Antibodies directed to drug epitopes to investigate the structure of drug-protein photoadducts. Recognition of a common photobound substructure in tiaprofenic acid/ketoprofen cross-photoreactivity.

Drug-induced photoallergy is an immune adverse reaction to the combined effect of drugs and light. From the mechanistic point of view, it first involves covalent binding of drug to protein resulting in the formation of a photoantigen. Hence, determination of the structures of drug-protein photoadducts is of great relevance to understand the molecular basis of photoallergy and cross-immunoreactivity among drugs. Looking for new strategies to investigate the covalent photobinding of drugs to proteins, we generated highly specific antibodies to drug chemical substructures. The availability of such antibodies has allowed us to discriminate between the different modes by which tiaprofenic acid (TPA), suprofen (SUP), and ketoprofen (KTP) photobind to proteins. The finding that the vast majority of the TPA photoadduct can be accounted for by means of antibody anti-benzoyl strongly supports the view that the drug binds preferentially via the thiophene ring, leaving the benzene ring more accessible. By contrast, selective recognition of SUP-protein photoadducts by antibody anti-thenoyl evidences a preferential coupling via the benzene ring leaving the thiophene moiety more distant from the protein matrix. In the case of KTP, photoadducts are exclusively recognized by antibody anti-benzoyl, indicating that the benzene ring is again more accessible. As a result of this research, we have been able to identify a common substructure that is present in TPA-albumin and KTP-albumin photoadducts. This is remarkable since, at a first sight, the greatest structural similarities can be found between TPA and SUP as they share the same benzoylthiophene chromophore. These findings can explain the previously reported observations of cross-reactivity to KTP (or TPA) in patients photosensitized to TPA (or KTP).

Mechanism of lipid peroxidation photosensitized by tiaprofenic acid: product studies using linoleic acid and 1,4-cyclohexadienes as model substrates.

A careful study of the linoleic acid hydroperoxide (LOOH) profile obtained upon peroxidation of linoleic acid (LA) photosensitized by tiaprofenic acid (TPA) and analogous ketones has been undertaken to distinguish between type-I and type-II photoperoxidation mechanisms. 1,4-Cyclohexadiene and 1,2-dimethylcyclohexa-2,5-dienecarboxylic acid (CHDCA) have also been used as models for LA since they also have double allylic systems. Coirradiation of LA with TPA and decarboxytiaprofenic acid (DTPA) in acetonitrile and micellar media produced significant amounts of conjugated dienic LOOH. The cis,trans to trans,trans ratio depended on the irradiation time; thus, this parameter is an ambiguous tool for mechanistic assignment. An interesting finding was the decrease of the LOOH level after long irradiation times in mixtures photooxidized by DTPA, which is attributed to quenching of the DTPA triplet by the generated dienic LOOH. High-performance liquid chromatography analyses confirmed that the main pathway operating in photodynamic lipid peroxidation sensitized by (D)TPA is a type-I mechanism. However, product studies using CHDCA have clearly shown that a type-II mechanism is also operating and might contribute to the overall photooxidation process in a significant way.

Involvement of type I and type II mechanisms in the linoleic acid peroxidation photosensitized by tiaprofenic acid.

Analysis of the photomixtures resulting from irradiation of aqueous solutions of linoleic acid sensitized by tiaprofenic acid (TPA) or its major photoproduct (DTPA) by HPLC has shown the formation of all the four possible conjugated dienic hydroperoxides. According to laser flash photolysis experiments the rate constants for hydrogen abstraction from linoleic acid by the excited triplet states of TPA and DTPA are 2 x 10(5) and 3.2x 10(5) M(-1) s(-1), respectively. These data, together with the known rate constants for oxygen quenching of triplet (D)TPA and for the reaction of singlet oxygen with linoleic acid, show that the mechanism is mixed type I/type II. Finally, typical radical scavengers such as BHA and singlet oxygen quenchers such as DABCO and sodium azide are efficient quenchers of the triplet excited state of DTPA. This shows the risk of assigning mechanisms based on indirect 'evidences' using 'specific' additives.

An In Vitro Approach to Drug Photoallergy: Use of Drug-directed Antibodies to Assess Photobinding of Non-steroidal Anti-inflammatories to Skin Cells.

Photobinding of drugs to biomolecules constitutes the early key event in the onset of photoallergy. This process generally involves excitation of the drug to an excited triplet state, which in turn can interact with cell constituents leading, in the case of proteins, to the formation of covalent photoadducts. The resulting photoantigens may trigger an immune response. In the present communication, we report the use of drug-directed antibodies to detect photoadduct formation in skin cells. This has been exemplified with tiaprofenic acid and suprofen as model compounds (two well known photoallergens) and human fibroblasts as representative skin cells. Upon irradiation of cells in the presence of these non-steroidal anti-inflammatories, time-dependent photoadduct formation was observed. This occurred predominantly at the cell membrane level. Most interestingly, the immunogenicity of cell photoadducts could be demonstrated by injection of Balb/c mouse fibroblasts into immunologically identical syngenic animals, where they triggered an immune response, as evidenced by the formation of specific antibodies and sensitized T-cells.

Photobinding of tiaprofenic acid and suprofen to proteins and cells: a combined study using radiolabeling, antibodies and laser flash photolysis of model bichromophores.

Drug photoallergy is a matter of current concern. It involves the formation of drug-protein photoadducts (photoantigens) that may ultimately trigger an immunological response. Tyrosine residues appear to be key binding sites in proteins. The present work has investigated the photobinding of tiaprofenic and (TPA) and the closely related isomer suprofen (SUP) to proteins and cells by means of radioactive labelling and drug-directed antibodies. To ascertain whether preassociation with the protein may play a role in photoreactivity, two model bichromophoric compounds (TPA-Tyr and SUP-Tyr) have been prepared and studied by laser flash photolysis. The results of this work show that (a) TPA and SUP photobind to proteins with similar efficiencies, (b) both drugs form photoadducts that share a basic common structure, as they are recognized by the same antibody and (c) drug-protein preassociation must play a key role in photoreactivity, as indicated by the dramatic decrease in the triplet state lifetimes of the model bichromophores compared to the parent drugs.

Drug-photosensitized protein modification: identification of the reactive sites and elucidation of the reaction mechanisms with tiaprofenic acid/albumin as model system.

Certain drugs can photosensitive the formation of protein modifications, which are thought to be responsible for the occurrence of photoallergy. In the present work, the UV irradiation of serum albumin in the presence of tiaprofenic acid has been studied as a model system for drug-photosensitized protein modifications. The photolysates evidenced that His, Tyr, and Trp are the reactive sites of the protein. The experimental results strongly suggest that formal hydrogen abstraction from the OH or NH groups of Tyr or Trp by the excited drug is the key photochemical process. Competition between cage escape and in cage recombination of the resulting radical pairs governs the final outcome: protein photo-cross-linking versus drug-protein adduct formation. These findings are highly relevant to understand the process of photohapten formation, the first event in the onset of photoallergy.

Mechanisms of photosensitization by drugs: Involvement of tyrosines in the photomodification of proteins mediated by tiaprofenic acid in vitro.

The photosensitizing potential of drugs must be related to their photoreactivity towards the target biomolecules. In this context, a representative photosensitizing drug (tiaprofenic acid) was co-irradiated with a model protein, bovine serum albumin (BSA). This led to a significant degree of protein crosslinking and to the formation of trace amounts of drug-BSA photoadducts. Amino acid analysis of the hydrolysed (HC1) protein showed that His and Tyr undergo a dramatic decrease (approx. 90%) as a consequence of drug-mediated photodynamic processes. When the drug was irradiated in the presence of the pure amino acids, extensive phototransformation of the latter was observed. Other photosensitizing drugs gave rise to similar processes when irradiated in the presence of BSA or the isolated amino acids. In conclusion, histidine and tyrosine appear to be key sites for the photosensitized damage to proteins. Photodegradation of the isolated amino acids in vitro may be an indicator of the photosensitizing potential of drugs.

Phototoxicity of non-steroidal anti-inflammatory drugs: in vitro testing of the photoproducts of Butibufen and Flurbiprofen.

In this work, the phototoxicity of two non-steroidal anti-inflammatory drugs, Butibufen and Flurbiprofen, was examined. Both were unstable to light, to give several photoproducts which were isolated and identified. The different photoproducts were formed by a primary photochemical mechanism which involves an initial cleavage of the C-C bond alpha to the carbonyl group, followed by several secondary processes. The cytotoxic effects of the xenobiotics were evaluated using two well-established biological in vitro tests: (a) enzyme leakage lactate dehydrogenase and glutamate-oxaloacetate transaminase from cultured fibroblasts and (b) lysis of red blood cells. The benzylic alcohols caused extensive leakage from cultured fibroblasts at the different concentrations assayed. The alcohol obtained from Butibufen was a potent lytic agent for human red blood cells. The other photoproducts, Butibufen and Flurbiprofen did not produce observable toxic effects on cells.

Photolytic degradation of benorylate: effects of the photoproducts on cultured hepatocytes.

The photodegradation of benorylate [4'-(acetamido)phenyl-2-acetoxybenzoate], a drug frequently used in rheumatoid arthritis therapy, has been examined under different sets of experimental conditions. Several photoproducts have been isolated and identified on the basis of their IR, NMR, and MS spectra. The most significant photochemical process is the photo-Fries rearrangement of benorylate, leading to 5-acetamido-2'-acetoxy-2-hydroxybenzophenone (1). This compound undergoes a rapid transacylation to the isomeric 5'-acetamido-2'-acetoxy-2-hydroxybenzophenone (2). A primary culture of rat hepatocytes has been used to evaluate the possible toxicity of these two benzophenones, keeping in mind the following criteria: leakage of cytosolic enzymes, attachment index to culture plates, gluconeogenesis from lactate and fructose, glycogen balance, and albumin synthesis. At the concentrations assayed, neither of the two major photoproducts of benorylate (benzophenones 1 and 2) had significant toxic effects on liver cells in culture.

Toxic effects of the photoproducts of chlorpromazine on cultured hepatocytes.

The photodegradation of chlorpromazine, a drug frequently used in psychotherapy, was examined under different sets of experimental conditions. A primary culture of rat hepatocytes was used to evaluate the possible hepatotoxicity of the chlorpromazine photoproducts, keeping in mind the following criteria: leakage of cytosolic enzymes; attachment index to culture plates, and albumin synthesis. Cells exposed to concentrations greater than 10(-4) M of the photomixtures showed extensive leakage of GOT and GPT into the culture medium and, at the same time, the cell attachment was seriously impaired. A concentration of 10(-7) M of the photoproducts proved capable of inhibiting the synthesis of albumin (20%). Photoproducts obtained after aerobic irradiations were as toxic for hepatocytes as those found in anaerobic conditions. The implications of our results in connection with the relevance of oxygen-dependent photoreactions of chlorpromazine to its phototoxicity, and the possible appearance of hepatic alterations in patients treated with the drug after exposure to the sunlight, are discussed.