Efficacy in sheep and pharmacokinetics in cattle that led to the selection of eprinomectin as a topical endectocide for cattle.

Eprinomectin (MK-397 or 4"-epi-acetylamino-4"-deoxy-avermectin B1) is a novel avermectin selected for development as a topical endectocide for all cattle, including lactating cows. The initial efficacy assessments were made in sheep to identify subclasses of the avermectin/milbemycins that possessed inherent activity against a spectrum of nematode parasites. This included examination of several hundred analogs each given orally to a single sheep experimentally infected with a range of parasitic nematodes. Representatives of several subclasses, most notably the 4"-epi-amino avermectin B1 subclass, were identified as possessing potent, broad-spectrum activity against the endoparasites, whereas subclasses such as those with a variety of synthetic substitutions at C-4a or oximes at C-5 were among the least potent. Eprinomectin, a member of the 4"-epi-amino subclass, possessed potent activity against the range of nematodes when tested at 0.025 mg kg-1 per os. Milk and plasma concentration profiles were also made for these and other selected avermectin/milbemycins following topical administration to lactating dairy cattle. The molecular structure of each compound had a significant effect on the milk to plasma ratio, but the ratio of each was constant over time, implying an equilibrium between the 2 compartments. Compounds that were saturated at the C-22,23 bond had milk to plasma ratios > or = 1.0, whereas those unsaturated at this bond were generally < or = 1.0. The milk to plasma ratio of eprinomectin was < or = 0.2. Therefore, not only is eprinomectin the most potent broad-spectrum avermectin/milbemycin identified to date, but it also possesses one of the lowest milk partitioning coefficients in this class of antiparasitics.