Target of Praziquantel

Published by MatTodd on 22 October 2007 - 1:26pm

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

Comments

Dear Dr. Todd,

 

This is my first visit on the blog. I'm a post-doc at the Dept. of Biochemical sciences at the university of Rome.

We demonstrated that praziquantel (and Ro 11-3128) is able to inhibits nucleoside uptake in cultured worms. This effect is evident at the same dosage and at the same time scale as calcium influx [Angelucci et al. (2007). Parasitol. 134, 1215].

We think that this effect might have relevance to the pharmacological effect of praziquantel, given that schistosomes as other praziquantel-sensitive parasites (e.g. Taenia and Echinococcus) are unable to synthetize purine de novo; moreover nematodes, that are insensitive to praziquantel, are able to synthetize purines.

 

Thank you for your attention

F Angelucci

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Dr Angelucci,
This is obviously very interesting work. (Link to the paper here). Are you able to spell out what you think PZQ is therefore doing?
I notice you overlay adenosine and PZQ in the paper - how good is the overlap with (S)-(+)-PZQ? Does this kind of comparison suggest certain PZQ analogues that ought to be synthesised and evaluated?
What can be learned from the effect on PZQ action of other adenosine antagonists?
Mat

Dear
Mat,

First of all I would like to stress the point that our initial observation is only a working hypothesis on the mechanism of action of PZQ.

The main point of our work is that PZQ has a clear effect on the inhibition of nucleoside uptake in cultured worms. To our knowledge this is the first effect, after calcium influx, that is detectable at sub-lethal doses and after few minutes of incubation with the drug. The effect is due only to the stereoisomer R and is not observed in mammalian cells and thus seems to be specific. Considering that PZQ requires a relatively long time to kill the worms in vitro, it may also be responsible for a sustained metabolic imbalance that may rise from the inhibition of nucleoside uptake.

The superimposition between adenosine and PZQ was performed with a low energy conformation of adenosine and the structure of PZQ found in the crystal structure of S. japonicum Glutathione transferase in complex with the drug. Using these models this superimposition between the S- stereoisomer is clearly worse than the R- active stereoisomer of PZQ. However, without a target site of interaction, this finding may not reflect the actual binding mode of PZQ.

We already tried to synthesize an amino-PZQ derivative, suggested by our superimposition, but even if the derivative paralyzes the worms at similar concentrations of PZQ, its schistomicidal effect is found at a concentration ten times higher than what reported for PZQ (unpublished results).

The implication that PZQ might interact with adenosine receptors or transporters is a speculation. Adenosine and some of its analogues are not able to counteract the effect of PZQ, even if some tested adenosine analogues are able to inhibit nucleoside uptake in cultured worms. On the other hand, the calcium antagonist nifedipine, that partially protects schistosomes from PZQ lethal effects (Pica-Mattoccia et al., 2007), inhibits adenosine uptake (as well as PZQ and other adenosine analogues). Indeed, nifedipine (and other 1,4-dihydropyridine), besides its effect on calcium channels, is also known to interact with adenosine receptors/transporters in mammalian cells (Van Rhee et al., 1996). Thus it seems that there is a correlation, in the mechanism of action of PZQ, between calcium influx and nucleoside uptake. Moreover, our findings may (at least) add some information on the mode of penetration of PZQ in the worm interior.

Francesco

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A paper was published in 2009 in which PZQ was modified with the dye BODIPY. "Annotation of genes differentially regulated by PZQ exposure suggests that schistosomes may undergo a transcriptomic response similar to that observed during oxidative stress." More details on the synthesis of this molecule can be found here.
 
More recently a paper came out indicating that PZQ binds to myosin light chain protein. Whether this is related to the mechanism of action is still unclear.