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Plasma Pharmacokinetics and Therapeutic Efficacy of Praziquantel and 4-Hydroxypraziquantel in Schistosoma japonicum-Infected Rabbits after Oral, Rectal, and Intramuscular Administration

The relationship between plasma level and therapeutic efficacy of praziquantel (PZQ) and its major human oxidative metabolite, 4-hydroxypraziquantel (4-OHPZQ), has been investigated in Schistosoma japonicum-infected rabbits using three different routes of PZQ administration. After intramuscular administration (20 mg/kg), the maximum level of PZQ in rabbit cardiac plasma was 1.6 ± 1.0 µg/ml (mean ± SD) 30 min after administration. After oral or rectal administration (40 mg/kg), maximum plasma levels were 0.1 ± 0.2 µg/ml (oral) and 0.5 ± 0.4 µg/ml (rectal). The corresponding maximum 4-OHPZQ concentrations in cardiac plasma were 4.6 ± 1.8 µg/ml (intramuscular), 1.7 ± 0.5 µg/ml (oral), and 4.1 ± 1.6 µg/ml (rectal) 2 hr after administration of PZQ. After administration of similar doses, maximum levels of PZQ in plasma from the femoral vein were 29.3 ± 27.5 µg/ml (intramuscular), 0.6 ± 1.0 µg/ml (oral), and 0.7 ± 0.6 µg/ml (rectal). However, 60 min after intramuscular administration, the maximum PZQ concentration in portal venous blood was only 1.0 ± 0.6 µg/ml, which is substantially less than corresponding maximum portal vein levels after oral (6.8 ± 6.5 µg/ml) or rectal (3.7 ± 4.6 µg/ml) administration. Therapeutically, in spite of the 4–6-fold lower levels of PZQ in portal venous plasma after intramuscular administration, adult worm reduction rates in infected rabbits using the above doses were 92.2% (intramuscular), 90.1% (rectal), and 72.5% (oral), respectively, four weeks after treatment. Thus, no direct correlation between levels of PZQ in peripheral or portal venous blood and therapeutic efficacy was observed in rabbits infected with S. japonicum.