Pharmaceutical companies need to test the efficacy and safety of drugs and therefore must deal with complex cause-effect relationships while screening for cytotoxicity. The complexity highlights the need for "crossover" studies that involve cytotoxicity, genotoxicity and pharmacokinetics. There are a number of global screening techniques available that cover most mechanisms of cytotoxicity and cell death. Measuring altered cell permeability, discussed below, is just one of these.
Cytotoxicity assays are used to quantify cell proliferation in response to growth factors, cytokines, mitogens, or nutrients, and to analyse cytotoxic compounds such as anticancer drugs. Most of these assays measure cell viability by plasma membrane permeability. Traditionally the release of radio-isotopic labels (3H or 51Cr) was measured, but these costly and hazardous manipulations are becoming less popular with scientists. Trypan blue exclusion has also been widely used, but the technique is unsuitable for large numbers of samples, and involves meticulous manual cell counting.
Newer permeability-based cytotoxicity methods measure the release of cytoplasmic enzymes. While glutamate-dependent transaminase assays are cumbersome and the enzymes themselves not very abundant, lactate dehydrogenase (LDH)-based assays have proven useful for measuring both cell-mediated and antibody-dependent cellular cytotoxicity. LDH release has been shown to correlate very well with 51Cr release and trypan blue staining.
Another family of assays focuses on biological functions inside intact cells as an alternative to permeability testing. Viable cells reduce tetrazolium salts to coloured formazan compounds, mainly using extramitochondrial dehydrogenases.
There are other laboratory techniques for measuring cytotoxicity, including:
· Cytotoxicity assays that rely on the ability of cells to make ATP
· The comet, or single-cell gel electrophoresis (SCGE), assay - designed for genotoxicity screening, but considered one of the most interesting methods for comprehensive cytotoxicity studies
· Assays that focus on tissue or cell-type specific cytotoxicity studies
These techniques are discussed in the Profile section of The Scientist. Suppliers of the assays are also provided on this page.