Pharmacokinetics of sulphadimethoxine in channel catfish (Ictalurus punctatus)

1. Plasma clearance, bioavailability, tissue disposition and elimination of 14C-sulphadimethoxine (SDM) were studied in channel catfish (Ictalurus punctatus) after intravenous (i.v.) and oral dosing (per os; p.o.) at 40 mg/kg body weight. 2. Analysis of blood SDM concentrations over time for intravascularly administered SDM showed that disposition and elimination were best described by a two-compartment pharmacokinetic model; estimated half-lives for SDM in blood were 0.09 and 12.6 h for the distribution and elimination phases, respectively. 3. SDM was found primarily in muscle tissue immediately after oral administration; however, clearance from muscle was rapid, with a half-life of 13.1 h. 4. With time, SDM-derived radioactivity became concentrated in the bile and was eliminated slowly (t 1/2 = 115.5 h). 5. Binding of SDM in channel catfish plasma was low (18%) and was non-specific and dose-independent. 6. With the exception of the initial, rapid clearance of SDM from blood, the pharmacokinetic parameters describing SDM distribution and elimination in channel catfish were similar to values reported for other vertebrate species; the rapid distribution of SDM from blood to the tissues in the catfish may be related to species differences in the plasma binding of SDM.