Assay treatment duration in cell panel screens can be an important factor in drug response profiling of slow acting therapeutics

Cell panel screening assays are an essential part of the drug discovery toolbox. However, fully resolving the drug response profiles of slow-acting therapeutics, such as those that target epigenetic pathways, remains challenging with conventional short term assay formats. Elucidation of the effects of these drug candidates requires much greater optimization of longer-term assay conditions.


High-throughput cell panel screening is used in drug discovery to rapidly test the effects of large numbers of potential therapeutic agents on different types of cancer cell lines. It allows researchers to efficiently and effectively screen for conditions that may have a specific effect on cells, such as proliferation or growth inhibition, and can be informative in identifying sensitivity and resistance mechanisms. These screens are used to aid selection of efficacious drugs for treatment of a specific disease, stratify patients for clinical trials, re-purpose drugs with clinically acceptable profiles and provide data to inform mechanism of action (MoA) studies.

Cell Panel Screening and Epigenetics

Cell panel screening assays routinely test compounds (and other modalities) for up to 6-days, and for many agents this time course is sufficient to obtain the required drug response data. However, the conventional 3-6-day assay window may not offer the resolution needed to identify or resolve the full cellular response for slower acting drugs such as those directed against epigenetic pathways.

There is growing evidence that epigenetic changes play a role in the development and progression of cancer. For example, changes in DNA methylation can silence the expression of genes that suppress tumour growth, leading to the development of cancer. Similarly, changes in histone modification can alter the accessibility of the genome and affect gene expression, potentially contributing to cancer development. There is also evidence that epigenetic changes may contribute to the resistance of cancer cells to chemotherapy and other forms of treatment, which can make it more difficult to effectively treat cancer. Therefore, epigenetics offers the opportunity to identify new therapeutic targets and is now considered to be a key area for cancer research. Consequently, having the right tools to identify the effects of potential therapeutic agents that target epigenetic pathways is becoming much more important.

Horizon’s Long-Term Assay

In order to analyze the sensitivity of cell lines to slow acting therapeutics (e.g. epigenetic drugs) over an extended time frame, Horizon has developed a long-term assay. The long-term assay is a cell panel screen that has been designed to run for 10-days to study the prolonged effects of a particular treatment on different cancer cell lines. As with any assay, high cell quality and a robust and standardized cell seeding process are essential for obtaining consistent and reproducible experimental results, but these are particularly critical in a long-term assay. Long-term assays require much greater optimization to provide the resolution needed over an extended cell culture timeframe. For example, during the 10-day period, due to the optimization of reduced seeding densities, it is not necessary for the cells to be split or have their media replenished, reducing experimental interference. Additionally, to be effective, long-term assays must be carefully designed and controlled to minimize the potential for variability associated with the use of these very low seeding densities and ensure reliable results. The data obtained from a long-term assay can provide important insights into the mechanisms of action of the drug candidate and help to identify potential therapeutic targets for the treatment of cancer. However, it is currently unclear if there are specific families of epigenetic enzymes or classes of compounds which would benefit from an extended assay and hence some decision making may be needed when deciding whether to screen with a conventional 6-day assay, or beyond at a longer timepoint.


Horizon’s OncoSignature™ 2D Long-Term Assay is part of a suite of cell-based assays available to support the drug discovery pipeline in combination with cell line engineering, functional genomics and immuno-oncology.