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J. Phys. Chem. B 2009, 113, 11306–11313
Photodegradation Mechanism of Nonsteroidal Anti-Inflammatory Drugs Containing Thiophene Moieties: Suprofen and Tiaprofenic Acid Klefah A. K. Musa and Leif A. Eriksson* ¨ rebro Life Science Center, O ¨ rebro UniVersity, 701 82 O ¨ rebro, Sweden School of Science and Technology and O ReceiVed: May 5, 2009; ReVised Manuscript ReceiVed: June 2, 2009
The photodegradation of nonsteroid anti-inflammatory drugs suprofen, 2-[4-(2-thienoyl)phenyl]propionic acid, and tiaprofenic acid, 2-(5-benzoyl-2-thienyl)propanoic acid, is studied by means of density functional theory. Besides the redox properties of the neutral species, we report on absorption spectra and degradation pathways involving excitation, intersystem crossing to the T1 state, and spontaneous decarboxylation of the deprotonated species of each drug. The energetics and properties of the suprofen and tiaprofenic acid systems are found to be very similar to those of the highly photolabile benzyl analogue ketoprofen. Mechanisms leading to the formation of a closed-shell decarboxylated ethyl species, as well as peroxyl radicals capable of initiating lipid peroxidation reactions, are discussed. Introduction Nonsteroidal anti-inflammatory drugs (NSAIDs) are in wide use for the treatment of inflammation and associated diseases, in spite of the occurrence of various adverse side effects, such as cutaneous phototoxic responses. Suprofen (SUP), tiaprofenic acid (TP), ketoprofen, and ibuprofen are NSAIDs derived from 2-arylpropionic acid with the most pronounced photosensitizing effects.1-3 NSAIDs are able to induce photosensitization, because of their ability to absorb radiation which is able to penetrate the skin (wavelengths longer than 310 nm) and since the resulting transient species are able to interact with biosubstrates.4 The pharmacological action of SUP and TP occurs by inhibition of the cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) isoenzymes.5,6 SUP is a potent tissue-selective inhibitor of prostaglandin biosynthesis,7,8 used to relieve the pain associated with primary dysmenorrhea9 and musculoskeletal and other forms of pain.10 Similar to SUP, TP is widely dispensed and administered as a racemic mixture for the treatment of acute and chronic arthritis and osteoarthritis.6 Topical pretreatment with NSAIDs such as suprofen is a common practice to maintain maximal pupil dilatation for cataract surgery, however often with associated eye irritation caused by the topical administration.11,12 Symptoms of suprofen overdose include bleeding in the eye or redness or swelling of the eye or the eyelid, blurred vision or other changes in vision, fever or chills, itching or tearing, nausea or vomiting, sticky or matted eyelashes, swelling of the face, throbbing pain, tightness in the chest, shortness of breath, troubled breathing, and sensitivity to light.5 Cutaneous photosensitization by drugs is a relatively common side effect, which involves the development of skin reactions through the combined effect of drugs and light.13-15 The relatively general term “photosensitivity reactions” may be more specifically categorized as phototoxic or photoallergic in nature, the former being much more common than the latter. SUP was shown to induce positive reactions to photopatch testing with ultraviolet A (UVA) and B (UVB) rays,16 introduced to the market as an ointment, and clinical evidence of photosensitivity * To whom correspondence should be addressed. E-mail: leif.eriksson@ oru.se.
Figure 1. Suprofen (left) and TP (right). The numbering shown is used throughout the study.
induced by suprofen was reported.17 The photosensitivity of TP has been investigated in vivo and in vitro and is reported in many studies.18-23 Both these drugs are frequently associated with a high incidence of phototoxic or photoallergic reactions; in many studies in vitro using proteins or whole cells it is demonstrated that the NSAID may provoke modifications in proteins and other cell constituents (e.g., DNA) after irradiation.24,25 The nature of the photobinding is as yet unknown.26 However, the investigation of both forms in the presence and absence of light is important to understand how light affects the mode, site, and mechanism of association with the protein. The photobinding to proteins can occur by two postulated pathways: (a) by association by weak van der Waals or hydrogen bonding, the drug binds to the protein and subsequently photobinds upon UV irradiation, or (b) the drug first decomposes in bulk solution, forming less polar photoproduct(s) which become more strongly associated to the protein and result in covalent bond formation once irradiation takes place.26,27 From a chemical structure point of view, SUP, 2-[4-(2thienoyl)phenyl]propionic acid, differs from the benzophenonelike ketoprofen by replacement of the nonsubstituted benzene ring by thiophene and the position of the propionic group in the para instead of meta position. TP, 2-(5-benzoyl-2-thienyl)propanoic acid, on the other hand, is closely related to SUP, with the propanoic acid placed on the thiophenyl ring instead of the phenolic moiety (Figure 1). The 2-arylpropionic acid derivative, or “profen” group, contains a chiral center (an asymmetric carbon atom) located at the C2 of the propionic moiety and, therefore, exists in two enantiomeric forms, R (-) and S (+). Only the S-enantiomer has significant pharmacological activity on cyclooxygenase.28,29 The structural characteristics of SUP can influence its biological fate, and the disposition of suprofen enantiomers may be
10.1021/jp904171p CCC: $40.75 2009 American Chemical Society Published on Web 07/14/2009
Photodegradation of Suprofen and Tiaprofenic Acid SCHEME 1: Schematic Presentation of the Photodecomposition of Suprofen (Upper Molecule) and Tiaprofenic Acid (Lower Molecule)
J. Phys. Chem. B, Vol. 113, No. 32, 2009 11307 degradation mechanisms for SUP and TP are constructed as shown in Scheme 1, which will be discussed in more detail below. Computational chemistry is a powerful tool to obtain a deeper insight into physicochemical properties of molecules. We herein report on a theoretical study of the SUP and TP photochemical mechanism, which may assist in the design of new drugs with improved therapeutic effects and/or reduced adverse side effects. Methodology
enantioselective, whereby metabolic chiral inversion transforms the inactive R-enantiomer into the pharmacologically active S-form.30 The key molecular basis for this mechanism involves the enantioselective formation of a coenzyme A (CoA) thioester by long-chain CoA ligase.31 Both SUP and TP have the capability to absorb UV radiation and produce radicals; they can furthermore act as photosensitizers by transferring the absorbed energy or an electron to oxygen, which results in the formation of reactive oxygen species (ROS) such as singlet oxygen and the superoxide radical anion.21,32 Upon UV exposure, both drugs can be transformed into different photodecarboxylated products (Scheme 1). The major photoproduct is the decarboxylated species (C), which is also an efficient photosensitizer capable of generating ROS.20-22 Trapping of oxygen or ROS by a UV-induced decarboxylated radical results in the formation of the oxygenated minor photoproducts decarboxylated alcohol or ketone derivatives. Previous experimental work suggests that, to reduce or prevent the phototoxic effects of suprofen and its decomposition process, powerful antioxidants such as vitamin C and cysteine derivatives should be used, as these have the ability to increase the bioavailability of suprofen and protect against UVA-induced decomposition thereof in topical application.33 On the basis of the experimental findings of SUP and TP photodegradation mechanisms22 and our previous work on related drugs ketoprofen, ibuprofen, and naproxen,34-36 photo-
All geometries of SUP and TP and their neutral, radical anion, radical cation, deprotonated anion, and decarboxylated forms were optimized at the B3LYP/6-31G(d,p) level of theory. Solvent effects were included implicitly, through single-point calculations on the optimized geometries at the same level of theory, including the integral equation formulation of the polarized continuum model (IEFPCM).37-39 Water was used as the solvent, through the value 78.31 for the dielectric constant in the IEFPCM calculations. Frequency calculations were performed on the optimized geometries at the same level of theory to ensure the systems to be local minima (no imaginary vibration frequencies) and to extract zero-point vibrational energies (ZPEs) and thermal corrections to the Gibbs free energies at 298 K. The numbering scheme of the atoms used throughout the study is given in Figure 1. The extension of the ground-state DFT methodology to the excited states is enabled by the time-dependent formalism (TD-DFT).40-42 Excitation spectra were thus calculated using the TD-DFT approach at the same level of theory. For these types of calculations, e.g., computing the UV-vis spectra, the hybrid B3LYP functional methodology which explicitly includes a fraction of Hartree-Fock exchange, in combination with a compact basis set such as 6-31(d,p) or 6-31+G(d,p), provides an attractive compromise between accuracy and computational cost.43-47 The UV-vis spectra of the molecules were obtained by plotting wavelengths of the excited states in nanometers against their corresponding oscillator strengths using a Gaussian line shape. In our previous studies of a similarly sized NSAID molecule, diclofenac and its photoproduct, test calculations were performed to investigate the suitable basis set to be used. The calculations show that the differences when using a range of basis sets (e.g., 6-31(d,p) and 6-311(d,p) with or without diffuse functions) are within a few nanometers, and we have thus chosen to use the B3LYP/6-31G(d,p) level of theory throughout.48 All calculations were performed with the GAUSSIAN 03 program package.49 Results and Discussion Redox Chemistry of Suprofen and Tiaprofenic Acid. We begin by investigating the proton affinity and redox properties of the parent compound A for both SUP and TP drugs. In Figures S1 and S2 in the Supporting Information we display the optimized structure of SUP and TP, respectively, including their corresponding radical anion and radical cation (A•- and A•+) and deprotonated acid (A-). For SUP the steric repulsion between two rings results in a dihedral angle (C8-C7-C10-C12) between them of -33.5°, -15.9°, -54.7°, and -27.7° (neutral, radical anion, radical cation, and deprotonated acid, respectively). This induced steric torsion reduces the delocalization conjugation over the molecule, and it is also reflected in elongated C-C bonds to the central carbonyl group (1.42-1.50 Å in compound A), compared to the C-C bond lengths 1.37-1.40 Å in the phenyl and thiophene rings. For TP, this
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Figure 2. Computed orbitals for SUP (left) and TP (right).
effect is slightly less, giving dihedral angles (C7-C6-C9-C11) of -18.3°, -10.6°, -41.6°, and -13.4°, respectively. Forming the radical anion or radical cation of SUP (TP) (Figure S1b,c and S2b,c in the Supporting Information, respectively), the main changes in optimized structures are found in the bonds to the carbonyl carbon C10 (C9), which can be rationalized by inspection of the orbitals of the parent compound (Figure 2). The highest occupied molecular orbital (HOMO) of A of SUP or TP is localized mainly on the central carbonyl oxygen and the thiophene ring, whereas the lowest unoccupied MO (LUMO) is a π* orbital antibonding between C10 and O11 (C9 and O10) and with tails out into the phenyl and (mainly) thiophene rings. Thus, both adding an electron to the LUMO and removal of an electron from the HOMO will primarily lead to an elongation of the CdO bond and a slight reduction in the C10-C bonds as displayed in Figures S2 and S3. Mulliken charges of SUP and TP of the different species are displayed in Tables S1 and S3 in the Supporting Information, respectively. For the radical anion A•-, only approximately 0.21 e is added to the carbonyl group, and for the cation 0.10 e is removed therefrom. Hence, in the relaxed structures, the charge delocalizes into the phenyl and thiophene rings, giving rise to slightly increased C-C bond lengths in these systems. The unpaired spin distributions of the radical anion (Tables S2 and S4, for SUP and TP, in the Supporting Information, respectively) show a delocalization over both rings and the central carbonyl unit, with the main component on O11 of SUP (0.26 e) and on O10 of TP (0.255 e). For the A•+ radical cation of SUP, the unpaired spin is found on the substituted (phenyl) ring and on the central carbonyl group, with the main component on O11 (0.47 e), whereas for TP the unpaired spin is found on the thiophenyl ring and on the central carbonyl group, with the main component on O10 only (0.29 e). Again, these pictures are consistent with shapes of the HOMO and LUMO orbitals of the neutral ground-state system as seen in Figure 2.
For the deprotonated species B- of both SUP and TP the C1-C2 bond increases from 1.524 to 1.624 Å (SUP) and from 1.531 to 1.656 Å (TP), indicating that the deprotonated system of these drugs may be conditioned to undergo decarboxylation. The carboxylic moiety holds approximately -0.65 (SUP) and -0.61 (TP) e of the negative charge, respectively, whereas the charges on the central carbonyl group are essentially unaltered. The orbitals of SUP and TP depicted in Figure 2 show that the HOMO, HOMO - 1 and HOMO - 2 orbitals of B- are all centered on the carboxylic moiety of the molecule, as opposed to the situation for the protonated species. Hence, it can be expected that the photochemistry of the form present at physiological pH (A-; pKa of SUP (TP) ) 3.9 (
