Nature has provided us with the basis of many medicines in clinical use today ( ). Indeed, the inspiration behind DSCG arises from the earlier data generated on Khellin, a plant-derived mast cell stabilizer from Amni visnaga. In the first section of this review, emphasis will be placed on recent studies with natural products with particular emphasis placed on flavonoids/phenolics, terpenoids, alkaloids and biologics. The discussion will then focus on work conducted with synthetic mast cell stabilizers whose structure was inspired from studies on plant-derived materials. The final section of the review will focus on recent developments on rationally designed specific inhibitors as well as studies with proprietary medications for other indications that also have be shown to stabilize mast cells ( ).

Mast cell stabilizing agents from natural sources

Flavonoids Flavonoids can be subdivided into many different classes including flavones, flavonols, flavonones, isoflavones and flavonol-3-ols and anthocyanidins. Regardless of the individual subdivisions, all contain the benzo-γ-pyrone architecture and are classified according to the presence of different substituents on the rings and to the degree of saturation of the benzo-γ-pyrone ring. Within the flavone class, the most active mast cell stabilizers are luteolin, disometin and apigenin. Using anti-IgE to elicit degranulation, luteolin inhibited the release of histamine, LTs, PG 2 and GM-CSF from human cultured mast cells (HCMCs) in a concentration-dependent manner (1–100 μM) (Kimata et al., 2000b). Luteolin also suppressed the production of proinflammatory cytokines TNF-α and IL-6 in bone marrow-derived cultured murine mast cells (BMMCs) (Kimata et al., 2000a). Luteolin and apigenin were strong inhibitors of production of IL-4 by purified basophils following combined challenge with anti-IgE and IL-3 (Hirano et al., 2006). Both luteolin and diosmetin showed potent inhibitory effects on the release of β-hexosaminidase from antigen-stimulated rat basophilic leukaemia (RBL)-2H3 cells with IC 50 values of 3.0 μM and 2.1 μM, respectively. Additionally, these three flavones inhibited anti–IgE-mediated production of TNF-α and IL-4 from this cell line (Mastuda et al., 2002). The structure of the flavonols differs from the flavones by the presence of an additional hydroxy-substituent at position 3. Examples of flavonols, which have demonstrated anti-allergic activity include kaempferol, fisetin, quercetin and morin. Kaempferol, fisetin and quercetin inhibited anti-IgE, phorbol-12-myristate-13-acetate, calcium ionophore A23187 and phorbol 12-myristate 13-acetate and calcium ionophore A23187 (PMACI)-induced histamine release in RBL-2H3 cells as well as the elevation of intracellular Ca2+ when evaluated at a relatively high concentration of 30 μM. Additionally, fisetin and quercetin decreased gene expression and production of proinflammatory cytokines such as TNF-α, IL-1β, IL-6 and IL-8 in PMACI-stimulated HMC-1 cells (Park et al., 2008). Fisetin was also shown to decrease gene expression of IL-4, inhibit the phosphorylation of p38 MAPK, ERK and JNK and suppress the activation of NF-κB (Park et al., 2007). Quercetin and kaempferol inhibited the secretion of mediators at concentrations of 1 and 10 μM from RBL-2H3 cells stimulated by anti-dinitrophenyl (DNP) and suppressed the mRNA expression of CD23 and p38 MAPK activation in Caco-2 cells stimulated by IL-4 (Lee et al., 2010). These flavonols also significantly inhibited the release of histamine and cytokines; TNF-α, IL-6 and IL-8 from human umbilical cord blood-derived cultured mast cells (hCBMCs) activated by anti-IgE and decreased the elevation of intracellular Ca2+ in this cell line (Kempuraj et al., 2005). Quercetin has been shown to down-regulate the mRNA transcription of histidine decarboxylase in HMC-1 cells, an enzyme involved in the synthesis of histamine (Kempuraj et al., 2006). Morin prevented the degranulation and the production of cytokines such as TNF-α and IL-4 in both RBL-2H3 cells and BMMCs stimulated by antigen at low concentrations (1–10 μM). Morin demonstrated inhibition of activating phosphorylation of Syk. In vivo, this flavonol suppressed IgE-mediated passive cutaneous anaphylaxis (PCA) in mice almost completely at a dose of 100 mg·kg−1 (Kim et al., 2009). The isoflavone, genistein inhibited degranulation of HCMCs challenged with anti-IgE in a dose-dependent manner with inhibition of histamine release by 92% at concentration of 100 μg·mL−1. Additionally, at this concentration, it inhibited phosphorylation of cellular proteins such as ERK-1 and ERK-2, which are involved in the downstream signalling cascade of activated mast cells (Suzuki et al., 1997). Similarly, genistein also inhibited histamine release and protein TK activation in BMMCs stimulated with antigen (Kawakami et al., 1992). The biflavone, ginkgetin, isolated from the leaves of Ginkgo biloba, demonstrated a dual COX-2/5-lipooxygenase inhibitory activity and was subsequently shown to inhibit the production of the de novo mediators, PGD 2 and LTC 4 in BMMCs stimulated with c-kit ligand (KL). Additionally, ginkgetin inhibited release of β-hexosaminidase from these cells stimulated with KL in a dose-dependent manner with IC 50 value of 6.52 μM (Son et al., 2005). Epigallocatechin gallate (EGCG), a constituent found in green tea is related to the flavonoid family, but differs from the core flavone and flavonol structure by the absence of the double bond and carbonyl group at positions 2–3 and 4, respectively. EGCG demonstrated anti-allergic activity in both in vitro and in in vivo models. It significantly inhibited the release of compound 48/80-induced degranulation of histamine in rat peritoneal mast cells (RPMCs). EGCG also suppressed compound 48/80-induced PCA reaction in rats (Li et al., 2005). EGCG also inhibited antigen-induced degranulation and LTC 4 secretion in RBL-2H3 cells across a concentration range of 10–100 μM and has been shown to block store-operated Ca2+ entry of this cell line, which is the main route of calcium influx in mast cells that leads to the degranulation of allergic mediators (Inoue et al., 2010). Silymarin is a mixture of polyphenolic flavonoids isolated from milk thistle (Silybum marianum) and is used primarily for its treatment of liver diseases such hepatitis, cirrhosis and jaundice. One of its primary constituents, silibinin was shown to inhibit the release of histamine from RPMCs stimulated by compound 48/80 and anti-DNP as well as the secretion of proinflammatory cytokines such as TNF-α and IL-6 from anti-DNP induced degranulation of RPMCs in a dose-dependent manner (10–100 μM). In vivo, silibinin inhibited compound 48/80-induced PCA reaction, in mice dose-dependently (10–100 mg·kg−1) (Choi and Yan, 2009c).

Coumarins Several reports exist that describe the mast cell stabilizing properties of coumarins. Scopletin (6-methoxy-7-hydroxycoumarin), which has been isolated from several plant species including Erycibe obtusifolia Benth inhibited the production of proinflammatory cytokines including TNF-α, IL-6 and IL-8 from the HMC-1 following challenge with PMACI. These cytokines play a role in triggering and sustaining allergic inflammation. However, scopletin did not affect the release of histamine induced by agents from HMC-1 cells (Moon et al., 2007). Interestingly, scaporone, the methylated analogues of scopletin dose-dependently decreased histamine release from rat peritoneal mast cells stimulated by anti-DNP IgE at concentrations ranging from 25 to 100 μM. It also inhibited PCA reaction in rats dose-dependently at concentrations of 10, 25 and 50 mg·kg−1. Scaporone also reduced the expression and secretion of proinflammatory cytokines such as TNF-α and IL-6 (Choi and Yan, 2009a). The coumarin, artekeiskeanol A, isolated from Artemisia keskeana Miq. suppressed degranulation of RBL-2H3 cells induced by antigen and calcium ionophore A23187 in a concentration-dependent manner (10–100 μM). Artekeiskeanol A also suppressed the mRNA levels of proinflammatory cytokines TNF-α and IL-13 and phosphorylation of signalling kinases such as p38 MAPK and JNK, which are involved in downstream signalling events (Hong et al., 2009). Selinidin, a couramin derived from Angelica keiskei attenuated the release of β-hexosaminidase from bone marrow-derived mast cells (BMMCs) stimulated by antigen and the production of proinflammatory mediators such as LT C 4 and TNF-α. Selinidin also decreased phosphorylation of PLCγ-1 and p38 MAPK, enzymes involved in the signalling pathway of degranulation (Kishiro et al., 2008). The furanocoumarin, 5-methoxy-8-(2-hydroxy-3-butoxy-3-methylbutyloxy)-psoralen, isolated from Angelica dahurica inhibited both COX-2 and 5-lipoxygenase activity and generation of the lipid mediators PGD 2 and LTC 4 . This furanocouramin also prevented degranulation of mice BMMCs activated with KL (Hua et al., 2008). Interestingly, cinnamic acid, which might be considered as a precursor to the coumarin structure, markedly suppressed antigen-stimulated degranulation of β-hexosaminidase from RBL-2H3 cells in a dose-dependent manner (10–100 μM) through inactivation of Syk and PLCγ pathways (Ninomiya et al., 2010). Thunberginols A and B are isocoumarin derivatives isolated from the processed leaves of Hydrangeae macrophylla var. thunbergii, which have shown anti-allergic activity by inhibiting histamine release from RPMCs stimulated by calcium ionophore A23187 and antigen (Matsuda et al., 1999). Thunberginol B demonstrated potent activity by completely inhibiting degranulation against both elicitors at a concentration of 30 μM. They both also inhibited degranulation of RBL-2H3 cells stimulated by calcium ionophore and antigen as well as the release of cytokines TNF-α and IL-4 (Wang et al., 2007). Thunberginol B was also shown to inhibit mRNA expression of several cytokines including IL-2, IL-3, IL-4 and IL-13, TNF-α and granulocyte/macrophage colony-stimulating factor (GM-CSF) in RBL-2H3 cells stimulated by antigen (Matsuda et al., 2008). Ellagic acid (2,3,7,8-tetrahydroxy[1]benzopyrano [5,4,3-cde][1]benzopyran-5,10-dione) is a polyphenolic compound found in fruits and nuts such as raspberries, strawberries, walnuts, longan seeds, mango kernel and pomegranate, which has shown to attenuate anti–IgE-mediated allergic response in vitro and in vivo. Ellagic acid dose-dependently inhibited histamine release as well as the secretion of proinflammatory cytokines such as TNF-α and IL-6 from anti-DNP IgE induced degranulation of RPMCs across a concentration range of 50–200 μM. Ellagic acid attenuated anti-DNP IgE-mediated PCA in rats (Choi and Yan, 2009b).

Phenols Magnolol and honokiol are phenolic structural isomers, isolated from the bark of Magnolia obovata that have shown to potently inhibit the degranulation of RBL-2H3 cells induced by IgE–antigen complex as well as the production of cytokines; IL-4 and TNF-α. Moreover, both compounds potently inhibited PCA reactions in mice induced by IgE–antigen complex dose-dependently at doses of 10 and 50 mg·kg−1 (Han et al., 2007). Resveratrol, is a phytoalexin, stilbene polyphenolic compound found in grapes, berries and peanuts. It suppressed the expression of inflammatory cytokines such as TNF-α, IL-6 and IL-8 in PMACI-induced–HMC-1 cells and decreased the levels of intracellular Ca2+ (Kang et al., 2009). Likewise, the anti-allergic activity of polydatin, a resveratrol glucoside, significantly decreased FcεRI-mediated degranulation in IgE-sensitized RBL-2H3 cells in a dose-dependent manner (1–100 μM) and inhibited the IgE-dependent PCA reaction in mice (300 mg·kg−1; El-Agamy, 2012). Curcumin is a polyphenolic compound found in Curcuma longa and related species. Curcumin has demonstrated anti-allergic activity in both in vitro and in vivo models. It significantly inhibited antigen-induced degranulation in a dose-dependent manner (1–10 μM) in both RBL-2H3 cells and BMMCs and moreover suppressed PCA reaction in mice at doses of 0.5–50 mg·kg−1. Curcumin significantly inhibited the expression of mRNA for cytokines; IL-4 and TNF-α in a dose-dependent manner as well as their secretion in antigen-stimulated RBL-2H3 cells (Lee et al., 2008). The xanthones; mangostin-α, -β and -γ isolated from the pericap of Garcinia mangostana L. inhibited the release of histamine from IgE-sensitized RBL-2H3 cells in response to antigen through suppression of the signalling transduction pathway involving Syk and PLCγ (Itoh et al., 2008). Within the flavonoid, coumarin and phenolic classes, it is evident from studies conducted to date that the precise mechanism by which these substances stabilize mast cells remains largely unknown. As with most planar molecules, as in these series, it is perhaps conceivable that many processes in the allergic cascade are targeted. This point is reinforced by the broad spectrum of biological actions exhibited by these classes of compounds and from the evidence presented above where they show effects against many different cell populations. The hypothesis that truly selective inhibitors of a given molecular target can only be generated from three-dimensional molecules is perhaps reinforced by findings within these studies. Nevertheless, as the allergic cascade may involve several inflammatory pathways, the idea of designing anti-allergic substances with multiple mechanisms of actions may have potential advantageous therapeutically.

Terpenoids Some recent examples of mast cell stabilizers from the terpenoid class include parthenolide (PTL), a sesquiterpene lactone isolated from the herb feverfew (Tanacetum parthenium), extracts from which exhibits anti-inflammatory properties and are used for the treatment of migraine (Murphy et al., 1988). PTL has also shown anti-allergic properties both in vitro and in vivo models. PTL inhibited antigen-IgE induced degranulation of both RBL-2H3 cells and BMMCs at low concentrations (0.6–5 μM) and strongly inhibited PCA reaction in mice by approximately 90% at a concentration of 10 mg·kg−1. PLT was also shown also to strongly suppress IgE–antigen-induced cytoskeletal rearrangement in RBL-2H3 cells, which is considered a critical step for the degranulation process in mast cells (Miyata et al., 2008). Dehydroleucodine, a sesquiterpene lactone isolated from Artemisia douglasiana Besser and xanthatin, a xanthanolide lactone isolated from Xanthium cavanillesii Schouw inhibited the release of the mediator serotonin from RPMCs induced by compound 48/80. Both substances showed more potent inhibitory activity than the established mast cell stabilizers, disodium cromoglycate and ketotifen (Penissi et al., 2009). Nine types of sesquiterpene lactones (SQLTs) isolated from Eupatorium chinense L. suppressed the degranulation from antigen-stimulated RBL-2H3 cells and furthermore were shown to suppress the elevation of intracellular Ca2+. In vivo, the sesquiterpene lactones-rich extract potently inhibited PCA reaction induced by antigen–IgE complex in mice in a dose-dependent manner (Itoh et al., 2009). Monoterpenes including; borneol and camphene, terpene alcohol; linalool and sesquiterpene; nerolidol in the form of an extract of Amomum xanthiodes (AXE) showed good anti-allergic activity both in both in vitro and in vivo screens. AXE reduced histamine release from RPMCs stimulated by compound 48/80 in a dose-dependent manner and also reduced the level of intracellular Ca2+. AXE suppressed compound 48/80-induced PCA reaction in mice (Kim et al., 2007).

Alkaloids Within the alkaloid class, sinomenine (SIN) (7,8-didehydro-4-hydroxy-3,7-dimethoxy-17-methylmorphinan-6-one), an alkaloid isolated from Sinomenium actum inhibited antigen-induced mast cell degranulation in RBL-2H3 cells in a dose-dependent manner from 0.5 to 2 mM. Similarly, SIN also inhibited the production of cytokines; IL-4 and TNF-α as well as the phosphorylation of various proteins involved in downstream signalling of activated mast cells such as Gab2 and p38 MAPK (Huang et al., 2008). Indoline (E,Z)-3-(3′,5′-dimethoxy-4-hydroxy-benzylidene)-2-indolinone, an alkaloid isolated from the medicinal plant Isatis tinctoria, demonstrated anti-allergic activity in vitro. Indoline-inhibited degranulation of BMMCs stimulated by antigen efficiently across a concentration range of 50–1000 nM. Indoline does not affect kinase activity directly downstream of FcεRI, but interrupts with granule exocytosis possibly by binding to proteins on the surface of granules such as soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptors, which play an essential role in the exocytosis of mast cells (Kiefer et al., 2010). Likewise, indoline inhibited compound 48/80-induced degranulation of RPMCs to a greater effect than that of the clinically used mast cell stabilizer, DSCG (Ruster et al., 2004). Xestospongin C is an alkaloid isolated from the sponge Xestospogia sp., which inhibited both antigen and thapsigargin induced degranulation of RBL-2H3 cells in a concentration-dependent manner (1–10 μM). It is suggested that xestospongin C exhibits its anti-allergic behaviour by crossing the mast cell membrane and blocking IP 3 receptors on the endoplasmic reticulum membrane. This action prevents Ca2+ store depletion and as a result inhibits the elevated levels of intracellular Ca2+ that is necessary for mast cell degranulation (Oka et al., 2002). Theanine is the major amino acid present in green tea. Recently, the anti-allergic activity of theanine has been elucidated in both in vitro and in vivo models. The amino acid inhibited compound 48/80-induced histamine release from both RMPCs and HMC-1 cells in a dose-dependent manner and showed significant activity at low concentration (1 μM). Additionally, theanine significantly suppressed the secretion of proinflammatory cytokines such as TNF-α, IL-1β, IL-6 and IL-8 by suppressing NF-κB activation in PMACI-stimulated HMC-1 cells. This activity was translated to the in vivo setting as theanine inhibited PCA reaction in mice at 1 mg·kg−1 concentration. It is suggested that it acts as a mast cell stabilizer by preventing perturbation of the lipid bilayer of mast cells (Kim et al., 2011).