On the photooxidation of the multifunctional drug niclosamide. A kinetic study in the presence of vitamin B₂ and visible light

Abstract

Objectives: The multifunctional drug niclosamide (NSD), extensively employed therapeutically, is a frequent pollutant of surface waters. Considering the environmental importance of photodegradative processes for this type of contaminant, the kinetic and mechanistic aspects of the possible visible-light-mediated photooxidation of NSD were studied under naturalistic conditions.

Methods: The visible-light absorber riboflavin (vitamin B₂) was employed as a photosensitizer. The vitamin can usually be found in natural waters and is the most common endogenous photosensitizer in mammals.

The interaction of NSD with electronically excited states of Rf and with photogenerated reactive oxygen species (ROS) was evaluated through conventional UV spectroscopy, laser flash photolysis, time-resolved phosphorescence detection of singlet molecular oxygen (O₂(¹Δ_g)), and polarographic dosage of dissolved oxygen.

Results: Ground state NSD quenched the long-lived triplet excited state of Rf (³Rf*) and the photogenerated ROS (O₂(¹Δ_g)) and superoxide radical anion . As a result, NSD was photooxidized. The rate constants for the interaction NSD–O₂(¹Δ_g) are particularly low, in the order of 10⁶/M/s, although the whole interaction is attributable to a pure reactive process. The O₂(¹Δ_g) quenching was faster in alkaline medium, favored by the ionization of the NSD phenolic group.

Under Rf-photosensitization, NSD was degraded very much more rapidly than phenol, the latter being considered a paradigmatic water-contaminant model compound.

NSD may behave as an antioxidant in bio-environments, as demonstrated employing the photooxidizable amino acid tryptophan as a relevant biological target.

Discussion: The results indicate that a -mediated process is the main route for the Rf-sensitized photooxidation of NSD. Photodegradation of the biocide in the presence and absence of phenol and tryptophan was quantitatively evaluated, discussed, and interpreted in terms of competitive quenching processes of ³Rf*, O₂(¹Δ_g), and by the substrates.

Keywords:

• Niclosamide
• Photodegradation
• Photooxidation
• Reactive oxygen species
• Riboflavin

Acknowledgment

The authors acknowledge M S Mabel Bregliani for helpful assistance in improving the use of English in the manuscript.

Disclaimer statements

Contributors None.

Funding Financial support from Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), Ministerio de Ciencia y Tecnología de Córdoba, Secretarías de Ciencia y Técnica of the Universidad Nacional de Río Cuarto (SECyT UNRC), of the Universidad Nacional de la Patagonia SJB (SECyT UNP-SJB), and of the Universidad Nacional de la Patagonia Austral (SECyT UNPA), all from Argentina, is gratefully acknowledged.

Conflicts of interest None.

Ethics approval Ethics approval is not applicable in this case.