Mechanism of photosensitive reactions induced by piroxicam

Sensitized photoxygenation of piroxicam in neat solvents and solvent mixtures

Detection of O(2)(1Delta(g)) phosphorescence emission, lambda(max)=1270 nm, following laser excitation and steady state methods were employed to determine the total rate constant, k(T), for the reaction between the non-steroidal anti-inflammatory drug piroxicam (PRX) and singlet oxygen in several solvents. Values of k(T) ranged from 0.048+/-0.003 x 10(6) M(-1) s(-1) in chloroform to 71.2+/-2.2 x 10(6) M(-1) s(-1) in N,N-dimethylformamide. The chemical reaction rate constant, k(R), was determined by using thermal decomposition of 1,4-dimethylnaphthalene endoperoxide as the singlet oxygen source. In acetonitrile, the k(R) value is equal to 5.0+/-0.4 x 10(6) M(-1) s(-1), very close to the k(T) value. This result indicates that, in this solvent, the chemical reaction corresponds to the main reaction path. Dependence of total rate constant on the solvent parameters pi* and beta can be explained in terms of a reaction mechanism that involves the formation of a perepoxide intermediate. Rearrangement of the perepoxide to dioxetane followed by ring cleavage and transacylation accounts for the formation of N-methylsaccharine and N-(2-pyridyl)oxamic acid, the main reaction products. Data obtained in dioxane-water (pH 4) mixtures with neutral enolic and zwitterionic tautomers of piroxicam in equilibrium show that the zwitterionic tautomer reacts with singlet oxygen faster than the enolic tautomer.

A cross-reaction between piroxicam-photosensitivity and thiosalicylate hypersensitivity in lymphocyte proliferation test

We examined the cross-reaction between photosensitivity to piroxicam (PXM) and contact sensitivity to thiosalicylate (TOS) by a lymphocyte proliferation test (LPT) in guinea pigs. The lymph node cells (LNCs) plus peritoneal exudate cells (PECs) from guinea pigs contact-sensitized with TOS remarkably cross-proliferated to PXM under UVA (4 J/cm2) irradiation. On the other hand, the PXM-photosensitized LNCs+PECs also cross-proliferated to TOS. From these results, the reciprocal cross-reaction between TOS-hypersensitivity and PXM-photosensitivity was reconfirmed by the in vitro LPT, indirectly indicating that the PXM-photosensitivity is a cell (probably T cell)-mediated PXM photoallergy in its nature. The TOS-primed LNCs+PECs did not cross-proliferate to UVA (4 J, 180 J or 500 J/cm2)-pretreated PXM (UVA-PXM) although it is supposed to contain several photoproducts of PXM. Furthermore, the TOS-primed LNCs developed a remarkable proliferative cross-response to the PECs pulsed with PXM under UVA (4 J/cm2) irradiation (photo-PXM-modified PECs), but not to the PECs pulsed with PXM or UVA-PXM. Therefore, it is presumed that the cross-reactive molecule, which is easily formed from PXM under UVA irradiation, is unstable, and that the formation of complete antigen by the generation of this molecule and its photobinding needs the coexistence of PECs, PXM and UVA irradiation at the same time in the culture.

Photosensitivity to piroxicam: absence of cross-reaction with tenoxicam

We studied 2 groups of patients. One group of 10 patients had a photosensitive eruption to piroxicam. Another group of 24 patients had positive patch test reactions to thimerosal and thiosalicylic acid and had never taken piroxicam or tenoxicam. Patients were patch tested with thimerosal 0.1% pet., thiosalicylic acid 0.1% pet., salicylic acid 2.0% pet., piroxicam 1 and 5% pet. and tenoxicam 1 and 5% pet. Photopatch tests were also performed with piroxicam and tenoxicam. All 10 patients with photosensitivity to piroxicam had positive patch tests to thimerosal and thiosalicylic acid and 9 of them had positive photopatch tests to piroxicam. 20 out of 24 patients with positive patch tests to thiosalicylic acid also had positive photopatch tests to piroxicam. All the patients tested with salicyclic acid were negative. Out of the 29 patients with positive photopatch tests to piroxicam, none reacted to tenoxicam. In countries with a high incidence of contact sensitivity to thimerosal/thiosalicylic acid, the use of piroxicam should be avoided and replaced by tenoxicam, a drug without reported photosensitivity.

Sensitivity to thimerosal and photosensitivity to piroxicam

17 patients allergic to thimerosal, with no previous history of photosensitivity or piroxicam ingestion, 2 patients with piroxicam-induced photosensitivity, and 10 controls were patch or photopatch tested, with 1 or more of the following: thimerosal, thiosalicylic acid, irradiated and non-irradiated solutions of piroxicam alone, L-cysteine alone and piroxicam plus L-cysteine, and piroxicam in petrolatum., The results of the tests supported the hypothesis that there are cross-reactions between thiosalicylic acid and prioxicam, a photosensitizer. The mechanism of the cross-reactions may involve photoproducts of piroxicam and L-cysteine, as patients allergic to thiosalicylic acid, who have positive photopatch tests to piroxicam, also had positive patch tests to the irradiated solution of piroxicam plus L-cysteine.

Piroxicam-induced photosensitivity and contact sensitivity to thiosalicylic acid

A photocontact dermatitis developed in three patients after the application of gel containing 0.5% piroxicam. Patch tests were positive to thiomersal and thiosalicylic acid. Photopatch tests with piroxicam at several concentrations were positive in the three patients but negative in 62 normal volunteer subjects. Patch tests performed on 14 patients with proved systemic photosensitivity to piroxicam were positive for thiomersal and thiosalicylic acid. Nine of 12 patients previously sensitized to thiosalicylic acid and with no history of exposure to piroxicam showed positive photopatch test reactions to this chemical. These results support a relation between piroxicam-induced photosensitivity and contact sensitivity to thiosalicylic acid. Contact allergic sensitivity to the latter is a marker for patients with a high risk of developing photosensitivity reactions to piroxicam. These reactions may be due to photoproducts of the drug rather than metabolites.