Introduction to Drug Stability
The ultimate goal of drug formulation is creating a product that remains safe and effective until its expiration date. This unit applies classic chemical reaction kinetics to track drug degradation over time. It teaches how to calculate Half-life (t1/2) and Shelf-life (t90) for zero and first-order reactions. Beyond math, it addresses the physical and chemical enemies of stability—like hydrolysis, oxidation, light, and temperature—and details the ICH protocols for ‘Accelerated Stability Testing’ used daily by the global pharmaceutical industry.
Syllabus & Topics
- 1Reaction kinetics: Zero, pseudo-zero, first, and second order reactions.
- 2Determination of order of reaction.
- 3Half-life (t1/2) and Shelf-life (t90) calculations.
- 4Physical factors influencing chemical degradation: Temperature, Light, Moisture.
- 5Chemical factors influencing degradation: Hydrolysis, Oxidation, Solvolysis.
- 6Accelerated stability testing in expiration dating.
- 7Arrhenius equation and its application in predicting shelf life.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What is the Difference Between Zero Order and First Order Kinetics?
In zero-order kinetics, a drug degrades at a constant amount per unit time, independent of its concentration, and a plot of concentration versus time yields a straight line. In first-order kinetics, a drug degrades at a constant fraction or percentage per unit time, meaning the rate depends on the remaining concentration; a plot of log(concentration) versus time produces a straight line.
Q2. How is Shelf-Life (t90) Defined?
Shelf-life (t90) is the time required for a drug product to degrade to 90% of its original active concentration. When the product reaches this level, it is considered to have reached its expiration date.
Q3. What is Accelerated Stability Testing?
Accelerated stability testing involves storing drug samples under exaggerated stress conditions, such as high temperature (e.g., 40°C) and high humidity (e.g., 75% RH), in stability chambers to speed up degradation. Since normal shelf-life may extend to 2–3 years, this method allows rapid prediction of long-term stability. The resulting data are extrapolated using the Arrhenius equation to estimate shelf-life at room temperature.
Q4. What Does the Arrhenius Equation Dictate?
The Arrhenius equation (k = A · e^(−Ea/RT)) describes the relationship between reaction rate and temperature. It states that as temperature increases, the degradation rate increases exponentially because a greater proportion of molecules acquire sufficient energy to overcome the activation energy barrier.