A stepwise tiered approach is applied in

A stepwise tiered approach is applied in regulatory mutagenicity testing [16]. In the first step, in vitro assays with a high sensitivity are used to identify test compounds that amiodarone hcl have high intrinsic genotoxic activity. In the second step, specific in vivo tests are performed to determine the relevance of the in vitro results for the in vivo situation. These in vivo mutagenicity studies are also included because some genotoxicants can only be detected in vivo after metabolic activation [17]. Compared to regulatory carcinogenicity testing, mutagenicity testing is relatively cheap and fast. Compounds without genotoxic liability can proceed first into clinical trials in humans. The carcinogenic potential is assessed later in the full developmental phase of drug development; however, the regulatory test strategy consists of a battery of core and ancillary tests for identifying the three forms of mutagenicity (i.e., gene mutations, clastogenicity, and aneugenicity), which cannot be detected in one single test.
Genotoxicity tests are used to detect genetic damage by various mechanisms in in vitro and in vivo systems. Several regulatory guidelines have been developed to provide various assays that are conducted for testing the genotoxicity. To date, most regulatory agencies and international authorities recommend a test scheme consisting of in vitro and in vivo methods to detect genotoxicity/mutagenicity induced by substances. The ICH recommends a standard battery test for pharmaceuticals to detect their genotoxicity. The ICH guidance optimizes the standard battery test for genetic toxicology and provides guidelines on the interpretation of results (Table 6) [15]. These guidelines help improve risk characterization for carcinogenic effects. In the following sections, some regulatory agencies or organizations are briefly described with regard to their own guidelines.
The Ames assay has a relatively high specificity, compared to other in vitro mutagenicity tests (Table 7). The sensitivity, specificity, and predictivity of the Ames assay calculated by Kirkland et al [18] were 58.8%, 73.9%, and 62.5%, respectively.
The chromosome aberration test is performed in vitro in cultured mammalian cells. It is also performed in the presence and in the absence of the S9 mixture [19,20]. Scoring the test requires specialized training and experience. The sensitivity and predictivity of carcinogenicity for this test are 65.6% and 59.8%, respectively. The specificity of this test is low (44.9%).
The fourth regulatory mutagenicity assay is the MN test. The sensitivity, specificity, and predictivity of the in vitro MN assay are 78.7%, 30.8%, and 67.8%, respectively. The specificity of the in vivo MN assay in bone marrow is much higher (75%). The sensitivity of the in vivo test is lower (40%) and the predictivity is 48% [21,22].
The toxicological relevance of the MN test is well defined: it is a multitarget genotoxic endpoint; it assesses clastogenic and aneugenic events; and it assesses some epigenetic effects, which is simple to score, accurate, and applicable in different cell types. In addition, it is predictive for cancer, amenable for automation, and allows good extrapolation for potential limits of exposure or thresholds. It is easily measured in experimental in vitro and in vivo systems. Implementation of in vitro micronucleus (IVMN) assays in the battery of tests for hazard and risk assessment of potential mutagens/carcinogens is therefore fully justified. The final draft of the OECD guideline is available for this test [20].
The presence of MN in cultured human cells was reported as early as the 1960s [21] and 1970s [22]. The in vitro micronucleus test (IVMNT) has evolved into a robust quantitative assay of chromosome damage by the development of the cytokinesis-block technique that eliminated the confounding effects on MN expression by the cytostatic effects caused by poor culture conditions, treatment effects, cell senescence, and variability in mitogen response in the lymphocyte test system [23].