The 5-lipoxygenase inhibitors ZD2138 and ZM230487 are potent and selective inhibitors of several antigen-induced guinea-pig pulmonary responses

Treatment of allergy: Overview of synthetic anti-allergy small molecules in medicinal chemistry

The prevalence of allergic diseases has been continuously increasing over the past few decades, affecting approximately 20-30% of the global population. Allergic reactions to infection of respiratory tract, digestive tract, and skin system involve multiple different targets. The main difficulty of anti-allergy research is how to develop drugs with good curative effect and less side effects by adopting new multi-targets and mechanisms according to the clinical characteristics of different allergic populations and different allergens. This review focuses on information concerning potential therapeutic targets as well as the synthetic anti-allergy small molecules with respect to their medicinal chemistry. The structure-activity relationship and the mechanism of compound-target interaction were highlighted with perspective to histamine-1/4 receptor antagonists, leukotriene biosynthesis, Th2 cytokines inhibitors, and calcium channel blockers. We hope that the study of chemical scaffold modification and optimization for different lead compounds summarized in this review not only lays the foundation for improvement of success rate and efficiency of virtual screening of antiallergic drugs, but also can provide valuable reference for the drug design of related promising research such as allergy, inflammation, and cancer.

Inflammation, Cancer and Oxidative Lipoxygenase Activity are Intimately Linked

Cancer and inflammation are intimately linked due to specific oxidative processes in the tumor microenvironment. Lipoxygenases are a versatile class of oxidative enzymes involved in arachidonic acid metabolism. An increasing number of arachidonic acid metabolites is being discovered and apart from their classically recognized pro-inflammatory effects, anti-inflammatory effects are also being described in recent years. Interestingly, these lipid mediators are involved in activation of pro-inflammatory signal transduction pathways such as the nuclear factor κB (NF-κB) pathway, which illustrates the intimate link between lipid signaling and transcription factor activation. The identification of the role of arachidonic acid metabolites in several inflammatory diseases led to a significant drug discovery effort around arachidonic acid metabolizing enzymes. However, to date success in this area has been limited. This might be attributed to the lack of selectivity of the developed inhibitors and to a lack of detailed understanding of the functional roles of arachidonic acid metabolites in inflammatory responses and cancer. This calls for a more detailed investigation of the activity of arachidonic acid metabolizing enzymes and development of more selective inhibitors.

Targeting Ca2+ release-activated Ca2+ channel channels and leukotriene receptors provides a novel combination strategy for treating nasal polyposis

BACKGROUND

Nasal polyposis is a chronic inflammatory disease of the upper respiratory tract that affects around 2% of the population and almost 67% of patients with aspirin-intolerant asthma. Polyps are rich in mast cells and eosinophils, resulting in high levels of the proinflammatory cysteinyl leukotrienes.

OBJECTIVES

To better understand the role of the proinflammatory leukotrienes in nasal polyposis, we asked the following questions: (1) How do nasal polyps produce leukotriene C(4) (LTC(4))? (2) Can LTC(4) feed back in a paracrine way to maintain mast cell activation? (3) Could a combination therapy targeting the elements of this feed-forward loop provide a novel therapy for allergic disease?

METHODS

We have used immunohistochemistry, enzyme immunoassay, and cytoplasmic calcium ion (Ca(2+)) imaging to address these questions on cultured and acutely isolated human mast cells from patients with polyposis.

RESULTS

Ca(2+) entry through store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels in polyps produced LTC(4) in a manner dependent on protein kinase C. LTC(4) thus generated activated mast cells through cysteinyl leukotriene type I receptors. Hence Ca(2+) influx into mast cells stimulates LTC(4) production, which then acts as a paracrine signal to activate further Ca(2+) influx. A combination of a low concentration of both a CRAC channel blocker and a leukotriene receptor antagonist was as effective at suppressing mast cell activation as a high concentration of either antagonist alone.

CONCLUSION

A drug combination directed against CRAC channels and leukotriene receptor antagonist suppresses the feed-forward loop that leads to aberrant mast cell activation. Hence our results identify a new potential strategy for combating polyposis and mast cell-dependent allergies.

Using guinea pigs in studies relevant to asthma and COPD

The guinea pig has been the most commonly used small animal species in preclinical studies related to asthma and COPD. The primary advantages of the guinea pig are the similar potencies and efficacies of agonists and antagonists in human and guinea pig airways and the many similarities in physiological processes, especially airway autonomic control and the response to allergen. The primary disadvantages to using guinea pigs are the lack of transgenic methods, limited numbers of guinea pig strains for comparative studies and a prominent axon reflex that is unlikely to be present in human airways. These attributes and various models developed in guinea pigs are discussed.

Development of 5-lipoxygenase inhibitors--lessons from cellular enzyme regulation

5-Lipoxygenase (5-LO) catalyzes the first steps in the conversion of arachidonic acid (AA) into leukotrienes (LTs) that are mediators of inflammatory and allergic reactions. Recently, the 5-LO pathway has also been associated with atherosclerosis and osteoporosis. Thus, in addition to the classical applications including asthma and allergic disorders, LT synthesis inhibitors might be of interest for the treatment of cardiovascular diseases and osteoporosis. Recently, it has been shown that cellular 5-LO activity is regulated in a complex manner that can involve different signalling pathways. 5-LO can be activated by an increase in intracellular Ca2+ concentration, diacylglycerols, phosphorylation by MAPKAP kinase-2 and ERK. Previous work could demonstrate that cellular 5-LO activity is repressed in a protein kinase A-dependent manner and by glutathione peroxidases. This comment focuses on the impact of these stimulatory and inhibitory pathways on the efficacy of 5-LO inhibitors and suggests additional criteria for the development of this class of compounds.

Role of intracellular Ca2+ in endothelium-dependent contraction and relaxation of rabbit intrapulmonary arteries

We examined whether Ca(2+) mobilizers induce endothelium-dependent contraction and relaxation (EDC and EDR) in isolated rabbit intrapulmonary arteries. Ionomycin (10(-7) M) and A-23187 (10(-7) M), both Ca(2+) ionophores, and thapsigargin (10(-6) M), an endoplasmic reticulum Ca(2+)-ATPase inhibitor, caused a contraction in the non-contracted preparations, and a transient relaxation followed by a transient contraction and sustained relaxation in the precontracted preparations. Endothelium-removal abolished the contraction and transient relaxation (EDC and EDR) but not sustained relaxation (endothelium-independent relaxation, EIR). In the noncontracted preparations, ionomycin-induced EDC was significantly attenuated by quinacrine (10(-5) M), manoalide (10(-6) M), both phospholipase A(2) inhibitors, indomethacin (10(-5) M) and aspirin (10(-4) M), both COX inhibitors, and ozagrel (10(-5) M), a TXA(2) synthetase inhibitor. In the precontracted arteries, EDR was markedly reduced by L-NAME (10(-4) M), a NOS inhibitor, and methylene blue (10(-6) M), a guanylate cyclase inhibitor, and was enhanced by indomethacin, aspirin and ozagrel, probably due to inhibition of EDC. ZM230487, a 5-lipoxygenase inhibitor, had no effect on EDR. EIR was not affected by L-NAME, indomethacin or ZM230487. Arachidonic acid (10(-6) M) evoked EDC sensitive to indomethacin and ozagrel. L-Arginine (10(-3) M) caused EDR sensitive to L-NAME in the ionomycin-stimulated preparations. In conclusion, Ca(2+) mobilizers cause EDC and EDR via production of TXA(2) and NO, respectively.

Thrombin-stimulated DNA synthesis in human cultured airway smooth muscle occurs independently of products of cyclo-oxygenase or 5-lipoxygenase

Arachidonic acid (AA) liberation and metabolism via cyclo-oxygenase or lipoxygenases may be an important regulatory pathway for mitogenic signalling in human cultured airway smooth muscle (ASM) cells. In cytokine-treated cells, thrombin markedly enhances production of the anti-mitogenic arachidonic acid metabolite, PGE(2). In this study, in the absence of cytokines, we examined the role of endogenous AA metabolism in thrombin-stimulated ASM DNA synthesis. Selective inhibitors of cyclo-oxygenase of 5-lipoxygenase metabolism had no significant effect on 0.3 U/ml thrombin-stimulated DNA synthesis. However, the non-selective, redox-active lipoxygenase inhibitors NDGA and BWA4C inhibited thrombin-stimulated DNA synthesis. Under basal conditions, and following stimulation by thrombin, the levels of the AA metabolites PGE(2), TxA(2), and LTC(4), remained below assay detection limits. Exogenous addition of AA, LTD(4), or 5-, 12-, and 15-HETE and HpETE metabolites had no consistent or substantial stimulatory effect on either basal or thrombin-stimulated DNA synthesis. These data suggest that the non-selective lipoxygenase inhibitors influence DNA synthesis via effects unrelated to lipoxygenase inhibition. The lack of detection of AA metabolites, the lack of influence of selective antagonists/inhibitors of the AA pathway, and the failure of selected AA metabolites to either enhance or directly stimulate DNA synthesis suggest that in the absence of cytokines, cyclo-oxygenase and lipoxygenase metabolism has little role in signalling of human ASM DNA synthesis by thrombin.

Role of arachidonic acid in leukotriene B(4)-induced guinea-pig eosinophil homotypic aggregation

The activation of eosinophils with the lipid mediator, leukotriene B(4), induces their homotypic aggregation. Upon activation with leukotriene B(4), eosinophils release a significant amount of arachidonic acid, a process dependent on the activation of phospholipase A(2). Here, we have evaluated whether arachidonic acid could induce aggregation of eosinophils and whether the release of arachidonic acid mediated the aggregation induced by leukotriene B(4). The exogenous administration of arachidonic acid induced a concentration-dependent eosinophil homotypic aggregation. Pretreatment of eosinophils with a 5-lipoxygenase inhibitor or a leukotriene B(4) receptor antagonist abrogated arachidonic-acid-induced aggregation. Arachidonic acid induced a significant increase in leukotriene B(4) levels and desensitised leukotriene B(4)-induced aggregation in a dose-dependent manner. Moreover, this desensitisation was effectively reversed by a 5-lipoxygenase inhibitor. However, arachidonic acid failed to induce a rise in intracellular Ca(2+) in eosinophils and failed to desensitise these cells to rises in intracellular Ca(2+) induced by leukotriene B(4). Pretreatment of eosinophils with the phospholipase A(2) inhibitor, mepacrine, inhibited the aggregation responses induced by 1 nM leukotriene B(4) by approximately 50% but had no significant effect on the other concentrations of leukotriene B(4) tested (0.1 to 100 nM). In conclusion, arachidonic acid stimulates eosinophil aggregation indirectly via the release of leukotriene B(4). Although a significant amount of arachidonic acid is released in response to activation of eosinophils with leukotriene B(4), the arachidonic acid released does appear to play a major role in mediating leukotriene B(4)-induced eosinophil aggregation.

Nonredox 5-lipoxygenase inhibitors require glutathione peroxidase for efficient inhibition of 5-lipoxygenase activity

Nonredox type 5-lipoxygenase (5-LO) inhibitors, such as ZM 230487, its methyl analogue ZD 2138, or the Merck compound L-739,010, suppress cellular leukotriene synthesis of ionophore stimulated granulocytes with IC50 values of about 50 nM. However, in cell homogenates or in preparations of purified enzyme, up to 150-fold higher concentrations are required for similar inhibition of 5-LO activity. This loss of 5-LO inhibition in cell homogenates was reversed by addition of glutathione or dithiothreitol, which increased the inhibitory potency of ZM 230487 or L-739,010 by about 100 to 150-fold so that 5-LO inhibition was comparable with that of intact cells. In the presence of thiols, addition of hydroperoxide [13(S)-HpODE], glutathione-peroxidase inhibition by iodacetate or selenium-deficiency lead to impaired 5-LO inhibition by ZM 230487 in cell homogenates. Moreover, addition of glutathione peroxidase was required for efficient inhibition of purified human 5-LO by ZM 230487. The data suggest that low hydroperoxide concentrations are important for efficient 5-LO inhibition by ZM 230487. The kinetic analysis revealed a noncompetitive inhibition of 5-LO by ZM 230487 at low hydroperoxide levels, whereas it acted as a competitive inhibitor with low affinity under nonreducing conditions in granulocyte homogenates. No such redox-dependent effects were observed with the 5-LO inhibitor BWA4C, the 5-LO activating protein-inhibitor MK-886 or the pentacyclic triterpene acetyl-11-keto-beta-boswellic acid. These data suggest that physiological conditions associated with oxidative stress and increased peroxide levels lead to impaired efficacy of nonredox type 5-LO inhibitors like ZM 230487 or L-739,010. This could explain the reported lack of activity of this class of 5-LO inhibitors in chronic inflammatory processes.

Leukotriene B4 Activates the NADPH Oxidase in Eosinophils by a Pertussis Toxin-Sensitive Mechanism That Is Largely Independent of Arachidonic Acid Mobilization

Experiments were designed to investigate whether leukotriene (LTB4) receptors can couple directly to phospholipase A2 (PLA2) in guinea pig eosinophils and the role of endogenous arachidonic acid (AA) in LTB4-induced activation of the NADPH oxidase. LTB4 (EC50 ∼ 16 nM) and AA (EC50 ∼ 6 μM) generated hydrogen peroxide (H2O2) in a concentration-dependent manner and at an equivalent maximum rate (5–6 nmol/min/106 cells). LTB4 stimulated PLA2 over a similar concentration range that activated the NADPH oxidase, although kinetic studies revealed that the release of [3H]AA (t1/2 ∼ 2 s) preceded H2O2 generation (t1/2 > 30 s). Pretreatment of eosinophils with pertussis toxin abolished the increase in inositol(1,4,5)trisphosphate mass, [Ca2+]c, [3H]AA release, and H2O2 generation evoked by LTB4. Qualitatively identical results were obtained in eosinophils in which phospholipase C (PLC) was desensitized by 4β-phorbol 12,13-dibutyrate with the exception that [3H]AA release was largely unaffected. Additional studies performed with the protein kinase C inhibitor, Ro 31-8220, and under conditions in which Ca2+ mobilization was abolished, provided further evidence that LTB4 released [3H]AA independently of signal molecules derived from the hydrolysis of phosphatidylinositol(4,5)bisphosphate by PLC. Pretreatment of eosinophils with the PLA2 inhibitor, mepacrine, abolished LTB4-induced [3H]AA release at a concentration that inhibited H2O2 by only 36%. Collectively, the results of this study indicate that agonism of LTB4 receptors on guinea pig eosinophils mobilizes AA by a mechanism that does not involve the activation of PLC. In addition, although LTB4 effectively stimulated PLA2, a central role for AA in the activation of the NADPH oxidase was excluded.