Photoprotection/photostability

The results obtained in this work indicate that both the structure of the stabilizer and the nature of the surrounding environment are important factors in determining the efficiency of the energy dissipation processes in these derivatives. Molecular structures in which the planar form is favoured and where the intramolecular hydrogen bond is protected from interactions with the medium by the incorpoation of bulky substituent groups, should exhibit highest photostability and impart improved photoprotection to polymer substrates. 

Thoma s interests included methods of testing (76-78), the influence of excipients and formulations on the photostability of products (71,79-83) and methods of photoprotection via packaging (67,71,84-88) or formulation (67,89-91). Several of his papers dealt with specific photostability problems such as antibiotics (92) antimitotics (93-96), adrenaline (97), molsidomin (98), phenothiazines (99), quinolines (100) and nifedipine (101-107). 

Cyclodextrins can form inclusion complexes with some drugs, modifying their physical and chemical properties. Because cyclodextrins are mainly used to increase the solubility of poorly soluble drugs, most investigations in this field focus on photostability of drugs in solution. Here, the positive effect of cyclodextrins on the photostability of colchicine (36), emetine and cephaeline (37) could be shown. However, these results show that the photoprotective effect depends on the particular cyclodextrin used. For some forms of cyclodextrin, an increase of the photodegradation rate can be obtained, as the example of molsidomine shows (13). 

Other benzoic acid derivatives also function as effective photoprotective agents. Methylparaben has a photostabilizing effect on solutions of riboflavin (69). Ethyl aminobenzoate significantly enhanced the photostability of menadione in aqueous solutions (107). 

Sodium benzoate was shown by Asker and Harris (108) to be an effective photostabilizer for physostigmine sulfate solutions. The photostability of minoxidil solution is also enhanced by sodium benzoate (48). Figure 17 shows the absorption curves of sodium benzoate and minoxidil. As can be seen in the figure, sodium benzoate absorbs radiation in the same range as minoxidil, thus acting as a photoprotective agent for the drug (109). 

Uric acid in very low concentrations, very strongly absorbs UV radiation and for this reason, has been used to protect various FD C colors against fading when they will be probably exposed to direct sunlight (112). Uric acid has been found to enhance the photostability of solutions of colchicine (70) and FD C Blue No. 2 (82,113). Sodium urate, the neutral salt of uric acid, has a photoprotective effect on solutions of metronidazole (68), doxorubicin hydrochloride (77), and physostigmine sulfate (108). In addition to its photon-absorbing property, uric acid has been reported to also possess antioxidant quality (114). 

Thiourea and dl-methionine are effective photoprotective agents for solutions of riboflavin (69), tetracycline hydrochloride (71), doxorubicin hydrochloride (77), reserpine (106), and potassium iodide (121). Thiourea enhances the photostability of solutions of minoxidil (48) and furosemide solution (74). Dl-methionine increases the photostability of ascorbic acid solutions (122). 

A number of photoprotective agents were found to enhance the photostability of demeclocycline hydrochloride solution exposed to fluorescent radiation from cool white tubes (76). These results are summarized in Table 8. 

The photostability of cosmetic sunscreens has been and still is a subject of discussion during use where the ICH guidelines do not apply. The major concern being that an important breakdown (photodegradation) could lead to a significant reduction of photoprotection and could potentially have fatal consequences. 

From these studies it is apparent that VIS radiation has little to no effect on DTIC photostability. But, DTIC is very sensitive to UV radiation. Exposure to direct sunlight should be strictly avoided and exposure to daylight should be minimized. During administration, the reservoirs and administration sets used for infusion should be photoprotected. Sets capable of excluding the UV component of all incident radiation are a convenient method of photoprotection in clinical practice. The utility of Amberset products, for this purpose has been experimentally proven. 

Certain vitamins if not photoprotected are very much degraded during administration. The photostability of parenteral vitamins is enhanced by their administration in fat emulsions. The stability of fat-soluble and water-soluble vitamins (Vitalipid N infant and Soluvit infant) in a soybean fat emulsion (Intralipid 10%), under conditions simulating the IV feeding of neonates and children without having to use photoprotection has been demonstrated. 

Depending on the type of ester used, losses of vitamin A by binding to the administration systems can be considerable and complicate the interpretation of the study results (91). Special emphasis should be put on delivering adequate doses and the photoprotection of vitamin A, when this active is employed in neonatal care. The effect of phototherapy radiation on vitamin A photostability, present in TPN admixtures, has been investigated (92). The addition of Intralipid fat emulsion to TPN admixtures significantly reduces losses of vitamin A (Fig. 10). 

When phytomenadione is added to a fat-free TPN admbcture and exposed to artificial daylight, it shows less than 10% decomposition after 4.5 hours of exposure. Decomposition is effectively prevented by photoprotection, achieved by the addition of a lipid emulsion to the TPN admixtures (93). It has also been reported that the addition of lipids to this same mixture had no effect on the photostability of phytomenadione. After three hours of exposure to window-glass-filtered sunlight, a 50% loss of the phytomenadione has been reported (94). Therefore, exposure to... 

Spirulina is high in protein and other nutrients, finding use as a food supplement and for malnutrition. Previous studies of the photoprotection of various pigments in S. platensis were carried out in the presence and the absence of biopterin-a-glucoside to evaluate the photostability of these photosynthetic pigments in the photosynthetic vesicles that imdergo UV irradiation (Noguchi et ah, 1999). 

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