Derquantel is the first member of the spiroindoles, a novel class of anthelmintics with a different mode of action from existing anthelmintic classes. It acts as an antagonist at nicotinic cholinergic receptors (nAChR). It prevents contraction of somatic muscle in the parasites by blocking ACh-induced activation of cation channels in the muscle membrane. This blockade results in flaccid paralysis in nematodes.

Abamectin is a member of the macrocyclic lactone (ML) family of anthelmintics. Abamectin exerts its anthelmintic effect by binding to glutamate-gated chloride (GluCl) channels expressed on nematode neurones and pharyngeal muscle cells. This leads to an increased permeability of the cell membrane to chloride ions with hyperpolarisation of nerve or muscle cells resulting in paralysis and death of the parasite.

After a single oral administration of Startect Dual Active, maximum concentrations of derquantel of 108 ng/ml were reached at 4.2 h. The terminal t1/2 of derquantel was 9.3 h and the absolute bioavailability was 56.3%. The maximum concentration of abamectin after oral administration of Startect Dual Active was 31.1 ng/ml and was reached at 24 h post-dose. The terminal t1/2 of abamectin was 28 h and the absolute bioavailability was 69.7%.

The metabolism of derquantel is extensive and complex. Derquantel undergoes biotransformation to a large number of metabolites over a short time period and as a result, extensive variation in metabolites has been found over tissues and time periods.

Highest concentrations of abamectin were found in liver and fat with much lower concentrations being present in kidney and muscle. By 10 and 14 days the concentrations in many kidney and fat samples were close to or below the limit of detection. Abamectin B1a was the major component in all tissues.

After oral administration, the majority of derquantel is eliminated in the faeces and a smaller part in the urine, while abamectin is almost entirely excreted via the faeces with elimination in urine being negligible.”

Pharmacokinetic studies in sheep have demonstrated that there are no negative interactions between the 2 active principles, derquantel and abamectin, in Startect Dual Active.