 is a direct-acting mutagen in mammalian cells with both gene and chromosomal level effects. The relative ineffectiveness in producing SCEs suggests that CP-67,015 may interfere with a DNA replicative/repair process, perhaps by alteration of one or more DNA polymerase activities. This suggestion is based in part on the known effect of the analog nalidixic acid on DNA gyrase in microbial cells and on topoisomerase in mammalian cells. The profile of genetic activity of CP-67,015, coupled with its inhibitory effect on topoisomerase function, gives rise to a model for SCE formation that is based on anomalies of topoisomerase activity during DNA synthesis.