Delivery of ivermectin by injectable microspheres

A bioabsorbable, injectable microsphere formulation containing ivermectin in poly-(lactide-co-glycolide) copolymer (PLA/PGA) was developed to provide long-lasting delivery of the drug for control of livestock pests. A solvent-evaporation technique was used to produce the spherical beads containing approximately 30% ivermectin and ranging in size from 25-250 microns. The pattern of delivery of the drug into the blood stream of Spanish goats was characterized for a 50:50 PLA/PGA, a 90:10 PLA/PGA copolymer formulation, and a PLA monomer formulation. When the 50:50 PLA/PGA formulation was used in cattle at the rate of 2 mg (AI)/kg body weight, 2 peaks of 45-50 ppb of ivermectin in serum were observed. The 1st peak was at approximately 1 wk after injection and the 2nd peak, which was broader than the 1st, occurred at approximately 6-7 wk after injection. Percentage of inhibition of estimated larvae for the lone star tick, Amblyomma americanum (L.), placed on treated cattle was 100% for the first 8 wk after injection and was 75, 57, 46 and 44% for wk 9, 10, 11, and 12, respectively. The treatment provided 98-100% control of larval horn flies, Haematobia irritans (L.), in the manure of treated cattle for 10 wk. The bioassay results against lone star ticks and larval horn flies were in agreement with the serum concentration data. The injectable microsphere formulation of ivermectin should be useful in a variety of other applications ranging from the control of Boophilus spp. ticks in south Texas to heartworms in pets.