High Content Screening and the Problem of Edge Effects

 By: Sinead Murphy
High content screening, or HCS, refers to the process in the biotechnology sector that focuses on using images of cells in drug discovery. Drug discovery is a hugely important part of the pharmaceutical industry and the constant quest to reach new solutions to medical problems and the search for new drugs.

Also known as cell based screening, high content screening has the advantage over 'high thoughput screening' in the level of detail possible. 'High throughput screening' is a faster method, but does not allow for the wealth of needed to understand drug effects.
The technology uses automated digital microscopy and flow cytometry. Combined with sophisticated IT systems, the technology is a powerful enabler of molecular research. Automation has resulted in the use of higher density well plates and the assay volumes have been reduced. There are clear cost savings to be made in such a strategy. However, this higher density approach can lead to environmental fluctuations between the wells and the plates.

Edge Effects
A common problem with high content screening lies in the area of 'edge effects'. Edge effects are caused by the differences between the cells at the edge and those at centre of the plates. This results in the deterioration of assay performance. In order to minimise the impact of these edge effects, laboratories and screening departments tend to avoid using the peripheral wells on plates. This is a costly exercise in that it avoids rather than solves the problem. For example, in a 96 Well Plate, 38% of the wells are at the edge of the plate and in a 384 Well Plate, 22% of the wells are at the edge of the plate.
Some of the edge effects issues are caused by:
- Fluctuations in temperature
- Osmolarity
- PH

Cell Based Assays
A microplate is a flat plate with multiple wells used as test tubes. Also known as a 'microtiter plate', or 'micro-titer' plate, the microplate is a typical tool used in testing laboratories and in analytical research generally. These are now available in a multitude of well densities, from 96 to 1536 wells per plate. A typical assay workflow for high content screening is performed with:
- cell seeding, Compound
- ligand dosing
- Fixation/Permeabilisation, washing, antibody and substrate addition

Then the images are acquired using high resolution digital imaging techniques which use multichannel fluorescent microscopy. The image analysis involves the selection and configuration of an image analysis algorithm. After that comes the data analysis and visualisation.

The next five years will see the use of the 1536 well plates. This allows for a greater number of tests to be carried out. This increasing miniaturisation saves time and also reduces the amount of reagents required. This ultimately is a key way to save costs. Biocroi's Advanced Microplate technology addresses the problem of low cell number and miniaturisation reduces the number of cells used per assay to the minimum required. Advanced Microplates technology images the entire well and captures the entire population of cells in a well.

The net effect will allow pharmaceutical companies to screen larger libraries with greater content, increasing the chances of finding good leads.
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