Flavonoids and hERG channels: Friends or foes?

The cardiac action potential is regulated by several ion channels. Drugs capable to block these channels, in particular the human ether-à-go-go-related gene (hERG) channel, also known as KV11.1 channel, may lead to a potentially lethal ventricular tachyarrhythmia called "Torsades de Pointes". Thus, evaluation of the hERG channel off-target activity of novel chemical entities is nowadays required to safeguard patients as well as to avoid attrition in drug development. Flavonoids, a large class of natural compounds abundantly present in food, beverages, herbal medicines, and dietary food supplements, generally escape this assessment, though consumed in consistent amounts. Continuously growing evidence indicates that these compounds may interact with the hERG channel and block it. The present review, by examining numerous studies, summarizes the state-of-the-art in this field, describing the most significant examples of direct and indirect inhibition of the hERG channel current operated by flavonoids. A description of the molecular interactions between a few of these natural molecules and the Rattus norvegicus channel protein, achieved by an in silico approach, is also presented.

Vasorelaxing Activity of R-(-)-3'-Hydroxy-2,4,5-trimethoxydalbergiquinol from Dalbergia tonkinensis: Involvement of Smooth Muscle CaV1.2 Channels.

Dalbergia species heartwood, widely used in traditional medicine to treat various cardiovascular diseases, might represent a rich source of vasoactive agents. In Vietnam, Dalbergia tonkinensis is an endemic tree. Therefore, the aim of the present work was to investigate the vascular activity of R-(-)-3'-hydroxy-2,4,5-trimethoxydalbergiquinol isolated from the heartwood of D. tonkinensis and to provide circular dichroism features of its R absolute configuration. The vascular effects of R-(-)-3'-hydroxy-2,4,5-trimethoxydalbergiquinol were assessed on the in vitro mechanical activity of rat aorta rings, under isometric conditions, and on whole-cell Ba2+ currents through CaV1.2 channels (IBa1.2) recorded in single, rat tail main artery myocytes by means of the patch-clamp technique. R-(-)-3'-Hydroxy-2,4,5-trimethoxydalbergiquinol showed concentration-dependent, vasorelaxant activity on both endothelium-deprived and endothelium intact rings precontracted with the α 1 receptor agonist phenylephrine. Neither the NO (nitric oxide) synthase inhibitor Nω-nitro-L-arginine methyl ester nor the cyclooxygenase inhibitor indomethacin affected its spasmolytic activity. R-(-)-3'-Hydroxy-2,4,5-trimethoxydalbergiquinol-induced vasorelaxation was antagonized by (S)-(-)-Bay K 8644 and unaffected by tetraethylammonium plus glibenclamide. In patch-clamp experiments, R-(-)-3'-hydroxy-2,4,5-trimethoxydalbergiquinol inhibited IBa1.2 in a concentration-dependent manner and significantly decreased the time constant of current inactivation. R-(-)-3'-Hydroxy-2,4,5-trimethoxydalbergiquinol likely stabilized the channel in its closed state, as suggested by molecular modelling and docking simulation to the CaV1.2 channel α 1c subunit. In conclusion, D. tonkinensis species may represent a source of agents potentially useful for the development of novel antihypertensive drugs.

Human Tuberculosis. III. Current and Prospective Approaches in Anti-Tubercular Therapy.

Ineffectively treated tuberculosis (TB) is associated with substantial morbidity and mortality. Cure of TB patients is hampered by the development of multidrug resistance in M. tuberculosis and the need of long-term treatment. The diarylquinoline derivative bedaquiline was approved in December 2012 under the accelerated-approval regulations of FDA as part of a combination therapy for treating adults with pulmonary MDR-TB for whom effective cures are not otherwise available. The bicyclic nitroimidazoles delamanid and its companion pretomanid inhibit mycolic acid synthesis via an unknown mechanism. In November 2013, delamanid received conditional approval by the European Medicines Agency for MDR-TB treatment. Use of both drugs, however, is limited owing to toxicity issues. If the aim to reduce treatment duration is pursued in order to limit costs and improve patient adherence, it is mandatory to demonstrate their noninferiority with fewer months of therapy. In three phase III clinical trials the efficacy of the most recent fluoroquinolones, gatifloxacin and moxifloxacin, has been investigated in a four-month treatment regimen of drug-susceptible TB. In all three studies, after two months the culture conversion rates of observed sputum indicated that fluoroquinolone-based therapies were likely to be superior. However, this feature did not reliably predict sterilizing activity or a risk of relapse. In other words, the shortened treatments were not noninferior to standard treatments. To counteract mycobacterial survival strategies and reduce the timelength of treatment with anti-TB drugs, other novel and powerful agents, as well as tuberculosis vaccines, are under intense clinical investigation for safety and efficacy assessment.

In vitro vasoactivity of zerumbone from Zingiber zerumbet.

The sesquiterpene zerumbone, isolated from the rhizome of Zingiber zerumbet Sm., besides its widespread use as a food flavouring and appetiser, is also recommended in traditional medicine for the treatment of several ailments. It has attracted great attention recently for its effective chemopreventive and therapeutic effects observed in various models of cancer. To assess the zerumbone safety profile, a pharmacology study designed to flag any potential adverse effect on vasculature was performed. Zerumbone was tested for vasorelaxing activity on rat aorta rings and for L-type Ba(2+) current blocking activity on single myocytes isolated from the rat-tail artery. The spasmolytic effect of zerumbone was more marked on rings stimulated with 60 mM than with 30 mM K(+) (IC50 values of 16 µM and 102 µM, respectively). In the presence of 60 mM K(+), zerumbone concentration-dependently inhibited the contraction induced by the cumulative additions of Ca(2+), this inhibition being inversely related to the Ca(2+) concentration. Phenylephrine-induced contraction was inhibited by the drug, though less efficiently and independently of the presence of an intact endothelium, without affecting Ca(2+) release from the intracellular stores. Zerumbone inhibited the L-type Ba(2+) current (estimated IC50 value of 458.7 µM) and accelerated the kinetics of current decay. In conclusion, zerumbone showed an overall weak in vitro vasodilating activity, partly attributable to the blocking of the L-type Ca(2+) channel, which does not seem to represent, however, a serious threat to its widespread use.

Reversion of nitrate tolerance in rat aorta rings by freeze-dried red wine.

Chronically administered organic nitrates induce nitrate tolerance and endothelial dysfunction, which limit their therapeutic use. eNOS uncoupling, ROS over-production, aldehyde dehydrogenase-2 as well as superoxide dismutase (SOD) oxidative inhibition, and cGMP desensitization are thought to play an important role. Natural polyphenols are effective antioxidants, which might counteract the mechanisms leading to nitrate tolerance. The aim of this work was to verify whether freeze-dried (dealcoholized) red wine (FDRW) was able to revert glyceryl trinitrate (GTN) tolerance and endothelial dysfunction induced in rat aorta rings with either GTN or diethyldithiocarbamate (DETCA), an irreversible inhibitor of Cu/Zn SOD. GTN induced a concentration-dependent relaxation of rings pre-contracted with phenylephrine. GTN spasmolysis was significantly reduced in rings pre-incubated with either GTN or DETCA. FDRW, at 2.8 µg of gallic acid equivalents (GAE)/mL concentration, was able to revert partially, though significantly, GTN-induced tolerance but not tolerance and endothelial dysfunction induced by DETCA. This work provides the first evidence in vitro that red wine components, at concentrations comparable to those achieved in human blood after moderate consumption of red wine, revert tolerance to nitrates with a mechanism possibly mediated by SOD.

Quercetin mitochondriotropic derivatives antagonize nitrate tolerance and endothelial dysfunction of isolated rat aorta rings.

Chronic use of glyceryl trinitrate is limited by serious side effects, inter alia tolerance and endothelial dysfunction of coronary and resistance arteries. The natural flavonoid quercetin has been shown to counteract the development of glyceryl trinitrate tolerance in vitro. Two mitochondriotropic, 4-O-triphenylphosphoniumbutyl derivatives of quercetin (QTA-3BTPI and Q-3BTPI) were compared to quercetin for protection against glyceryl trinitrate-induced tolerance and endothelial dysfunction of isolated rat aorta rings. Both QTA-3BTPI and Q-3BTPI significantly counteracted the reduced vascular responsiveness to both glyceryl trinitrate and acetylcholine caused by prolonged exposure of the vessel to glyceryl trinitrate itself, their potency being much greater than that of quercetin. QTA-3BTPI, however, turned out to cause endothelial dysfunction per se. Since Q-3BTPI antagonized in vitro nitrate tolerance and endothelial dysfunction of vessels, this encourages assessing whether this effect is displayed also in vivo during long-term glyceryl trinitrate treatment.

Mechanism of osthole inhibition of vascular Ca(v)1.2 current.

Osthole is a coumarin extracted from Cnidium monnieri (L.) Cusson. The medicinal plant is widely used in Vietnamese as well as Chinese traditional medicine as a vasodilating and antihypertensive agent. Here we have tested the proposition that the block of Ca(v)1.2 channels is mainly responsible for its vascular activity. An in-depth analysis of the effect of osthole on Ca(v)1.2 current (I(Ca1.2)) was performed in rat tail artery myocytes using the whole-cell patch-clamp method. Osthole decreased I(Ca1.2) in a concentration- and voltage-dependent manner. At holding potentials of -50 and -80mV, the pIC(50) values were 4.78±0.07 and 4.36±0.08, respectively; the latter corresponded to the drug apparent dissociation constant for resting channels, K(R), of 47.8µM. Osthole speeded up the inactivation kinetics of I(Ca1.2) and shifted the voltage dependence of the inactivation curve to more negative potentials in a concentration-dependent manner, with an apparent dissociation constant for inactivated channels (K(I)) of 6.88µM. Block of I(Ca1.2) was frequency-dependent and the rate of recovery from inactivation was slowed down. In conclusion, osthole is a vascular Ca(v)1.2 channel antagonist stabilizing the channel in its inactivated state. This mechanism may account for the systolic blood pressure reduction induced by the drug in animal models of hypertension and points to osthole as a lead for the development of novel antihypertensive agents.

Vascular myorelaxing activity of isolates from South African Hyacinthaceae partly mediated by activation of soluble guanylyl cyclase in rat aortic ring preparations.

The vasorelaxing effect of isolates (compounds 1, 2, 3, and 4 (homoisoflavanones), compound 5 (sesquiterpenoid), compounds 6 and 7 (bufadienolides)) from the South African Hyacinthaceae has been assessed using rat aortic ring preparations. Compounds 2, 3, and 4 inhibited the tonic contraction induced by both 60 mM K(+) (K60) and phenylephrine, compound 3 being the most potent. Compounds 5, 6, and 7 caused a modest concentration-dependent relaxation, whereas compound 1 was ineffective. Under K25- or K60-induced depolarization, compound 3 displayed antispasmodic effects not reversed by tetraethylammonium. Under precontraction induced with phenylephrine, compound 3 shifted to the left the concentration-relaxation curves of either isoprenaline or sodium nitroprusside. 1 H-[1,2,4] oxidazolol [4,3-a] quinoxalin-1-one shifted to the right the concentration-relaxation curve of compound 3, while 3'-isobutyl-1-methylxanthine had no effect. In the absence of extracellular Ca(2+), compound 3 (estimated pIC50 = 4.66) and ryanodine reduced the response to phenylephrine. Phenylephrine-stimulated influx of extracellular Ca(2+) was markedly reduced when tissues were pretreated with compound 3 (pIC50 = 5.14) or nifedipine, but stimulated by ryanodine. Compound 3 partially antagonized the contraction induced by phorbol 12-myristate-13-acetate. To our knowledge, this has been the first account describing the vasodilating activity of homoisoflavonoids: compound 3 proved an effective vasorelaxing agent, partly acting via the activation of soluble guanylyl cyclase.