Macrocyclic lactones (MLs) are the cornerstone of parasite control in livestock due to their broad-spectrum activity against endo (nematodes) and ecto (lice, ticks, mites) parasites. These molecules, introduced into the veterinary pharmaceutical market 40 years ago, have substantially improved animal welfare and productivity by offering extended high efficacy, reducing treatment frequency, and displaying a favorable safety profile. However, their widespread and intensive use has led to a significant challenge nowadays: the development of parasite resistance. This review focuses on the critical link between drug pharmacokinetics (variation in concentration profiles and exposure over time) and pharmacodynamics (drug efficacy) and the ability of both avermectin and milbemycin MLs families to control livestock ectoparasites. This review discusses the integrated assessment of drug behavior in the host, its diffusion into target parasites, and the impact of different pharmaceutical formulations on enhancing drug delivery to infection sites. These are considered critical research/development areas to optimize the use of MLs, preventing treatment failures and finally extending the lifespan of these essential pharmaceutical ingredients. Finally, the importance of the rational use of MLs, guided by parasite epidemiology and pharmacological knowledge, is emphasized as a key strategy to preserve the antiparasitic efficacy of these still very useful molecules.

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.

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.

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.