Investigation of transformation products of selected veterinary drugs generated by electrochemistry, microsomal assays, hydrolysis & photolysis
Authors: Knoche Lisa , Schwerdtle Tanja 
Published: June 2023

The knowledge of transformation pathways and transformation products of veterinary drugs is important for health, food and environmental matters. Residues (original veterinary drug and transformation products) are found in food products of animal origin and also in the environment (e.g., soil or surface water). Several transformation processes can alter the original veterinary drug, ranging from biotransformation in living organism to environmental degradation processes like photolysis, hydrolysis, or microbial processes. In this thesis, four veterinary drugs were investigated, three ionophore antibiotics Monensin, Salinomycin and Lasalocid and the macrocyclic lactone Moxidectin. Ionophore antibiotics are mainly used to cure and prevent coccidiosis in poultry especially prophylactic in broiler farming. Moxidectin is an antiparasitic drug and is used for the treatment of internal and external parasites in food‐ producing and companion animals. The main objective of this work was the usage of different laboratory approaches to generate and identify transformation products. The identification was conducted using high‐resolution mass spectrometry (HR‐MS). A major focus was put on the application of electrochemistry for simulation of transformation processes. The electrochemical reactor, equipped with a three‐electrode flow‐through cell, enabled the oxidation or reduction by applying a potential. Derived transformation products were analyzed by online coupling of the electrochemical reactor and a HR‐MS and offline by liquid chromatography (LC) combined with HR‐MS. The main modification reaction of the identified transformation products was different for each investigated veterinary drug. Monensin was showing decarboxylation and demethylation as main modification reactions, for Salinomycin mostly decarbonylation was occurring and for Lasalocid methylation was prevalent. For Moxidectin I observed oxidation (hydroxylation) reaction and adduct formation with solvent. In general, for salinomycin and Lasalocid more transient transformation products (online measurement) than stable transformation products (offline measurements) were detected. In contrast, the number of transformation products using online and offline measurements were identical for monensin and moxidectin. As a complementary approach, metabolism tests with rat or human liver microsomes were made for the ionophore antibiotics. Monensin was investigated by using rat liver microsomes and identified transformation products were based on decarboxylation and demethylation. Salinomycin and Lasalocid were converted by human and rat liver microsomes. For both substances were more transformation products found by using human liver microsomes. The transformation products of the rat liver microsome conversion were redundant, the transformation products were also found at the human liver microsome assay. Oxidation (hydroxylation) was found to be the main modification reaction for both. In addition, a frequent ion‐exchange between sodium and potassium was identified. The last two experiments were performed for one substance each, the hydrolysis of monensin and the photolysis of moxidectin was investigated. The transformation products of the pH‐ dependent hydrolysis were based on ring‐opening and dehydration. Moxidectin formed several transformation products by irradiation with UV‐C light and main modification reactions were isomeric changes, (de‐)hydration and changes of the methoxime moiety. In summary, transformation products of the four investigated veterinary drugs were generated by the different laboratory approaches. Most of the identified transformation products were identified for the first time. The resulting findings provide an understanding for clarifying the transformation behavior.