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Pharmacological particulars
Pharmacotherapeutic group: Avermectins
ATC Vet Code: QP54AA01.
Pharmacodynamic Properties
Ivermectin is a 22,23-dihydro derivative of an avermectin (which is a fermentation product produced by Streptomyces avermitilis) and consists of 2 homologues: B1a and B1b. It is a parasiticide with nematocidal, insecticidal and acaricidal activity documented in a wide range of domesticated animals. Ivermectin is not effective in liver fluke and cestode infestations.
Avermectins bind selectively with glutamate-gated chloride ion channels, which occur in invertebrate nerve or muscle cells. This leads to an increase of the cell membrane permeability to chloride ions of the nerve or muscle cells, causing irreversible neuromuscular blockade in the parasite, followed by paralysis and death.
Compounds of this class may also interact with other ligand-gated chloride channels, such as those gated by the neurotransmitter gamma-aminobutyric acid (GABA). Ivermectin stimulates GABA liberation at presynaptic nerve terminations (in Nematodes) or the neuromuscular junctions (in Arthropodes), that leads to the paralysis and death of the relevant parasites.
Resistance to ivermectin in horses has not been reported, however it is possible that frequent and repeated use may lead to the development of resistance.
Pharmacokinetic Properties
After oral administration of the recommended dose to horses, the following parameters were observed: Cmax of 29 ng/ml, Tmax of 7 h, AUC of 1485 ng/ml.h and t½ of 55 h. Ivermectin is highly lipophilic and has good ability to penetrate to the location of parasites. It is stored in and slowly released from fat after which it is converted by the liver to less lipid soluble metabolites by oxidative biotransformation. The excretion route of the active substance occurs mainly in the bile and faeces. Less than 2% is eliminated via urine. Ivermectin is highly protein bound and clearance is slow.