A molecular insight into the phototoxic reactions observed with vemurafenib, a first-line drug against metastatic melanoma

The electronic properties of vemurafenib (VB) provide a rational basis for understanding its strong UVA-induced phototoxicity. Thus, solvation of hydrophobic VB by hydrogen bonding solvents controls its photophysical, photochemical and photosensitizing properties. Addition of phosphate buffered saline (PBS) to methanol (MeOH) induces a bathochromic shift of the VB absorbance spectrum and a fluorescence emission (λmax = 450 nm, quantum yield (Φ) = 0.011). Phosphorescence (λmax = 461 nm) is observed at 77 K in MeOH. 308 nm laser flash spectroscopy demonstrates that the lifetimes (τ) and quantum yields of the VB triplet state ((3)T(*)(1)) in deaerated MeOH (τMeOH = 0.41 μs, λmax ∼ 380 nm), MeOH-PBS and HSA solutions markedly depend on the microenvironment. A long-lived radical (half-life >200 μs) is also formed. The state (3)T(*)(1) is quenched by O2 and electron donors (Cys and 2'-deoxyguanosine) at a rate constant >1 × 10(9) M(-1) s(-1). UVA-irradiation of VB in air-saturated MeOH or MeOH-PBS solutions produces a UVA-absorbing photoproduct (Φ ∼ 5 × 10(-4)). VB photosensitizes Trp destruction by type I (radical formation) and type II (singlet oxygen ((1)O2) formation) photodynamic reactions (Φ = 0.005). Singlet oxygen production is further demonstrated by the VB-photosensitized His oxidation (ΦMeOH = 0.006).

Drug–tubulin interactions interrogated by transient absorption spectroscopy

The triplet excited state of complexed COL and MTC gives well defined transient spectra undetectable in the absence of TU.

Photoreactivity of the nonsteroidal anti-inflammatory 2-arylpropionic acids with photosensitizing side effects

The photoreactivity of the nonsteroidal anti-inflammatory 2-arylpropionic acids benoxaprofen, carprofen, naproxen, ketoprofen, tiaprofenic acid, and suprofen is reviewed with special emphasis on fundamental photophysical and photochemical properties. The absorption and emission properties of the excited states of these drugs as well as their main photodegradation routes are summarized. The photochemical mechanisms are discussed on the basis of product studies and detection of short-lived intermediates by means of laser flash photolysis. After dealing with the unimolecular processes, attention is focused on the photosensitized reactions of key biomolecules, such as lipids, proteins or nucleic acids. Finally, a short section on the photobiological effects on simple biological models is also included. Although some earlier citations are included, the literature coverage is in general limited to the last decade.

Photochemistry of 2-hydroxy-4-trifluoromethylbenzoic acid, major metabolite of the photosensitizing platelet antiaggregant drug triflusal

Triflusal is a platelet antiaggregant drug with photoallergic side effects. However, it is considered a prodrug since it is metabolized to 2-hydroxy-4-trifluoromethylbenzoic acid (HTB)--the pharmacologically active form. HTB was found to be photolabile under various conditions. Its major photodegradation pathway appears to be the nucleophilic attack at the trifluoromethyl moiety. The involvement of the triplet state in the photodegradation has been unequivocally proved by direct detection of this transient in laser flash photolysis and by quenching experiments with oxygen, cyclohexadiene and naphthalene. Finally, the photobinding of HTB to proteins such as bovine serum albumin has been demonstrated using ultraviolet-visible (UV-Vis) and fluorescence spectroscopy. Nucleophilic groups present in the protein appear to be responsible for the formation of covalent drug photoadducts, which is the first step involved in the photoallergy shown by triflusal.

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.

Photobinding of carprofen to protein

Carprofen is a non-steroidal antiinflammatory drug with marked photosensitising properties. In order to elucidate the mechanisms underlying the phenomenon of drug-protein photobinding, mixtures of the drug and human serum albumin were irradiated under different experimental conditions. After irradiation and subsequent gel-filtration chromatography of the photomixture, the eluting protein fraction was analysed by means of fluorescence spectroscopy. The formation of drug-protein photoadducts could be evidenced by the characteristic emission properties of the carbazole chromophore. The photobinding of the drug to human serum albumin appears to involve the formation of aryl radicals resulting from carbon-halogen photocleavage. This mechanistic interpretation is supported by the observed variations in the intensity of the fluorescence spectra, which can be correlated with the lower quantum yield emission of chlorocarbazoles as compared to non-halogenated analogues. The results from laser flash photolysis studies are also in agreement with this proposal.

Tiaprofenic acid-photosensitized damage to nucleic acids: a mechanistic study using complementary in vitro approaches

In order to determine whether or not tiaprofenic acid (TPA) could cause cellular DNA damage, human fibroblasts were irradiated in the presence of the drug and subsequently examined by means of the comet assay. This led to the observation that TPA actually sensitizes cellular DNA to the subsequent irradiation. When TPA was irradiated in the presence of supercoiled plasmid DNA, it produced large amounts of single-strand breaks (SSB); this is consistent with the effects observed on cellular genomic DNA by the comet assay. More importantly, low concentrations of TPA, unable to produce direct SSB, caused photo-oxidative damage to DNA as revealed by the use of excision-repair enzymes. The fact that TPA-irradiated DNA was a substrate of formamidopyrimidine glycosylase as well as endonuclease III revealed that both purine and pyrimidine bases were oxidized. This was further supported by the TPA-photosensitized oxidation of 2'-deoxyguanosine which led to a product mixture characteristic of mixed type-I/II mechanisms. Thymidine was less reactive under similar conditions, but it also decomposed to give a typical type-I product pattern. Accordingly, the TPA triplet was quenched by the two nucleosides with clearly different rate constants (10(8) vs 10(7) M-1 s-1, respectively). As cellular RNA also contains oxidizable bases, it could be the target of similar processes, thus interfering with the biosynthesis of proteins by the cells. Extraction of total RNA from TPA-irradiated human fibroblasts, followed by gel electrophoresis and PCR analysis, confirmed this hypothesis. Finally, photosensitization experiments with Saccharomyces cerevisiae showed that, in spite of an efficient drug-yeast interaction leading to cytotoxicity, neither intergenic recombination nor gene conversion took place. Thus, while TPA-photosensitized damage to nucleic acids can result in genotoxicity, the risk of mutagenicity does not appear to be significant.

A photophysical and photochemical study of 6-methoxy-2-naphthylacetic acid, the major metabolite of the phototoxic nonsteroidal antiinflammatory drug nabumetone

Nabumetone is a phototoxic nonsteroidal antiinflammatory drug used for the treatment of osteoarthritis. However, nabumetone is considered a prodrug with its metabolite 6-methoxy-2-naphthylacetic acid the active form. Photophysical and photochemical studies on this metabolite have been undertaken. It undergoes photodecarboxylation in aerated aqueous and organic solvents. In addition to the accepted photodegradation pathway for related molecules, a new mechanism that implies generation of the naphthalene radical cation from the excited singlet and addition of O2 prior to the decarboxylation process has been demonstrated. Evidence for the involvement of the excited singlet state in this mechanism have been obtained by steady-state and time-resolved fluorescence experiments. The fluorescence quenching by O2 and the shorter singlet lifetime in aerated solvents support this assignment. Laser flash photolysis also supports this mechanism by showing the noninvolvement of the triplet in the formation of the naphthalene radical cation. Finally, the well-known electron acceptor CCl4 acts as an efficient singlet quencher, enhancing the route leading to the radical cation, preventing intersystem crossing to the triplet and thus resulting in a dramatic increase in the yield of 6-methoxy-2-naphthaldehyde, the major oxidative decarboxylation product; this constitutes unambiguous proof in favor of the new mechanistic proposals.

Photosensitizing drugs containing the benzophenone chromophore

The nonsteroidal anti-inflammatory agents ketoprofen, tiaprofenic acid, suprofen and tolmetin, together with the anti-hyperlipoproteinemic drug fenofibrate and the anti-arrhythmic amiodarone can be included in the group of benzophenone-derived photosensitizing drugs. They contain a diaryl ketone chromophore and mediate the development of phototoxic reactions. In some cases, photoallergic responses have been reported. These properties have been substantiated in clinical reports, as well as by means of in vivo and in vitro assays. Tolmetin is phototoxic in vitro, however there are no reports on photosensitization by this drug in humans. In general, photochemical and photobiological studies strongly suggest that photosensitization involves formal hydrogen abstraction (either in a single step or via electron transfer followed by proton transfer) by the benzophenone-like chromophore from the excited triplet state. In the case of amiodarone, the radicals generated by photodehalogenation from the triplet are responsible for the photosensitivity side-effects.

Photochemical and photobiological properties of ketoprofen associated with the benzophenone chromophore

Irradiation of ketoprofen in neutral aqueous medium gave rise to 3-ethylbenzophenone as the major photoproduct. Its formation is justified via protonation of a benzylic carbanion or hydrogen abstraction by a benzylic radical. Minor amounts of eight additional compounds were isolated. Four of them are derived from the benzylic radical: 3-(1-hydroperoxyethyl)benzophenone, 3-(1-hydroxyethyl)benzophenone, 3-acetylbenzophenone and 2,3-bis-(3-benzoylphenyl)butane. The other four products involve initial hydrogen abstraction by the excited benzophenone chromophore of ketoprofen: 1,2-bis-(3-ethylphenyl)-1,2-diphenyl-1,2-ethanediol, 2-(3-benzoylphenyl)-1-(3-ethylphenyl)-1-phenylpropan-1-ol, alpha-(3-ethylphenyl)phenylmethanol, 1,2-bis-[3-(2-hydroxycarbonylethyl) phenyl]-1,2-diphenyl-1,2-ethanediol. The latter process was found to mediate the photoperoxidation of linoleic acid through a type I mechanism, as evidenced by the inhibition produced by the radical scavengers butylated hydroxyanisole and reduced glutathione. The major photoproduct, which contains the benzophenone moiety but lacks the propionic acid side chain, also photosensitized linoleic acid peroxidation. Because lipid peroxidation is indicative of cell membrane lysis, the above findings are highly relevant to explain the photobiological properties of ketoprofen.

Photosensitization by fenofibrate. II. In vitro phototoxicity of the major metabolites

Fenofibric acid, the major metabolite of fenofibrate, was found to be photolabile. Its irradiation in aqueous solution gave rise to two photoproducts, whose formation involves photodecarboxylation of the dissociated acid to an aryloxy-substituted carbanion, which is directly protonated or, alternatively, undergoes a Wittig rearrangement. A comparative in vitro phototoxicity study has been carried out on the anti-hyperlipoproteinemic drug fenofibrate, its metabolites and the photoproducts of fenofibric acid. Fenofibrate, fenofibric acid and its two photoproducts were found to be active when examined by the photohemolysis test and were able to photosensitize peroxidation of linoleic acid, as evidenced by the UV monitoring of dienic hydroperoxides. In summary, the major metabolite of fenofibrate (fenofibric acid), as well as its photoproducts, are phototoxic in vitro. This behavior can be attributed to the fact that the four compounds retain the benzophenone chromophore present in fenofibrate and is indicative of free radical-mediated photosensitization. In agreement with this rationalization, the metabolites with a reduced ketone functionality exhibit no detectable in vitro phototoxicity.

In vitro phototoxicity of clofibrate. Photochemical and photohemolytic studies on its metabolite clofibric acid

Aqueous or methanolic solutions of clofibrate and clofibric acid are photolabile towards UVB light under aerobic as well as anaerobic conditions. Nine photoproducts have been identified; their formation involves primary cleavage of the carbon-halogen bond or of the aryloxy-carbon bond, followed by hydrogen abstraction and/or radical recombination. Clofibric acid is phototoxic in vitro as indicated by the photohemolysis test, under both oxygen and argon atmospheres, although the photohemolysis rate is markedly higher under aerobic conditions. Partial inhibition of this process on addition of butylated hydroxyanisole (BHA), reduced glutathione (GSH), sodium azide (NaN3) or 1,4-diazabicyclo[2.2.2]octane (DABCO) suggests the involvement of type I as well as type II mechanisms.

Photodegradation and in vitro phototoxicity of fenofibrate, a photosensitizing anti-hyperlipoproteinemic drug

The phototoxic anti-hyperlipoproteinemic drug fenofibrate was found to be photolabile under aerobic and anaerobic conditions. Irradiation under argon of a methanol solution of this drug produced the photoproducts isopropyl 4-(1-[4-chlorophenyl]-1,2-dihydroxy)ethylphenoxyisobutyrate, 1,2-bis(4-chlorophenyl)-1,2bis (4-[isopropoxycarbonylisopropoxy]phenyl)ethane-1,2-diol and 4-(4-chlorobenzoyl)phenol, while under oxygen the photoproducts were 4-chloroperbenzoic acid, methyl 4-chlorobenzoate, 4-chlorobenzoic acid and singlet oxygen, as evidenced by trapping with 2,5-dimethylfuran. These results can be rationalized through hydrogen abstraction by excited fenofibrate, to afford a free radical as key intermediate. Biologically active antioxidants such as glutathione and cysteine efficiently reduced 4-chloroperbenzoic acid to 4-chlorobenzoic acid. The involvement of an electron transfer mechanism is suggested by detection (UV-vis spectrophotometry) of the radical cation TMP+. during the oxidation of tetramethylphenylenediamine (TMP) with 4-chloroperbenzoic acid. Fenofibrate was phototoxic in vitro when examined by the photohemolysis test, both under oxygen and argon atmosphere, although the photohemolysis rate was markedly lower under anaerobic conditions. The photoproducts 4-(1-[4-chlorophenyl]-1,2-dihydroxy)ethylphenoxyisobutyrate and 4-chloroperbenzoic acid induced hemolysis in the dark; however, this effect was quantitatively less important than photohemolysis by fenofibrate. On the other hand, fenofibrate photosensitized peroxidation of linoleic acid, monitored by the UV detection of dienic hydroperoxides. Based on the inhibition of this process upon addition of butylated hydroxyanisole, a radical chain (type I) mechanism appears to operate. In summary, fenofibrate is phototoxic in vitro. This behavior can be explained through the involvement of free radicals, singlet oxygen and stable photoproducts.

Oxidative decarboxylation of naproxen

The decarboxylation of naproxen (1H) and its salt (1-) was achieved by means of chemical [Ce(IV) or S2O8(2-)] and electrochemical oxidation. The product patterns were compatible with mechanisms involving single-electron transfer from the pi-system or the carboxylate moiety. The results are discussed in connection with the involvement of electron-transfer processes in the reported phototoxicity of naproxen.

Photochemistry of tiaprofenic acid, a nonsteroidal anti-inflammatory drug with phototoxic side effects

The phototoxic nonsteroidal anti-inflammatory drug tiaprofenic acid (1) is photolabile under aerobic conditions. Irradiation of a methanol solution of 1 under oxygen produces the photoproducts 2, 3, 4, and 5, and also produces a singlet oxygen as evidenced by trapping with 2,5-dimethylfuran.