Phytotherapy represents a promising field for developing new therapeutic strategies, either by acting as antiparasitic agents themselves or by enhancing the efficacy of synthetic drugs. The present study evaluated the in vivo pharmacokinetic and pharmacodynamic interactions following the administration of doramectin, either alone or in combination with cinnamaldehyde and pink grapefruit essential oil (CNM–PGF). Additionally, ex vivo P-glycoprotein-mediated interactions were assessed at the intestinal level. Higher efficacy against Haemonchus contortus was observed with the combined treatment of doramectin plus phytochemicals or loperamide. Although phytochemicals and loperamide reduced the ex vivo efflux of rhodamine 123, no pharmacokinetic interactions were detected in vivo. The enhanced efficacy is most likely attributable to localized drug–drug interactions and increased drug availability at the parasite interface. Based on the results obtained, the best treatment would be the combination of DRM with phytochemicals such as CNM–PGF and loperamide. However, a pharmaceutical formulation that allows sustained in vivo interaction between these compounds is needed.

Fenbendazole (FBZ), a benzymidazole (BZD) anthelmintic drug, is used for in-feed medication in pigs. BZD-containing drugs may induce cytochrome P450 isozymes (CYPs), particularly those members of the CYP1A subfamily. This research aimed to evaluatein vitrothe effect of thein vivosustained administration of FBZ on the catalytic activities of xenobiotic metabolizing enzymes in pig liver. The availability of FBZ and its metabolites in plasma and liver tissue was also assessed. Five Landrace piglets remained untreated (controls), and other six were treated with a pre-mix of FBZ, combined with food, for 9 consecutive days as usually is recommended by practitioners. Blood samples were collected from each treated animal up to day 9 and analyzed by HPLC; both control and treated animals were slaughtered for preparation of liver microsomes. Plasma concentration ratios OFZ/FBZ and FBZSO₂/OFZ increased significantly (p<0.05) from the beginning to the end of drug exposure, which may indicate an enhanced conversion of FBZ into its metabolites. FBZ represented 45.8±3.4% of the total anthelmintic molecules in liver tissue. Increased CYP1A-dependent 7-ethoxy (24.5-fold, p=0.0032) and 7-methoxyresorufin (17.2-fold, p=0.0006) O-dealkylase activities was observed in liver microsomes from FBZ-treated animals. The continuous FBZ administration may accelerate its ownin vivohepatic metabolism through the CYP1A pathway, which may have a negative impact on its clinical efficacy. CYP1A induction in pig liver may also affect the biotransformation of other xenobiotics such as aflatoxin B1 present in certain pig foodstuffs.

Background/Objectives: The widespread development of anthelmintic resistance in gastrointestinal nematodes constitutes a major production-limiting factor in grazing ruminants. Resistance mechanisms often involve drug efflux transporters like P-glycoprotein (P-gp). This study aimed to evaluate the potential of the phytochemicals cinnamaldehyde (CNM) and thymol (TML) to modulate P-gp activity and enhance the pharmacokinetic profile and efficacy of levamisole (LVM) in lambs. Methods: An ex vivo diffusion assay using sheep ileum was conducted to assess the influence of CNM, TML, and LVM on the transport of the P-gp substrate Rhodamine 123 (Rho123). Subsequently, a clinical trial was performed in lambs naturally infected with resistant nematodes. Animals received LVM (3.75 mg/kg) subcutaneously, either alone or co-administered with CNM or TML (80 mg/kg). Plasma LVM concentrations were analyzed by HPLC, and anthelmintic efficacy was determined via the Fecal Egg Count Reduction (FECR) test. Results: Ex vivo assays demonstrated that CNM, TML and LVM significantly reduced the efflux ratio of Rho123, confirming P-gp inhibition. The pharmacokinetic parameters of LVM did not differ significantly in the co-administered groups. However, the combination of LVM + TML tended to increase the total systemic exposure of LVM. Although all experimental groups showed a significant reduction in EPG between day 0 and day 7 (FECR 50–58%), the magnitude of this reduction did not differ significantly among treatments. Conclusions: While CNM and TML effectively inhibited P-gp activity ex vivo and slightly modified LVM pharmacokinetics, these effects were insufficient to yield clinically meaningful improvements in its efficacy against nematodes under the tested conditions. Future strategies should focus on optimizing delivery systems to maximize phytochemical–drug interactions.

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.