Stability Of Drugs:Photolysis
From Pharmpedia
Students,
we have seen in the introductory note on chemical stability that when molecules absorb energy and are energized or activated they go to a high energy state and then release that energy in a chemical reaction and come back to their original position. When this energy of activation is supplied by light and is absorbed by the compound the decomposition reaction is called photolytic. The activated species then returns to ground state by either (a) emitting light of a different frequency (this is fluorescene or phosphorescene) or (b) decomposition (Photolysis).
An excellent example for photolysis is the photolysis is the photo degradation of sodium nitroprusside in aqueous solution.
Sodium nitroprusside is administered by intravenous infusion for the management of acute hypertension. If the solution is
protected from light, it is stable for at least 1 year; if exposed to normal room light, it has a shelf life of only 4 hours.
The energy associated with the radiation increases as its wavelength decreases, so that the energy of U.V. is greater than that of visible which is greater than that of J.R.Look at the following table which I have reproduced from Aulton’s book on Pharmaceutics.
Relationship between wavelength and associated energy of various forms of light.
|
Type of radiation |
Wavelength |
Energy |
|
U.V.
Visible
i.r.
|
50 – 400
400 – 750
750 – 10,000 |
Kcal mol-1
287 – 72 72 – 36
36 - 1 |
The higher energy range of visible light and U.V. light cause photolysis. Conventional tungsten filament light bulbs are safe and
do not contribute to photolysis.
Oxidation to a great extent and hydrolysis to some extent are catalysed by light. Photo– induced reactions are common in steroids; an example is the formation of 2 – benzoyl cholestan – 3 – one following is radiation of
Cholest – 2 en -3 –ol benzoate
Photolysis is prevented by suitable packing in amber
coloured bottles, cardboard outers and in aluminium foil over wraps and blisters.
Jen, T. Cartensen in his book on Drug Stability, Principles and Practices gives extensive examples of photolysis. In
pharmaceutical systems most reported photolysis reactions have been first order. A good example is the photolysis of cefatoxime.
The wavelength of the irradiating light plays an important part in photolysis.
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