Mibefradil: a selective T-type calcium antagonist

Ion Channels as Therapeutic Targets in High Grade Gliomas

Simple Summary

Glioblastoma multiforme is an aggressive grade IV lethal brain tumour with a median survival of 14 months. Despite surgery to remove the tumour, and subsequent concurrent chemotherapy and radiotherapy, there is little in terms of effective treatment options. Because of this, exploring new treatment avenues is vital. Brain tumours are intrinsically electrically active; expressing unique patterns of ion channels, and this is a characteristic we can exploit. Ion channels are specialised proteins in the cell’s membrane that allow for the passage of positive and negatively charged ions in and out of the cell, controlling membrane potential. Membrane potential is a crucial biophysical signal in normal and cancerous cells. Research has identified that specific classes of ion channels not only move the cell through its cell cycle, thus encouraging growth and proliferation, but may also be essential in the development of brain tumours. Inhibition of sodium, potassium, calcium, and chloride channels has been shown to reduce the capacity of glioblastoma cells to grow and invade. Therefore, we propose that targeting ion channels and repurposing commercially available ion channel inhibitors may hold the key to new therapeutic avenues in high grade gliomas.

Abstract

Glioblastoma multiforme (GBM) is a lethal brain cancer with an average survival of 14–15 months even with exhaustive treatment. High grade gliomas (HGG) represent the leading cause of CNS cancer-related death in children and adults due to the aggressive nature of the tumour and limited treatment options. The scarcity of treatment available for GBM has opened the field to new modalities such as electrotherapy. Previous studies have identified the clinical benefit of electrotherapy in combination with chemotherapeutics, however the mechanistic action is unclear. Increasing evidence indicates that not only are ion channels key in regulating electrical signaling and membrane potential of excitable cells, they perform a crucial role in the development and neoplastic progression of brain tumours. Unlike other tissue types, neural tissue is intrinsically electrically active and reliant on ion channels and their function. Ion channels are essential in cell cycle control, invasion and migration of cancer cells and therefore present as valuable therapeutic targets. This review aims to discuss the role that ion channels hold in gliomagenesis and whether we can target and exploit these channels to provide new therapeutic targets and whether ion channels hold the mechanistic key to the newfound success of electrotherapies.

Cognitive benefits of angiotensin IV and angiotensin-(1-7): A systematic review of experimental studies

OBJECTIVES

To explore effects of the brain renin-angiotensin system (RAS) on cognition.

DESIGN

Systematic review of experimental (non-human) studies assessing cognitive effects of RAS peptides angiotensin-(3-8) [Ang IV] and angiotensin-(1-7) [Ang-(1-7)] and their receptors, the Ang IV receptor (AT4R) and the Mas receptor.

RESULTS

Of 450 articles identified, 32 met inclusion criteria. Seven of 11 studies of normal animals found Ang IV had beneficial effects on tests of passive or conditioned avoidance and object recognition. In models of cognitive deficit, eight of nine studies found Ang IV and its analogs (Nle¹-Ang IV, dihexa, LVV-hemorphin-7) improved performance on spatial working memory and passive avoidance tasks. Two of three studies examining Ang-(1-7) found it benefited memory. Mas receptor removal was associated with reduced fear memory in one study.

CONCLUSION

Studies of cognitive impairment show salutary effects of acute administration of Ang IV and its analogs, as well as AT4R activation. Brain RAS peptides appear most effective administered intracerebroventricularly, close to the time of learning acquisition or retention testing. Ang-(1-7) shows anti-dementia qualities.

T-type Ca2+ channels are required for enhanced sympathetic axon growth by TNFα reverse signalling

Tumour necrosis factor receptor 1 (TNFR1)-activated TNFα reverse signalling, in which membrane-integrated TNFα functions as a receptor for TNFR1, enhances axon growth from developing sympathetic neurons and plays a crucial role in establishing sympathetic innervation. Here, we have investigated the link between TNFα reverse signalling and axon growth in cultured sympathetic neurons. TNFR1-activated TNFα reverse signalling promotes Ca²⁺ influx, and highly selective T-type Ca²⁺ channel inhibitors, but not pharmacological inhibitors of L-type, N-type and P/Q-type Ca²⁺ channels, prevented enhanced axon growth. T-type Ca²⁺ channel-specific inhibitors eliminated Ca²⁺ spikes promoted by TNFα reverse signalling in axons and prevented enhanced axon growth when applied locally to axons, but not when applied to cell somata. Blocking action potential generation did not affect the effect of TNFα reverse signalling on axon growth, suggesting that propagated action potentials are not required for enhanced axon growth. TNFα reverse signalling enhanced protein kinase C (PKC) activation, and pharmacological inhibition of PKC prevented the axon growth response. These results suggest that TNFα reverse signalling promotes opening of T-type Ca²⁺ channels along sympathetic axons, which is required for enhanced axon growth.

Effect of Mibefradil on CYP3A4 In Vivo

Mibefradil, a calcium channel blocker, was removed from the market because of adverse drug interactions with coadministered CYP3A4 substrates. This study examined the effect of mibefradil on the activity of hepatic and intestinal CYP3A4 in vivo, employing the erythromycin breath test (EBT) and oral midazolam pharmacokinetics. This was a two-period, single-blind, placebo-controlled crossover study in which 8 male volunteers were randomized to the order of receiving placebo and a single 100-mg oral dose of mibefradil. Oral midazolam was coadministered with intravenous [14C N-methyl] erythromycin 1 hour after mibefradil/placebo administration. The EBT was performed 20 minutes following erythromycin administration. Blood and urine were collected during the 36 hours following probe drug administration for analysis of midazolam pharmacokinetics. Coadministration of mibefradil increased the Cmax of midazolam 3-fold, the AUC 8- to 9-fold, and the t1/2 4-fold. Mibefradil coadministration decreased the amount of exhaled 14CO2 in 6 of 8 subjects, with a mean decrease of 25%. It was concluded that a single oral dose of mibefradil significantly inhibits CYP3A4 in intestine and liver. These data support that adverse drug interactions involving mibefradil reflect inhibition of CYP3A4 in intestine and liver. Also, they suggest that the EBT, while a valid probe of in vivo hepatic CYP3A4 activity, is a single time point measurement and may be less sensitive than oral midazolam pharmacokinetics in detecting CYP3A4 inhibition.

Pyridyl amides as potent inhibitors of T-type calcium channels

A novel series of amide T-type calcium channel antagonists were prepared and evaluated using in vitro and in vivo assays. Optimization of the screening hit 3 led to identification of the potent and selective T-type antagonist 37 that displayed in vivo efficacy in rodent models of epilepsy and sleep.

Neuroprotective effects of blockers for T-type calcium channels

Cognitive and functional decline with age is correlated with deregulation of intracellular calcium, which can lead to neuronal death in the brain. Previous studies have found protective effects of various calcium channel blockers in pathological conditions. However, little has been done to explore possible protective effects of blockers for T-type calcium channels, which forms a family of FDA approved anti-epileptic drugs. In this study, we found that neurons showed an increase in viability after treatment with either L-type or T-type calcium channel antagonists. The family of low-voltage activated, or T-type calcium channels, comprise of three members (Ca_v3.1, Ca_v3.2, and Ca_v3.3) based on their respective main pore-forming alpha subunits: α1G, α1H, and α1I. Among these three subunits, α1H is highly expressed in hippocampus and certain cortical regions. However, T-type calcium channel blockers can protect neurons derived from α1H-/- mice, suggesting that neuroprotection demonstrated by these drugs is not through the α1H subunit. In addition, blockers for T-type calcium channels were not able to confer any protection to neurons in long-term cultures, while blockers of L-type calcium channels could protect neurons. These data indicate a new function of blockers for T-type calcium channels, and also suggest different mechanisms to regulate neuronal survival by calcium signaling pathways. Thus, our findings have important implications in the development of new treatment for age-related neurodegenerative disorders.

The mibefradil derivative NNC55-0396, a specific T-type calcium channel antagonist, exhibits less CYP3A4 inhibition than mibefradil

A novel mibefradil derivative, NNC55-0396, designed to be hydrolysis-resistant, was shown to be a selective T-type Ca(2+) channel inhibitor without L-type Ca(2+) channel efficacy. However, its effects on cytochromes P450 (P450s) have not previously been examined. We investigated the inhibitory effects of NNC55-0396 toward seven major recombinant human P450s--CYP3A4, CYP2D6, CYP1A2, CYP2C9, CYP2C8, CYPC19, and CYP2E1--and compared its effects with those of mibefradil and its hydrolyzed metabolite, Ro40-5966. Our results show that CYP3A4 and CYP2D6 are the two P450s most affected by mibefradil, Ro40-5966, and NNC55-0396. Mibefradil (IC(50) = 33 +/- 3 nM, K(i) = 23 +/- 0.5 nM) and Ro40-5966 (IC(50) = 30 +/- 7.8 nM, K(i) = 21 +/- 2.8 nM) have a 9- to 10-fold greater inhibitory activity toward recombinant CYP3A4 benzyloxy-4-trifluoromethylcoumarin-O-debenzylation activity than NNC55-0396 (IC(50) = 300 +/- 30 nM, K(i) = 210 +/- 6 nM). More dramatically, mibefradil (IC(50) = 566 +/- 71 nM, K(i) = 202 +/- 39 nM) shows 19-fold higher inhibition of CYP3A-associated testosterone 6beta-hydroxylase activity in human liver microsomes compared with NNC55-0396 (IC(50) = 11 +/- 1.1 microM, K(i) = 3.9 +/- 0.4 microM). Loss of testosterone 6beta-hydroxylase activity by recombinant CYP3A4 was shown to be time- and concentration-dependent with both compounds. However, NNC55-0396 (K(I) = 3.87 microM, K(inact) = 0.061/min) is a much less potent mechanism-based inhibitor than mibefradil (K(I) = 83 nM, K(inact) = 0.048/min). In contrast, NNC55-0396 (IC(50) = 29 +/- 1.2 nM, K(i) = 2.8 +/- 0.3 nM) and Ro40-5966 (IC(50) = 46 +/- 11 nM, K(i) = 4.5 +/- 0.02 nM) have a 3- to 4-fold greater inhibitory activity toward recombinant CYP2D6 than mibefradil (IC(50) = 129 +/- 21 nM, K(i) = 12.7 +/- 0.9 nM). Our results suggest that NNC55-0396 could be a more favorable T-type Ca(2+) antagonist than its parent compound, mibefradil, which was withdrawn from the market because of strong inhibition of CYP3A4.

Follicle-stimulating hormone-induced Galphah/phospholipase C-delta1 signaling mediating a noncapacitative Ca2+ influx through T-type Ca2+ channels in rat sertoli cells

Our previous study demonstrated that FSH-induced immediate Ca(2+) influx in rat Sertoli cells (SCs) is mediated by the Galphah/phospholipase C-delta1 (PLC-delta1) signaling pathway. As to which Ca(2+) channel is responsible for such Ca(2+) influx was not understood. In this study, thapsigargin triggered an in-store calcium release and evoked a 1.5-fold elevation of intracellular Ca(2+) in Ca(2+)-free media, whereas FSH exhibited no effect. The readdition of CaCl(2) (2.5 mm) to FSH-pretreated or thapsigargin-sensitized SCs in Ca(2+)-free media immediately elicited a rapid Ca(2+) influx or a 2-fold increase of second intracellular Ca(2+) elevation, respectively. The addition of Ca(2+) chelator EGTA (0.2 mm) reduced the FSH-induced elevation of intracellular Ca(2+) in SCs incubated with CaCl(2). However, pretreatment with dantrolene (25 microM), which inhibits in-store calcium release, did not affect the FSH-induced elevation of intracellular Ca(2+). NiCl(2) (10 microM), a T-type calcium channel blocker, abolished the FSH-induced SC Ca(2+) influx. Furthermore, mibefradil (10 and 100 microm), another specific blocker for T-type Ca(2+) channels, dose-dependently suppressed the FSH-induced Ca(2+) influx. In contrast, nifedipine (10 and 50 microm) or omega-conotoxin GVIA (100 and 500 nm), blocker of L- or N-type Ca(2+) channels, respectively, did not affect the FSH-induced SC Ca(2+) influx. On the other hand, FSH-induced Ca(2+) influx was significantly reduced by pretreatment of SCs with myristoylated synthetic peptide (0.1 and 1 microm) of PLC-delta1 fragment TIPWNSLKQGYRHVHLL but not affected by 2',5'-dideoxyadenosine (3 and 15 microm), a selective inhibitor of adenylate cyclase. In conclusion, the FSH-induced Galphah/PLC-delta1 pathway-dependent Ca(2+) influx of rat SCs is mediated by T-type Ca(2+) channels and independent of in-store calcium release.

Actions of Mibefradil, Efonidipine and Nifedipine Block of Recombinant T- and L-Type Ca2+ Channels with Distinct Inhibitory Mechanisms

We compared detailed efficacy of efonidipine and nifedipine, dihydropyridine analogues, and mibefradil using recombinant T- and L-type Ca2+ channels expressed separately in mammalian cells. All these Ca2+ channel antagonists blocked T-type Ca2+ channel currents (I(Ca(T))) with distinct blocking manners: I(Ca(T)) was blocked mainly by a tonic manner by nifedipine, by a use-dependent manner by mibefradil, and by a combination of both manners by efonidipine. IC50s of these Ca2+ channel antagonists to I(Ca(T)) and L-type Ca2+ channel current (I(Ca(L))) were 1.2 micromol/l and 0.14 nmol/l for nifedipine; 0.87 and 1.4 micromol/l for mibefradil, and 0.35 micromol/l and 1.8 nmol/l for efonidipine, respectively. Efonidipine, a dihydropyridine analogue, showed high affinity to T-type Ca2+ channel.

The role of voltage gated T-type Ca2+ channel isoforms in mediating "capacitative" Ca2+ entry in cancer cells

The mechanism by which Ca2+ enters electrically non-excitable cells is unclear. The sensitivity of the Ca2+ entry pathway in electrically non-excitable cells to inhibition by extracellular Ni2+ was used to direct the synthesis of a library of simple, novel compounds. These novel compounds inhibit Ca2+ entry into and, consequently, proliferation of several cancer cell lines. They showed stereoselective inhibition of proliferation and Ca2+ influx with identical stereoselective inhibition of heterologously expressed Cav3.2 isoform of T-type Ca2+ channels. Proliferation of human embryonic kidney (HEK)293 cells transfected with the Cav3.2 Ca2+ channel was also blocked. Cancer cell lines sensitive to our compounds express message for the Cav3.2 T-type Ca2+ channel isoform, its delta25B splice variant, or both, while a cell line resistant to our compounds does not. These observations raise the possibility that clinically useful drugs can be designed based upon the ability to block these Ca2+ channels.

Contribution of nifedipine-insensitive voltage-dependent Ca2+ channel to diameter regulation in rabbit mesenteric artery

We investigated a possible role of nifedipine-insensitive high voltage-activated (NI-HVA) Ca2+ channels in arterial diameter regulation in the semi-terminal branches of rabbit mesenteric artery (RMA). From these branches, NI-HVA Ca2+ currents showing almost identical properties to those previously identified in a similar region of guinea-pig [Circulation Research 1999;85:596-605] were recorded with whole-cell patch clamp recording. With video-microscopic measurement, the diameter of RMA segments perfused intraluminally at a constant rate (2-6 mL/h; 269 +/- 9 micro m, n = 27) decreased by 50-60% by raising the external K+ concentration ([K+]o) to 75 mM, a substantial part of which remained after addition of 1-10 micro M nifedipine (44 +/- 5% of initial diameter, n = 27). This nifedipine-insensitive diameter decrease (NI-DD) appeared to consist of initial transient and subsequent tonic phases (this separation was, however, not always clear), was resistant to tetrodotoxin, and was completely abolished in Ca2+-free or 100 micro M Cd2+-containing bath solutions. The magnitude of NI-DD increased depending on [K+]o with a threshold concentration of 20-40 mM. Raising the external Ca2+ concentration dose-dependently increased the magnitude of NI-DD, the extent being more prominent in the late tonic phase. Combined application of caffeine (10 mM) with ryanodine (3 micro M) produced a large transient NI-DD, which strongly attenuated the NI-DD evoked by a subsequent elevation in [K+]o. Using the fura-2 spectrofluorimetric Ca2+ imaging technique, a nifedipine-insensitive [Ca2+]i increase showing similar [K+]o-dependence and Cd2+ sensitivity to NI-DD was observed. These properties of NI-DD accord with those of NI-HVA Ca2+ channels, thus suggesting their contribution to small arterial diameter regulation in RMA.

T-channel-like pharmacological properties of high voltage-activated, nifedipine-insensitive Ca2+ currents in the rat terminal mesenteric artery

1. Pharmacological properties of nifedipine-insensitive, high voltage-activated Ca(2+) channels in rat mesenteric terminal arteries (NICCs) were investigated and compared with those of alpha1E and alpha1G heterologously expressed in BHK and HEK293 cells respectively, using the patch clamp technique. 2. With 10 mM Ba(2+) as the charge carrier, rat NICCs (unitary conductance: 11.5 pS with 110 mM Ba(2+)) are almost identical to those previously identified in a similar region of guinea-pig, such as in current-voltage relationship, voltage dependence of activation and inactivation, and divalent cation permeability. However, these properties are considerably different when compared with alpha1E and alpha1G. 3. SNX-482(200 nM and sFTX3.3 (1 micro M), in addition to omega-conotoxin GVIA (1 micro M) and omega-agatoxin IVA (100 nM), were totally ineffective for rat NICC currents, but significantly suppressed alpha1E (by 82% at 200 nM; IC(50)=11.1 nM) and alpha1G (by 20% at 1 micro M) channel currents, respectively. A non-specific T-type Ca(2+) channel blocker nimodipine (10 micro M) differentially suppressed these three currents (by 40, 3 and 85% for rat NICC, alpha1E and alpha1G currents, respectively). 4. Mibefradil, the widely used T-type channel blocker, almost equally inhibited rat NICC and alpha1G currents in a voltage-dependent fashion with similar IC(50) values (3.5 and 0.3 micro M and 2.4 and 0.14 micro M at -100 and -60 mV, respectively). Furthermore, other organic T-type channel blockers such as phenytoin, ethosuximide, an arylpiperidine derivative SUN N5030 (IC(50)=0.32 micro M at -60 mV for alpha1G) also exhibited comparable inhibitory efficacies for NICC currents (inhibited by 22% at 100 micro M; IC(50)=27.8 mM; IC(50)=0.53 micro M, respectively). 5. These results suggest that despite distinctive biophysical properties, the rat NICCs have indistinguishable pharmacological sensitivities to many organic blockers compared with T-type Ca(2+) channels.

Randomized comparison of T-type versus L-type calcium-channel blockade on exercise duration in stable angina: results of the Posicor Reduction of Ischemia During Exercise (PRIDE) trial

BACKGROUND

Mibefradil is a T-type calcium-channel antagonist and arterial vasodilator with negative chronotropic effects. It is not known if T-type calcium-channel blockade is superior to L-type calcium-channel blockade in patients with stable angina pectoris.

METHODS

A multicenter, randomized, double-blind trial was conducted in patients with documented coronary disease and stable angina to compare a 360 mg dose of diltiazem CD with 100 mg dose of mibefradil. The primary end point was change in time to symptom-limited exercise termination from baseline to 8 weeks. Secondary efficacy parameters included time to onset of persistent ST-segment depression, time to awareness of angina, and change in exercise duration from baseline to 2 and 4 weeks of treatment.

RESULTS

A total of 121 patients were randomized to mibefradil and 113 to diltiazem CD. At 8 weeks, the increase in exercise duration was 24.5 seconds greater in the mibefradil group (P =.017; 95% CI 4.4-44.7 seconds). At 8 weeks, time to development of > or =1 mm ST-segment depression was greater by 45.3 seconds (P =.0025; 95% CI 16.2-74.5) with mibefradil, but time to development of angina was not significantly different.

CONCLUSION

T-type calcium-channel antagonism with mibefradil improved treadmill exercise parameters compared with diltiazem in patients with chronic stable angina. Further investigation and development of antagonists of T-type calcium channels with fewer adverse drug interactions is warranted and may be promising in the management of ischemic heart disease.

Effects of Ca2+ channel blockers on Ca2+ loading induced by metabolic inhibition and hyperkalemia in cardiomyocytes

The effects of the L-type (nifedipine and verapamil) and the T-type (mibefradil) Ca2+ channel blockers on the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by NaCN metabolic inhibition and hyperkalemia were examined in chicken cardiomyocytes using fluorescence imaging with Fura-2. NaCN induced a slow and sustained rise in [Ca2+]i, which was not affected by pretreating the cells for 5 min with nifedipine, verapamil, or mibefradil at 100 nM or 10 microM. Pretreatment of the cells with 10 microM nifedipine, verapamil, or mibefradil for 5 min remarkably inhibited the K+-induced increase in [Ca2+]i. These inhibitory effects diminished after 48-h pretreatment with nifedipine or verapamil but not with mibefradil. Ryanodine also induces an increase in [Ca2+]i, and this effect was enhanced by 48-h pretreatment of the cells with 10 microM verapamil but not with 10 microM mibefradil. We conclude that the NaCN-induced increase in [Ca2+]i is independent of the Ca2+ influx though the L-type or T-type Ca2+ channels. Chronic inhibition of the L-type Ca2+ channels but not T-type channels may enhance the ryanodine receptor-mediated Ca2+ release, which may be responsible for the development of tolerance to their inhibitory effects on K+-induced increase in [Ca2+]i.

Calcium channel blockers in cardiac failure

Congestive cardiac failure is an increasingly prevalent syndrome associated with a high morbidity and mortality. The role of calcium channel blockers in the treatment of heart failure is unclear. The potential benefits of these agents derive not only from their vasodilator properties, but also from anti-ischemic effects, beneficial effects on endothelial function and the development of atherosclerosis, and favorable effects on calcium cycling at a molecular level. Pitted against this array of potential benefits are direct negative inotropic effects and the potential for neuroendocrine activation. Treatment with short-acting dihydropyridine agents has not resulted in long-term clinical benefits in patients with cardiac failure. Diltiazem may be beneficial in patients with nonischemic heart failure, and verapamil has a neutral effect in cardiac failure, although it may have a role in combination with ace inhibition. To date, amlodipine has been associated with the most promising results, with evidence of a mortality benefit in nonischemic heart failure. Mibefradil is of no benefit in the management of heart failure, although the trend toward increased mortality in the treatment arm of the Mortality Assessment in Congestive Heart Failure (MACH)-1 trial may have been due to drug interactions. The potential role of calcium blockers in diastolic dysfunction and in combination with ace-inhibition requires further study.