Summary of Major Conclusions From the 8th International Workshop on Genotoxicity Testing ( <scp>IWGT</scp> ), Ottawa, Canada

International Workshops on Genotoxicity Testing (IWGT) use a round-table workshop format that convenes about every 4 years. The workshops are usually held as a satellite meeting to the International Conference on Environmental Mutagens (ICEM), which are generally held on different continents each time. The purpose of IWGT is to convene expert working groups (WGs) on current topics of interest in the genotoxicity field, and to publish data or evidence-based recommendations that reflect the state of the science. The focus is on technologies and approaches relevant for regulatory testing but may include topics beyond immediate practical relevance. Each WG, where possible, includes members from academia, industry, and regulatory agencies from the Americas, Europe, and Asia (Martus et al. 2023). Each WG has a leadership team consisting of a chair, co-chair, and rapporteur that lead and manage the discussions. Since the IWGT process is data-driven, a major part of the effort lies in an extensive preparatory phase of each of the discussions. This is needed to collect relevant data and information and prepare for the face-to-face discussion, which is the culmination point of the exercise. The aim is to discuss the pre-agreed questions that need to be addressed and try to reach agreement on consensus statements. Reaching consensus requires flexibility, and it is to the credit of all WG members at all workshops that they were willing to achieve compromise. When consensus cannot be reached, an anonymous minority view may be included in the report, with, if possible, recommendations on what additional data might be required to be able to reach consensus in the future. The discussions at the face-to-face workshop meeting are public, which means that an audience is present. If feasible, input from the audience can be obtained and implemented, but the general view is that the statements are developed by the WGs. As customary, topics for the Ottawa meeting, held from August 23 to 26, 2022, were selected by the IWGT Steering Committee based on a process of gathering priority topics from within the scientific community of genetic toxicologists. In Ottawa, five WGs were convened, complemented by two plenary sessions where presentations of invited speakers to the whole audience were followed by a discussion. Since some of the topics addressed were considered too extensive to be reasonably discussed in a single publication, some WGs decided to split up the consensus recommendations into several papers. A description of the groups and their detailed recommendations is provided in the individual WG reports in this Special Issue, whereas summaries of their major conclusions are given below. In the first of the WGs, it was explored how much the outcome of a typical combination of in vitro tests, which form the core of a standard genotoxicity testing battery in various regulations, can be analyzed and assessed for the contribution of each test to the overall call. The work employed advanced mathematical methods, notably Bayesian algorithms. The WG focused on the bacterial reverse mutation (Ames) test, the mammalian in vitro gene mutation (Hprt or mouse lymphoma Tk [MLA]) test, and a test for mammalian cytogenetic damage (chromosome aberration or micronucleus test [MNT]). In the second WG, the status of using transcriptomic biomarkers to predict genotoxicity was analyzed. Due to the complexity of the topic, the WG decided to separate the publication into three sub-papers, preceded by an overview publication summarizing the topic (Froetschl et al. 2025). The sub-publications focused on transcriptomic biomarkers obtained in human cell lines in vitro (Li et al. 2025), data obtained from in vivo studies (Corton et al. 2025), and statistical considerations for using such biomarkers (Meier et al. 2025). Another WG continued discussions initiated in a previous IWGT on in vivo genotoxicity testing strategies (Beevers et al. 2025). This time, the WG focused on the acceptable maximum dose for a negative erythrocyte MNT, the validation and implementation status of MNTs in non-hematopoietic tissues, and the impact of nuisance factors on the alkaline single cell electrophoresis (comet) assay. The next WG discussed the status of the use of quantitative methods to interpret genotoxicity test results. Due to the complexity of this topic, the recommendations are presented by four sub-papers elaborating on specific sub-aspects of using quantitative methods to analyse and interpret genotoxicity data. One publication addressed the use of the benchmark dose (BMD) approach for effective analysis of continuous genotoxicity dose–response data, and determination of point-of-departure (PoD) values that can be used for potency ranking and risk assessment (Haber et al. in press). A second publication discussed the determination of appropriate critical effect size (CES) values for interpretation of in vivo genotoxicity dose–response data and appropriate use of uncertainty factor (UF) values for calculation of human exposure limit values (e.g., HBGV or Health-based Guidance Values) (Pfuhler et al. in press). A third publication addressed data derived from in vitro tests (Beal et al. 2025), reviewing the approaches and computational methods employed for analysis of in vitro dose–response data, and interpretation of PoD metrics in a risk assessment context. A fourth publication focused on effect severity (Parsons et al. 2025), proposing adjustment factor values that could be employed when interpreting in vivo genotoxicity dose–response data for human health risk assessment and regulatory decision-making. In the last WG, the use of historical negative control (HNC) data for the interpretation of genotoxicity results was examined. The WG focused on acquisition and maintenance of historical control databases for various in vivo study types, and factors that might influence their use. Finally, this IWGT meeting included two plenary sessions where invited speakers provided updates on specific aspects of each topic (see below). Unlike the regular WGs, these two groups did not derive consensus statements. Instead, their discussions and overarching insights were summarized in their publications. In the first of those plenary sessions, the use of epigenetic information for genotoxicity assessment was discussed (Godschalk et al. 2024). This session included a talk on recommended methods for obtaining epigenetic data and their strategic use alongside conventional genotoxicity data. In addition to variations in the DNA sequence or chromosome structures, it was explored whether epigenetic data could enhance risk assessment, particularly when considering non-genotoxic carcinogenesis. Other topics included the chemical mechanisms involved in inducing epigenetic modifications and the correlation of these changes with developmental The second plenary session addressed genotoxicity with gene where conventional test developed for are et al. 2024). included the use of and and the of relevant regulatory Genotoxicity testing for is using of standard Due to the of DNA and in vitro tests a correlation the mechanisms of damage and the of human risk derived from such tests based on for some of the mechanisms (e.g., gene A combination of tests is of the of mechanisms to and the that single is able to all of those in vivo tests are used for and to derive a of for from the appropriate or of but the to use means a focus on the of in vitro data. a combination of a bacterial reverse mutation (Ames) test and a in vitro for cytogenetic damage has to be a of and in vivo genotoxicity et al. The WG in vitro databases of bacterial reverse mammalian gene mutation (Hprt and and chromosome damage (chromosome aberration and micronucleus for overall that were or and that were considered for a of The results were into bacterial mammalian and chromosome damage and data on all three In vivo data to the of those in vitro test results were obtained from chromosome and et al. 2025). that gene changes that predict specific mechanisms of or are used in some of particularly when information is In a of to such these not This IWGT WG focused on relevant transcriptomic biomarkers for genotoxicity testing that a status of validation that their use in a of approach to (Froetschl et al. 2025). Due to the complexity of the it was decided to focused on in vitro (Li et al. or in vivo (Corton et al. and an additional to the in relevant for the and use of the transcriptomic biomarkers (Meier et al. 2025). a approach and the WG a that as for to the This approach reports with information considered relevant in this context. 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