Phenotypic Screening Gives the Big Picture
High-content screening (as in automated microscopy with automated image analysis) was initially developed in the late 1990s for screening those targets that were difficult to come by with more traditional detection methods. However, while it enabled certain target-based screens, it directly provided additional insights beyond the simple quantified effect, such as toxicity or unexpected changes of cellular phenotype. While classical screens with a single readout only allow you to quantify the measured effect, high-content screens promise more: Additional phenotypic changes can be observed and also quantified, allowing in principle a distinction of the chemical responses into three classes: similar to untreated cells; similar to control-treated cells; or dissimilar to both.
By definition, high-content screening thus is phenotypic screening, as the cells’ phenotypes are quantified. It represents the classical biological research: Change condition and observe the outcome, albeit at a new level of scale. Also, it allows automated examination of more complex subjects than cells, such as whole organisms, tissues or even the formation of tissues and cellular interactions over time. And finally, it inherited from high-throughput screening the quality control and standardization requirements for all screening experiments: the experiment must be standardized to optimize reproducibility at defined conditions—typical for treated and untreated cells.
Besides its place in target-based and phenotypic screening, high-content screening is still the biologist’s workhorse in genetic screening: All genes of the organism under investigation (e.g. yeast) are systematically silenced and the effect on this “gene knockout” on the phenotype is observed using image-based high-content screening. Similarly, high-content screening is applied in compound combination studies to measure the effect of combined substances on model organisms.
While there is a significant intersection between phenotypic and high-content screening, there are distinctions. For instance, label-free screening by biosensors is an alternative technology, which often can detect the smallest physiological changes in organisms, such as the change of the center of mass in a cell without biochemical or genetic engineering of the cells (as would be needed to, e.g., apply an antibody-bound dye or introduce a receptor).
Nevertheless, high-content screening is the primary tool for all phenotypic screening today, and most new chemical entities have been discovered by phenotypic screening. This does not, however, preclude target-based screening from the drug discovery process. Target-based screening yields more optimized entities quicker and allows a more easy and efficient optimization of a compound toward a target. Additionally, the FDA requires the target of a new drug to be identified and proven before it can be released to the market.
All the advances in natural sciences have led to a sophisticated set of tools and instruments to segment and understand the effect of a given substance on model organisms. Successful phenotypic screening increases the mechanistic understanding of the underlying biology. With new experimental technologies, higher throughput, and advanced diagnostic tools, researchers can observe phenotypes and make a quantum leap forward in more relevant disease models.