In the pharmaceutical world, the term photostability is frequently encountered—and for good reason. It plays a crucial role in ensuring the safety, efficacy, and quality of drug products. Recognizing its importance, the International Council for Harmonisation (ICH) has dedicated a specific guideline to this topic: ICH Q1B . This guideline outlines the requirements for photostability testing of new drug substances and products.
But what exactly is photostability, and why does it deserve such focused attention? In this article, we’ll delve deep into the concept of photostability, exploring its significance, testing approaches, regulatory expectations, and how it impacts the overall development and lifecycle of pharmaceutical products. Let’s begin our exploration into the world of photostability.
What is Photostability?
Photostability refers to the stability of a pharmaceutical substance or product when exposed to light. It is an essential part of stability testing to determine how light exposure affects the quality, potency, and safety of a drug.
Photostability testing is required as per ICH Q1B guidelines, ensuring that pharmaceutical products remain effective throughout their shelf life and do not degrade into harmful byproducts when exposed to light.
Why Perform Photostability Testing?
Regulatory Requirement – Compliance with ICH Q1B and national regulatory guidelines (e.g., FDA, EMA).
Quality Assurance – Ensures that the drug maintains its efficacy, potency, and safety over time.
Formulation Development – Helps in selecting appropriate packaging and storage conditions.
Degradation Profiling- Identifies degradation products formed due to light exposure.
Labeling Requirements – Determines if the product requires protection from light.
How to Perform Photostability Testing?
1. Selection of Samples:
Drug Substance (API) – Pure active pharmaceutical ingredient.
Drug Product – Finished dosage form (tablet, capsule, solution, etc.).
Placebo – Without API (to check if excipients degrade).
Marketed Package – If applicable, to evaluate light protection.
2. Light Sources Used
As per ICH Q1B guidelines, samples must be exposed to:
Fluorescent Light (cool white light): Mimics indoor storage conditions.
Near UV Light (320–400 nm): Mimics sunlight exposure.
Intensity Requirement:
1.2 million lux hours (visible light)
200 watt-hours/m² (UV light).
3. Exposure Conditions:
Place the samples in a photostability chamber under controlled temperature and humidity.
Expose the samples to light for a specified duration to meet ICH requirements.
If degradation occurs quickly, an accelerated study can be done.
4. Post-Exposure Testing:
After light exposure, test samples for:
Appearance changes (color change, precipitation, melting).
Assay of API using HPLC or UV Spectroscopy.
Degradation product profiling using HPLC, LC-MS, or GC-MS.
Calculations in Photostability Testing
1. Light Exposure Calculation
The required light exposure is:
Visible light (lux hours)
Exposure= lluminance (lux)×Time (hours)
Example: If a sample is placed in 6000 lux for 200 hours:
6000×200=1.2million lux hours
UV Light (watt-hours/m²)
Exposure=UV Intensity (W/m²)×Time (hours)
Example: If a UV source provides 2 W/m² and the exposure lasts 100 hours:
2×100=200watt-hours/m²
2. Degradation Calculation
Percentage Degradation
%Degradation=(Initial Assay−Final Assay)/
Initial Assay)×100
Example:
Initial Assay: 100%.
Final Assay after exposure: 95%
=(100−95)/100×100=5% degradation
If degradation is more than 5%, formulation or packaging changes may be required.
What is rationale Behind 1.2 Million Lux-Hours and 200 W·h/m² in Photostability Testing (ICH Q1B).
The limits set in ICH Q1B guidelines—1.2 million lux-hours (visible light) and 200 W·h/m² (UV light)—are designed to simulate real-world light exposure over a product's shelf life. These values were derived based on worst-case scenarios for typical pharmaceutical storage and handling conditions.
Definition:
Lux is a measure of illuminance (light intensity per unit area).
Lux-hours is a cumulative measure of light exposure over time.
Rationale:
The 1.2 million lux-hours simulates the amount of light a pharmaceutical product may experience during normal storage, transportation, and use over its shelf life.
Many pharmaceuticals are stored in warehouses, pharmacies, and hospitals where fluorescent or ambient light exposure is common.
This limit ensures that products remain stable under practical light conditions without excessive degradation.
Example Calculation:
If a product is exposed to 6000 lux of light in a pharmacy for 200 hours:
6000×200=1.2million lux-hours
This simulates real-world exposure over several months to years in well-lit conditions.
2. Why 200 W·h/m² for UV Light?
Definition:
Watts per square meter (W/m²) measures UV light intensity.
Watt-hours per square meter (W·h/m²) measures total UV energy received over time.
Rationale:
UV light (320–400 nm) can cause photodegradation of pharmaceuticals by breaking chemical bonds.
200 W·h/m² represents the cumulative UV exposure from sunlight and artificial sources during product distribution and storage.
This value simulates exposure a product might receive if stored near windows, under artificial UV light, or exposed to sunlight during transportation.
Example Calculation:
If a UV source provides 2 W/m², the exposure time needed to reach 200 W·h/m² would be:
200/2=100 hours.
3.What is normal average white light intensity in the room?
The normal average white light intensity in a room depends on the type of lighting and its purpose. Here are typical values:
Indoor Light Intensity Levels (in Lux)
Home (Living Room, Bedroom): 100–300 lux
Office, Classroom: 300–500 lux
Supermarket, Retail Stores: 500–1000 lux
Hospital, Laboratories: 1000–2000 lux
Surgical Room, Inspection Areas: 2000–10000 lux.
Comparison with ICH Q1B Requirement
Typical room lighting (300–500 lux) would take 2400 to 4000 hours (~3–6 months) to reach 1.2 million lux-hours.
This confirms that ICH Q1B testing simulates long-term real-world light exposure in a much shorter time (e.g., a few days in a high-intensity photostability chamber).
4. How to calculate Time Required to Reach 1.2 Million Lux-Hours in a Normal Room
Formula
Lux-Hours=Illuminance (lux)×Time (hours)
To determine how long it takes for a product in a typical room to reach 1.2 million lux-hours, we solve for time:
Time (hours)=1,200,000 lux-hours/
Room Light Intensity (lux)
Example: Suppose we have kept about 2 g sample in a petridish for 15 hours in photo stability chamber which visible light intensity is about 5000 lux and uv light intensity is 3.1 Watt/m2
So calculate the total exposLet's calculate both visible light exposure (lux-hours) and UV exposure (W·h/m²) based on the given conditions.
Let's calculate both visible light exposure (lux-hours) and UV exposure (W·h/m²) based on the given conditions.
Given Data:
-
Sample weight: 2 g (not needed for light exposure calculation)
-
Exposure time: 15 hours
-
Visible light intensity: 5000 lux
-
UV light intensity: 3.1 W.h/m²
Visible Light Exposure (Lux-Hours Calculation)
Lux-Hours=Illuminance (lux)×Time (hours)
Lux-Hours=5000×15=75,000 lux-hours
UV Light Exposure (Watt-Hours per Square Meter Calculation)
UV Exposure (W/m²)=
UV Intensity (W.h/m²)×Time (hours)
=3.1×15=46.5W/m²
Final Answer:
Total Visible Light Exposure = 75,000 lux-hours
Total UV Exposure = 46.5 W·h/m2
Comparison with ICH Q1B Requirements:
Required visible light exposure = 1.2 million lux-hours
Our exposure (75,000 lux-hours) is only 6.25% of the required amount.
Required UV exposure = 200 W·h/m²
Your exposure (46.5 W·h/m²) is 23.25% of the required amount.
Conclusion: Our current exposure is much lower than ICH Q1B requirements. To meet full ICH Q1B criteria, We would need to extend the exposure time.
But how much time we need to meet the requirements
We can calculate with below formula that we discussed before.
Required Time for Visible Light Exposure
Using the formula:
Time=ICH Required Luxurious / Lux Intensity
Time=1,200,000/5000 =240hours
So, we need to expose the sample for 240 hours or if calculate in day terms it would be 10 days to meet the visible light requirement
Required Time for UV Exposure:
Using the formula:
Time=ICH Required UV Exposure (W/hm²)/
UV Intensity (W/m²)
Time=200÷3.1=64.5 hours
So, We need to expose the sample for 64.5 hours or almost 2.6 days.
Types of Degradation and Chemical Reactions in Photostability Testing
1)Photodecomposition (Direct Absorption)
The drug absorbs light and breaks chemical bonds, leading to decomposition.
Eg. Riboflavin, Amphotericin B
2) Photooxidation
Light energy transfers to oxygen, generating reactive oxygen species (ROS) that oxidize the drug.
Eg. Ascorbic acid, Adrenaline, Simvastatin
3) Photoreduction
Light-induced reduction reactions alter the oxidation state of the drug.
4) Photocyclization
Light induces a rearrangement or ring formation in the chemical structure
Eg. Tetracyclines, Ciprofloxacin
5) Photoisomerization.
The drug undergoes isomerization (cis–trans conversion) due to light absorption.
Eg. Retinoids, Diclofenac, Omeprazole
6) Photohydrolysis
Light exposure causes hydrolysis in the presence of water.
Some Example Drugs and Degradation Products:
Nifedipine → Converts to an inactive nitroso derivative.
Furosemide → Forms colored impurities, reducing potency.
Amiodarone → Forms toxic iodine-containing degradation products.
Tetracyclines → Undergo photoisomerization, forming epitetracyclines, which are toxic.
Chlorpromazine → Forms sulfoxide, reducing potency.
Vitamin B2 (Riboflavin) → Undergoes photooxidation, leading to inactive compounds.
Ibuprofen → Forms hydroperoxides, affecting stability.
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