Plant tannins have been widely studied for their antiparasitic properties. They also possess other beneficial effects for animal health and environment, such as controlling ruminant bloating and reducing the emission of greenhouse gases from the rumination process. Combined with commercial anthelmintics, tannins could offer a new type of strategy to control the development of anthelmintic resistance. However, more information on the interactions between tannins and commercial anthelmintics is needed as in vivo, different tannin-rich forages have demonstrated either synergistic or antagonistic effects when combined with oral anthelmintics, depending on the types of tannins and the analytical approaches involved.

In this work, the interactions of two tannin classes, proanthocyanidins (PAs) and hydrolysable tannins (HTs), with commercial anthelmintics were studied utilizing isothermal titration calorimetry (ITC) and nuclear magnetic resonance (NMR) spectroscopy. ITC studies were conducted using one commercial anthelmintic, thiabendazole (TBZ), while the NMR studies included two anthelmintics, TBZ and ivermectin. The ITC studies indicated that the strength of the interaction is greatly affected by the structural characteristics of tannins, the strongest interactions arising from the presence of galloyl groups and increased molecular weight of the tannin. In general, stronger interactions with TBZ were observed for PAs than for HTs of the same molarity. The results from the NMR study supported the information gained from ITC and in addition elucidated the molecular mechanisms of the interactions.

Differences in the interaction mechanisms were observed for the tannin classes studied, aligning with the in vivo findings on the effect of forage composition. The structural features of tannins involved in the interactions with commercial anthelmintics also contribute to the anthelmintic effects of tannins. This necessitates further research on the possible implications on the bioavailability of commercial anthelmintics if structurally the most potent tannins are used in combination with anthelmintics used as oral drenches.