Mechanism of vasorelaxation of thoracic aorta caused by xanthone

Xanthones as P-glycoprotein modulators and their impact on drug bioavailability

Introduction: P-glycoprotein (P-gp) is an important efflux pump responsible for the extruding of many endogenous and exogenous substances out of the cells. P-gp can be modulated by different molecules - including xanthone derivatives - to surpass the multidrug resistance (MDR) phenomenon through P-gp inhibition, or to serve as an antidotal strategy in intoxication scenarios through P-gp induction/activation.Areas covered: This review provides a perspective on P-gp modulators, with particular focus on xanthonic derivatives, highlighting their ability to modulate P-gp expression and/or activity, and the potential impact of these effects on the pharmacokinetics, pharmacodynamics and toxicity of P-gp substrates.Expert opinion: Xanthones, of natural or synthetic origin, are able to modulate P-gp, interfering with its protein synthesis or with its mechanism of action, by decreasing or increasing its efflux capacity. These modulatory effects make the xanthonic scaffold a promising source of new derivatives with therapeutic potential. However, the mechanisms beyond the xanthones-mediated P-gp modulation and the chemical characteristics that make them more potent P-gp inhibitors or inducers/activators are still understudied. Furthermore, a new window of opportunity exists in the neuropathologies field, where xanthonic derivatives with potential to modulate P-gp should be further explored to optimize the prevention/treatment of brain pathologies.

Synthetic Chiral Derivatives of Xanthones: Biological Activities and Enantioselectivity Studies

Many naturally occurring xanthones are chiral and present a wide range of biological and pharmacological activities. Some of them have been exhaustively studied and subsequently, obtained by synthesis. In order to obtain libraries of compounds for structure activity relationship (SAR) studies as well as to improve the biological activity, new bioactive analogues and derivatives inspired in natural prototypes were synthetized. Bioactive natural xanthones compromise a large structural multiplicity of compounds, including a diversity of chiral derivatives. Thus, recently an exponential interest in synthetic chiral derivatives of xanthones (CDXs) has been witnessed. The synthetic methodologies can afford structures that otherwise could not be reached within the natural products for biological activity and SAR studies. Another reason that justifies this trend is that both enantiomers can be obtained by using appropriate synthetic pathways, allowing the possibility to perform enantioselectivity studies. In this work, a literature review of synthetic CDXs is presented. The structures, the approaches used for their synthesis and the biological activities are described, emphasizing the enantioselectivity studies.

Chiral Derivatives of Xanthones with Antimicrobial Activity

According to the World Health Organization, the exacerbated use of antibiotics worldwide is increasing multi-resistant infections, especially in the last decade. Xanthones are a class of compounds receiving great interest in drug discovery and development that can be found as natural products or obtained by synthesis. Many derivatives of xanthones are chiral and associated with relevant biological activities, including antimicrobial. The aim of this review is to compile information about chiral derivatives of xanthones from natural sources and their synthesized examples with antimicrobial activity.

Interaction effects on cytochrome P450 both in vitro and in vivo studies by two major bioactive xanthones from Halenia elliptica D. Don

The major components, 1-hydroxy-2,3,5-trimethoxy-xanthone (HM-1) and 1,5-dihydroxy-2,3-dimethoxy-xanthone (HM-5) isolated from Halenia elliptica D. Don (Gentianaceae), could cause vasodilatation in rat coronary artery with different mechanisms. In this work, high-performance liquid chromatography coupled to ion trap time-of-flight mass spectrometry (LCMS-IT-TOF) was used to clarify the metabolic pathways, and CYP450 isoform involvement of HM-1 and HM-5 were also studied in rat. At the same time, in vivo inhibition effects of HM-1 and ethyl acetate extracts from origin herb were studied. Three metabolites of HM-5 were found in rat liver microsomes (RLMs); demethylation and hydroxylation were the major phase I metabolic reactions for HM-5. Multiple CYP450s were involved in metabolism of HM-1 and HM-5. The inhibition study showed that HM-5 inhibited Cyp1a2, 2c6 and 2d2 in RLMs. HM-1 inhibited activities of Cyp1a2, Cyp2c6 and Cyp3a2. In vivo experiment demonstrated that both HM-1 and ethyl acetate extracts could inhibit Cyp3a2 in rats. In conclusion, the metabolism of xanthones from the origin herb involved multiple CYP450 isoforms; in vitro, metabolism of HM-5 was similar to that of its parent drug HM-1, but their inhibition effects upon CYP450s were different; in vivo, Cyp3a2 could be inhibited by HM-1 and ethyl acetate extracts.

Vasorelaxation Effect of Estrone Derivate EA204 in Rabbit Aorta

Estrogen and its derivatives exert vascular protective effects, but the underlying mechanisms remain to be studied fully. Objective. To investigate the vasorelaxation effect and related mechanisms of an estrone derivate EA204[3-(2-piperidin-1-yl)-ethoxy-estra-1, 3, 5 (10)-trien-17-one] on isolated arterial preparation from rabbit thoracic aorta. Methods. Aortic rings from rabbit thoracic aorta were prepared and held in small organ bath filled with Krebs solution; tension change was recorded by a multichannel physiological signal collection and handling system. Results. EA204 (10(-5) to 10(-3) M) induced a concentration-dependent relaxation of aortic rings with endothelium and without endothelium. In denuded arterial preparations, EA204 had a potent relaxing effect on isolated arterial preparations contracted with phenylephrine, norepinephrine, and high-K(+) solution or BaCl2. Mechanism study indicates that EA204 relaxes aortic rings by inhibiting Ca(2+) channels (both receptor-operating Ca(2+) channels and the voltage-dependent Ca(2+) channels were involved) to decrease extracellular Ca(2+) influx and intracellular Ca(2+) release. EA204 is different from verapamil, which is a noncompetitive inhibitor of Ca(2+) channels. In addition, K(+) channels opening may contribute to this vasorelaxation effect. Conclusion. EA204 had a potent endothelium-independent relaxing effect on isolated arterial preparation by inhibiting Ca(2+) channels and opening K(+) channels. The results suggest that EA204 is a potential compound for treatment of cardiovascular diseases in postmenopausal women.

Cardiovascular activity of the chiral xanthone derivatives

A series of 6 derivatives of xanthone were synthesized and evaluated for cardiovascular activity. The following pharmacological experiments were conducted: the binding affinity for adrenoceptors, the influence on the normal electrocardiogram, the effect on the arterial blood pressure, the effect on blood pressor response and prophylactic antiarrhythmic activity in adrenaline induced model of arrhythmia (rats, iv). Two compounds revealed nanomolar affinity for α1-adrenoceptor which was correlated with the strongest cardiovascular (antiarrhythmic and hypotensive) activity in animals' models. They were enantiomers of previously described (R,S)-4-(2-hydroxy-3-(4-(2-methoxyphenyl)piperazin-1-yl)propoxy)-9H-xanthen-9-one hydrochloride and revealed similar antiarrhythmic potential in adrenaline induced model of arrhythmia in rats after intravenous injection (ED50=0.53 mg/kg and 0.81 mg/kg, respectively). These values were lower than values obtained for reference drug urapidil. These compounds were more active in this experiment than urapidil (ED50=1.26 mg/kg). The compound 5 administered iv at doses of 0.62-2.5 mg/kg at the peak of arrhythmia prevented and/or reduced the number of premature ventricular beats in a statistically significant manner. The ED50 value was 1.20 mg/kg. The S-enantiomer (6) given at the same doses did not show therapeutic antiarrhythmic activity in this model. These compounds significantly decreased the systolic and diastolic blood pressure throughout the whole observation period in anesthetized, normotensive rats. The studied enantiomers showed higher toxicity than urapidil, but imperceptibly higher that another cardiovascular drugs, that is, carvedilol or propranolol. They were also evaluated for mutagenic potential in the Ames (Salmonella) test. It was found that at the concentrations tested the compounds were non mutagenic when compared to solvent control. Results were quite promising and suggested that in the group of xanthone derivatives new potential antiarrhythmics and hypotensives might be found.

Biofunctional studies of new 2-methoxyphenylpiperazine xanthone derivatives with α₁-adrenolytic properties

BACKGROUND

The aim of this study was to assess the selectivity of the studied xanthone derivatives for α1-adrenoceptor subtypes (α1A, α1B, α1D, α1L) in functional experiments in order to verify if they possess any selectivity for a distinct subtype of α1-adrenoceptor. Moreover, several pharmacological tests were carried out to assess whether they reveal other than α1-adrenoceptor blocking properties such as: antagonistic for 5-HT2 receptors, vasorelaxant or spasmolytic.

METHODS

The influence on α1A-adrenoceptors was examined in biofunctional studies employing isolated rat vas deferens, on α1B-adrenoceptors in guinea-pig spleen, on α1D-adrenoceptors in rat aorta, and on α1L-adrenoceptors in rabbit spleen. Affinity for 5-HT2 receptors was measured in radioligand binding assay, whereas antagonistic potency for 5-HT2 receptors was studied on isolated rat aorta. Vasorelaxant effect of tested compounds was assessed in functional study employing rat aorta, whereas direct spasmolytic activity was investigated using the isolated rabbit small intestine.

RESULTS

The present study provides evidences that the tested 2-methoxyphenylpiperazine xanthone derivatives are non-selective α1-adrenoceptor blockers. However, at higher concentrations the direct spasmolytic effect could enhance their hypotensive activity. The obtained results indicate that the studied xanthones possessed weak calcium entry blocking activity, as well as antagonistic properties for 5-HT2A receptors.

CONCLUSIONS

The results of the present study support the idea that the hypotensive activity of the studied compounds is related to their α1-adrenolytic properties.

Vasodilator compounds derived from plants and their mechanisms of action

The present paper reviews vasodilator compounds isolated from plants that were reported in the past 22 years (1990 to 2012) and the different mechanisms of action involved in their vasodilator effects. The search for reports was conducted in a comprehensive manner, intending to encompass those metabolites with a vasodilator effect whose mechanism of action involved both vascular endothelium and arterial smooth muscle. The results obtained from our bibliographic search showed that over half of the isolated compounds have a mechanism of action involving the endothelium. Most of these bioactive metabolites cause vasodilation either by activating the nitric oxide/cGMP pathway or by blocking voltage-dependent calcium channels. Moreover, it was found that many compounds induced vasodilation by more than one mechanism. This review confirms that secondary metabolites, which include a significant group of compounds with extensive chemical diversity, are a valuable source of new pharmaceuticals useful for the treatment and prevention of cardiovascular diseases.

Synthesis and preliminary evaluation of pharmacological properties of some piperazine derivatives of xanthone

A series of 9 piperazine derivatives of xanthone were synthesized and evaluated for cardiovascular activity. The following pharmacological experiments were conducted: the binding affinity for adrenoceptors, the influence on the normal electrocardiogram, the effect on the arterial blood pressure and prophylactic antiarrhythmic activity in adrenaline induced model of arrhythmia (rats, iv). Three compounds revealed nanomolar affinity for α(1)-adrenoceptor which was correlated with the strongest cardiovascular (antiarrhythmic and hypotensive) activity in animals' models. The most promising compound was 4-(3-(4-(2-methoxyphenyl)piperazine-1-yl)propoxy)-9H-xanthen-9-one hydrochloride (12) which revealed antiarrhythmic activity with ED(50) value of 0.69 mg/kg in adrenaline induced arrhythmia (rats, iv). Other synthesized xanthone derivatives, that is, (R,S)-4-(2-hydroxy-3-(4-(2-methoxyphenyl)piperazine-1-yl)propoxy)-9H-xanthen-9-one hydrochloride (10) and (R,S)-4-(2-acetoxy-3-(4-(2-methoxyphenyl)piperazine-1-yl)propoxy)-9H-xanthen-9-one hydrochloride (11) also acted as potential antiarrhythmics in adrenaline induced model of arrhythmia in rats after intravenous injection (ED(50) = 0.88 mg/kg and 0.89 mg/kg, respectively). These values were lower than values obtained for reference drugs such as propranolol and urapidil, but not carvedilol. Results were quite promising and suggested that in the group of xanthone derivatives new potential antiarrhythmics and hypotensives might be found.

Mechanism of the vasodilator effect of Euxanthone in rat small mesenteric arteries

In the present work we investigated the mechanism involved in the vasodilator effect induced by euxanthone in rat small mesenteric arteries. We observed that euxanthone induced concentration-dependent vasodilatation in arteries by a mechanism independent on the release of endothelial factors, such as nitric oxide (NO) and cyclooxygenase-derived factors. In addition our results also suggest that euxanthone induced its vasodilator effect through inhibition of calcium-sensitive mechanisms activated by protein kinase C, rather than by inhibition of contractions dependent on the release of the intracellular calcium stores or by inhibition of voltage-operated calcium channels.

Vasodilatory actions of xanthones isolated from a Tibetan herb, Halenia elliptica

In this study, six major xanthones, isolated and identified from Halenia elliptica were investigated for their vasodilatory actions in isolated rat coronary artery. The xanthones, including 1-hydroxy-2,3,5-trimethoxy-xanthone (HM-1), 1-hydroxy-2,3,4,7-tetramethoxy-xanthone (HM-2), 1-hydroxy-2,3,4,5-tetramethoxy-xanthone (HM-3), 1,7-dihydroxy-2,3,4,5-tetramethoxy-xanthone (HM-4), 1,5-dihydroxy-2,3-dimethoxy-xanthone (HM-5) and 1,7-dihydroxy-2,3-dimethoxy-xanthone (HM-7) caused vasodilation in the coronary artery pre-contracted with 1 microM 5-hydroxytryptamine (5-HT), with EC(50) values ranging from 1.4+/-0.1 microM (HM-1) to 6.6+/-1.4 microM (HM-2). The EC(50) values of the other xanthones were between those of HM-1 and HM-2. Removal of endothelium of the coronary artery led to decreases in the vasorelaxant effects of HM-1, HM-7 but not HM-2, HM-3, HM-4 and HM-5. Our results showed that xanthones isolated from Halenia elliptica are vasoactive substances which exhibit either endothelium-dependent or endothelium-independent mechanisms in rat coronary artery. The potency and mechanism(s) of the vasorelaxant effects of these xanthones may be relevant to the structure-activity differences in the level and the position of the substituent groups with the primary xanthone structure.

Ameliorative prospective of alpha-mangostin, a xanthone derivative from Garcinia mangostana against beta-adrenergic cathecolamine-induced myocardial toxicity and anomalous cardiac TNF-alpha and COX-2 expressions in rats

Altered membrane integrity and inflammation play a key role in cardiovascular damage. We investigated the salubrious effect of exogenously administered alpha-mangostin against beta-adrenergic cathecolamine-induced cardiovascular toxicity with special reference to membrane ATPases, lysosomal hydrolases and inflammatory mediators TNF-alpha and Cyclooxygenase-2 (COX-2) expressions in albino rats. Induction of rats with isoproterenol (150mg/kg body wt, i.p.) for 2 days resulted in a significant increase in the activities of serum and cardiac lysosomal hydrolases (beta-d-glucuronidase, beta-d-galactosidase, beta-d-N-acetylglucosaminidase, acid phosphatase and cathepsin-D). A significant increase in cardiac levels of sodium, calcium with a decrease in the level of potassium paralleled by abnormal activities of membrane-bound phosphatases (Na(+)-K(+) ATPase, Ca(2+) ATPase and Mg(2+) ATPase) were observed in the heart of ISO-administered rats. Cardiac TNF-alpha and COX-2 expressions were assessed by Western blotting. Cardiac TNF-alpha and COX-2 expressions were significantly elevated in ISO-intoxicated rats. Pre-co-treatment with alpha-mangostin (200mg/kg body wt.) orally for 8 days significantly attenuated these abnormalities and restored the levels to near normalcy when compared to ISO intoxicated group of rats. In conclusion, alpha-mangostin preserves the myocardial membrane integrity and extenuates anomalous TNF-alpha and COX-2 expressions by mitigating ISO-induced oxidative stress and cellular damage effectively. Restoration of cellular normalcy accredits the cytoprotective role of alpha-mangostin.

Mechanisms of the vasorelaxant effect of 1-hydroxy-2, 3, 5-trimethoxy-xanthone, isolated from a Tibetan herb, Halenia elliptica, on rat coronary artery

1-Hydroxy-2, 3, 5-trimethoxyxanthone (HM-1) is a xanthone isolated from Halenia elliptica, a Tibetan medicinal herb. HM-1 (0.33-42.1 microM) produced a concentration-dependent relaxation in rat coronary artery rings pre-contracted with 1 microM 5-hydroxytryptamine (5-HT), with an EC(50) of 1.67+/-0.27 microM. Removal of the endothelium significantly affected the vasodilator potency of HM-1, resulting in 46% decrease in E(max) value. The endothelium-dependent effects of HM-1 was confirmed when its vasorelaxant effect was inhibited after addition of nitric oxide synthase (NOS) inhibitor N(omega)-nitro-l-arginine methyl ester (100 microM) or the soluble guanylate cyclase inhibitor 1H-[1, 2, 4] oxadiazolo [4,3-alpha] quinoxalin-1-one (ODQ, 10 microM). Atropine (100 nM), flurbiprofen (10 microM), propranolol (100 microM), pyrilamine (10 microM), cimetidine (10 microM) and SQ22536 (100 microM) had no effect on the vasorelaxant activity of HM-1 indicated the non-involvement of other receptor/enzyme systems. In endothelium-denuded coronary artery rings, the vasorelaxant effect of HM-1 was unaffected by potassium channel blockers such as tetraethylammonium (10 mM), iberiotoxin (100 nM), barium chloride (100 microM) and 4-aminopyridine (1 mM). The involvement of Ca(2+) channel in 5-HT-primed artery ring preparations incubated with Ca(2+)-free buffer was confirmed when HM-1 (9.93 microM) partially abolished the CaCl(2)-induced vasoconstriction (87% inhibition in intact-endothelium artery rings; 50% inhibition in endothelium-denuded rings). In the KCl-primed preparations incubated with Ca(2+)-free buffer, HM-1 (9.93 microM) produced a 27.3% inhibition in endothelium-denuded rings. HM-1 (3.31-33.1 microM) had minimal relaxant effects (14.4%-20.3%) on the contractile response generated by 10 microM phorbol 12,13-diacetate (PDA) in Ca(2+)-free solutions, suggesting minimal effects on intracellular Ca(2+) mechanisms. These findings suggest the vasodilator action of HM-1 involved both an endothelium-dependent mechanism involving NO and an endothelium-independent mechanism by inhibiting Ca(2+) influx through L-type voltage-operated Ca(2+) channels; a minor contribution to the effects of HM-1 may be related to inhibition of the protein kinase C-mediated release of intracellular Ca(2+) stores.

Vasodilator effect of the extracts and some coumarins from the stem bark of Mammea africana (Guttiferae)

CH(2)Cl(2) fraction obtained from the stem bark of Mammea africana inhibited noradrenaline (NA) or KCl-induced contraction in isolated guinea pig and rat aorta. The vasorelaxant potency of the CH(2)Cl(2) fraction of Mammea africana was diminished by a pre-treatment with Nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase, which was however not affected by indomethacin pre-treatment. These findings indicated that the vasorelaxant effect of Mammea africana may be partially endothelium dependent, mediated by nitric oxide and that vasoactive prostanoids might not be contributing to the vasorelaxation effect. Three bioactive compounds were isolated from this CH(2)Cl(2) fraction and identified as 4-n-propylcoumarins (1) (mammea B/BB), 4-phenylcoumarins (2) (mammea A/AA or mammeisin) and (B/BA) (3) and might involved in the vasorelaxant effect of the extract. The mechanisms of the vasorelaxant effect might therefore be multiple, including endothelium dependence and the mechanisms, which interfere with the liberation of Ca(2+) into the muscle cell.

Quantitative and qualitative determination of six xanthones in Garcinia mangostana L. by LC-PDA and LC-ESI-MS

A new method was developed for the simultaneous analysis of six naturally occurring xanthones (3-isomangostin, 8-desoxygartanin, gartanin, alpha-mangostin, 9-hydroxycalabaxanthone and beta-mangostin). The quantitative determination was conducted by reversed phase high performance liquid chromatography with photodiode array detector (LC-PDA). Separation was performed on a Phenomenex Luna C18(2) (150 mm x 3.00 mm, 5 microm) column. The xanthones were identified by retention time, ultraviolet (UV) spectra and quantified by LC-PDA at 320 nm. The precision of the method was confirmed by the relative standard deviation (R.S.D.), which was < or =4.6%. The recovery was in the range from 96.58% to 113.45%. A good linear relationship was established in over two orders of magnitude range. The limits of detection (LOD) for six xanthone compounds were < or =0.248 microg/mL. The identity of the peaks was further confirmed by high performance liquid chromatography with time-of-flight mass spectrometry (LC-TOF MS) system coupled with electrospray ionization (ESI) interface. The developed methods were applied to the determination of six xanthones in Garcinia mangostana products. The satisfactory results showed that the methods are effective for the analysis of real samples.

Pharmacological Effects of Xanthones as Cardiovascular Protective Agents

Many epidemiological studies indicate that consumption of dietary polyphenolic compounds is beneficial in the prevention of cardiovascular diseases. Xanthones are a class of polyphenolic compounds that commonly occur in plants and have been shown to have extensive biological and pharmacological activities. Recently, the pharmacological properties of xanthones in the cardiovascular system have attracted great interest. Xanthones and xanthone derivatives have been shown to have beneficial effects on some cardiovascular diseases, including ischemic heart disease, atherosclerosis, hypertension and thrombosis. The protective effects of xanthones in the cardiovascular system may be due to their antioxidant, antiinflammatory, platelet aggregation inhibitory, antithrombotic and/or vasorelaxant activities. In particular, the antagonism of endogenous nitric oxide synthase inhibitors by xanthones may represent the basis for improved endothelial function and for reduction of events associated with atherosclerosis.

Alpha-naphthoflavone induces vasorelaxation through the induction of extracellular calcium influx and NO formation in endothelium

The effect of alpha-naphthoflavone (alpha-NF) on vascular function was studied in isolated ring segments of the rat thoracic aorta and in primary cultures of human umbilical vein endothelial cells (HUVECs). alpha-NF induced concentration-dependent relaxation of the phenylephrine-precontracted aorta endothelium-dependently and -independently at lower and higher concentrations, respectively. The cGMP, but not cAMP, content was increased significantly in alpha-NF-treated aorta. Pretreatment with N(omega)-nitro- l-arginine methyl ester (L-NAME) or methylene blue attenuated both alpha-NF induced vasorelaxation and the increase of cGMP content significantly. The increase of cGMP content induced by alpha-NF was also inhibited by chelating extracellular Ca(2+) with EGTA. These results suggest that the endothelium-dependent vasorelaxation induced by alpha-NF is mediated most probably through Ca(2+)-dependent activation of NO synthase and guanylyl cyclase. In HUVECs, alpha-NF induced concentration-dependent formation of NO and Ca(2+) influx. alpha-NF-induced NO formation was abolished by removal of extracellular Ca(2+) and by pretreatment with the Ca(2+) channel blockers SKF 96365 and Ni(2+), but not by the L-type Ca(2+) channel blocker verapamil. The Ca(2+) influx, as measured by (45)Ca(2+) uptake, induced by alpha-NF was also inhibited by SKF 96365 and Ni(2+). Our data imply that alpha-NF, at lower concentrations, induces endothelium-dependent vasorelaxation by promoting extracellular Ca(2+) influx in endothelium and the activation of the NO-cGMP pathway.