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Assay Development: An Overview

Illustration of a molecule seen through a magnifying glass.
Credit: Technology Networks
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The purpose of assay development and screening in drug discovery

Assays are investigative procedures that qualitatively assess a compound or examine a compound’s effects on identified molecular, cellular, or biochemical targets. The first steps in drug development are the identification and validation of potential drug targets involved in human disease. These targets are typically either a cellular structure or specific protein. Targets include:

  •          Receptors
  •          Enzymes
  •          Hormones and factors
  •          Nuclear receptors
  •          DNA
  •          Ion channels

In the following stage of drug development, biological assays and compound screening assays are created. These assays are used to identify compounds that have a desired activity at the drug target. These compounds are referred to as “hit” molecules. During the initial phase of hit compound identification, termed high throughput screening (HTS), a compound library that contains many potential hit molecules is tested to identify any compounds with the desired activity towards the target. Further assays are required to retest the hit molecule’s activity at the target. Finally, cell-based assays are used to examine a drug’s toxicity, safety profile, and efficacy. Every drug that is developed undergoes a unique series of assays that are specifically designed and organized for the drug target and compound in question. This process is termed assay development.

Important factors in assay development

Developing top-notch assays is pivotal to drug discovery and development. The better the developed assays, the fewer potential problems in subsequent stages of the research and development process. Proper assay development requires carefully considering multiple factors, including relevance, reproducibility, quality, interference and cost.


Importance in Assay Development


Research should be conducted to examine the ability of the assay to:

1.       Predict the specific disease state.

2.       Identify compounds that exhibit an appropriate mechanism of action and strength.


In a compound screening environment, an assay must be reproducible. This means that feasible reproducibility exists across assay plates, screen days and the full duration of the specific drug discovery program.


Z’-factor is calculated to measure the power or quality of an HTS assay. The signal window and variance of negative and positive signals is used in this calculation. The Z’-factor must be >0.4 to be considered acceptable for use, with some researchers preferring a Z’-factor of >0.6.


Assays must be designed that consider the effects of compounds found in the assay, such as the solvents used.


Measures should be taken to reasonably limit assay cost.

Assay types and technologies

Assays in drug discovery fall into two main categories: biochemical assays and cell-based assays. Biochemical assays are often the first type of assay used. Biochemical assays are valuable for evaluating and examining the target protein and identifying the compounds that possess the desired activity at the target. These assays:

  •         Can be applied to enzyme or receptor targets
  •         Are consistent and reliable
  •         Are simpler than cell-based assays

Cell-based assays elucidate compound activity via a functional read-out of said activity. Cell-based assays often follow biochemical assays. These assays:

  •         Can be applied to ion channels, nuclear receptors or membrane receptors
  •         Report on the toxicity, efficacy and other properties of the hit compound
  •         Are more complex than biochemical assays

The following table lists some common examples of biochemical and cell-based assays.

Biochemical Assays

Cell-based Assays

ADP Hunter Assay for Kinase Activity

Cell Proliferation Assays

Amplified Luminescent Proximity Homogenous Assay (AlphaScreen Technology) for Protein-Protein Interaction

Viability Assays

Assay for Protease Cleavage Activity

Migration Assays

Reporter Gene Assays

G Protein-Coupled Receptor Assays