(C) PLOS One
This story was originally published by PLOS One and is unaltered.
. . . . . . . . . .



Progress interrogating TRPMPZQ as the target of praziquantel [1]

['Jonathan S. Marchant', 'Department Of Cell Biology', 'Neurobiology', 'Anatomy', 'Medical College Of Wisconsin', 'Milwaukee', 'Wisconsin', 'United States Of America']

Date: 2024-10

Collectively, these 10 lines of evidence provide strong support for TRPM PZQ acting as the therapeutic target of PZQ, with the experimental data discussed above proving consistent with correct target validation. However, caution is always merited, and further questions remain. One wryly notes that even for cancer drugs undergoing clinical trials in humans, their assumed targets have often retrospectively been shown to be false [51,52]. This underscores the importance of coalescing multiple lines of evidence to underpin target validation [52,53]. In this regard, 3 areas merit further attention [13].

Insight from functional genetic approaches is needed. Results from knockdown or knockout analyses, to ablate TRPM PZQ expression in parasites, have yet to be reported. Neither of these approaches are trivial to execute: Knockdown by RNA interference (RNAi) can be finicky depending on the target, how abundant it is and where it is expressed in the worm. Stable transgenesis in schistosomes is also an active focus for optimization. TRPM PZQ is not abundantly expressed at the surface of the worm but is found within excitable cells. The large cation flux mediated by TRPM PZQ would likely necessitate a highly penetrant knockdown of TRPM PZQ for RNAi data to be interpretable, as residual expression of TRPM PZQ could still support a robust depolarization response to PZQ. Challenges related to off-target effects with RNAi, and the adequacy of controls for many commonly scored phenotypes, also persist [ 54 ]. But provided TRPM PZQ is not crucial for parasite viability, these genetic loss-of-function approaches will provide critical insight as to the essentiality of TRPM PZQ for PZQ action. The availability of small molecule blockers of TRPM PZQ (see point #10) will complement these genetic loss-of-function approaches as pharmacological blockade of TRPM PZQ should phenocopy and thereby validate RNAi effects. Clearly, if PZQ-evoked depolarization, contraction, and surface damage phenotypes persist in the absence of TRPM PZQ , then other targets must mediate these effects. TRPM PZQ , despite the aforementioned evidence, would then be a “false” target in relation to the anthelmintic activity of PZQ.

Other targets?

PZQ inevitably has more than one target, consistent with the polypharmacological profile expected with any small molecule [55,56]. Many of these will be “secondary” targets, with these interactions not recapitulating the high sensitivity and stereoselectivity displayed by TRPM PZQ (the “primary” target). For example, in humans, where the process of target identification is more facile, PZQ has been shown to regulate multiple TRP channels [57,58] and several GPCRs [59]. However, these interactions exhibit lower sensitivities (micromolar at best) and often different stereochemistry (for example, hTRPM8 is only activated by (S)-PZQ [58]). For the human 5-HT 2B receptor, where a (R)-PZQ binding pose has been defined and validated, the lower sensitivity of 5-HT 2B toward PZQ (EC 50 in low micromolar range [59,60]) can be explained by the loss of specific binding interactions that been shown to anchor PZQ within the schistosome TRPM PZQ binding pocket. For example, whereas hydrogen-bonding interactions occur in TRPM PZQ to both the carbonyl groups of PZQ, only a single hydrogen-bond interaction is predicted in the human 5-HT 2B binding pocket [60]. Loss of optimal hydrogen-bond interactions will decrease binding affinity [61], likely explaining the shift from the “hundreds of nanomolar” to the “micromolar” sensitivity range, even though “selective” binding (5-HT 2B compared with 5-HT 2A or 5-HT 2C ) is still evident. These host targets may be relevant to several side effects associated with PZQ (5-HT 2B : smooth muscle contraction underpinning nausea, abdominal pains; TRPM8: poor taste), and, potentially, also therapeutic efficacy (vascular contraction in mesenteric vessels) by aiding the hepatic shift of contracted worms [59,60].

Just as with the discovery of such “secondary” targets in humans, secondary parasite targets for PZQ will be discovered. Indeed, several PZQ-interacting proteins in schistosomes have already been proposed including myosin light chain [62], actin [62,63] (but see [64]), voltage-operated Ca2+ channels [65], multidrug-resistant transporters [66], adenosine transporters [67], glutathione S-transferase (GST) [68], and several members of the tegumental allergen (TAL) family of proteins [69]. However, for the majority of these candidates, quantitative characterization of PZQ binding and the selectivity of the ligand binding site is lacking. Also, acknowledging the tight SAR of PZQ and the reciprocally tight SAR of the TRPM PZQ binding pocket, it is worth pointing out that many conjugated PZQ analogs utilized in prior target discovery strategies would poorly interact with TRPM PZQ , if at all. Whether any of these reported interactions contribute to the therapeutic efficacy of PZQ remains the critical question, and this will require careful validation. Three fundamental criteria, outlined in the preceding sections for TRPM PZQ , must be met. First, is there reasonable congruence between the affinity for PZQ at the proposed target versus PZQ efficacy against the worm? Second, is there a similar SAR for PZQ analogs at the proposed target versus the parasite? Third, is there a clear functional outcome consequent to PZQ engaging these targets that is consistent with the triad of phenotypic outcomes (depolarization, worm contraction, tegument damage)? For example, with Sm.TAL1, where careful efforts have been made to characterize PZQ binding, the resolved affinity is low (K d of Sm.TAL1 for PZQ = 140 μM [69]) compared with PZQ action on worms. For schistosome GST, where PZQ was cocrystallized with the enzyme, the binding site lies within an amphipathic groove at the dimer interface, which promiscuously accommodates many hydrophobic chemotypes [21] not reflecting the established SAR. Further, PZQ binding does not affect GST function [70]. Therefore, many of these proposed interactors may not stand up to scrutiny as a “primary” target.

[END]
---
[1] Url: https://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0011929

Published and (C) by PLOS One
Content appears here under this condition or license: Creative Commons - Attribution BY 4.0.

via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/plosone/

You are viewing proxied material from magical.fish. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.