The use of dermatological preparations or other drugs in the treatment of these disorders by application as a thin film on the skin can lead to rapid photodegradation on exposure to light. UVC radiation is absorbed by ozone and is thus absent at the sea level. Some of the dealkylation reactions are shown in Figure 3. The degradation products have been identified by 1 H— and 13 C— NMR studies as 4- 2-nitrosophenyl pyridine 15 and its oxidation product 4- 2-nitrophenyl pyridine 16 derivatives [ 54 ] [Figure 5 ]. This occurs in the presence of an electron acceptor as a result of photoionization see 4 , particularly in molecules having an anionic state.
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The degradation is fastest in aqueous solutions at pH 2.
International Journal of Photoenergy
The elucidation of the mechanisms of these pathways requires a thorough understanding of the nature and type of the photochemical photostability of drugs and drug formulations second edition involved. View at Google Scholar T. View at Google Scholar M. The response of the drug to light absorption and excitation can be considered in terms of photodegradation photolysis reactions through the formation of free radicals or photosensitization reactions by intermolecular energy transfer.
This results in the absorption of light by the excipients at the expense of the drug and thus protecting it from photodegradation. This may happen by internal conversion IC see 2a nonradiative transition to the ground state, or photon emission fluorescence leading back to the ground state see 3.
The factors that influence the rate of a photochemical reaction include the concentration of the drug, the volume of sample, the quantum yield of the photochemical reaction, and the intensity and spectral distribution of the light source.
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Photostability of Drugs and Drug Formulations – CRC Press Book
A careful consideration of the factors that promote the photodegradation of drugs and their control may improve photostability. In particular, the effect of the dielectric constant on the kinetics of the photodegradation of methylisoalloxazine [ 62 ], formylmethylflavin [ 65 ], riboflavin [ 72— ], moxifloxacin [ ], and norfloxacin  in aqueous and organic solvents has been studied.
Oxygen present in a formulation may be removed by purging with nitrogen or by photoxtability addition of a suitable antioxidant depending on the susceptibility of the drug to photooxidation. It highlights the role of the photochemistry in the photostability studies, describes the functional groups important for the photoreactivity of drugs, explains photophysical processes, and deals with the kinetics of photochemical reactions.
Solvents can exert considerable influence on the stability of the photosensitive drugs in the pharmaceutical formulations. The photodegradation rates may be affected by the polarity and formultaions of the medium.
Photostability and Photostabilization of Drugs and Drug Products
In the free radical chain process, a sensitizer photostability of drugs and drug formulations second edition a hydrogen atom from the drug molecule see The photodegradation of the drug substances may occur by zero-order, first-order, and second-order reactions or by simultaneous parallel reactions to give two or three products and by consecutive reactions involving intermediate species to give the final product.
The photostability of drugs in liposomal formulations may be affected by the composition of the liposomes, the entrapment efficiency, and the drug-phospholipid interaction [ ]. The light sensitivity of the drug substances requires the use of an effective packaging system to protect them from photochemical damage. The kinetics of the photoaddition reactions of RF has been studied [ 43 ]. Therefore, care should be exercised photostabiltiy the selection of the sunscreen agents by consultation with a dermatologist.
The photoprotective effect of these pigments depends on their particle size which influences the light absorption and scattering capacity. They undergo degradation reactions in aqueous and organic solvents by various pathways upon exposure to light.
Learn More about VitalSource Bookshelf. The excess energy in the excited state may also be dissipated as heat on collision with neighboring molecules by vibrational relaxation VR. This occurs in the presence of an electron acceptor as a result of photoionization see 4particularly in molecules drhgs an anionic state. This may imply the involvement of a polar intermediate or the reaction pathway [ 62 ]. A detailed study of the photoreduction reaction of riboflavin RF 1 in aqueous and organic solvents has been made [ 62 — 74 ].
View at Google Scholar G. Objectives of the Photostability Studies In view of the photosensitivity and photoinstability of drugs and adjuvants, knowledge of the photostability of these substances and their formulated products is necessary to evaluate the following: Add to Wish List.
Several studies have been conducted to evaluate the effect of pH on the stability of the drug substances [ 14] and to assess the optimum pH from the rate-pH relationships to achieve stabilization [ 1445, photostabiliity, ]. The application of the photostability testing in the pharmaceutical industry is necessary to ensure the potency, the efficacy, and the safety of the manufactured products in their clinical use.
These aspects involve the study of the kinetics of the photodegradation reactions.
The presence of the following chemical functional groups in the drug molecules [ 10 ] is usually necessary for the occurrence of photochemical reactions: