# Effects of Newer Veterinary Macrolide Antimicrobials on the CYP3A-Dependent Metabolism in Cattle Liver Microsomes: Potential Metabolic Drug–Drug Interaction with Monensin

## TL;DR

Newer macrolide antibiotics in cattle are unlikely to cause significant drug interactions with monensin through liver enzyme inhibition.

## Contribution

Demonstrates that newer macrolides weakly inhibit CYP3A-dependent metabolism of monensin in cattle, reducing interaction risk.

## Key findings

- Tilmicosin, tulathromycin, and gamithromycin weakly inhibit CYP3A-dependent monensin metabolism.
- Molecular docking confirms these newer macrolides have lower binding affinity to CYP3A than traditional ones.
- Drug–drug interactions under standard dosing are unlikely but require further in vivo study.

## Abstract

The concurrent administration of multiple drugs in cattle feedlot systems can lead to pharmacological drug–drug interactions, particularly those associated with the inhibition of hepatic xenobiotic-metabolizing enzymes. A well-recognized example is the incompatibility between monensin—an ionophore with a narrow safety margin—and traditional macrolide antimicrobials. All these drugs are metabolized by cytochrome P450 (CYP) 3A in bovine liver. We hypothesized that this metabolic interaction could also occur with more recently introduced macrolides, such as tilmicosin, tulathromycin, and gamithromycin. Using an integrated in vitro and in silico approach, the current work shows that these newer macrolides are comparatively weak inhibitors of CYP3A-dependent monensin metabolism. Therefore, the likelihood of a clinically relevant drug–drug interaction under standard dosing conditions is low. Nevertheless, this potential interaction warrants further investigation in vivo, particularly under unintended monensin overexposure.

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.