A cell-based assay can be used in basic life science research as well as in drug discovery for screening purposes. In the biomedical field, the use is typically limited to a low to medium sample throughput, whereas in drug screening, cell-based assays are often used in high-throughput procedures.
Life science research
In basic life science research, cell-based assays aim to unravel all mechanisms on which biochemical processes are based – in both the physiological and the pathological state. Furthermore, cells are used as building blocks in biomanufacturing of sophisticated cell-based constructs, mimicking the in vivo situation as closely as possible. These can then in turn be used for test purposes.
Cell-based assays are used in various stages of drug development. It is a prerequisite to test a drug candidate in defined cell-based in vitro experiments before moving on to live animals or later stage clinical trials. The central question in drug discovery naturally revolves around the biological activity of the test substances. This ‘potency’ of drug products can also be tested with a cell-based assay during drug development. Cell-based assays can be used for the initial ‘hit finding’ to identify a drug with confirmed activity against a biological target. This step of drug discovery depends on knowledge of complex biology, comprehensive screening libraries and innovative drug screening platforms.
Furthermore, an appropriately adapted cell-based assay can of course also be used for lead optimisation during drug development. In drug development, it is important to exclude general adverse reactions from the screened drugs such as a cytotoxic effect. Drug developers use cell-based assays mainly to quantify cytotoxicity and cell death caused by tested compounds. Screened drugs are cycled through a series of complementary cell-based assays in drug discovery campaigns to maximise potency and selectivity while minimising toxicity.
In addition to the evaluation of the biological activity of a drug, cell-based assays can also be used for more detailed investigations during the drug development, allowing the mechanisms of action (MOA) to be unravelled more precisely.
Finally, it is not sufficient to proof the biological activity of a drug. It must also be ensured that there are no off-target interactions. Many drugs fail in the late stage of clinical trials during drug development due to unanticipated side effects in more complex systems. A cell-based assay can’t prevent this completely, but they can help to identify problematic drugs early in drug development campaigns. Therefore, drug targets are validated in a second round of disease-relevant cell-based assays during drug discovery.
What are benefits of working with cell-based assays?
The greatest benefit of using cell-based assays is obvious: In contrast to simple biochemical assays, they better reflect the complexity of biological systems, as they do not only contain an isolated receptor or enzyme but all organelles of a cell, organised in a functional cellular unit. This complexity allows to accurately model cellular behaviour in real-life and thereby brings more physiological relevance to in vitro assays and higher reliability e.g., to the identification of potentially effective drugs. Furthermore, cell-based assays provide the opportunity to evaluate different properties of substances at the same time.
A cell-based assay can be used to measure the cellular behaviour over time (up to months) and to understand the influence of environmental factors. Advanced systems for analysing living cells can provide necessary and unique insights into dynamic life processes.
Since cell-based assays can also be miniaturized and evaluated using multiplexing, they are also increasingly used in high-throughput screening (HTS). Many compounds can be tested in parallel under different conditions, in small test volumes, thereby circumventing multi-step screening methods³.
The bottom line is, cell-based assays save a lot of resources, time and money not only in drug development. That is why their use is so popular in various fields.
Different types of cell-based assays and how they work
A cell-based assay can either be based on isolated primary cells or performed with immortalised cell lines. Cell lines do not only exhibit lower demands on cell culture conditions, but also provide more robust results due to smaller heterogeneity in the population. However, the risk that the obtained results may differ from the in vivo model is much lower when using primary cells. These are, however, not easy to get and handle4. The most common cell-based assays can be divided into the following main categories:
Cell viability assays
Cell viability describes the ability of a cell population to live and is regularly expressed as percentage. Essential cellular functions are assessed to estimate the viability of a population. Cell viability assays can be based either on metabolic activity, the cell’s reduction potential or cellular ATP production. The cell’s reduction potential is regularly measured via resazurin or tetrazolium-based assays like MTT or WST. ATP production is most frequently measured with kits using the very ATP as fuel to produce a luminescent signal (figure 2). This AppNote comprises a comparison of different viability assays in terms of lower limit of detection and user-friendliness. Using sophisticated solutions like BMG LABTECH’s Atmospheric Control Unit (ACU), long term culture and accompanying evaluation of cellular reaction in real-time are made possible as shown in this AppNote, studying cell viability and cytotoxicity over 72h.