PHOTOSTABILIZATION OF DRUGS IN DOSAGE FORMS WITHOUT PROTECTION FROM PACKAGING MATERIALS

The number of new drugs sensitive to light has been steadily increasing during the course of the last two decades. When enwrapped by the packaging material, the pharmaceutical dosage form is normally well protected from the influence of photodegradation. However, the manufacturer must also take into consideration those periods of time when the dosage forms are not covered by packaging, i.e. both during the process of manufacturing itself and during handling by the consumer/patient on application at home or in hospital. In this respect, the kinetics of degradation are strongly dependent on light intensity and spectral distribution of the light source used; for example, nifedipine solutions undergo 3-fold faster degradation in 'normal' daylight than under exposure to a 40 W light bulb. The instability of nifedipine corresponds absolutely with its absorption at about 450 nm and beyond. Stabilization with preselected colourants or other appropriate excipients has been successfully demonstrated. This mode of action applies to many drugs, e.g. daunorubicin, dihydroergotamine, haloperidol, furosemide (frusemide) and nitrofurazone, and it is not only feasible for drugs in solution, but also effective in other dosage forms such as tablets or topical preparations. The principle of photoprotection by spectral overlapping is described.