Target of Praziquantel

One of the enduring research problems associated with Praziquantel is that its mechanism of action is unknown, which is to say the biological target is unknown. If we were to find this target, we could design drugs rationally. There is evidence in the literature that the parasite's calcium ion channel is involved in praziquantel's mode of action. Robert Greenberg reviewed this area in 2005. Conor Caffrey has also summarised this research in a recent article on schistosomiasis chemotherapy. In general, the chain of events between PZQ contact, calcium entry and parasite death is still unknown. The hypothesis of direct binding between PZQ and ion channel proteins is unsatisfactory because: a) Resistant strains of the parasite show no changes in this protein; Immature worms are almost completely refractory to PZQ, although they express the same calcium channel beta-subunit proteins; the functional aspects of PZQ effects on calcium channels have been documented only indirectly by current changes in oocytes expressing single components of the schisto channel; no indication exists as to whether calcium channels would interact directly with PZQ or through other intermediates. Paper b) The existence of voltage gated calcium channels at the worm surface has not been demonstrated. c) Calcium influx and muscle contraction are not always followed by worm death. Indeed, it is still an assumption that calcium influx is the cause of schistosome death. Paper Rather than me sifting through the literature, I'd like to ask you all for help in collating links to relevant papers here, so suggestions for and against calcium channels? Possible other target proteins will form separate project pages of their own linking off this one. Mat Todd

Actin

Does praziquantel bind actin? Tallima and El Ridi recently proposed actin as the molecular target of PZQ, based on affinity chromatography studies. Todd and Cioli performed similar experiments (with a different support-bound PZQ) and found actin bound to PZQ-free support, implying that actin is a false positive, identified due to its cellular abundance. Other evidence? Required projects?

Role of Calcium Flux

Another interesting PZQ paper has come out of the cell biology group at the CNR in Rome, concerning the mode of action of PZQ. Using radioactive calcium, the group have demonstrated that calcium influx in itself is not enough to kill parasites. Pre-incubation with cytochalasin D promoted calcium influx but protected the worms from PZQ's schistosomicidal effects.

Strategies to Identify the Target of Praziquantel

There are three obvious methods to identify PZQ's in vivo target: 1) Radiolabelled PZQ (tritiated probably best). It's likely that the interaction between PZQ and its target is not long-lived, which means purely radiolabelled PZQ is not the best strategy. 2) Solid-supported PZQ. It may be possible to attach PZQ to a solid support and pass the parasite proteome down a column of this support to identify retarded proteins. For preliminary results in this direction see the actin project page. 3) A strong approach is the attachment of a photoaffinity probe to PZQ, e.g. an azide. Incubation of photoaffinity-PZQ ("hv-PZQ") with schistosome extract followed by irradiation with UV light should cross-link the PZQ with the target protein. Naturally such proteins need to be identified, implying a radioactive photoaffinity probe is needed, which is more difficult and expensive to make. For approaches 2 and 3, a suitable place to attach things to PZQ must be found, which links to the search for effective analogs of PZQ where variation in the structure of the molecule is tolerated.