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  • Evaluation of Photodegradation Pathways

    Photostability study provides useful information about the handling and packing of drug substances and drug products.

    Most drug substances are light-sensitive hence during formulation they may degrade at the time of processing, manufacturing, and storage. Due to photodegradation, there may be a chance of 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 regions of light can differ with its photoreactivity, chemical structure, and type of dosage form.

    Purposes of the Photostability Studies

    Photostability of drugs and their 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 in 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 the 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 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 for an 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 formulations.


    1. ICH Q1B, Photostability testing of new drug substances and products,
    2. Photostability of Drugs and Drug Formulations, Second edition, Edited by, Hanne Hjorth T√łnnesen,
    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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