Cysteinyl leukotriene receptor-1 antagonists as modulators of innate immune cell function

Abstract

Cysteinyl leukotrienes (cysLTs) are produced predominantly by cells of the innate immune system, especially basophils, eosinophils, mast cells, and monocytes/macrophages. Notwithstanding potent bronchoconstrictor activity, cysLTs are also proinflammatory consequent to their autocrine and paracrine interactions with G-protein-coupled receptors expressed not only on the aforementioned cell types, but also on Th2 lymphocytes, as well as structural cells, and to a lesser extent neutrophils and CD8(+) cells. Recognition of the involvement of cysLTs in the immunopathogenesis of various types of acute and chronic inflammatory disorders, especially bronchial asthma, prompted the development of selective cysLT receptor-1 (cysLTR1) antagonists, specifically montelukast, pranlukast, and zafirlukast. More recently these agents have also been reported to possess secondary anti-inflammatory activities, distinct from cysLTR1 antagonism, which appear to be particularly effective in targeting neutrophils and monocytes/macrophages. Underlying mechanisms include interference with cyclic nucleotide phosphodiesterases, 5'-lipoxygenase, and the proinflammatory transcription factor, nuclear factor kappa B. These and other secondary anti-inflammatory mechanisms of the commonly used cysLTR1 antagonists are the major focus of the current review, which also includes a comparison of the anti-inflammatory effects of montelukast, pranlukast, and zafirlukast on human neutrophils in vitro, as well as an overview of both the current clinical applications of these agents and potential future applications based on preclinical and early clinical studies.

Figure 1

Cysteinyl leukotriene receptor-1-independent and -dependent anti-inflammatory activities of montelukast, pranlukast, and zafirlukast. The intracellular signalling pathways are depicted with the targets of the cysLTR1 antagonists indicated by means of a rectangle. Phospholipase A₂ (PLA₂) activation that is represented in pathway 1 during which membrane phospholipids are converted to arachidonic acid (AA), which is subsequently metabolized to either leukotrienes (LTA₄, LTB₄, and LTC₄) or prostaglandins (PGH₂) by 5-lipoxygenase (5-LO) and COX-1/2 enzymes, respectively. PGH₂ is converted to PGE₂ by PGE₂ synthase. Phospholipase (PLC) activation is represented in pathway 2 with the generation of inositol triphosphate (IP₃) which releases cytosolic Ca²⁺ from storage vesicles with subsequent downstream activation of PKC, NADPH oxidase, 5-LO, and NFκB, with release of proinflammatory cytokines. ATP binds to purinergic receptors (P2YR) and is also converted to cyclic AMP by adenylate cyclase (AC) which is degraded by intracellular phosphodiesterases (PDE). Cyclic AMP downregulates the Ca²⁺-mediated activation of 5-LO and NADPH oxidase. Inhibition of cysLTR1 receptors attenuates the receptor-dependent effects of cysteinyl leukotrienes.

Figure 2

Effects of montelukast, pranlukast, and zafirlukast (0.25–2 μM) on the lucigenin-enhanced chemiluminescence responses of neutrophils activated by N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP, 1 μM). The results are expressed as the mean peak chemiluminescence values in relative light units measured at 30–50 s after addition of fMLP and vertical lines show SEM (n = 6 with 2 replicates for each drug concentration and control system in each experiment). The absolute values for unstimulated neutrophils and for cells activated with fMLP in the absence of the drugs were 2099 ± 223 and 10594 ± 660 relative light units, respectively. *P < 0.05 for comparison with the fMLP-activated, drug-free control system.

Figure 3

Effects of montelukast, pranlukast, and zafirlukast (0.25–2 μM) on the release of elastase from neutrophils activated with N-formyl-L-methionyl-L-leucyl-L-phenylalanine (1 μM)/cytochalasin B (1 μM) (fMLP/CB). The results (n = 6 with 5 replicates for each drug concentration and control system in each experiment) are expressed as the mean values for total extracellular elastase (milliunits/10⁷ cells) and vertical lines show SEM. The absolute values for the unstimulated control system and for cells activated with fMLP/CB were 33 ± 1 and 937 ± 9 milliunits elastase/10⁷ cells. *P < 0.05 for comparison with the drug-free control system.

Figure 4

Effects of montelukast, pranlukast, and zafirlukast (0.5 and 1 μM) on the production of LTB₄ by PAF-activated neutrophils. The results are presented as the mean values for total extracellular LTB₄ (pg/mL) and vertical lines show SEM (n = 9 with 2 replicates for each drug concentration and control system in each experiment). The absolute values for the unstimulated control system and for cells activated were 21 ± 5 and 3092 ± 1669 pg LTB₄/mL, respectively. *P < 0.05 for comparison with the drug-free control system.

Figure 5

Effects of montelukast, pranlukast, and zafirlukast (0.5–20 μM) on cAMP- (a) and cGMP- (b) phosphodiesterase (PDE) activities in neutrophil cytosol. The results are presented as the mean enzyme activities, and vertical lines show SEM (n = 2–5, with three to four replicates for each drug concentration and control system in each experiment). *P < 0.05 for comparison with the corresponding drug-free control system.