IC50
IC50 — The half-maximal inhibitory concentration, representing the concentration of a compound needed to inhibit a biological process by 50%.
What Is IC50?
IC50 (half-maximal inhibitory concentration) is the concentration of an inhibitor required to reduce a biological response by 50%. It is the standard measure of inhibitory potency for antagonists, enzyme inhibitors, and cytotoxic compounds. Lower IC50 values indicate more potent inhibitors. IC50 is determined from dose-response curves by fitting data to the Hill equation.
IC50 vs. Ki
IC50 is an empirical measurement that depends on assay conditions (substrate concentration, incubation time). Ki is a thermodynamic constant independent of assay conditions, calculated from IC50 using the Cheng-Prusoff equation: Ki = IC50 / (1 + [S]/Km) for competitive inhibitors. Ki enables valid comparisons between studies performed under different conditions.
How to Measure IC50
- Test 8-10 inhibitor concentrations spanning 4 log units around the expected IC50
- Include uninhibited control (100% activity) and fully inhibited control (0% or background)
- Normalize data as percent inhibition: (1 - [treated/untreated]) x 100
- Fit to 4-parameter logistic curve; the inflection point = IC50
Context in Peptide Research
IC50 is reported for peptide protease inhibitors, receptor antagonists in competitive binding assays, and antimicrobial peptides (minimum inhibitory concentration, MIC, is a related concept). Always report assay conditions alongside IC50 values, as the number is meaningless without context.
Frequently Asked Questions
What is IC50?
The half-maximal inhibitory concentration, representing the concentration of a compound needed to inhibit a biological process by 50%.
Why is IC50 important in peptide research?
IC50 is a fundamental concept in pharmacology as it relates to peptide science. It directly influences experimental design, compound characterization, and the reliability of research outcomes across biochemistry and molecular biology disciplines.