Cytogenetic effects of chloroquine in human lymphocyte cultures

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Effects of low dose radiation and vitamin C treatment on chloroquine-induced genotoxicity in mice

Chloroquine (CHQ) is a commonly used antimalarial agent. We evaluated the genotoxic potential of CHQ using chromosome aberration (CA), micronucleus (MN), and sperm head abnormality (SA) assays in vivo in Swiss albino mice. The interaction between a low dose of radiation and CHQ, as well as the effect of vitamin C on CHQ-induced genotoxicity, was also evaluated. It was observed that CHQ induced CA, as well as MN, in the bone marrow cells under certain treatment conditions. Further, CHQ induced significant increase in the frequency of SA both at 24 hr and 21 days of the treatment. In the present study vitamin C pretreatment apparently reduced the frequency of CA, MN, and SA induced by CHQ. In the combination studies with radiation and CHQ, we found that exposure to low doses of radiation (0.5 Gy) either prior to or following CHQ treatment, in the dose ranges tested, has little or no synergistic effect in the mutagenic evaluations in somatic cells. However, radiation exposure along with CHQ treatment resulted in significant increase in the frequency of SA as compared to the groups receiving CHQ alone at 21 days of the treatment. In summary, CHQ has the potential to induce genotoxicity in mammalian cells. Further, germ cells may be relatively more sensitive as compared to the somatic cells.

DNA intercalative potential of marketed drugs testing positive in in vitro cytogenetics assays

We have previously noted that the Physicians' Desk Reference (PDR) contains over 80 instances in which a drug elicited a positive genotoxic response in one or more in vitro assays, despite having no obvious structural features predictive of covalent drug/DNA interactive potential or known mechanistic basis. Furthermore, in most cases, these drugs were "missed" by computational genotoxicity-predicting models such as DEREK, MCASE and TOPKAT. We have previously reported the application of a V79 cell-based model and a 3D DNA docking model for predicting non-covalent chemical/DNA interactions. Those studies suggested that molecules that are very widely structurally diverse may be capable of intercalating into DNA. To determine whether such non-covalent drug/DNA interactions might be involved in unexpected drug genotoxicity, we evaluated, using both models where possible, 56 marketed pharmaceuticals, 40 of which were reported as being clastogenic in in vitro cytogenetics assays (chromosome aberrations/mouse lymphoma assay). As seen before, the two approaches showed good concordance (62%) and 26 of the 40 (65%) drugs exhibiting in vitro clastogenicity were predicted as intercalators by one or both methods. This finding provides support for the hypothesis that non-covalent DNA interaction may be a common mechanism of clastogenicity for many drugs having no obvious structural alerts for covalent DNA interaction.