Irreversible photo-oxidation of propranolol triggered by self-photogenerated singlet molecular oxygen

Ketoprofen products induced photosensitization of sulfonamide antibiotics: The cocktail effects of pharmaceutical mixtures on their photodegradation

Indirect photolysis induced by naturally occurring sensitizers constitutes an important pathway accounting for the transformation and fate of many recalcitrant micropollutants in sunlit surface waters. However, the photochemical transformation of micropollutants by photosensitizing pharmaceuticals has been less investigated. In this study, we demonstrated that the non-steroidal anti-inflammatory drug ketoprofen (KTF) and its photoproducts, 3-acetylbenzophenone (AcBP) and 3-ethylbenzophenone (EtBP), could sensitize the photodegradation of coexisting sulfonamide antibiotics, e.g., sulfamethoxazole (SMX), under artificial 365 nm ultraviolet (UV) and sunlight irradiation. Key reactive species including triplet excited state and singlet oxygen (1O2) responsible for photosensitization were identified by laser flash photolysis (LFP) and electron paramagnetic resonance (EPR) techniques, respectively. High-resolution mass spectrometry (HRMS) and structure-related reactivity analyses revealed that the sensitized photolysis of SMX occurred mainly through single electron transfer. The rate constants of sulfonamides sensitized by AcBP photolysis followed the order of sulfisoxazole (SIX)＞sulfathiazole (STZ)＞SMX＞sulfamethizole (SMT). Exposure to sunlight also enhanced the photolysis of SMX in the presence of KTF or AcBP, and water matrix had limited impact on such process. Overall, our results reveal the feasibility and mechanistic aspects of photosensitization of coexisting contaminants by pharmaceuticals (or their photoproducts) and provide new insights into the cocktail effects of pharmaceutical mixtures on their photochemical behaviors in aqueous environment.

Photodegradation of propranolol in surface waters: An important role of carbonate radical and enhancing toxicity phenomenon

Antihypertensive propranolol (PRO) is frequently detected in surface waters and has adverse effects on aquatic organisms. In this study, its photochemical fate in surface water with the aspect of kinetics, products and toxicity were investigated employing steady-state photochemistry experiments and ecotoxicity tests. The results showed that photodegradation of PRO was enhanced in river water than that in phosphate buffer where dissolved organic matter (DOM), NO₃^-, and HCO₃^- played important roles. DOM accelerated the photodegradation mainly through generation of excited triplet-state DOM while NO₃^- played dual roles in the photodegradation. The reaction between excited triplet-state PRO and HCO₃^- can generate carbonate radical (CO₃^·-) to promote the photodegradation. The second-order reaction rate constant between PRO and CO₃^·- was determined to be (3.4 ± 0.8) × 10⁸ M^-1 s^-1. Eight photodegradation products were identified in the studied river water sample. Finally, the toxicity evaluated by Vibrio fischeri increased after photodegradation and three photodegradation products were responsible for the increasing toxicity, which was concluded from the significant correlation between toxicity parameters and quantity of the photodegradation products.

Dichlorine radicals (Cl2•-) promote the photodegradation of propranolol in estuarine and coastal waters

Propranolol (PRO) is frequently detected in estuarine and coastal waters, which has adverse effects on estuarine and coastal ecosystems. In this study, the effects of halide ions and DOM from estuarine and coastal waters on the photochemical transformation of PRO were investigated. The results demonstrated that the presence of Br^- alone exhibited slight effect on photochemical transformation of PRO, while photodegradation rates of PRO increased with the addition of 0.1-0.54 M Cl^-. The quenching experiments and the laser flash photolysis experiments together demonstrated the generation of Cl₂^•- in the photolytic systems. Cl₂^•- is possibly produced through the charge separation of exciplex of ³PRO* and Cl^- rather than via direct oxidation of Cl^-. Additional experiments indicated that addition of seawater DOM inhibited the halide ions-sensitized photodegradation rates of PRO, which may be due to the quenching of Cl₂^•- by phenolic substances in DOM molecules. Compared with pure water, three new photochemical intermediates were identified in the presence of DOM or Cl^-. The direct photolysis of PRO mainly reacted by hydroxyl additions, hydroxyl elimination and de-propylation, whereas electron transfer coupled with H-abstraction by Cl₂^•- and ³DOM* was proposed as the primary role for PRO degradation in the presence of Cl^- or DOM.

Comparative study on degradation of propranolol and formation of oxidation products by UV/H2O2 and UV/persulfate (PDS)

The frequent detection of propranolol, a widely used β-blocker, in wastewater effluents and surface waters has raised serious concern, due to its adverse effects on organisms. UV/hydrogen peroxide (UV/H₂O₂) and UV/persulfate (UV/PDS) processes are efficient in eliminating propranolol in various waters, but the formation of oxidation products in these processes, as well as the assessment of their toxicity, has not been systematically addressed. In this study, we identified and compared transformation products of propranolol produced by hydroxyl radical (^•OH) and sulfate radical (SO₄^•-). The electrostatic attraction enhances the reaction between SO₄^•- and the protonated form of propranolol, while ^•OH shows non-selectivity toward both protonated and neutral propranolol species. The hydroxylation of propranolol by ^•OH occurs at either amine moiety or naphthalene group while SO₄^•- favors the oxidation of the electron-rich naphthalene group. Further oxidation by ^•OH and SO₄^•- results in ring-opening products. Bicarbonate and chloride exert no effect on propranolol degradation. The generation of CO₃^•- and Cl-containing radicals is favorable to oxidizing naphthalene group. The acute toxicity assay of Vibrio fischeri suggests that SO₄^•- generates more toxic products than ^•OH, while CO₃^•- and Cl-containing radicals produce similar toxicity as SO₄^•-. High concentrations of bicarbonate in UV/H₂O₂ increase the toxicity of treated solution.

Effects of inorganic ions on the photolysis of propranolol in FA solution

Photolysis of the widely used beta-blocker propranolol (PRO) was investigated in the presence of fulvic acid (FA) and inorganic ions under simulated solar irradiation. PRO undergoes direct photolysis proceeding mainly via degradation of the triplet excited state, ³PRO*. FA and inorganic ions inhibited photolysis of PRO in the order of FA > Fe³⁺ > Cl^- > Ca²⁺ > Mg²⁺ > NO₃^- > K⁺. An antagonistic effect between FA and inorganic ions toward the suppression of PRO photolysis was exhibited. The binding behaviors of PRO, FA, and inorganic ions were examined through fluorescence quenching experiments, which showed that inorganic ions affected the binding between FA and PRO through competing for the binding sites of FA or PRO. The correlation analysis demonstrated a positive correlation between the binding constant (K_OC) of FA-PRO and the inhibition rate of FA on PRO photolysis. The observed rate constants of photolysis (k_obs) have opposite correlations with the concentration of singlet oxygen (¹O₂). These findings strongly suggest that inorganic ions decrease the inhibition effect of FA on PRO photolysis via restraining the complexation of FA-PRO and production of ¹O₂.

Transformation products in the water cycle and the unsolved problem of their proactive assessment: A combined in vitro/in silico approach

Transformation products (TPs) emerging from incomplete degradation of micropollutants in aquatic systems can retain the biological activity of the parent compound, or may even possess new unexpected toxic properties. The chemical identities of these substances remain largely unknown, and consequently, the risks caused by their presence in the water cycle cannot be assessed thoroughly. In this study, a combined approach for the proactive identification of hazardous elements in the chemical structures of TPs, comprising analytical, bioanalytical and computational methods, was assessed by the example of the pharmaceutically active micropollutant propranolol (PPL). PPL was photo-transformed using ultraviolet (UV) irradiation and 115 newly formed TPs were monitored in the reaction mixtures by LC-MS analysis. The reaction mixtures were screened for emerging effects using a battery of in vitro bioassays and the occurrence of cytotoxic and mutagenic activities in bacteria was found to be significantly correlated with the occurrence of specific TPs during the treatment process. The follow-up analysis of structure-activity-relationships further illustrated that only small chemical transformations, such as the hydroxylation or the oxidative opening of an aromatic ring system, could substantially alter the biological effects of micropollutants in aquatic systems. In conclusion, more efforts should be made to prevent the occurrence and transformation of micropollutants in the water cycle and to identify the principal degradation pathways leading to their toxicological activation. With regard to the latter, the judicious combination of bioanalytical and computational tools represents an appealing approach that should be developed further.

Mechanistic study of fulvic acid assisted propranolol photodegradation in aqueous solution

Laser flash (355 nm) and stationary (365 nm) photolysis were used to study the mechanisms of propranolol photolysis in the presence of fulvic acid in aqueous solutions. The FA-assisted photodegradation of propranolol was observed using UV-A irradiation (where propranolol is stable). Direct evidence indicated that the photodegradation resulted from the static quenching of the FA triplet state by propranolol via the electron transfer mechanism. The triplet state yield (ϕT≈0.6%) and the T-T absorption coefficient (ɛT(620 nm)≈5×10(4) M(-1) cm(-1)) were estimated for the first time by modeling the yields of the FA triplet state in the presence of propranolol. Thus, fulvic acid is a promising agent for accelerating propranolol photodegradation in aqueous solutions under UV-A light irradiation.

Selective oxidative degradation of organic pollutants by singlet oxygen-mediated photosensitization: tin porphyrin versus C60 aminofullerene systems

This study evaluates the potential application of tin porphyrin- and C(60) aminofullerene-derivatized silica (SnP/silica and aminoC(60)/silica) as (1)O(2) generating systems for photochemical degradation of organic pollutants. Photosensitized (1)O(2) production with SnP/silica, which was faster than with aminoC(60)/silica, effectively oxidized a variety of pharmaceuticals. Significant degradation of pharmaceuticals in the presence of the 400-nm UV cutoff filter corroborated visible light activation of both photosensitizers. Whereas the efficacy of aminoC(60)/silica for (1)O(2) production drastically decreased under irradiation with λ > 550 nm, Q-band absorption caused negligible loss of the photosensitizing activity of SnP/silica in the long wavelength region. Faster destruction of phenolates by SnP/silica and aminoC(60)/silica under alkaline pH conditions further implicated (1)O(2) involvement in the oxidative degradation. Direct charge transfer mediated by SnP, which was inferred from nanosecond laser flash photolysis, induced significant degradation of neutral phenols under high power light irradiation. Self-sensitized destruction caused gradual activity loss of SnP/silica in reuse tests unlike aminoC(60)/silica. The kinetic comparison of SnP/silica and TiO(2) photocatalyst in real wastewater effluents showed that photosensitized singlet oxygenation of pharmaceuticals was still efficiently achieved in the presence of background organic matters, while significant interference was observed for photocatalyzed oxidation involving non-selective OH radical.

Photolysis of beta-blockers in environmental waters

Many drugs such as beta-blockers have been shown to occur in aquatic environments. Even if adequate ecotoxicity data are not available, it is of primary importance to get informations about their fate in environmental waters, particularly about their photofate in sewage treatment plant effluents (STP). The main difficulties when studying pharmaceutical photochemical behaviour in environmental waters, are linked to the very low environmentally relevant concentrations (ng L(-1) to microg L(-1)) which can generate problems in terms of analytical sensitivity. Moreover, the complexity of environmental matrices can modify micropollutants degradation kinetics. The photodegradation of beta-blockers has been compared at two concentration levels (10 microg L(-1) and 10 mg L(-1)) and in two different matrices (pure water and STP effluent). It has been shown that the concentration does not influence beta-blockers degradation pathways, thus allowing the identification of degradation compounds using the 10 mg L(-1) solutions. Although environmental waters speed up the degradation process, the same photoproducts were appeared in both matrices. Using LC-MS/MS, hydroxyl radical additions have been identified as an important degradation pathway for especially pindolol, propranolol and timolol, leading to several positional isomers, corresponding to mono-, di- or tri-hydroxylations. Kinetics of appearance/disappearance of these photoproducts have been studied in STP effluents.

Kinetics and degradation products for direct photolysis of beta-blockers in water

Related to improving persistence assessment of active pharmaceutical ingredients (APIs), direct aqueous photolysis of beta-blockers: propranolol (hydrochloride salt), atenolol, and metoprolol (succinate salt) were investigated by exposing the samples (0.0003-10 mg L(-1)) to a solar irradiator (filtered xenon lamp: 290-800 nm) at 20-26 degrees C. Results suggested that direct photolysis in optically dilute solutions followed pseudo first-order kinetics. The measured half-lives of propranolol, atenolol, and metoprolol were approximately 16, 350, and 630 h, respectively. These were 3-5 orders of magnitude slower than the estimated minimum half-lives. The measured half-lives were related to day light surface conditions by comparing the light intensity of the lamp and the sun at different latitudes and seasons. Major direct photolysis products were identified from propranolol that led to a proposed reaction pathway, involving ring oxidation, rearrangement, and deoxygenation. Electron paramagnetic resonance (EPR) spectroscopy results confirmed that at least one carbon-based radical intermediate was formed during the direct photolysis of propranolol in aqueous solutions. The overall results demonstrated that with fast direct photolysis half-lives, propranolol is unlikely to be persistent in natural waters. Further work is needed to investigate indirect photolysis of atenolol and metoprolol in surface waters in order to understand the overall persistence of these APIs in the environment.

Photodegradation products of propranolol: The structures and pharmacological studies

Recently, single-dose drug packaging systems, allowing the administration of multiple drugs in a single pill, have become popular for the convenience of the patient. The quality of drugs and an accurate measurement of their photostabilities within this system, however, have not been carefully addressed. Drugs that are unstable in light should be carefully handled to protect their potency and ensure their safety. Propranolol (1), a beta-adrenergic receptor antagonist, is widely used for angina pectoris, arrhythmia, and hypertension. Due to its naphthalene skeleton, this drug may be both light unstable and a photosensitizing agent. In this study, we isolated three photodegraded products of propranolol (1): 1-naphthol (2), N-acetylpropranolol (3), and N-formylpropranolol (4). The structures of these compounds were determined by spectroscopic methods and chemical syntheses. We also examined the acute toxicities of these substances in mice and their binding to beta-adrenergic receptors using rat cerebellum cortex membranes. Although the photoproducts isolated in this study did not exhibit any acute toxicity or significant binding to beta-adrenergic receptors, these results serve as a warning to single-dose packaging systems, as propranolol (1) must be handled carefully to protect the compound from light-induced degradation.

Singlet oxygen: the relevance of extracellular production mechanisms to oxidative stress in vivo

Since the physiological relevance of 1O2 independent of photosensitization has been controversial, we review proposed reaction mechanisms for its extracellular production in vivo and discuss the relevance of this production to oxidative stress. We conclude that extracellular 1O2 production by the spontaneous dismutation of O2*- does have physiological relevance. Also, extracellular 1O2 production by the eosinophil peroxidase-H2O2-bromide system could have physiological relevance. As regards the other reactions discussed in this review, the evidence is not sufficient to warrant any conclusions as to the physiological relevance of these to extracellular 1O2 production. What is evident is that the microenvironment will have a significant influence on the success or failure of extracellular 1O2 production. To date, most demonstrations of 1O2 production by physiologically relevant mechanisms have used conditions that minimize competitive reactions. More research demonstrating how physiologically relevant competitive reactions influence extracellular 1O2 production is needed.