Spectroscopic, electrochemical, and molecular docking studies of the interaction between the antihistamine drug desloratadine and dsDNA.

Through the utilization of fluorescence spectroscopy, electrochemical, and molecular docking methods, this research investigates the interaction between the antihistamine drug desloratadine and calf thymus double-stranded DNA (ct-dsDNA). Deoxyguanosine (dGuo) and deoxyadenosine (dAdo) oxidation signals were diminished by incubation with varying concentrations of desloratadine, as determined by differential pulse voltammetry (DPV). This change was ascribed to desloratadine's binding mechanism to ct-dsDNA. The binding constant (Kb) between desloratadine and ct-dsDNA was determined to be 2.2 × 105 M-1 throughout electrochemical experiments. In order to further develop our comprehension of the interaction mechanism between desloratadine and ct-dsDNA, a series of spectroscopic experiments and molecular docking simulations were conducted. The Kb value was found to be 8.85 × 104 M-1 at a temperature of 25 °C by the use of fluorescence spectroscopic techniques. In summary, the utilization of electrochemical and spectroscopic techniques, alongside molecular docking investigations, has led to the prediction that desloratadine has the capability to interact with ct-dsDNA by groove binding.

Azelastine inhibits viropexis of SARS-CoV-2 spike pseudovirus by binding to SARS-CoV-2 entry receptor ACE2.

A recent study have reported that pre-use of azelastine is associated with a decrease in COVID-19 positive test results among susceptible elderly people. Besides, it has been reported that antihistamine drugs could prevent viruses from entering cells. The purpose of this study is to investigate whether azelastine have antiviral activity against SARS-CoV-2 in vitro and the possible mechanism. Here, we discovered antihistamine azelastine has an affinity to ACE2 by cell membrane chromatography (CMC); Then we determined the equilibrium dissociation constant (KD) of azelastine-ACE2 as (2.58 ± 0.48) × 10-7 M by surface plasmon resonance (SPR). The results of molecular docking showed that azelastine could form an obvious hydrogen bond with Lys353. The pseudovirus infection experiments showed that azelastine effectively inhibited viral entry (EC50 = 3.834 µM). Our work provides a new perspective for the screening method of drug repositioning for COVID-19, and an attractive and promising drug candidate for anti-SARS-CoV-2.

Differential Regulation of Bilastine Affinity for Human Histamine H1 Receptors by Lys 179 and Lys 191 via Its Binding Enthalpy and Entropy.

Bilastine, a zwitterionic second-generation antihistamine containing a carboxyl group, has higher selectivity for H1 receptors than first-generation antihistamines. Ligand-receptor docking simulations have suggested that the electrostatic interaction between the carboxyl group of second-generation antihistamines and the amino group of Lys179ECL2 and Lys1915.39 of human H1 receptors might contribute to increased affinity of these antihistamines to H1 receptors. In this study, we evaluated the roles of Lys179ECL2 and Lys1915.39 in regulating the electrostatic and hydrophobic binding of bilastine to H1 receptors by thermodynamic analyses. The binding enthalpy and entropy of bilastine were estimated from the van 't Hoff equation using the dissociation constants. These constants were obtained from the displacement curves against the binding of [3H] mepyramine to membrane preparations of Chinese hamster ovary cells expressing wild-type human H1 receptors and their Lys179ECL2 or Lys1915.39 mutants to alanine at various temperatures. We found that the binding of bilastine to wild-type H1 receptors occurred by enthalpy-dependent binding forces and, more dominantly, entropy-dependent binding forces. The mutation of Lys179ECL2 and Lys1915.39 to alanine reduced the affinity of bilastine to H1 receptors by reducing enthalpy- and entropy-dependent binding forces, respectively. These results suggest that Lys179ECL2 and Lys1915.39 differentially contribute to the increased binding affinity to bilastine via electrostatic and hydrophobic binding forces.

Testing of the inhibitory effects of loratadine and desloratadine on SARS-CoV-2 spike pseudotyped virus viropexis.

Currently, there is an urgent need to find a treatment for the highly infectious coronavirus disease (COVID-19). However, the development of a new, effective, and safe vaccine or drug often requires years and poses great risks. At this critical stage, there is an advantage in using existing clinically approved drugs to treat COVID-19. In this study, in vitro severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) spike pseudotyped viral infection experiments indicated that histamine H1 antagonists loratadine (LOR) and desloratadine (DES) could prevent entry of the pseudotyped virus into ACE2-overexpressing HEK293T cells and showed that DES was more effective. Further binding experiments using cell membrane chromatography and surface plasmon resonance demonstrated that both antagonists could bind to ACE2 and that the binding affinity of DES was much stronger than that of LOR. Molecular docking results elucidated that LOR and DES could bind to ACE2 on the interface of the SARS-CoV-2-binding area. Additionally, DES could form one hydrogen bond with LYS31 but LOR binding relied on non-hydrogen bonds. To our knowledge, this study is the first to demonstrate the inhibitory effect of LOR and DES on SARS-CoV-2 spike pseudotyped virus viropexis by blocking spike protein-ACE2 interaction. This study may provide a new strategy for finding an effective therapeutic option for COVID-19.

Distinct binding of cetirizine enantiomers to human serum albumin and the human histamine receptor H1.

Cetirizine, a major metabolite of hydroxyzine, became a marketed second-generation H1 antihistamine that is orally active and has a rapid onset of action, long duration of effects and a very good safety record at recommended doses. The approved drug is a racemic mixture of (S)-cetirizine and (R)-cetirizine, the latter being the levorotary enantiomer that also exists in the market as a third-generation, non-sedating and highly selective antihistamine. Both enantiomers bind tightly to the human histamine H1 receptor (hH1R) and behave as inverse agonists but the affinity and residence time of (R)-cetirizine are greater than those of (S)-cetirizine. In blood plasma, cetirizine exists in the zwitterionic form and more than 90% of the circulating drug is bound to human serum albumin (HSA), which acts as an inactive reservoir. Independent X-ray crystallographic work has solved the structure of the hH1R:doxepin complex and has identified two drug-binding sites for cetirizine on equine serum albumin (ESA). Given this background, we decided to model a membrane-embedded hH1R in complex with either (R)- or (S)-cetirizine and also the complexes of both ESA and HSA with these two enantiomeric drugs to analyze possible differences in binding modes between enantiomers and also among targets. The ensuing molecular dynamics simulations in explicit solvent and additional computational chemistry calculations provided structural and energetic information about all of these complexes that is normally beyond current experimental possibilities. Overall, we found very good agreement between our binding energy estimates and extant biochemical and pharmacological evidence. A much higher degree of solvent exposure in the cetirizine-binding site(s) of HSA and ESA relative to the more occluded orthosteric binding site in hH1R is translated into larger positional fluctuations and considerably lower affinities for these two nonspecific targets. Whereas it is demonstrated that the two known pockets in ESA provide enough stability for cetirizine binding, only one such site does so in HSA due to a number of amino acid replacements. At the histamine-binding site in hH1R, the distinct interactions established between the phenyl and chlorophenyl moieties of the two enantiomers with the amino acids lining up the pocket and between their free carboxylates and Lys179 in the second extracellular loop account for the improved pharmacological profile of (R)-cetirizine.

Bacterial metabolism rescues the inhibition of intestinal drug absorption by food and drug additives.

Food and drug products contain diverse and abundant small-molecule additives (excipients) with unclear impacts on human physiology, drug safety, and response. Here, we evaluate their potential impact on intestinal drug absorption. By screening 136 unique compounds for inhibition of the key intestinal transporter OATP2B1 we identified and validated 24 potent OATP2B1 inhibitors, characterized by higher molecular weight and hydrophobicity compared to poor or noninhibitors. OATP2B1 inhibitors were also enriched for dyes, including 8 azo (R-N=N-R') dyes. Pharmacokinetic studies in mice confirmed that FD&C Red No. 40, a common azo dye excipient and a potent inhibitor of OATP2B1, decreased the plasma level of the OATP2B1 substrate fexofenadine, suggesting that FD&C Red No. 40 has the potential to block drug absorption through OATP2B1 inhibition in vivo. However, the gut microbiomes of multiple unrelated healthy individuals as well as diverse human gut bacterial isolates were capable of inactivating the identified azo dye excipients, producing metabolites that no longer inhibit OATP2B1 transport. These results support a beneficial role for the microbiome in limiting the unintended effects of food and drug additives in the intestine and provide a framework for the data-driven selection of excipients. Furthermore, the ubiquity and genetic diversity of gut bacterial azoreductases coupled to experiments in conventionally raised and gnotobiotic mice suggest that variations in gut microbial community structure may be less important to consider relative to the high concentrations of azo dyes in food products, which have the potential to saturate gut bacterial enzymatic activity.

Metabolism of desloratadine by chimeric TK-NOG mice transplanted with human hepatocytes.

1. Desloratadine is an antiallergic drug with species-dependent metabolic profiles in mice, rats, monkeys and humans. We investigated whether humanized-liver mice could reproduce the reported human-specific in vivo metabolic profile for desloratadine in terms of the formation of 3-hydroxydesloratadine and its O-glucuronide.2. Hepatocytes prepared from humans and humanized-liver mice both preferentially catalyzed the formation of 3-hydroxydesloratadine and its O-glucuronide in vitro.3. After a single oral administration of desloratadine, plasma levels of desloratadine and its metabolites (3-hydroxydesloratadine and its O-glucuronide) in humanized-liver mice were lower and higher, respectively, than those in control mice.4. The amounts of 3-hydroxydesloratadine and its O-glucuronide excreted in humanized-liver mouse feces and urine were higher than those of the control mice, whereas 5- and 6-hydroxydesloratadine formation were predominant in the feces and urine samples from control mice. A significant correlation (r = 0.68) for the dose percentage of urinary and fecal metabolites of desloratadine was only observed between the humanized-liver mice and the reported values for humans.5. These results indicated that urinary 3-hydroxydesloratadine O-glucuronide and fecal desloratadine, 3-hydroxydesloratadine and 5-hydroxydesloratadine were the major excretion pathways of desloratadine in humanized-liver mice, which is reasonably similar to that reported for humans.

Investigation of the effect of hepatic metabolism on off-target cardiotoxicity in a multi-organ human-on-a-chip system.

Regulation of cosmetic testing and poor predictivity of preclinical drug studies has spurred efforts to develop new methods for systemic toxicity. Current in vitro assays do not fully represent physiology, often lacking xenobiotic metabolism. Functional human multi-organ systems containing iPSC derived cardiomyocytes and primary hepatocytes were maintained under flow using a low-volume pumpless system in a serum-free medium. The functional readouts for contractile force and electrical conductivity enabled the non-invasive study of cardiac function. The presence of the hepatocytes in the system induced cardiotoxic effects from cyclophosphamide and reduced them for terfenadine due to drug metabolism, as expected from each compound's pharmacology. A computational fluid dynamics simulation enabled the prediction of terfenadine-fexofenadine pharmacokinetics, which was validated by HPLC-MS. This in vitro platform recapitulates primary aspects of the in vivo crosstalk between heart and liver and enables pharmacological studies, involving both organs in a single in vitro platform. The system enables non-invasive readouts of cardiotoxicity of drugs and their metabolites. Hepatotoxicity can also be evaluated by biomarker analysis and change in metabolic function. Integration of metabolic function in toxicology models can improve adverse effects prediction in preclinical studies and this system could also be used for chronic studies as well.

Predictive Performance of Physiologically Based Pharmacokinetic (PBPK) Modeling of Drugs Extensively Metabolized by Major Cytochrome P450s in Children.

The accuracy of physiologically based pharmacokinetic (PBPK) model prediction in children, especially those younger than 2 years old, has not been systematically evaluated. The aim of this study was to characterize the pediatric predictive performance of the PBPK approach for 10 drugs extensively metabolized by CYP1A2 (theophylline), CYP2C8 (desloratidine, montelukast), CYP2C9 (diclofenac), CYP2C19 (esomeprazole, lansoprazole), CYP2D6 (tramadol), and CYP3A4 (itraconazole, ondansetron, sufentanil). Model performance in children was evaluated by comparing simulated plasma concentration-time profiles with observed clinical results for each drug and age group. PBPK models reasonably predicted the pharmacokinetics of desloratadine, diclofenac, itraconazole, lansoprazole, montelukast, ondansetron, sufentanil, theophylline, and tramadol across all age groups. Collectively, 58 out of 67 predictions were within 2-fold and 43 out of 67 predictions within 1.5-fold of observed values. Developed PBPK models can reasonably predict exposure in children age 1 month and older for an array of predominantly CYP metabolized drugs.

Terfenadine metabolism of human cytochrome P450 2J2 containing genetic variations (G312R, P351L and P115L).

The human cytochrome P450 2J2 is involved in several metabolic reactions, including the oxidation of important therapeutics and epoxidation of endogenous arachidonic acid. At least ten genetic variations of P450 2J2 have been identified, but their effects on enzymatic activity have not been clearly characterized. Here, we evaluated the functional effects of three genetic variations of P450 2J2 (G312R, P351L, and P115L). Recombinant enzymes of wild-type and three variant P450 2J2 were heterologously expressed in Escherichia coli and purified. P450 expression levels in the wild-type and two variants (P351L and P115L) were 142-231 nmol per liter culture, while the G312R variant showed no holoenzyme peak in the CO-binding spectra. Substrate binding titrations to terfenadine showed that the wild-type and two variants displayed Kd values of 0.90-2.2 µM, indicating tight substrate binding affinities. Steady-state kinetic analysis for t-butyl methyl hydroxylation of terfenadine indicated that two variant enzymes had similar kcat and Km values to wild-type P450 2J2. The locations of mutations in three-dimensional structural models indicated that the G312R is located in the I-helix region near the formal active site in P450 2J2 and its amino acid change affected the structural stability of the P450 heme environment.

Citric Acid Suppresses the Bitter Taste of Olopatadine Hydrochloride Orally Disintegrating Tablets.

Orally disintegrating tablets (ODTs) are formulated to disintegrate upon contact with saliva, allowing administration without water. Olopatadine hydrochloride, a second-generation antihistamine, is widely used for treating allergic rhinitis. However, it has a bitter taste; therefore, the development of taste-masked olopatadine ODTs is essential. Some studies have suggested that citric acid could suppress the bitterness of drugs. However, these experiments were performed using solutions, and the taste-masking effect of citric acid on ODTs has not been evaluated using human gustatory sensation tests. Thus, this study evaluated citric acid's taste-masking effect on olopatadine ODTs. Six types of olopatadine ODTs containing 0-10% citric acid were prepared and subjected to gustatory sensation tests that were scored using the visual analog scale. The bitterness and overall palatability of olopatadine ODTs during disintegration in the mouth and after spitting out were evaluated in 11 healthy volunteers (age: 22.8±2.2 years). The hardness of the ODTs was >50 N. Disintegration time and dissolution did not differ among the different ODTs. The results of the gustatory sensation tests suggest that citric acid could suppress the bitterness of olopatadine ODTs in a dose-dependent manner. Olopatadine ODTs with a high content of citric acid (5-10%) showed poorer overall palatability than that of those without citric acid despite the bitterness suppression. ODTs containing 2.5% citric acid, yogurt flavoring, and aspartame were the most suitable formulations since they showed low bitterness and good overall palatability. Thus, citric acid is an effective bitterness-masking option for ODTs.

Cellular Uptake of Levocetirizine by Organic Anion Transporter 4.

The pharmacokinetics of cetirizine, a nonsedating antihistamine, is profoundly affected by transporter-mediated membrane transport in the kidney. In this study, we aimed to investigate the transport mechanism of levocetirizine, the pharmacologically active enantiomer of cetirizine, via human organic anion transporter 4 (OAT4) expressed in the apical membrane of renal proximal tubules and the basal plasma membrane of placental syncytiotrophoblasts. In cells expressing human OAT4 under the control of tetracycline, levocetirizine uptake was increased by tetracycline treatment. On the other hand, OAT4 expression did not facilitate efflux of preloaded levocetirizine from the cells, either in the presence or absence of extracellular Cl-. The OAT4-mediated levocetirizine uptake was concentration-dependent with a Km of 38 µM. The uptake rate of levocetirizine via OAT4 was approximately twice that of racemic cetirizine, indicating stereoselective uptake of levocetirizine. On the other hand, OAT4-mediated [3H]dehydroepiandrosterone sulfate uptake was inhibited by dextrocetirizine and levocetirizine. Overall, our findings indicate that OAT4 mediates levocetirizine uptake but is unlikely to mediate the efflux.

Virtual Clinical Trial Toward Polytherapy Safety Assessment: Combination of Physiologically Based Pharmacokinetic/Pharmacodynamic-Based Modeling and Simulation Approach With Drug-Drug Interactions Involving Terfenadine as an Example.

A Quantitative Systems Pharmacology approach was utilized to predict the cardiac consequences of drug-drug interaction (DDI) at the population level. The Simcyp in vitro-in vivo correlation and physiologically based pharmacokinetic platform was used to predict the pharmacokinetic profile of terfenadine following co-administration of the drug. Electrophysiological effects were simulated using the Cardiac Safety Simulator. The modulation of ion channel activity was dependent on the inhibitory potential of drugs on the main cardiac ion channels and a simulated free heart tissue concentration. ten Tusscher's human ventricular cardiomyocyte model was used to simulate the pseudo-ECG traces and further predict the pharmacodynamic consequences of DDI. Consistent with clinical observations, predicted plasma concentration profiles of terfenadine show considerable intra-subject variability with recorded Cmax values below 5 ng/mL for most virtual subjects. The pharmacokinetic and pharmacodynamic effects of inhibitors were predicted with reasonable accuracy. In all cases, a combination of the physiologically based pharmacokinetic and physiology-based pharmacodynamic models was able to differentiate between the terfenadine alone and terfenadine + inhibitor scenario. The range of QT prolongation was comparable in the clinical and virtual studies. The results indicate that mechanistic in vitro-in vivo correlation can be applied to predict the clinical effects of DDI even without comprehensive knowledge on all mechanisms contributing to the interaction.

BRET-based ß-arrestin2 recruitment to the histamine H1 receptor for investigating antihistamine binding kinetics.

Ligand residence time is thought to be a critical parameter for optimizing the in vivo efficacy of drug candidates. For the histamine H1 receptor (H1R) and other G protein-coupled receptors, the kinetics of ligand binding are typically measured by low throughput radioligand binding experiments using homogenized cell membranes expressing the target receptor. In this study, a real-time proximity assay between H1R and ß-arrestin2 in living cells was established to investigate the dynamics of antihistamine binding to the H1R. No receptor reserve was found for the histamine-induced recruitment of ß-arrestin2 to the H1R and the transiently recruited ß-arrestin2 therefore reflected occupancy of the receptor by histamine. Antihistamines displayed similar kinetic signatures on antagonizing histamine-induced ß-arrestin2 recruitment as compared to displacing radioligand binding from the H1R. This homogeneous functional method unambiguously determined the fifty-fold difference in the dissociation rate constant between mepyramine and the long residence time antihistamines levocetirizine and desloratadine.

Advances in high-resolution MS and hepatocyte models solve a long-standing metabolism challenge: the loratadine story.

BACKGROUND: Loratadine (LOR, Claritin(®)) is a long-acting antihistamine used to treat allergic rhinitis. The major active human metabolite, desloratadine (DL, Clarinex(®)), is extensively metabolized to 3-hydroxydesloratadine (3-OH-DL) (M40) and subsequently glucuronidated before elimination. This study revealed the ability of a novel, long-term hepatocyte micropatterned co-culture (MPCC) model to generate in vivo metabolites. Metabolites were detected and characterized using non-targeted MS/MS(ALL) with SWATH™ acquisition by a UHPLC-Q-TOF system. Results & methodology: Human MPCCs extensively metabolized LOR and formed 3-OH-DL-glucuronide (M13). Cross-species comparisons revealed monkey- and rat-specific metabolites with gender-specific DL-pyridine-N-oxide formation in male rats. These results demonstrate a first for an in vitro hepatocyte model to generate circulating metabolites of LOR and detect species-specific differences. Early focus on human metabolites could have spared characterization of nonhuman metabolites in preclinical species.

Marmoset cytochrome P450 2J2 mainly expressed in small intestines and livers effectively metabolizes human P450 2J2 probe substrates, astemizole and terfenadine.

1. Common marmoset (Callithrix jacchus), a New World Monkey, has potential to be a useful animal model in preclinical studies. However, drug metabolizing properties have not been fully understood due to insufficient information on cytochrome P450 (P450), major drug metabolizing enzymes. 2. Marmoset P450 2J2 cDNA was isolated from marmoset livers. The deduced amino acid sequence showed a high-sequence identity (91%) with cynomolgus monkey and human P450 2J2 enzymes. A phylogenetic tree revealed that marmoset P450 2J2 was evolutionarily closer to cynomolgus monkey and human P450 2J2 enzymes, than P450 2J forms in pigs, rabbits, rats or mice. 3. Marmoset P450 2J2 mRNA was abundantly expressed in the small intestine and liver, and to a lesser extent in the brain, lung and kidney. Immunoblot analysis also showed expression of marmoset P450 2J2 protein in the small intestine and liver. 4. Enzyme assays using marmoset P450 2J2 protein heterologously expressed in Escherichia coli indicated that marmoset P450 2J2 effectively catalyzed astemizole O-demethylation and terfenadine t-butyl hydroxylation, similar to human and cynomolgus monkey P450 2J2 enzymes. 5. These results suggest the functional characteristics of P450 2J2 enzymes are similar among marmosets, cynomolgus monkeys and humans.

Crystal structure of equine serum albumin in complex with cetirizine reveals a novel drug binding site.

Serum albumin (SA) is the main transporter of drugs in mammalian blood plasma. Here, we report the first crystal structure of equine serum albumin (ESA) in complex with antihistamine drug cetirizine at a resolution of 2.1Å. Cetirizine is bound in two sites--a novel drug binding site (CBS1) and the fatty acid binding site 6 (CBS2). Both sites differ from those that have been proposed in multiple reports based on equilibrium dialysis and fluorescence studies for mammalian albumins as cetirizine binding sites. We show that the residues forming the binding pockets in ESA are highly conserved in human serum albumin (HSA), and suggest that binding of cetirizine to HSA will be similar. In support of that hypothesis, we show that the dissociation constants for cetirizine binding to CBS2 in ESA and HSA are identical using tryptophan fluorescence quenching. Presence of lysine and arginine residues that have been previously reported to undergo nonenzymatic glycosylation in CBS1 and CBS2 suggests that cetirizine transport in patients with diabetes could be altered. A review of all available SA structures from the PDB shows that in addition to the novel drug binding site we present here (CBS1), there are two pockets on SA capable of binding drugs that do not overlap with fatty acid binding sites and have not been discussed in published reviews.

Prevalence of Desloratadine Slow-metabolizer Phenotype and Food-dependent Pharmacokinetics of Desloratadine in Healthy Chinese Volunteers.

BACKGROUND AND OBJECTIVES: Desloratadine, the major active metabolite of loratadine, is a non-sedating long-acting antihistamine that is widely used in the treatment of allergic rhinitis and chronic idiopathic urticaria. This study aimed to investigate the prevalence of desloratadine slow-metabolizer (DSM) phenotype and the effects of food on the pharmacokinetics of desloratadine and its active metabolite 3-OH-desloratadine in healthy Chinese volunteers. METHODS: A total of 46 healthy Chinese male volunteers were included in this investigation. All subjects received a single dose of a 5-mg desloratadine tablet under fasting or fed conditions and the plasma concentrations of desloratadine and 3-OH-desloratadine were measured by liquid chromatography-tandem mass spectrometry. The pharmacokinetic profiles were analyzed using a non-compartmental method in the Phoenix WinNonlin program. The individuals with a 3-OH-desloratadine-to-desloratadine exposure ratio lower than 10 % or a desloratadine half-life (t 1/2) of ≥50 h were supposed to be DSM. RESULTS: There was only one DSM among the 46 volunteers, with a prevalence of 2.2 %. Moreover, administration in a fed state resulted in 34.07 and 32.06 % decreases in maximum plasma concentration and area under the concentration-time curve from time zero to infinity for desloratadine and 47.26 and 48.46 % for 3-OH-desloratadine compared with those values under fasting conditions. CONCLUSIONS: Taken together, these results indicated that the incidence of the DSM phenotype in the Chinese population was low and that food intake could significantly decrease the absorption rate and extent of desloratadine.

Similar substrate specificity of cynomolgus monkey cytochrome P450 2C19 to reported human P450 2C counterpart enzymes by evaluation of 89 drug clearances.

Cynomolgus monkeys are used widely in preclinical studies as non-human primate species. The amino acid sequence of cynomolgus monkey cytochrome P450 (P450 or CYP) 2C19 is reportedly highly correlated to that of human CYP2C19 (92%) and CYP2C9 (93%). In the present study, 89 commercially available compounds were screened to find potential substrates for cynomolgus monkey CYP2C19. Of 89 drugs, 34 were metabolically depleted by cynomolgus monkey CYP2C19 with relatively high rates. Among them, 30 compounds have been reported as substrates or inhibitors of, either or both, human CYP2C19 and CYP2C9. Several compounds, including loratadine, showed high selectivity to cynomolgus monkey CYP2C19, and all of these have been reported as human CYP2C19 and/or CYP2C9 substrates. In addition, cynomolgus monkey CYP2C19 formed the same loratadine metabolite as human CYP2C19, descarboethoxyloratadine. These results suggest that cynomolgus monkey CYP2C19 is generally similar to human CYP2C19 and CYP2C9 in its substrate recognition functionality.

Why are second-generation H1-antihistamines minimally sedating?

H1-antihistamines are widely used in treating allergic disorders, e.g., conjunctivitis, urticaria, dermatitis and asthma. The first-generation H1-antihistamines have a much greater sedative effect than the second-generation H1-antihistamines. Researchers could not offer a satisfactory explanations until late 1990s when studies showed that second-generation H1-antihistamines were substrates of P-glycoprotein. P-glycoprotein, expressed in the blood-brain barrier, acts as an efflux pump to decrease the concentration of H1-antihistamines in the brain, which minimizes drug effects on the central nervous system and results in less sedation. P-glycoprotein is found in the apical side of the epithelium. It consists of transmembrane domains that bind substrates/drugs and nucleotide-binding domains that bind and hydrolyze ATP to generate energy for the drug efflux. This review mainly discusses interactions between P-glycoprotein and commonly used second-generation H1-antihistamines. In addition, it describes other possible determining factors of minimal sedating properties of second-generation H1-antihistamines.