Photoactivation of corticosteroids in UVB-exposed skin.

The photodegradation of flumethasone (FM) and fluocinolone acetonide (FC) was studied in solution and in the pig skin. Both glucocorticosteroids applied to the pig skin were unstable under UVB light. The photoproducts formed in the skin were the lumi-, photolumi- and andro-derivatives for FM, the same found in vitro. Instead, FC hydroperoxide formed in solution was not found in the skin: the reactivity and oxidative ability of this photoproduct towards biological substrates (lipids, proteins) seems the reason of the lack of its detection in the ex vivo model. In fact, it demonstrated to quickly oxidize amino acids and peptides, and to react with BSA both in the dark and under irradiation. Moreover, the presence in the irradiated pig skin of the FC andro-derivative, which usually forms in H-donating environment, seems consistent with the mechanism of Norrish I fragmentation followed by H-abstraction, likely from the surrounding biological substrates. These findings indicate that photoreactivity of these compounds may take place in the skin of patients exposing themselves to sunlight and is a warning about possible skin damage as a result of that. Furthermore, photolability of these drugs in the skin might cause loss of their therapeutic activity.

Photoreactivity of chloramphenicol in vitro and in vivo.

The negative side effects of chloramphenicol (CAP) mostly involve blood dyscrasias (e.g. irreversible non-dose-dependent aplastic anemia), allergic skin reactions and eye damage. To learn the cause of these side effects, most research focuses on metabolically formed nitroso- and hydroxylamino derivatives in the predisposed patient. In previous investigations it was demonstrated that photochemical decomposition of CAP in vitro by UV-A leads to formation of p-nitrobenzaldehyde (pNB), p-nitrobenzoic acid (pNBA) and p-nitrosobenzoic acid (pNOBA); the latter comprises up to 45 mol% of the starting amount of CAP. Incubation of these photoproducts in rat blood showed that pNB and pNOBA rapidly react and that PNBA is stable under these conditions. Reaction products from pNB (half-life 1.7 min) proved to be pNBA and p-nitrobenzyl alcohol (pNBOH) while pNOBA (half-life 3.7 min) was converted into p-aminobenzoic acid (pABA). Exposure of CAP in rat blood to UV-A yielded the same end products: pNBA, pABA and pNBOH. To estimate the amount of oxidative stress generated in vivo by these compounds, the ability to form methemoglobin (MetHb) in erythrocytes was tested; only pNOBA and p-hydroxylaminobenzoic acid (pHABA), a possible intermediate in the decomposition of pNOBA, proved to be reactive. Ultraviolet-A exposure of rats, after intraperitoneal injection of CAP, led to 3.6 times the basic level of MetHb. In addition, covalent binding of 3H-labeled CAP photoproducts to the skin of the back and to the ears was found, which was 9.1 and 3.2 times higher, respectively, than the dark values.(ABSTRACT TRUNCATED AT 250 WORDS)

UV-radiation protecting efficacy of thiols, studied with UVA-induced binding of 8-MOP and CPZ to rat epidermal biomacromolecules in vivo.

The following topically-applied thiols were investigated with regard to their possible UV-radiation protective properties: captopril, cysteamine, ergothioneine, mesna, mercaptopropionylglycine, N-acetyl-cysteine and penicillamine. As a measure for protection the inhibition of in vivo irreversible photobinding of the labelled phototoxic drugs chlorpromazine (CPZ) and 8-methoxypsoralen (8-MOP) to rat epidermal biomacromolecules was used. Ergothioneine, mesna and penicillamine did not show any effect; probably, as a result of their charge they are not able to enter the stratum corneum. Captopril, cysteamine, mercaptopropionylglycine and N-acetylcysteine showed a considerable inhibition of CPZ and 8-MOP photobinding. Captopril and N-acetylcysteine were clearly the most potent whereas cysteamine was the least effective. Captopril, mercaptopropionylglycine and N-acetylcysteine appeared to have a wider action range and to be a more effective protector than dl-alpha-tocopherol and di-butyl-hydroxytoluene. Cysteamine and mercaptopropionylglycine were only capable of protecting the stratum corneum. Captopril and N-acetylcysteine on the other hand showed an additional dose-dependent inhibition of photobinding to the viable epidermis. Gradually with increasing time after application, the protecting efficacy with regard to the viable layer of the epidermis decreased; the duration of protection depending on the dose.

Thiols as potential UV radiation protectors: an in vitro study.

The following thiols were investigated with regard to their possible UV-radiation protective properties: captopril, cysteamine, ergothioneine, mesna, mercaptopropionylglycine, N-acetylcysteine, and penicillamine. As a measure for protection, the inhibition of in vitro irreversible photobinding of the labeled phototoxic drugs chlorpromazine (CPZ) and 8-methoxypsoralen (8-MOP) to protein and DNA was used. Besides photobinding to biomacromolecules, the photodegradation of CPZ and the formation of promazine (PZH) and hydroxypromazine (PZOH) were measured as well. Because of the H-atom and electron donating capacity of the thiols, the ratio [PZOH]/[PZH] was expected to be decreased and the photodegradation of CPZ was expected to be higher in the presence of thiols. Maximum inhibition of CPZ photobinding ranged for the different thiols between 21-100% (DNA) and 17-87% (human serum albumin). All thiols enhanced the photodegradation of CPZ (19-84%) and inhibited the ratio [PZOH]/[PZH] (90-97%). 8-MOP photobinding to human serum albumin was also clearly inhibited (75-96%), but remarkably less to DNA (2-41%). This study indicates that thiols are able to cope with a variety of reactive species. Scavenging of radicals, quenching of singlet molecular oxygen species and reaction with excited states seem to be essential mechanisms involved with this process.

Photochemical reactions of quindoxin, olaquindox, carbadox and cyadox with protein, indicating photoallergic properties.

Quindoxin (quinoxaline-1,4-dioxide), a former 'growth promoter' used in animal husbandry, has been taken from the market because of its photoallergic properties. Nowadays its derivatives olaquindox, carbadox and cyadox are frequently applied for the same purpose. Recent reports show that olaquindox too, can induce photoallergic skin reactions in stockmen. From the present investigation it appeared that all compounds mentioned, form a reactive oxaziridine upon exposure to light, just like many other imino-N-oxides. Photoreactivity with protein, which is considered as an important condition for a compound to be a potential photoallergen, was also studied. Quindoxin and olaquindox proved to meet this condition, as was expected. But carbadox and cyadox also react and were shown to be even more reactive towards human serum albumin.

Phototoxicity of olaquindox in the rat.

Because of its photoallergic properties quindoxin, a growth promoting feed additive used for example in cattle raising, has been taken off the market. Recently, it appeared that the quindoxin derived olaquindox (OLAQ), used for the same purpose, also produces these light-induced side effects. In this study, phototoxicity of OLAQ was investigated by comparing four groups of rats with a different daily regime: 1 = light plus OLAQ, 2 = dark plus OLAQ, 3 = light (no OLAQ), 4 = dark (no OLAQ). It appeared that rats of group 2 excreted per day approximately 60 per cent of their daily dose as OLAQ and approximately 2 per cent as OLAQ-4-monoxide, whereas these figures were approximately 19 per cent and 30 per cent, respectively, for the light-exposed animals. Besides, rats of group 1 only, suffered from severe erythema, oedema and necrosis of the ears after four days treatment. It is probable that OLAQ, photoactivated in the skin, forms a reactive oxaziridine, analogous to what has been found with the tranquilliser chlordiazepoxide (Librium; Hoffmann La Roche), another imino-N-oxide.