The IC50 is a widely used quantitative measure in pharmacology and biochemical research that indicates the concentration of a drug, inhibitor, or other test substance required to reduce a specified biological response by 50 percent. In practical terms, an IC50 describes how much of a compound is needed to cut activity — for example enzyme catalysis, receptor signaling, cell proliferation, or microbial growth — to half of its maximal level under the conditions of the assay. IC50 values are typically reported in molar concentration units such as nanomolar (nM) or micromolar (µM).

How IC50 is obtained and what it represents

Researchers determine IC50 by measuring the response of a biological system across a range of concentrations of the test compound and fitting a dose–response curve. Common fits include the four-parameter logistic model; the fitted curve yields the concentration at which response is reduced to 50% of the control. The shape of the curve is often described by a Hill slope, which reflects cooperative or noncooperative interactions. Because the IC50 depends on experimental details — such as enzyme or receptor expression level, incubation time, substrate concentration, and assay readout — it is an operational parameter rather than a fixed intrinsic constant of a molecule.

Investigators sometimes convert IC50 to pIC50 by taking the negative base-10 logarithm of the IC50 expressed in molar units; higher pIC50 values indicate greater potency on a logarithmic scale. For competitive inhibitors in enzyme assays, the IC50 can be related to the inhibitor dissociation constant (Ki) by equations (for example, the Cheng–Prusoff relationship) that account for substrate concentration and affinity. For agonists, EC50 is an analogous quantity describing the concentration needed to reach 50% of a maximal effect; see EC50 for comparison.

Common uses and examples

IC50 values are a routine tool in drug discovery and toxicology for ranking compound potency, screening libraries, and guiding lead optimization. They are used across fields: small-molecule enzyme inhibitors, receptor antagonists, antiviral agents, and antimicrobial susceptibility testing. Regulatory and guidance documents often refer to IC50 in the context of in vitro potency assessments; for example, agencies provide definitions and context for laboratory studies of drug action — see regulatory guidance.

Limitations and important caveats

  • Assay dependence: IC50 values vary with experimental conditions, so comparisons are meaningful only when conditions are similar.
  • Not a direct affinity measure: IC50 is not identical to binding affinity (Ki) unless specific assumptions are met.
  • Kinetics and mode of action: slow-binding inhibitors, irreversible inhibitors, or allosteric modulators require different analyses.
  • Units and reporting: always report units and assay details to allow interpretation and reproducibility.

Further reading and resources

For practical examples and target-specific considerations, many laboratories provide protocols for enzyme inhibition assays, receptor binding, and cell-based screens. Background on reagents and biological targets is available in basic pharmacology and biochemistry texts; for introductory descriptions of target types see resources on drugs (drug targets), enzymes (enzyme inhibitors), and microorganisms (antimicrobial testing).