A summary of the efficacy of praziquantel against schistosomes in animal experiments and notes on its mode of action.

2-Cyclohexylcarbonyl-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isoquinolin-4-one (praziquantel, EMBAY 8440, Biltricide) is broadly effective in mice, Syrian hamsters and Mastomys natalensis experimentally infected with Schistosoma mansoni, S. haematobium, S. japonicum, S. intercalatum, and S. mattheei. In vitro it is equally effective against all developmental stages of S. mansoni whilst in vivo it is especially effective against young schistosomules and mature worms. 14C-Praziquantel is rapidly taken up by S. mansoni and not transformed metabolically. In the mammal, however, metabolism of praziquantel is rapid and 80% of the orally administered dose is excreted in the urine within 24 h. Clinico-chemical and haematological studies have shown that praziquantel is well tolerated by mice heavily infected with S. mansoni. Praziquantel induces a rapid contraction of the schistosomes by a specific effect on the permeability of the cell membrane. In vivo it induces a rapid liver shift of the schistosomes. Praziquantel further results in a vacuolization and disintegration of the schistosome tegument.