Related Experiment Video
Updated: Aug 6, 2026
09:42
Unraveling Entropic Rate Acceleration Induced by Solvent Dynamics in Membrane Enzymes
Published on: January 16, 2016
Two Routes to the Same Equation: Kinetic and Statistical-Thermodynamic Derivations of the Classical Drug-Receptor
Robert B Raffa1,2,3
1Pharmacology, Temple University, Philadelphia, USA.
Cureus
|July 19, 2026
Summary
A new thermodynamic derivation explains the drug-receptor dissociation constant (KD). This reveals KD as a Boltzmann factor, linking drug affinity to binding free energy and unifying pharmacology with thermodynamics.
Area of Science:
- Pharmacology
- Statistical Thermodynamics
- Physical Chemistry
Background:
- The hyperbolic relationship between drug concentration, dissociation constant (KD), and receptor occupancy is fundamental to pharmacology.
- Current understanding often treats KD as an empirical ratio of rate constants, lacking deeper thermodynamic meaning.
Purpose of the Study:
- To present an independent derivation of the receptor occupancy equation using equilibrium statistical thermodynamics.
- To elucidate the fundamental thermodynamic nature of the equilibrium dissociation constant (KD).
Main Methods:
- Utilized equilibrium statistical thermodynamics and the grand canonical ensemble.
- Modeled receptors as two-state systems in equilibrium with a bulk drug reservoir.
- Derived the occupancy equation from first principles.
Main Results:
- Reproduced the standard occupancy equation exactly.
- Revealed the dissociation constant (KD) as a Boltzmann factor, representing binding free energy.
- Demonstrated that affinity (1/KD) is directly related to free energy, with enthalpic and entropic contributions.
Conclusions:
- The thermodynamic approach provides a fundamental physical basis for KD and drug affinity.
- This unifies concepts in receptor pharmacology, surface adsorption, and molecular switches.
- Clarifies the exponential relationship between affinity differences and binding energy.
Related Concept Videos
01:30Quantitative Aspects of Drug-Receptor Interaction
The receptor occupancy theory connects a drug's response to the number of occupied receptors. With higher drug concentrations, more receptors are occupied, leading to increased responses. The formation of drug-receptor complexes involves association and dissociation rates, which reach equilibrium when the forward and backward reactions are equal. The equilibrium association constant (Ka) and its inverse, the equilibrium dissociation constant (Kd), indicate drug affinity. Higher Ka and lower Kd...
01:25Drug Distribution as One-Compartment Model and Elimination by Nonlinear Pharmacokinetics: Overview
Drug administration can occur through various routes, each of which may result in a different process of elimination. This process is often mixed with nonlinear and linear processes. It's important to understand that a single drug can be metabolized into different metabolites through parallel processes.
For instance, consider the metabolism of sodium salicylate. This compound is metabolized into two distinct substances: a glucuronide and a glycine conjugate. The rate of conjugation depends on...
For instance, consider the metabolism of sodium salicylate. This compound is metabolized into two distinct substances: a glucuronide and a glycine conjugate. The rate of conjugation depends on...
01:18Pharmacodynamic Models: Emax Drug–Concentration Effect Model
The Emax drug-concentration effect model is central to pharmacodynamics in drug discovery and development. This model is predicated on the receptor occupancy theory, which posits that the effect of a drug is directly related to the number of receptors occupied by the drug and the resultant complex formation.The model describes the reversible interaction between a drug (C) and a receptor (R) to form a drug-receptor complex (RC). The kinetics of this interaction are quantified by an equation that...
01:28Pharmacokinetic–Pharmacodynamic Relationship: Dose to Pharmacological Effect
A drug’s dosage and pharmacokinetic properties determine how quickly it acts, how intense its effects are, and how long it lasts. Higher doses increase drug concentration at receptor sites, producing a hyperbolic curve when pharmacologic response is plotted against drug dose. Converting this scale to a log-linear format results in a sigmoidal curve, better representing dose–response relationships.For drugs following a one-compartment model, the pharmacologic response is directly proportional to...
01:12Two-Compartment Open Model: Extravascular Administration
The two-compartment model for extravascular administration represents a drug's absorption and distribution process. It features a central compartment, where the drug is first absorbed, and a peripheral compartment, which illustrates the drug's distribution throughout the body. The rate of change in drug concentration in the central compartment is calculated by three exponents: absorption, distribution, and elimination.
The absorption exponent (ka) indicates the speed at which the drug is...
The absorption exponent (ka) indicates the speed at which the drug is...
01:24Pharmacokinetic–Pharmacodynamic Relationship: Problems
The empirical approach to drug therapy optimization relies on correlating pharmacological response with administered dosage. Such an approach can be costly, time-consuming, and often yields poor correlation due to variables like formulation factors and drug elimination characteristics. A more precise approach correlates response with plasma drug concentration or the amount of drug in the body, rather than dosage. This is achieved through pharmacokinetic-pharmacodynamic (PK/PD) modeling, which...