In vitro biotransformation and investigation of metabolic enzymes possibly responsible for the metabolism of bisdesoxyolaquindox in the liver fractions of rats, chicken, and pigs

In vivo studies to highlight possible illegal treatments of rabbits with carbadox and olaquindox

For the treatment of rabbit dysentery and bacterial enteritis, veterinary practitioners often adopt veterinary medicinal products authorised for other food-producing species, but in some cases non-authorised drugs frequently used in the past, such as carbadox and olaquindox, might be illegally adopted. To verify the carbadox and olaquindox distribution and persistence in rabbit tissues, two independent in vivo studies were carried out. In the first study, 24 healthy rabbits received water medicated with carbadox at 100 mg l(-1) over a period 28 days, whereas in the second one, 24 healthy rabbits were administered water containing olaquindox at 100 mg l(-1). In each study rabbits were randomly assigned to four groups to be sacrificed respectively at 0, 5, 10 and 20 days from treatment withdrawal, for depletion studies. A control group of six animals was adopted for control and as a reservoir of blank tissues. Muscle and liver samples collected from each treated animal were stored at -20°C pending the analysis. Sensitive and robust liquid chromatography-tandem mass spectrometry analytical methods were set up for the parent compounds and their main metabolites quinoxaline-2-carboxylic acid, desoxycarbadox and 3-methylquinoxaline-2-carboxylic acid to verify their residual. Data collected demonstrate that the combination of liver as target matrix, quinoxaline-2-carboxylic acid and 3-methylquinoxaline-2-carboxylic acid as marker residue and enzymatic digestion is strategic to evidence carbadox and/or olaquindox illegal treatments in rabbits, even 20 days after treatment withdrawal at concentration levels higher than 0.5 µg kg(-1). This findings suggests that liver should be proposed as target matrix for official control in national monitoring plan.

Identification of the rat liver cytochrome P450 enzymes involved in the metabolism of the calcium channel blocker dipfluzine hydrochloride

This study aimed to identify the specific cytochrome P450 (CYP450) enzymes involved in the metabolism of dipfluzine hydrochloride using the combination of a chemical inhibition study, a correlation analysis and a panel of recombinant rat CYP450 enzymes. The incubation of Dip with rat liver microsomes yielded four metabolites, which were identified by liquid chromatography-coupled tandem mass spectrometry (LC/MS/MS). The results from the assays involving eight selective inhibitors indicated that CYP3A and CYP2A1 contributed most to the metabolism of Dip, followed by CYP2C11, CYP2E1 and CYP1A2; however, CYP2B1, CYP2C6 and CYP2D1 did not contribute to the formation of the metabolites. The results of the correlation analysis and the assays involving the recombinant CYP450 enzymes further confirmed the above results and concluded that CYP3A2 contributed more than CYP3A1. The results will be valuable in understanding drug-drug interactions when Dip is coadministered with other drugs.

In vitro and in vivo metabolite profiling of valnemulin using ultraperformance liquid chromatography-quadrupole/time-of-flight hybrid mass spectrometry

Valnemulin, a semisynthetic pleuromutilin derivative related to tiamulin, is broadly used to treat bacterial diseases of animals. Despite its widespread use, metabolism in animals has not yet been fully investigated. To better understand valnemulin biotransformation, in this study, metabolites of valnemulinin in in vitro and in vivo rats, chickens, swines, goats, and cows were identified and elucidated using ultraperformance liquid chromatography-quadrupole/time-of-flight hybrid mass spectrometry (UPLC-Q/TOF-MS). As a result, there were totally 7 metabolites of valnemulin identified in vitro and 75, 61, and 74 metabolites detected in in vivo rats, chickens, and swines, respectively, and the majority of metabolites were reported for the first time. The main metabolic pathways of valnemulin were found to be hydroxylation in the mutilin part (the ring system) and the side chain, oxidization on the sulfur of the side chain to form S-oxides, hydrolysis of the amido bond, and acetylization in the amido of the side chain. In addition, hydroxylation in the mutilin part was proposed to be the primary metabolic route. Furthermore, the results revealed that 2β-hydroxyvalnemulin (V1) and 8α-hydroxyvalnemulin (V2) were the major metabolites for rats and swines and S-oxides (V6) in chickens.

An introduction to hybrid ion trap/time-of-flight mass spectrometry coupled with liquid chromatography applied to drug metabolism studies

Metabolism studies play an important role at various stages of drug discovery and development. Liquid chromatography combined with mass spectrometry (LC/MS) has become a most powerful and widely used analytical tool for identifying drug metabolites. The suitability of different types of mass spectrometers for metabolite profiling differs widely, and therefore, the data quality and reliability of the results also depend on which instrumentation is used. As one of the latest LC/MS instrumentation designs, hybrid ion trap/time-of-flight MS coupled with LC (LC-IT-TOF-MS) has successfully integrated ease of operation, compatibility with LC flow rates and data-dependent MS(n) with high mass accuracy and mass resolving power. The MS(n) and accurate mass capabilities are routinely utilized to rapidly confirm the identification of expected metabolites or to elucidate the structures of uncommon or unexpected metabolites. These features make the LC-IT-TOF-MS a very powerful analytical tool for metabolite identification. This paper begins with a brief introduction to some basic principles and main properties of a hybrid IT-TOF instrument. Then, a general workflow for metabolite profiling using LC-IT-TOF-MS, starting from sample collection and preparation to final identification of the metabolite structures, is discussed in detail. The data extraction and mining techniques to find and confirm metabolites are discussed and illustrated with some examples. This paper is directed to readers with no prior experience with LC-IT-TOF-MS and will provide a broad understanding of the development and utility of this instrument for drug metabolism studies.

Cytotoxicity and genotoxicity of 1,4-bisdesoxyquinocetone, 3-methylquinoxaline-2-carboxylic acid (MQCA) in human hepatocytes

Quinoxaline-1,4-dioxides, widely used as medicinal feed additives as antibacterial growth promoters, have been shown to exert diverse toxicities. Their toxicities are hypothesized to be closely related to the formation of N-oxide reductive metabolites. 1,4-Bisdesoxyquinocetone and MQCA are important N-oxide reductive metabolites of quinocetone or olaquindox. In this study, we evaluated the cytotoxicity and genotoxicity of the metabolites, 1,4-bisdesoxyquinocetone and MQCA, as well as their parental drugs (quinocetone and olaquindox) in two human hepatocyte cell lines, L-02 and Chang liver cells. All these compounds inhibited the growth of cells in a dose-dependent and time-dependent manner by the MTT assay. Hormesis effects were found in L-02 cells treated with quinocetone at low doses. In the comet assay, although the two metabolites induced dose-related DNA damage in both cell lines, the levels of damage were less than that demonstrated for the parent drugs. The flow cytometric analysis showed that only the two metabolites induced cell cycle arrest at the S phase, and a decrease in the G0/G1, G2/M phase of Chang liver cells, which was not found for the L-02 cells treated with any compounds. The results indicate that 1,4-bisdesoxyquinocetone and MQCA are toxic to L-02 and Chang liver cells, and provide important new information towards understanding the olaquindox and quinocetone toxic mechanisms.