| Pharmacogenomics of antihistamines

Pharmacogenomics is a science to study the relationship between gene sequence polymorphism and drug effect diversity35. Pharmacogenomics aims at drug effect and safety, it has contributed not only to the acknowledgment of gene polymorphism responsible for variation in drug response but also to the new drugs or new drug delivery methods development process 36. In recent decades, the rapid development of pharmacogenomics has endowed a new connotation and brought new opportunities for individualized drug use, which makes today’s clinical individualized drug use more far-reaching 37.
Antihistamine is a drug with a marked therapy effect that is widely used in allergic diseases, while individual variability in drug use has been reported. Individual differences may be mainly manifested as differences in pharmacodynamics and adverse reactions. Antihistamine pharmacogenomics has found that gene polymorphism plays an important role in its application (Fig 1). The genetic variation of metabolic enzyme, transporters, and targeted molecules influences the distribution, metabolism, and excretion of antihistamines through molecular mechanisms. Also, genes variation related disease like urticaria or allergic rhinitis may affect the application of antihistamine. A summary of the effect of drug metabolism enzyme, transporters, and receptors polymorphism on antihistamine is listed in Table 1. This paper summarizes the current researches of pharmacogenomics on antihistamines, hoping to have a certain significance for further research in this field and guiding clinical medication in the future.

| Drug metabolism enzyme polymorphism

The cytochromes P450(CYPs) is the main enzyme system of drug metabolism, which contains many subgroups. Human CYPs are mainly membrane-associated proteins that are widely expressed in most tissues, especially in liver tissue with specific CYPs types distributed38, 39. The gene variation of CYPs can change its susceptibility to induction and inhibition, then influence the pharmacokinetics of drugs. Therefore, CYPs play an important role in inter-individual drug response that is closely linked to the efficacy and side effects of drugs 40. Most antihistamines are metabolized by the liver CYP450 enzyme system.
Cytochrome P450 (CYP) 3A is an important group of the CYP450 enzyme system, which is most abundant in the liver and gastrointestinal tract41. However, previous studies have shown that due to the single nucleotide polymorphisms (SNPs) in gene sequences, the enzyme encoded by the CYP3A allele has little or no activity42. The members of the CYP3A subfamily mainly include four genes that lie within a 231 kb region of chromosome 7q22.1, namely CYP3A4(MIM124010), CYP3A5(MIM605325), CYP3A7(MIM605340), and CYP3A43(MIM606534)41. Human cytochrome P450(CYP)3A4 is involved in the metabolism of about 50% of commonly used drugs 43, 44, among which the relationship between genetic polymorphism of CYP450 and antihistamines has been reported. Rupatadine (RUP) is an oral antihistamine and platelet activator antagonist that metabolizes desloratadine and 3-Hydroxydesloratadin primarily through CYP3A4 mediated metabolism. CYP3A5*3 can reduce the expression of CYP3A5, and the polymorphism of the CYP3A5 gene may significantly affect the activity pattern of CYP3A42. Yuqing Xiong et al45 found thatCYP3A5*1/*1 carrier had high metabolic activity and low transporter activity, while CYP3A5*1 /*3 carriers had the opposite effect, which may lead to different concentrations of rupatadine (RUP) in the gastrointestinal tract and blood. Differences in concentration will lead to various treatment effectiveness and toxicity. The underlying mechanism of CYP3A5 polymorphism in individual differences in antihistamine application needs to be further demonstrated. Maekawa et al found that variants of CYP3A4 such as CYP3A4*16 and CYP3A4*18 alleles showed different catalytic activities for different CYP3A4 substrates46. In another study, possible genetic variants of CYP3A4, such as CYP3A4*2, *7, *16, and *18, may lead to drug-drug interaction individual-mediated variation by CYP3A4 inhibitor cimetidine47. Therefore, when antihistamines are used together with other drugs, the polymorphism of CYP3A4 in patients may be taken into account and the drug should be reasonably applied.
The CYP2 family is the largest group of the human P450 enzyme family. The CYP2J subfamily contains a single human gene, CYP2J2, which was originally proposed by Kikuta and his colleagues48. Human cytochrome CYP2J2 is highly expressed in the heart and is involved in a variety of metabolic reactions, including oxidation of important drugs and epoxidation of endogenous arachidonic acid49, 50. It is involved in the metabolism of a variety of antihistamine drugs, including ebastine, terfenadine, and astemizole 51, 52. But the contribution of CYP2J2 to overall clearance is not clear and may strongly depend on the tissue53, 54. At least 10 genetic variants of CYP2J2 have been listed on the CYP alleles website. Jeong et al55used the antihistamine terfenadine as a substrate to analyze the biochemical characteristics of CYP2J2*8, *9, and *10 alleles (G312R, P351L, and P115L) containing non-synonymous SNPs and biochemical characteristics of P351L and P115L. It was found that the metabolic activity of the two mutant enzymes P351L and P115L was similar to that of the wild type, and the changes of G312R amino acids affect the structural stability of the P450 heme environment. This may affect the metabolism of antihistamines like terfenadine and cause corresponding adverse reactions, but the specific mechanism of influence and the manifestation of adverse reactions are not described in the paper.
CYP2D6 is highly correlated with drug metabolism. CYP2D6 is involved in the metabolism of about 25% of clinically used drugs, and its gene polymorphism changes the pharmacokinetics of those with poor or extensive metabolism 56, 57. CYP2D6 is encoded by a highly polymorphic gene 58. Based on the activity of CYP2D6 isozymes, it identifies four types of carriers with different gene polymorphisms: normal (extensive), poor, intermediate, and hypertrophic metabolites59. Extensive metabolite (EM): The main alleles were *1 and *2 ; Poor metabolite (PM): The main allele was *3~*6 ; Intermediate metabolite (IM): The primary alleles were *10 and*41 . Wild-type allelic hypervelocity metabolites included individuals with gene variants (CYP2D6*1)xN and(CYP2D6*2)xN duplication and multiplication60. The most common allelic variants associated with the poor phenotype of metabolites wereCYP2D6*3, CYP2D6*4, CYP2D6*5, and CYP2D6*660. People with poor metabolism are more likely to be affected by poor substrate metabolism. CYP2D6 is the main metabolic pathway for many sedative drugs in the central nervous system56, 61. Antihistamine effects are also known to occur with antidepressants and antipsychotic drugs62, such as mirtazapine, esmirtazapine, mianserin, and meclizine. Mirtazapine and mianserin have an H1 antagonist effect obviously, studies have shown that people with dysmetabolic CYP2D6 phenotypes are more sensitive to the potential damage of esmirtazapine and are more likely to be exposed to high levels of drugs or metabolites63. Besides CYP2D6, another study has found that mirtazapine metabolism may also be influenced differently by the CYP3A5 genotype and by multiple combinations (three or more)64, so there may also be a greater chance of adverse reactions. Meclizine has long been a widely accepted antihistamine used in the preventative treatment and management of nausea, vomiting, and dizziness associated with exercise disorders 65. Zhijun Wang et al 65found that CYP2D6 was the dominant enzyme in meclizine metabolism, and its polymorphism might be related to the elimination of variation of meclizine.

| Polymorphism of drug transporters

Drug transporters are integral membrane proteins localized to tissue and cellular membrane domains in the human body, mediating the transport of drug molecules through active and passive mechanisms66. Influx and efflux transporters play an important role in the absorption, tissue distribution, and excretion of drugs67. It has been reported that non-sedating antihistamines are characterized by the efflux function of P-glycoprotein(P-gp) through the blood-brain barrier 68. P-glycoprotein is an ATP-dependent carrier protein first discovered in tumor cells and expressed by multi-drug resistance genes69. It exists in a variety of tissues in the human body, such as the liver, kidney, brain, and so on 69-73. P-gp is encoded by the MDR1(ABCB1) gene74. P-gp gene polymorphism, including 1236C>T, 2677G>A/T, and 3435C>T mutation produced, and conflicting results have been obtained in the study of the pharmacokinetics of fexofenadine. Persons with 2677AA/3435CCgenotype of MDR1(ABCB1) have lower plasma concentrations of fexofenadine after a single oral taking compared to the person with other genotypes75. On the contrary, Drescher et al found that there was no significant correlation ofC3435T polymorphism with the H1 receptor occupancy by the P-glycoprotein substrate fexofenadine76. In addition to P-gp, organic anion transport polypeptide (OATPs) that encoded by the SLCO gene 77 and multidrug resistance protein 2(MRP2) that encoded by the ABCC2genes 78 may also work on fexofenadine pharmacokinetics. OATP1B1, 1B3, and 2B1 involved in fexofenadine liver metabolism in OATP drug transporters79-81 and Akamine et al82studied the effect of drug transporter polymorphism on the pharmacokinetics of fexofenadine and found that the pharmacokinetics of S-fexofenadine was related to the single polymorphism of SLCO2B1 and multiple polymorphism combinations of ABCB1 C1236T, C3435T, and ABCC2 C-24T. In the small intestine and liver, the binding of multiple transporters involved in OATP, P-GP, and multidrug resistance protein 2(MRP2) may strongly respond to exposure to fexofenadine, resulting in different configurations between enantiomers. In another study, the relationship between SLCO2B1 , the encoding gene of OATP transporter, and the antihistamine drug fexofenadine was also pointed out. Through in vitro kinetics studies, this study showed that SLCO2B1 c.[1457C>T]2 allele reduce OATP2B1 transport function and the bioavailability of fexofenadine83, which may affect the efficacy and toxicity of the drug.

| Receptor polymorphism

As a drug target for antihistamines, the change of the histamine receptor, especially the polymorphism of the receptor gene, will also affect the effect of drugs and lead to the individual difference. Therefore, the study of the variation of the receptor gene is of great significance. Gu et al84found that among HRH4 genes, rs77485247 mutant (TA+AA) and rs77041280 mutant (TA+TT) may reduce the efficacy of H1A orally, which may increase the incidence of adverse reactions. The polymorphism of HRH4 rs77485247 and rs77041280 may affect the therapeutic effect of antihistamines. A study on histamine H1 receptor gene polymorphism and efficacy in Chinese Han patients with allergic rhinitis showed that the efficacy of H1 antihistamines was enhanced in AR patients with HRH1 wild-type gene type, while the heterozygous mutant (CT) and homozygous mutant (TT) genes have a higher risk of AR and may affect the efficacy of antihistamines and increase their adverse reactions85.

| Disease susceptibility and drug efficacy, side effects

Antihistamines are the first-line therapeutic drugs for urticaria, which is an allergic disease mainly involved with mast cells. The polymorphisms of the genes involved in the development and progression of urticaria may also affect the efficacy and toxicity of antihistamines. In recent years, the research in this field has been increasing and relevant research reports have provided us with some ideas. Reports in this area include FCER1A, CACNA1C, ORAI1, C5AR1, and CRTH2, as shown in Table 2. FCER1A is an important immune-related gene that encodes ligand-binding subunit a chain (FceRIɑ) of high-affinity IgE receptor (FcɛRI) to trigger IgE-mediated allergic reactions, related to the total level of IgE serum86-88. High total serum IgE levels are closely correlated with the clinical expression and severity of allergic diseases89-91. However, no association between FCER1A and allergic rhinitis was found in one study 92. For urticaria, Guo et al of our group have found that among the rs2298805, rs10908703, and rs2494262 genotypes of FCER1A , rs2298805 polymorphism is associated with the risk of CSU and serum total IgE concentration, and CSU patients with rs2298805A allele show a better response to non-sedate H1-antihistamine93. Further functional studies are needed to elucidate the mechanism of rs2298805 affecting the pathogenesis of urticaria and the therapeutic effect of non-sedated H1-antihistamine.CACNA1C codes for the pore-forming α1C subunit of the L-type voltage-gated calcium channel (LTCC). Our group’s study on genetic polymorphism of the CACNA1C gene and susceptibility and prognosis of chronic spontaneous urticaria showing that genetic polymorphisms of the CACNA1C gene can affect the mast cell activation and degranulation process, thus affecting the onset of the CSU94. A/G mutation at rs58619945 in patients may be related to the onset of chronic spontaneous urticaria, different alleles at rs216008 may be related to the efficacy of desloratadine, and mutations at rs7316246 may indicate differences in the severity and prognosis of the disease94. The degranulation process of mast cells is mainly mediated by Ca2+influx mediated by calcium channels activated by calcium releasing enzymes. ORAI1 is a plasma membrane protein, encoded by the ORAI1 gene, and calcium-release-activated calcium (CRAC) channels pore formation subunit95. Studies on ORAI1 knockout mice have shown that ORAI1 plays an important role in mast cell degranulation, leukotriene (LTC4) secretion, histamine release, and TNF-ɑ secretion 96, 97. Our group reported98 that rs3741595, rs3741596, rs12320939, and rs12313273, especially the rs3741596A allele, were significantly increased in CSU patients and rs12320939T allele was associated with increased expression of the ORAI1 gene. The ORAI1 gene allele rs3741596A enhanced the degranulation activity of mast cells and the rs3741595C may be associated with the therapeutic effects of desloratadine. C5a receptor (C5aR) plays an important role in mast cell activation and degranulation by regulating the complement C5a pathway, thus enhancing the release of histamine. C5a/C5aR pathway promotes histamine release and leads to CSU inflammatory response, human C5aR is encoded by C5AR1 (also known as C5aR) gene, and Yan et al99 in our group founded that C5AR1 SNP -1330T/G can be used as an effective pharmacodynamics predictor of the efficacy of non-sedating H1-antihistamine drugs in CSU patient. The chemoattractant receptor-homologous molecule expressed on Th2 Lymphocytes (CRTH2) is a member of the G protein-coupled, seven-transmembrane receptor family and it has been reported to be associated with mast cell-related allergic diseases 100, 101. Palikhe NS et al102 founded that CRTH2 gene polymorphism(-466T>C and-129C>A ) may not be associated with susceptibility in Korean patients with chronic urticaria, but the CRTH2-466T>C gene polymorphism was associated with the required antihistamine dose and CU patients with the CRTH2 -466Tgenotype need a higher dose.
There have been a few clear reports on gene polymorphism and the exact adverse effects of antihistamines. Sedating side effects is a major adverse effect of first-generation and some second-generation antihistamines. Juan et al in our team103 evaluated the association between HRH1 gene rs901865 polymorphism and sedating side effects severity in 114 Chinese CSU patients treated with desloratadine. HRH1 rs901865 G/G polymorphism was found to be associated with an increase in desloratadine induced lethargy but did not affect drug efficacy or increase the risk of CSU. This study will be important for predicting the severity of narcolepsy adverse reactions after the desloratadine application. Cardiotoxicity is a serious adverse reaction induced by antihistamines. Among second-generation antihistamines, terfenadine and astemizole have been reported to cause tip torsion ventricular tachycardia with prolonged QT interval and even death due to drug overdose and drug interactions104-107. Antihistamines can cause cardiotoxicity by blocking the current of a human Etherago-go-related gene (HERG). A614v-HERG mutations have been shown to cause syncope in patients with a first-generation H1 receptor antagonist and hydroxyzine can inhibit the channel current of the gene108. The correlation between gene polymorphism and adverse cardiotoxicity induced by antihistamines is scarce, more research needs to be done in the future.