Values for the rate constants for the interactions of each NSAID by quenching with electronically excited singlet
Abstract
The present chapter deals on the interaction of nonsteroidal anti-inflammatory drugs (NSAIDs), diflunisal, indomethacin, meloxicam, tenoxicam and piroxicam with reactive oxygen species (ROS) photogenerated in aqueous solution by the vitamin riboflavin employed as a dye sensitizer. Simple techniques as substrate and oxygen consumption and more sophisticated time-resolved spectroscopic methods were employed for the kinetic and mechanistic evaluation of the deactivation of the in situ generated ROS singlet molecular oxygen (O2(Δ1g)), superoxide radical anion (O2·− ) and hydrogen peroxide (H2O2) by the mentioned NSAIDs. Results could be prudently extrapolated to a possible action of NSAIDs in the retardation or inhibition of neuroinflammatory disorders, in which oxidative agents such as ROS were found to be upregulated. Despite the potential benefit, some adverse effects in humans reported in relation with high doses of NSAIDs alert about the cares that have to be taken about their use.
Keywords
- antioxidants
- NSAIDs
- photosensitization
- riboflavin
- ROS
1. Introduction
In the last decades, it has been a widespread use of an increasing number of chemical compounds with analgesic, antipyretic, and anti-inflammatory properties. In order to remark their differences with other group of medicines which presents known bad side effects, they were labeled as nonsteroidal anti-inflammatory drugs with the acronym NSAIDs [1–3].
At the same time, many neuroinflammatory mediators, including oxidative agents such as reactive oxygen species (ROS), were found to be upregulated in neurodegenerative disorders (ND) that affect human brain areas [4, 5]. This fact immediately allows the proposal of some kind of cause-effect link between the presence of ROS, oxidation processes, neuroinflammation, and ND pathogenesis [4, 5].
Oxidative stress is a process that occurs in early stages of ND and is considered an identifier mark for their detection as could be evaluated by DNA, RNA, lipids, and protein oxidation levels [6–8]. Simultaneously, several studies have observed an inverse correspondence between prolonged NSAID administration and the development of some ND in humans, (for review, see Ref. [9]). So, it is now accepted that NSAIDs could play a protective role on many ND and one of the reasons of the great interest for getting more insight into the elucidation of the pathways and mechanisms of the oxidative processes in which several NSAIDs and different ROS take part.
The present chapter will analyze the results presented in two relatively recent papers that have been dedicated to evaluate the possible action of some NSAIDs as protectors against ROS-mediated oxidation/deterioration of biological targets [10, 11]. Those research works are focused on NSAIDs from different chemical structure classes, one salicylic acid derivative, diflunisal (DFN), an indolic acid derivative, indomethacin (IMT) (Figure 1) and the enolic acid derivatives, oxicams, represented by meloxicam (MEL), tenoxicam (TEN) and piroxicam (PIR) (Figure 2).
2. Oxidation processes
Many compounds in the presence of oxygen and any electron donor can generate different ROS—by energy and/or electron transfer processes—like singlet molecular oxygen,
The generated
The
In living organisms, a great number of biomolecules essential to life such as DNA, RNA, lipids, and proteins, can be oxidized by the generated ROS producing oxidative stress [6–8, 14]. Among other substrates, NSAIDs are compounds that can be oxidized in the presence of Rf-generated ROS and as shown can act as quenchers of electronically excited states of Rf (Eqs. (4) and (5)).
The protonation of
Its bimolecular decay through a disproportionation reaction can yield the ground state of the vitamin and fully reduced Rf (Eq. (7)).
The last product, in the presence of ground state oxygen, is reoxidized to Rf radical and superoxide radical anion
The electron transfer process, in Eq. (8) is relevant as a source of
In parallel, the generated
Another possible pathway for
The
or by a substrate, as happens in the presence of NSAIDs (Eq. (14)).
Finally, Eq. (15) represents the main pathway of substrate disappearance in
In order to get more insight into the behavior of NSAIDs toward Rf-generated ROS several
2.1. Stationary photolysis: riboflavin-photosensitization
In complex biological structures, Rf and NSAIDs may occupy the same locations. Kinetic and mechanistic aspects of their mutual interaction constitute the crucial information for understanding the behavior of NSAIDs toward Rf-generated ROS and the potential
Using a home-made photolyzer, aerated neutral aqueous solutions of each of the following NSAIDs DFN, IMT, MEL, TEN, and PIR, were irradiated with the light of a 150W quartz-halogen lamp, in the presence of Rf as a sensitizer. All the NSAIDs used as substrates are transparent to visible light. Nevertheless, in order to assure that they do not absorb any incident radiation, a cut-off filter at 400 nm was employed. The processes were followed by the absorption spectra using a diode array spectrophotometer (Hewlett Packard 8452A). The light irradiation induced changes in the absorption spectra of the mixtures 0.05 mM DFN + 0.04 Rf (Figure 3), 0.05 mM IMT + 0.04 mM Rf (Figure 3, inset A) and 0.05 mM MEL + 0.04 mM Rf (Figure 4). The processes could be monitored from the absorbance decay at the respective absorption maxima for each substrate. In this way, the rates of sensitized photoxygenation for each NSAID were determined.
In parallel experiments, using a specific oxygen electrode (Orion 97-08) the oxygen concentration was measured during irradiation of the same mixtures in aqueous solutions in a closed Pyrex cell [10]. Under these conditions, all the NSAIDs under study showed oxygen consumption. Regarding the oxicams family, TEN and PIR presented the lowest rate of oxygen consumption. It was a little bit higher for MEL (Figure 4, inset B). In the corresponding set for DFN and IMT, the rate of oxygen uptake was significantly higher for the latter (Figure 3, inset B).
From all these preliminary findings, we assume that the transformations in NSAIDs can be attributed to interactions with electronically excited states of Rf with the possible participation of photogenerated ROS.
2.1.1. Kinetics and mechanism
The xantenic dye Rose Bengal (RB) is one of the most frequently employed photosensitizers that exclusively generate
The combination of stationary and time-resolved experiments unambiguously demonstrates the participation of
NSAID | ||||
---|---|---|---|---|
(1.1) (d) | (0.61) (d) | |||
| | (1.6) (e) |
NSAID | |||
---|---|---|---|
~1 | 1 | 1 | |
0.11 | 0.26 | 0.07 | |
0.63 | 1 | 1 | |
0.52 | 0.48 | 0.68 | |
~1.00 | 0.47 | 0.67 |
2.2. Interaction of NSAIDs with photogenerated ROS
Some compounds that are specific ROS quenchers have been used to elucidate which species are effectively involved in a given oxidative event [20, 21]. Catalase from bovine liver (
The enzyme superoxide dismutase from bovine erythrocytes (SOD) dismutates the species
Meanwhile, sodium azide (
Similar experiments were performed using solutions 0.5 mM of the three oxicams and
3. Photoprotective effect of NSAIDs toward amino acids and peptides oxidation
In order to evaluate an eventual antioxidant/protective effect of NSAIDs towards biologically relevant substrates, amino acids (AA) and peptides may be employed as typical oxidizable targets in a proteinaceous medium.
Tryptophan (Trp) and tyrosine (Tyr) are AAs that can be affected by photo-damages through photodynamic activity [23, 24]. They are known quenchers of
PIR and Trp, as isolated substrates, are efficient
In neutral pH, Tyr is present in a very low reactive form. The interaction Tyr with
A relevant result was that PIR in the presence of Rf showed an interesting degree of protection against Trp or Tyr oxidation by the
The dipeptide Trp-Tyr in a 0.5 mM aqueous solution was employed as a biologically relevant model compound, with RB or Rf as photosensitizers and IMT or DFN as potential photo-protective substrates. The
Meanwhile, the rate for the mixture Trp-Tyr + DFN decreased more than 50% of the one for the isolated dipeptide. Upon Rf-sensitization, similar results were obtained for DFN and IMT (Figure 10). This fact suggested that the photoxidation occurs mainly by reaction with the Rf-photogenerated
4. Conclusions
The results presented for the NSAIDs under study pointed out their efficiency as quenchers of photogenerated
DNF could be considered as an ideal scavenger of
Based on the discussed results, the NSAIDs studied herein present, in principle, promising properties for medicinal use as bio-antioxidants against
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