Karen Larsen

Traditional macrolide antimicrobials are inhibitors of cytochrome P4503A (CYP3A) in cattle liver. Monensin (MON), an ionophore with a narrow safety margin, undergoes CYP3A-dependent O-demethylation, and its incompatibility with macrolides is well recognized in livestock animals. This study evaluated the effects of newer macrolides—tilmicosin (TIL), tulathromycin (TUL), and gamithromycin (GAM)—on CYP3A-dependent metabolism in bovine liver microsomes and examined how these drugs influence MON hepatic metabolism. Molecular docking studies were also performed to predict their interactions with CYP3A enzymes. The CYP3A-dependent enzyme activity, testosterone 6β-hydroxylase, was inhibited in the presence of triacetyl-oleandomycin (used as a reference macrolide), as well as with MON. None of the other macrolides tested affected this enzymatic activity. All macrolides inhibited MON metabolism, but the extent of inhibition observed with triacetyl-oleandomycin was higher than that produced by TIL, TUL, and GAM. Molecular docking analyses indicated that triacetyl-oleandomycin and MON exhibited the highest binding affinities for the active site of CYP3A isozymes, compared with TIL, TUL, and GAM. The agreement between enzymatic data and in silico predictions indicates that TIL, TUL, and GAM are weaker inhibitors of CYP3A-mediated MON metabolism. The modest reduction in MON hepatic metabolism caused by these macrolides—commonly used in cattle feedlots—suggests a low likelihood of clinically relevant drug–drug interactions under typical dosing conditions.

Organophosphates (OPs), pyrethrins and fipronil, are acaricides commonly used in cattle, mainly as pour on formulations. Scant information is available on their potential interactions with hepatic xenobiotic metabolizing enzymes. This work aimed to evaluate in vitro the potential inhibitory effects of widely employed acaricides on catalytic activities mediated by hepatic cytochrome P450 (CYP) and flavin‐monooxygenase (FMO) enzymes in cattle. Bovine (n = 4) liver microsomes were incubated in the absence (control assays) and in presence of different OPs (fenthion, chlorpyrifos, ethion, diazinon and dichlorvos), fipronil and cypermethrin at 0.1–100 μm. Five oxidative enzyme activities were assayed by spectrofluorimetric or HPLC methods: 7‐ethoxyresorufin O‐deethylase (for CYP1A1), methoxyresorufin O‐demethylase (for CYP1A2), benzyloxyresorufin O‐debenzylase (for CYP2B), testosterone 6‐beta hydroxylase (for CYP3A) and benzydamine N‐oxidase (for FMO). All acaricides, particularly phosphorothionate‐containing OPs, inhibited to some extent more than one enzyme activity. The most frequent inhibitor was fenthion, which inhibited (p < .05) all enzyme activities tested (from 22% at 1 μm to 72% at 100 μm). However, low inhibitory potencies (IC50s higher than 7 μm) of all acaricides studied were observed against the catalytic activities assayed. Therefore, the risk of in vivo metabolic interactions due to inhibition of monooxygenases would be low under common husbandry conditions.

Glyphosate (GLY) is one of the most commonly used herbicides worldwide. Both GLY and aminomethylphosphonic acid (AMPA), its main degradation product, may be present in feedstuffs offered to dairy cows. Although the major proportions of ingested GLY and AMPA are eliminated with faeces, a potential degradation of GLY to AMPA in the rumen of dairy cows has been suggested. Considering that the rumen plays a central role in the pre-systemic metabolism of xenobiotics, this research aimed to investigate whether or not GLY and AMPA are metabolised in the ruminal environment of cattle. The distribution of both compounds between the fluid and solid phases of the ruminal content (RC) was also evaluated. RC from 3 steers were collected in an abattoir. Aliquots were incubated (3–6 h) in anaerobiosis with GLY (15 µg/mL) and AMPA (1.5 µg/mL). Metabolic viability of RC was assessed by the measurement of the sulpho-reduction of the anthelmintic derivative albendazole sulphoxide (ABZSO) into albendazole (ABZ) in the absence (controls) or in presence of GLY and AMPA. Incubations of boiled (inactive) RC were used as controls. Samples were analysed by HLPC with fluorescence detection. Neither GLY nor AMPA were metabolised in metabolically active RC from cattle. Both compounds were predominantly found in the fluid phase compared to the solid (particulate) matter of RC. Neither GLY nor AMPA had a negative effect on the metabolic production of ABZ. A high metabolic stability of both compounds within the ruminal environment would be expected in vivo. Their presence in high proportion in the fluid phase of the ruminal content may give rise to a rapid flow of both GLY and AMPA to the posterior gastrointestinal tract. Negative effects on the ruminal biotransformation of therapeutically used drugs would not be expected when the herbicide and its degradation product are consumed with food.