Leukotriene modifiers pathway, Pharmacodynamics

Leukotrienes are linked to inflammatory diseases such as asthma, allergic rhinitis, psoriasis, and rheumatoid arthritis [Article:2166915]. Leukotrienes were identified as important mediators of bronchoconstriction and inflammatory cellular infiltration [Articles:7063880, 9017783, 17394438].

Leukotrienes are generated from arachidonic acid (AA) in multiple inflammatory cells, particularly mast cells, basophils, eosinophils, neutrophils and macrophages [Article:17394438]. Transient calcium increase caused by antigen stimulation activates protein kinase cascades and results in the release of AA from membrane phospholipids by cytosolic phospholipase A2 (PLA2G4) [Article:9403692]. AA binds to 5-lipoxygenase activating protein (ALOX5AP) and in this complex AA is converted to 5-S-hydroperoxy-6,8-trans-11,14-cis-eicosatetraenoic acid (5-HPETE) by the enzyme arachidonate 5-lipoxygenase (ALOX5) [Article:6324182]. ALOX5 also catalyzes the conversion of 5-HPETE to leukotriene A4 (LTA4) [Article:2166915]. ALOX5 is selectively expressed in myeloid cells, while enzymes responsible for the conversion of LTA4 in other leukotrienes are more widely distributed [Article:2166915]. LTA4 is converted to leukotriene B4 (LTB4) by leukotriene A4 hydrolase (LTA4H) in neutrophils [Article:9895400]. In eosinophils, mast cells, and macrophages, LTA4 is conjugated with reduced glutathione to leukotriene C4 (LTC4) by leukotriene C4 synthase (LTC4S) either in the LTA4 producing or other cells (see figure) [Articles:6490615, 2166915, 2995976]. LTB4 and LTC4 are transported out of the myeloid cell through specific transmembrane transporters [Articles:2753893, 2166027]. LTC4 undergoes extracellular metabolism to leukotriene D4 (LTD4) by gamma-glutamyltransferase 1 (GGT1) and further to leukotriene E4 (LTE4) by extracellular dipeptidases [Article:9895400]. LTC4, LTD4, and LTE4 are called cysteinyl leukotrienes.

LTA4 can also be transferred between cells, a process that is known as transcellular biosynthesis of leukotrienes. This transfer allows the production of LTB4 in red blood cells and LTC4 in endothelial cells, even if those cells are missing the first few steps of the 5-lipoxygenase cascade (see figure) (reviewed in [Article:16968946]).

The cysteinyl leukotriene receptors are involved in the pathophysiology of bronchial asthma, allergic rhinitis, atopic dermatitis, urticaria, cardiovascular system disorders and tumors. The cysteinyl leukotrienes are binding on the cysteinyl leukotriene receptor 1 (CYSLTR1) and induce physiological effects such as constriction of airway smooth muscle, increased vascular permeability, edema, decreased mucociliary clearance and mucus hypersecretion [Article:17394438]. The cysteinyl leukotriene receptor 2 (CYSLTR2) is also activated by LTC4, LTD4, and LTE4. CYSLTR2 mediates increased vascular permeability and chemokine transcription [Articles:18779380, 15328359, 18048362]. LTB4 stimulates leucocyte chemotaxis, chemokinesis and vascular endothelium adherence, delays neutrophil apoptosis and prolongs neutrophil survival mediated by the high-affinity LTB4 receptor (LTB4R) [Article:17394438].

Two classes of leukotriene modifiers are used for the treatment of asthma; 5-lipoxygenase inhibitors and leukotriene receptor antagonists. Zileuton is suppresses the leukotriene formation by selective inhibition of ALOX5. Zileuton has no or minimal effect on ALOX5-related enzymes, such as arachidonate 12-lipoxygenase, arachidonate 15-lipoxygenase, and cyclooxygenase [Article:17394438].

While zileuton suppresses the formation of leukotrienes, the cysteinyl leukotriene receptor antagonists montelukast, pranlukast and zafirlukast inhibit the CYSLTR1, which is a target of cysteinyl leukotrienes.

Asthmatic patients treated with the above drugs showed substantial interindividual variability in their therapeutic response. Most pharmacogenetics association studies have focused on variants in genes in the leukotriene pathway (see figure). Genetic variants in ALOX5 and LTC4S are most widely studied and have been associated with altered therapeutic response to asthma drugs although most studies have been small (less than 100 patients), measured different phenotypes, drug doses or treatments durations and have not yet been validated.

The ALOX5 repeat promoter polymorphism is characterized by deletion or addition of (GGGCGG) sequence repeats which affects number of Sp1 and Egr-1 transcription factor binding motifs in the promoter. The most common (wild-type) allele has 5 repeats in this region. The variant alleles are determined by variant numbers of repeats occurring in the region, which can range from 3 to 8 in humans. This addition/deletion variant is studied in association with cysteinyl leukotriene receptor antagonist response but with inconsistent results. The mutant ALOX5 repeat polymorphism was associated with decreased exacerbation rates in 61 White asthmatic patients treated with montelukast [Article:16293801], while other studies show a decrease number of asthma exacerbations, improvement of forced expiratory volume at 1 second (FEV1) and decreased use of beta2 agonists in Spanish patients with wild-type allele or heterozygous variant allele [Article:18339529] or no significant difference in terms of bronchodilator response or bronchial hyperresponsiveness between wild-type allele and heterozygous variant allele [Article:12107604].

Rs2115819 [Article:16293801], rs4987105 and rs4986832 [Article:17460547] are further single nucleotide polymorphisms (SNP) in the ALOX5 gene identified in association with cysteinyl leukotriene receptor antagonist response (see Table1).

Table 1 summarizes association studies which highlight relevance of additional SNPs in genes involved in the leukotriene pathway, such as LTC4S rs730012 [Article:16293801], LTA4H rs2660845 [Articles:16293801, 21385196], ABCC1 rs119774 [Article:16293801], CYSLTR2 rs912277 and rs912278 [Article:17460547] in response to montelukast.

Studies in cell lines expressing OATP2B1, coded for by the gene SLCO2B1, showed an increased permeability of montelukast [Article:19151602]. Plasma concentrations of montelukast were significantly higher in individuals homozygous for the Arg312 variant of OATP2B1 (rs12422149; Arg312Gln) [Articles:19151602, 20974993]. These observations suggest that pharmacokinetic variations may affect montelukast treatment. In fact, the Arg312 variant of OATP2B1 was significantly associated with improvement in asthma symptoms, as evidenced by an increase in the mean Asthma Symptom Utility Index score after one and six months of treatment, respectively (p<0.0001) in those subjects homozygous for this variant [Article:19151602]. The heterozygous Arg312Gln variant was associated with no significant change in symptoms. This is consistent with a clinical effect being mediated, at least in part, via difference in drug transport.

Tantisira et al. investigated the effect of variants on the ALOX5 inhibitor zileuton based on the above described finding of SNP influenced variation in montelukast treatment response [Article:19214143]. Six SNP are associated with zileuton response based on changes in FEV1 in 577 asthmatics: ALOX5 rs215819, ALOX5 rs892690, ALOX5 rs2029253, ABCC1 rs119774, ABCC1 rs215066, and LTC4S rs272431 [Article:19214143].

The limitations of the presented leukotriene modifiers pharmacogenomics studies are small sample sizes and missing replication in independent populations. Further studies in larger cohorts and replication across multiple populations are needed to consider gene variants in the leukotriene pathway to personalize the asthma therapy with leukotriene modifier agents (see also reviews [Articles:19077707, 19665766, 9925425]).

Table1: Variants associated with response to cysteinyl leukotriene receptor antagonists.

PEF= peak expiratory flow; FEV1= forced expiratory volume at 1 second; FE(NO)=fraction of exhaled nitric oxide