Evaluation of Photodegradation Pathways

Drug Metabolite Impurities, Pathways & Analytical Techniques
August 5, 2020
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August 7, 2020
Evaluation of Photodegradation Pathways
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Photostability study provides useful information about the handling and packing of drug substance and drug products.

Most of the drug substances are light-sensitive hence during formulation it may degrade at the time of processing, manufacturing, and storage. Due to photodegradation, there may be a chance loss of potency, impact on efficiency, and harmful biological properties.

Drugs can be protected from the harmful effects of light by using proper containers and packaging materials. Drug sensitivity for some spectral region of light can differ with its photoreactivity, chemical structure, and type of dosage form.

Purposes of the Photostability Studies

Photostability of drugs and its formulated products is necessary to evaluate the characteristics such as physical and chemical changes, route and mechanisms of photodegradation, shelf life, the efficiency of the stabilizing agents for photo stabilization and require changes of the formulation processing parameters, packaging, labeling, transportation, and storage.

Photodegradation Pathways:

Drug substances or drug products may degrade by one or more routes to form different products. Photodegradation mostly depends on the presence of certain functional groups, physical characteristics, and mode of the compound. The main mode of photodecomposition reactions of drugs is as per below:

  1. Addition
  2. Cyclization
  3. N-Dealkylation
  4. Decarboxylation
  5. Decarboxylation
  6. Dehalogenation
  7. Dimerization
  8. Oxidation
  9. Reduction
  10. Isomerization
  11. Rearrangement
  12. Hydrolysis

Photodegradation reactions are very challenging because many pathways are reported for most of the drugs. Additionally, the irradiation environments are dissimilar in laboratories such as wavelength range, exposure time, and drug concentration.

Also, in various cases oxidation may be responsible for further secondary or tertiary degradation products when exposed to extended time.


The objective of these studies is to ensure product quality along with all of the necessary attributes during their storage and use. Photostability study of the drug substance and drug products is a very much important part that provides valued information about its stability, safety, and efficacy. The photostability data of drugs and degradation pathways provide vital evidence to formulation scientists to develop robust, stable, safe, and effective formulation.


  1. ICH Q1B, Photostability testing of new drug substances and products,
  2. Photostability of Drugs and Drug Formulations, Second edition, Edited by, Hanne Hjorth Tønnesen,
  3. Analysis of drug impurities, Edited by Richard J. Smith and Michael L. Webb,
  4. Photochemical degradation of components in drug formulations, Pharm Technol Eur.,
  5. Thatcher S.R., Mansfield R.K., Miller R.B., Davis C.W., Baertschi S.W. Pharmaceutical photostability: A technical and practical interpretation of the ICH Guideline and its application to pharmaceutical stability: Part I. 

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