EXPLORATORY BIOAVAILABILITY AND PHARMACOKINETIC STUDIES OF SULFADIMETHOXINE AND ORMETOPRIM IN THE PENAEID SHRIMP, PENAEUS-VANNAMEI

The pharmacokinetics and bioavailability of sulphadimethoxine (SDM) an d ormetoprim (OMP) were examined, following simultaneous administratio n in penaeid shrimp. The hemolymph concentration versus time data foll owing intra-sinus injection for both SDM (42 mg/kg) and OMP (8.6 mg/kg ) were well described by a multiexponential equation suggesting multic ompartmental behavior. The SDM:OMP parameter estimates of systemic cle arance, steady-state volume of distribution, and disposition half-life were 215:1765 ml/kg.h, 1319:34 382 ml/kg, and 9.0:17.8 h, respectivel y. Hemolymph protein binding of SDM and OMP was 5.2% and 12.1%, respec tively. The bioavailabilities of SDM and OMP were 30% and 32%, respect ively. Peak hemolymph concentration (C-max) and time of occurrence of that value (T-max) for SDM following a single oral dose (210 mg/kg) we re 25.0 mu g/ml at 4 h, while the corresponding values for OMP were (d ose, 42 mg/kg) 0.70 mu g/ml at 4 h, The percent of the available oral dose 2 h post-administration for SDM:OMP in the hemolymph, muscle, and hepatopancreas were 5.0:0.6%, 7.8:3.4%, and 2.4:24.8%, respectively. Hemolymph and muscle tissue concentrations were below detectable limit s after 48 h for SDM and 24 h for OMP. The SDM:OMP drug combination ha s good potential as a shrimp chemotherapeutant in that they were rapid ly and moderately absorbed, have relatively short half-lives and low h emolymph protein binding which translates to the bulk of absorbed drug s being available for a therapeutic response.