Experiences with the in vivo and in vitro comet assay in regulatory testing

The in vivo comet assay has recently been implemented into regulatory genotoxicity testing of pharmaceuticals with inclusion into the ICH S2R1 guidance. Regulatory genotoxicity testing aims to detect DNA alterations in form of gene mutations, larger scale chromosomal damage and recombination and aneuploidy. The ICH S2R1 guideline offers two options of standard batteries of tests for the detection of these endpoints. Both options start with an AMES assay and option 1 includes an in vitro mammalian cell assay and an in vivo micronucleus assay in rodent, whereas option 2 includes an in vivo micronucleus assay in bone marrow in rodent and a second in vivo assay in a second tissue with a second endpoint. The test recommended as second in vivo test is the comet assay in rat liver. The in vivo comet assay is considered as mature enough to ensure reliable detection of relevant in vivo genotoxicants in combination with the micronucleus test in bone marrow and the AMES assay. Although lots of research papers have been published using the in vitro comet assay, the in vitro version has not been implemented into official regulatory testing guidelines. A survey of the years 1999-2014 revealed 27 in vivo comet assays submitted to BfArM with market authorisation procedures, European and national advice procedures and clinical trial applications. In three procedures, in vitro comet assays had been submitted within the genetic toxicology packages.