Loperamide modifies the tissue disposition kinetics of ivermectin in rats

ABSTRACT Ivermectin (IVM) is a broad‐spectrum antiparasitic drug extensively used in human and veterinary medicine that is largely excreted in bile and faeces. Loperamide (LPM) is an opioid derivative that reduces gastrointestinal secretions and motility. Both IVM and LPM have been reported to act as P‐glycoprotein substrates (P‐GP). The goal of the present work was to study the LPM‐induced modifications to the pattern of tissue distribution for IVM. Thirty‐six Wistar male rats were randomly allocated to two groups (n = 18) and treated subcutaneously with IVM alone or co‐administered with LPM. Rats were killed at different times post‐treatment and samples (blood and tissues) were collected and analyzed by HPLC. The presence of LPM induced a marked enhancement in the IVM plasma concentrations, resulting in a significantly higher area under concentration time curve (AUC) value (P < 0.01) than that obtained after the administration of IVM alone. Significantly higher IVM availabilities in the liver tissue and small intestine wall (P < 0.05) were obtained in the presence of LPM. There were no statistically significant differences in drug availability in the large intestinal wall after both treatments. However, LPM induced a marked decrease in the amount of IVM recovered in the large intestinal lumen content. The ratio between IVM concentrations in the large intestinal luminal content and plasma at day 1 post‐treatment was 4.64‐fold higher in the absence of LPM. The delayed intestinal transit time caused by LPM accounting for an extended plasma–intestine recycling time, and a potential competition between IVM and LPM for the P‐GP‐mediated bile–intestinal secretion processes, may account for the enhanced IVM systemic availability reported in the current study.