Praziquantel, marketed as Biltricide, is an anthelmintic used in humans and animals for the treatment of tapeworms and flukes. Specifically, it is effective against schistosoma, Clonorchis sinensis the fish tape worm Diphyllobothrium latum. Praziquantel works by causing severe spasms and paralysis of the worms' muscles. This paralysis is accompanied - and probably caused - by a rapid Ca 2+ influx inside the schistosome. Morphological alterations are another early effect of praziquantel. These morphological alterations are accompanied by an increased exposure of schistosome antigens at the parasite surface. The worms are then either completely destroyed in the intestine or passed in the stool. An interesting quirk of praziquantel is that it is relatively ineffective against juvenile schistosomes. While initially effective, effectiveness against schistosomes decreases until it reaches a minimum at 3-4 weeks. Effectiveness then increases again until it is once again fully effective at 6-7 weeks. Glutathione S-transferase (GST), an essential detoxification enzyme in parasitic helminths, is a major vaccine target and a drug target against schistosomiasis. Schistosome calcium ion channels are currently the only known target of praziquantel. The antibiotic rifampicin decreases plasma concentrations of praziquantel. Carbamazepine and phenytoin are reported to reduce the bioavailability of praziquantel. Chloroquine reduces the bioavailability of praziquantel. The drug cimetidine heightens praziquantel bioavailability.

Praziquantel, (-)- is oxopyrazinoisoquinoline derivative and a levorotated isomer of Praziquantel patented by Merck Patent G.m.b.H. as anthelmintics agent. In rabbits infested with S. japonicum, the therapeutic effect of Praziquantel, (-)-was greater than that of dl-praziquantel as rated by the number of the worms in tissues and by liver damage. Histopathological examination showed that liver egg granulomas in the levo-praziquantel group were fewer in no. and smaller in size and were predominantly composed of Pseudotubercles instead of eosinophilic abscesses. Levo-praziquantel is therapeutically superior to praziquantel, while dextro-praziquantel is almost ineffective.

Praziquantel, (+)- is the dextrorotated (+) isomer of Praziquantel. Praziquantel (PZQ) is the drug of choice against schistosomiasis. Since exposure of schistosomes to the drug is associated with calcium influx and muscular contraction, calcium channels have been suggested as the target. It is a specific pharmacological effect seen exclusively with the active levo-R(-)stereo isomer of the drug. Praziquantel, (+)- apparently contributes little to the therapeutic efficacy of Praziquantel. In vivo, single 400-mg/kg oral doses of Praziquantel, (-)- and Praziquantel, (+)- achieved worm burden reductions of 100 and 19%, respectively. Moreover, worms treated in vivo with Praziquantel, (+)- displayed an only transient hepatic shift and returned to the mesenteric veins within 24 h.