Putative Synthetic Cannabinoids MEPIRAPIM, 5F-BEPIRAPIM (NNL-2), and Their Analogues Are T-Type Calcium Channel (CaV3) Inhibitors

Synthetic cannabinoid receptor agonists (SCRAs) are a large and growing class of new psychoactive substances (NPSs). Two recently identified compounds, MEPIRAPIM and 5F-BEPIRAPIM (NNL-2), have not been confirmed as agonists of either cannabinoid receptor subtype but share structural similarities with both SCRAs and a class of T-type calcium channel (Ca_V3) inhibitors under development as new treatments for epilepsy and pain. In this study, MEPIRAPIM and 5F-BEPIRAPIM and 10 systematic analogues were synthesized, analytically characterized, and pharmacologically evaluated using in vitro cannabinoid receptor and Ca_V3 assays. Several compounds showed micromolar affinities for CB₁ and/or CB₂, with several functioning as low potency agonists of CB₁ and CB₂ in a membrane potential assay. 5F-BEPIRAPIM and four other derivatives were identified as potential Ca_V3 inhibitors through a functional calcium flux assay (>70% inhibition), which was further confirmed using whole-cell patch-clamp electrophysiology. Additionally, MEPIRAPIM and 5F-BEPIRAPIM were evaluated in vivo using a cannabimimetic mouse model. Despite detections of MEPIRAPIM and 5F-BEPIRAPIM in the NPS market, only the highest MEPIRAPIM dose (30 mg/kg) elicited a mild hypothermic response in mice, with no hypothermia observed for 5F-BEPIRAPIM, suggesting minimal central CB₁ receptor activity.

Inhibition of human recombinant T-type calcium channels by phytocannabinoids in vitro

BACKGROUND AND PURPOSE

T-type Ca channels (I_Ca ) regulate neuronal excitability and contribute to neurotransmitter release. The phytocannabinoids Δ⁹ -tetrahydrocannabinol and cannabidiol effectively modulate T-type I_Ca , but effects of other biologically active phytocannabinoids on these channels are unknown. We thus investigated the modulation of T-type I_Ca by low abundance phytocannabinoids.

EXPERIMENTAL APPROACH

A fluorometric (FLIPR) assay was used to investigate modulation of human T-type I_Ca (Ca_V 3.1, 3.2 and 3.3) stably expressed in FlpIn-TREx HEK293 cells. The biophysical effects of some compounds were examined using whole-cell patch clamp recordings from the same cells.

KEY RESULTS

In the FLIPR assay, all eleven phytocannabinoids tested modulated T-type I_Ca , with most inhibiting Ca_V 3.1 and Ca_V 3.2 more effectively than Ca_V 3.3. Cannabigerolic acid was the most potent inhibitor of Ca_V 3.1 (pIC₅₀ 6.1 ± 0.6) and Ca_V 3.2 (pIC₅₀ 6.4 ± 0.4); in all cases phytocannabinoid acids were more potent than their corresponding neutral forms. In patch clamp recordings, cannabigerolic acid inhibited Ca_V 3.1 and 3.2 with similar potency to the FLIPR assay, the inhibition was associated with significant hyperpolarizing shift in activation and steady state inactivation of these channels. In contrast, cannabidiol, cannabidivarin and cannabigerol only affected channel inactivation.

CONCLUSION AND IMPLICATIONS

Modulation of T-type calcium channels is a common property of phytocannabinoids, which all increase steady state inactivation at physiological membrane potentials, with some also affecting channel activation. Thus, T-type I_Ca may be a common site of action for phytocannabinoids, and the diverse actions of phytocannabinoids on channel gating may provide insight into structural requirement for selective T-type I_Ca modulators.

Modulation of Recombinant Human T-Type Calcium Channels by Δ9-Tetrahydrocannabinolic Acid In Vitro

Introduction: Low voltage-activated T-type calcium channels (T-type I_Ca), Ca_V3.1, Ca_V3.2, and Ca_V3.3, are opened by small depolarizations from the resting membrane potential in many cells and have been associated with neurological disorders, including absence epilepsy and pain. Δ⁹-tetrahydrocannabinol (THC) is the principal psychoactive compound in Cannabis and also directly modulates T-type I_Ca; however, there is no information about functional activity of most phytocannabinoids on T-type calcium channels, including Δ⁹-tetrahydrocannabinolic acid (THCA), the natural nonpsychoactive precursor of THC. The aim of this work was to characterize THCA effects on T-type calcium channels. Materials and Methods: We used HEK293 Flp-In-TREx cells stably expressing Ca_V3.1, 3.2, or 3.3. Whole-cell patch clamp recordings were made to investigate cannabinoid modulation of I_Ca. Results: THCA and THC inhibited the peak current amplitude Ca_V3.1 with pEC₅₀s of 6.0±0.7 and 5.6±0.4, respectively. THC (1 μM) or THC produced a significant negative shift in half activation and inactivation of Ca_V3.1, and both drugs prolonged Ca_V3.1 deactivation kinetics. THCA (10 μM) inhibited Ca_V3.2 by 53%±4%, and both THCA and THC produced a substantial negative shift in the voltage for half inactivation and modest negative shift in half activation of Ca_V3.2. THC prolonged the deactivation time of Ca_V3.2, while THCA did not. THCA inhibited the peak current of Ca_V3.3 by 43%±2% (10 μM) but did not notably affect Ca_V3.3 channel activation or inactivation; however, THC caused significant hyperpolarizing shift in Ca_V3.3 steady-state inactivation. Discussion: THCA modulated T-type I_Ca currents in vitro, with significant modulation of kinetics and voltage dependence at low μM concentrations. This study suggests that THCA may have potential for therapeutic use in pain and epilepsy through T-type calcium channel modulation without the unwanted psychoactive effects associated with THC.

Modulation of human T-type calcium channels by synthetic cannabinoid receptor agonists in vitro

BACKGROUND AND PURPOSE

Consumption of Synthetic Cannabinoid Receptor agonists (SCRAs) is associated with severe adverse reactions including seizures, arrhythmias and death, but the molecular mechanisms surrounding SCRA toxicity are not yet established. These disease-like symptoms are also synonymous with altered T-type calcium channel activity which controls rhythmicity in the heart and brain. This study examined whether SCRAs alter T-type activity and whether this represents a possible mechanism of toxicity.

EXPERIMENTAL APPROACH

Fluorescence-based and electrophysiology assays were used to screen 16 structurally related synthetic cannabinoids for their ability to inhibit human T-type calcium channels expressed in HEK293 cells. The most potent compounds were then further examined using patch clamp electrophysiology.

KEY RESULTS

MDMB-CHMICA and AMB-CHMINACA potently blocked Cav3.2 with IC50 values of 1.5 and 0.74 μM respectively. Current inhibition increased from 47 to 80% and 45-87% respectively when the channel was in slow-inactivated state. Both SCRAs had little effect on steady state inactivation, however MDMB-CHMICA significantly shifted the half activation potential by -7mV. Neither drug produced frequency dependent block, in contrast to the phytocannabinoid Δ9-THC.

CONCLUSIONS AND IMPLICATIONS

SCRAs are potent agonists of CB1 receptors and can be extremely toxic, but observed toxicity also resembles symptoms associated with altered Cav3.2 activity. Many SCRAs tested were potent modulators of Cav3.2, raising the possibility that SC toxicity may be due in part to Cav3.2 modulation. This potent T-type channel modulation suggests the possibility of SCRAs as a new drug class with potential to treat diseases associated with altered T-type channel activity. This article is part of the special issue on 'Cannabinoids'.