Methods of DNA adduct determination and their application to testing compounds for genotoxicity.
At the International Workshop on Genotoxicity Test Procedures (IWGTP) held in Washington, DC (March 25-26, 1999), a working group considered the uses of DNA adduct determination methods for testing compounds for genotoxicity. When a drug or chemical displays an unusual or inconsistent combination of positive and negative results in in vitro and in vivo genotoxicity assays and/or in carcinogenicity experiments, investigations into whether or not DNA adducts are formed may be helpful in assessing whether or not the test compound is a genotoxin. DNA adduct determinations can be carried out using radiolabeled compounds and measuring radioactive decay (scintillation counting) or isotope ratios (accelerator mass spectrometry) in the isolated DNA. With unlabeled compounds adducts may be measured by (32)P-postlabeling analysis of the DNA, or by physicochemical methods including mass spectrometry, fluorescence spectroscopy, or electrochemical detection, or by immunochemical methods. Each of these approaches has different strengths and limitations, influenced by sensitivity, cost, time, and interpretation of results. The design of DNA binding studies needs to be on a case-by-case basis, depending on the compound's profile of activity. DNA purity becomes increasingly important the more sensitive, and less chemically specific, the assay. While there may be adduct levels at which there is no observable biological effect, there are at present insufficient data on which to set a threshold level for biological significance.