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Related Concept Videos

Calculating the Equilibrium Constant02:46

Calculating the Equilibrium Constant

The equilibrium constant for a reaction is calculated from the equilibrium concentrations (or pressures) of its reactants and products. If these concentrations are known, the calculation simply involves their substitution into the Kc expression.
For example, gaseous nitrogen dioxide forms dinitrogen tetroxide according to this equation:
Pharmacodynamic Models: Linear Concentration–Effect Model01:15

Pharmacodynamic Models: Linear Concentration–Effect Model

The linear concentration–effect model, underpinned by the principle that pharmacological effect (E) is directly proportional to plasma drug concentration (C), emerges as a pivotal simplification of the Emax model for conditions where C is significantly less than EC50. This model portrays a linear trajectory of the concentration–effect relationship when drug levels are markedly below the EC50 threshold.Despite its inherent assumption of continuous effect augmentation with increasing drug...
Chemical Equilibria: Redefining Equilibrium Constant01:20

Chemical Equilibria: Redefining Equilibrium Constant

The effect of an inert salt on the solubility of a sparingly soluble salt is known as the salt effect. The degree of the salt effect varies with the ionic strength of the solution, which in turn depends on the activity of the species in the solution. The activity is expressed as the product of concentration and the activity coefficient of the species.
To calculate the equilibrium constants of solutions of moderately high ionic strength, one must account for the salt effect. This redefined...
Thermodynamics: Activity Coefficient01:24

Thermodynamics: Activity Coefficient

Activity is the measure of the effective concentration of the species in solution. It can be expressed as the product of the molar concentration of the species and its activity coefficient. The activity coefficient is a dimensionless quantity and depends on the total ionic strength of the solution.
The activity coefficient is a measure of the deviation from ideal behavior. When the ionic strength of the solution is minimal, the activity coefficient of an ionic species is close to unity, making...
Factors Affecting Activity Coefficient01:17

Factors Affecting Activity Coefficient

The extended Debye-Hückel equation indicates that the activity coefficient of an ion in an aqueous solution at 25°C depends on three partially interdependent properties: the ionic strength of the solution, the charge of the ion, and the ion size. 
The activity coefficient value for an ion is close to one when the solution has almost zero ionic strength, i.e., when the solution shows close to ideal behavior. As the ionic strength of the solution increases from 0 to 0.1 mol/L, a decrease in the...
Dose-Response Relationship: Potency and Efficacy01:22

Dose-Response Relationship: Potency and Efficacy

The potency of a drug is the measure of its ability to produce a biological response and can be compared by looking at the half-maximum effective concentration or EC50 values of different drugs. A lower EC50 value indicates higher potency of the drug. In the dose–response curve of two antihypertensive drugs, candesartan and irbesartan, a significant difference is observed in their EC50 values. A lower EC50 value for candesartan indicates that it is more potent than irbesartan, as it produces...

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Related Experiment Video

Updated: May 22, 2026

A Streamlined, Label-Free Real-Time 50% Tissue Culture Infectious Dose (TCID50) Assay using Impedance for Automated Viral Titer Quantification
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A Streamlined, Label-Free Real-Time 50% Tissue Culture Infectious Dose (TCID50) Assay using Impedance for Automated Viral Titer Quantification

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The IC(50) concept revisited.

Gary W Caldwell1, Zhengyin Yan, Wensheng Lang

  • 1Community of Research Excellence & Advanced Technology (CREATe), Janssen Pharmaceutical Companies of Johnson and Johnson, Welsh and McKean Roads, Spring House, PA 19477-0776, USA. gcaldwel1@its.jnj.com

Current Topics in Medicinal Chemistry
|May 11, 2012
PubMed
Summary
This summary is machine-generated.

Accurately measuring the half maximal inhibitory concentration (IC50) is crucial for ranking enzyme inhibitors in drug design. Understanding experimental design and mathematical modeling, including handling outliers, is key to reliable IC50 values.

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Area of Science:

  • Biochemistry
  • Enzymology
  • Pharmacology

Background:

  • Enzyme inhibition is a key strategy in drug design.
  • Accurate determination of half maximal inhibitory concentration (IC50) is vital for compound evaluation.
  • IC50 is extensively used for reversible enzymatic inhibition studies.

Purpose of the Study:

  • To elucidate the importance of experimental design and mathematical modeling for IC50 determination.
  • To provide guidance on selecting appropriate models and handling data outliers.
  • To discuss the quantitative and mechanistic information derivable from IC50 experiments.

Main Methods:

  • Measuring product formation rate during the linear phase of enzymatic reactions.
  • Verifying the reversibility of enzyme-catalyzed reactions.
  • Applying appropriate mathematical models for data analysis.
  • Implementing strategies for outlier detection and management in data sets.

Main Results:

  • Accurate IC50 determination relies on precise measurement of reaction rates.
  • Experimental reversibility confirmation is essential for valid IC50 calculations.
  • Correct model selection and robust outlier handling significantly impact IC50 reliability.

Conclusions:

  • Sound experimental design and rigorous mathematical modeling are paramount for accurate IC50 values.
  • Understanding these factors enables more reliable ranking of enzyme inhibitors.
  • The study highlights the critical interplay between experimental execution and data analysis in biochemical assays.