Activation of host transient receptor potential (TRP) channels by praziquantel stereoisomers

Abstract

The anthelmintic praziquantel (±PZQ) serves as a highly effective antischistosomal therapy. ±PZQ causes a rapid paralysis of adult schistosome worms and deleterious effects on the worm tegument. In addition to these activities against the parasite, ±PZQ also modulates host vascular tone in blood vessels where the adult worms reside. In resting mesenteric arteries ±PZQ causes a constriction of basal tone, an effect mediated by (R)-PZQ activation of endogenous serotoninergic G protein coupled receptors (GPCRs). Here, we demonstrate a novel vasodilatory action of ±PZQ in mesenteric vessels that are precontracted by high potassium-evoked depolarization, an effect previously reported to be associated with agonists of the transient receptor potential melastatin 8 channel (TRPM8). Pharmacological profiling a panel of 17 human TRPs demonstrated ±PZQ activity against a subset of human TRP channels. Several host TRP channels (hTRPA1, hTRPC3, hTRPC7) were activated by both (R)-PZQ and (S)-PZQ over a micromolar range whereas hTRPM8 showed stereoselective activation by (S)-PZQ. The relaxant effect of ±PZQ in mesenteric arteries was caused by (S)-PZQ, and mimicked by TRPM8 agonists. However, persistence of both (S)- PZQ and TRPM8 agonist evoked vessel relaxation in TRPM8 knockout tissue suggested that canonical TRPM8 does not mediate this (S)-PZQ effect. We conclude that (S)-PZQ is vasoactive over the micromolar range in mesenteric arteries although the molecular mediators of this effect remain to be identified. These data expand our knowledge of the polypharmacology and host vascular efficacy of this clinically important anthelmintic.

Introduction

Schistosomiasis is a socioeconomically devastating helminth infection afflicting over 200 million people worldwide [1]. The resulting disease burden of chronic schistosomiasis is estimated to encumber third world economies with an annual loss of 70 million disability-adjusted life years [2, 3]. In infected individuals, the prolific egg laying capacity of paired adult worms (>1000 eggs/day deposited in tissues, [4]) triggers localized inflammatory responses around eggs trapped within host tissues. Chronic infections progress toward fibrosis and obstructive disease in gastrointestinal tissues and liver (S. mansoni, S. japonicum), genitourinary disease (S. haematobium), anemia, undernutrition and a heightened risk for other comorbidities. Effective drug therapy for schistosomiasis is therefore a healthcare priority [1–3]. The drug praziquantel (±PZQ) has served as the stalwart antischistosomal therapy since the 1980s and the need for ±PZQ is significant [5]. Thankfully, the drug has remained effective over three decades of clinical use, although there are certainly features of ±PZQ that are less than optimal. These include anxiety over the emergence of drug resistance in face of selective pressures imposed by mass distribution efforts, a refractoriness of juvenile worms to PZQ, our lack of understanding over the molecular target(s) of PZQ and an inability to improve on PZQ by chemical derivatization of the drug [6, 7]. Certainly, a better understanding of how ±PZQ works would catalyze future drug development efforts toward the next generation of antischistosomal compounds.

Addition of ±PZQ to adult schistosomes causes an acute Ca2+ influx, rapid paralysis of the musculature and a more chronic tegumental damage that aids immunological elimination of worms from the host. Efficacy in vitro and in vivo is associated with the action of (R)-PZQ as the more active enantiomer (eutomer) in the clinical formulation [8, 9], underpinning effort to develop an enantiopure clinical formulation [10]. ±PZQ also displays activity against target(s) in the host [11, 12], including vasoconstriction of the mesenteric blood vessels inhabited by the adults worms, an effect caused by (R)-PZQ stimulation of endogenous serotoninergic GPCRs [13]. The distomer (S)-PZQ also exhibits host bioactivity: it is associated with an unpleasant bitter taste effect [14] and effects a transient translocation (‘hepatic shift’) of S. mansoni worms from the splanchnic beds to the liver on administration [9] despite the appreciated lack of efficacy of (S)-PZQ against worms in vitro. Recent work has revealed activity of ±PZQ against the human transient receptor potential melastatin 8 channel (TRPM8, [15]), although the efficacy of the individual enantiomers at regulating TRPM8 are undefined. TRP channels belong to a superfamily of ion channels that respond to a broad diversity of stimuli and chemotypes underpinning many elements of our sensory physiology [16, 17]. Schistosome TRPs are themselves promising targets for their druggability [18, 19]. Collectively, both recent reports underscore considerable progress in defining activities and target(s) of ±PZQ action in the human host [13, 15].

Here, we report a novel vasodilatory action of (S)-PZQ in contracted mesenteric vessels. Based on previously published data implicating TRPM8 channels in this vasodilatory effect in rat mesenteric arteries [20], further prioritized by the work of Babes et al. [15] showing activation of TRPM8 by ±PZQ, activity of ±PZQ on endogenous TRPs that regulate myogenic tone was suspected. This study was designed to investigate the interaction of (R)-PZQ and (S)-PZQ with human TRPs, and test the possibility that such interactions regulate mesenteric vessel tone.

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