What is screening?
In broad terms, ‘screening’ is used to describe a search for chemicals with specific desirable properties within a larger compound collection. In a genetic toxicology context, it can be applied to testing performed prior to the regulatory safety assessment. Screening tests help to reduce the number of chemicals of interest by providing an early insight into potential liabilities e.g. genotoxic hazard.
Primary screens typically assess the designed characteristics in a novel chemical series, and are used to select promising candidate chemistries or ‘hits’, based on efficacy, target binding, affinity or inhibition studies.
The diagram shows a generalised view of the development of new compounds across a range of industries.
Later qualification screening is often also used to concentrate on the reduced numbers of hits identified from the primary screen. These secondary screens are often designed to provide an early insight into potential liabilities within the designed chemistry, such as cytotoxicity, genotoxic hazard or even the mechanisms of genotoxic hazard. This tiered screening approach identifies ‘hits’ within the chemical series that have the highest degree of intended effect, coupled with reduced liabilities.
The use of screening in genetic toxicology is logical
- To identify low liability chemicals to carry forward for further development
- To inform chemists of the need for structural amendments or for the generation of different chemical series, to remove genotoxic liability
- To alert safety assessment teams that a more comprehensive risk assessment might be required
- To economise! Use small amount of precious chemical to find liabilities early and then act swiftly to save development costs
Types of in vitro genotoxicity screening assay
Screening assays are generally operated outside the bounds of GLP compliance, only in vitro, and function with lower test chemical requirements than safety assessment tests. Dependent upon the industrial context, assay throughput and hence format can be significant considerations. The in vitro genotoxicity screening tests can be thought of in 2 sub-groups.
- High throughput, microplate-based, pan-mechanistic assays, e.g. GreenScreen HC and BlueScreen HC
- Adapted versions of mechanism-specific regulatory assays, e.g. Ames MPF and flow cytometric in vitro MNT
- Non-GLP protocols of the OECD regulatory assays, Ames and Micronucleus tests. Providing a regulatory assay endpoint with reduced compound and time requirements.
In lower throughput screening, the tests might also be performed in the presence of an exogenous metabolising system, such as the commonly used ‘S9’ liver extracts.