Tissue Distribution and Renal Excretion of Ormetoprim after Intravascular and Oral Administration in the Channel Catfish (Ictalurus punctatus)

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Abstract
Ormetoprim is used to potentiate sulfadimethoxine in treating certain bacterial diseases of aquatic species. The tissue disposition and renal excretion of ormetoprim and metabolites were examined after intravascular and oral administration (4 mg∙kg−1) in channel catfish (Ictalurus punctatus). Peak plasma level (0.66 μg∙mL−1) of14C-ormetoprim occurred at 6 h after oral dosing. The oral bioavailability was estimated at 52%. Ormetoprim and metabolites were widely distributed in the tissues. The tissue concentrations were highest in the liver, trunk kidney, head kidney, and spleen. Clearance of the radiolabel from tissues was rapid. The muscle contained 49.3% of the intravascularly administered dose at 2 h; however, at 72 h, less than 1% of the dose remained in this tissue.14C-Ormetoprim was more persistent in the skin than in the muscle. Ormetoprim was extensively metabolized in catfish. After intravascular administration, 21.1% of the dose of14C-ormetoprim was eliminated in the urine in 48 h, predominantly as polar metabolites; less than 4% of the dose was eliminated as the parent compound. Biliary excretion was a minor route of elimination (5–6% of the dose). The data suggest branchial excretion of ormetoprim and/or metabolites.