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.

Molecular Origin of the Distinct Tabletability of Loratadine and Desloratadine: Role of the Bonding Area - Bonding Strength Interplay.

PURPOSE: To explain the different tabletability of two structurally similar H1-receptor antihistamine drugs, loratadine (LOR) and desloratadine (DES), based on the molecular basis of bonding area and bonding strength. METHODS: LOR and DES were characterized by powder X-ray diffractometry, thermal analysis, and dynamic water sorption. The compressibility, tabletability, compactibility, and Heckel analysis of their bulk powders and formulations were evaluated. A combined energy framework and topological analysis was used to characterize the crystal structure - mechanical property relationship. Surface energy of bulk powder was assessed by contact angle measurement using the Owens/Wendt theory. RESULTS: Both LOR and DES bulk powders are phase pure and stable under compaction. The superior tabletability of LOR is attributed to both larger bonding area (BA) and higher interparticle bonding strength (BS). The larger BA of LOR results from its experimentally established higher plasticity, which is explained by the presence of more densely packed molecular layers with smooth surface topology. The higher BS of LOR corresponded to its significantly higher dispersive component of the surface energy. CONCLUSIONS: This work provides new insights into the molecular origins of BA and BS, which can be applied to improve mechanical properties and tableting performance of drugs through appropriate crystal engineering.

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.

Late-Stage Lead Diversification Coupled with Quantitative Nuclear Magnetic Resonance Spectroscopy to Identify New Structure-Activity Relationship Vectors at Nanomole-Scale Synthesis: Application to Loratadine, a Human Histamine H1 Receptor Inverse Agonist.

An experimental approach is described for late-stage lead diversification of frontrunner drug candidates using nanomole-scale amounts of lead compounds for structure-activity relationship development. The process utilizes C-H bond activation methods to explore chemical space by transforming candidates into newly functionalized leads. A key to success is the utilization of microcryoprobe nuclear magnetic resonance (NMR) spectroscopy, which permits the use of low amounts of lead compounds (1-5 µmol). The approach delivers multiple analogues from a single lead at nanomole-scale amounts as DMSO-d6 stock solutions with a known structure and concentration for in vitro pharmacology and absorption, distribution, metabolism, and excretion testing. To demonstrate the feasibility of this approach, we have used the antihistamine agent loratadine (1). Twenty-six analogues of loratadine were isolated and fully characterized by NMR. Informative SAR analogues were identified, which display potent affinity for the human histamine H1 receptor and improved metabolic stability.

Development of medicated chewing gum of taste masked levocetirizine dihydrochloride using different gum bases: in vitro and in vivo evaluation.

Objective: The aim of this study was to develop medicated chewing gum (MCG) formulation for taste-masked levocetirizine dihydrochloride (LCZ) that can provide fast drug release into the salivary fluid.Methods: Taste-masked LCZ was first prepared by two methods: cyclodextrin complexation using Kleptose or Captisol and formation of drug resin complex using Kyron T-154 or Kyron T-314 to overcome poor LCZ palatability. MCGs were then prepared using the taste-masked drug, gum base (Artica-T, Chicle, or Health In Gum (HIG), plasticizer (glycerol or soy lecithin at 6 or 8% of the final gum weight). The developed MCGs were evaluated for physical properties, content uniformity, and drug release. Best release MCGs were evaluated thermally to investigate the plasticizer effectiveness and for ex vivo chew out study to confirm adequate drug release. Drug bioavailability was determined for selected formula compared to commercial tablets.Results: Based on taste-masking efficiency, drug/Kleptose complex (1:3 molar ratio) was chosen for incorporation into chewing gums. Physical properties and drug release showed that gum base type, plasticizer type, and level affected not only physical properties but also drug release from MCGs. Thermal study showed decreased glass transition temperature (Tg) with increased plasticizer level. Chew out study confirmed almost complete drug release after a few minutes of chewing. Pharmacokinetic results showed shorter tmax (0.585 vs. 1.375 h) and higher Cmax (0.113 vs. 0.0765 µg/mL) for MCGs than conventional tablets.Conclusion: Results provided evidence that MCGs could be a better alternative to conventional tablet formulations with improved bioavailability and enhanced palatability.

Antihistamines for Allergic Rhinitis Treatment from the Viewpoint of Nonsedative Properties.

Antihistamines targeting the histamine H1 receptor play an important role in improving and maintaining the quality of life of patients with allergic rhinitis. For more effective and safer use of second-generation drugs, which are recommended by various guidelines, a classification based on their detailed characteristics is necessary. Antihistamines for first-line therapy should not have central depressant/sedative activities. Sedative properties (drowsiness and impaired performance) are associated with the inhibition of central histamine neurons. Brain H1 receptor occupancy (H1RO) is a useful index shown to be correlated with indices based on clinical findings. Antihistamines are classified into non-sedating (<20%), less-sedating (20⁻50%), and sedating (≥50%) groups based on H1RO. Among the non-sedating group, fexofenadine and bilastine are classified into "non-brain-penetrating antihistamines" based on the H1RO. These two drugs have many common chemical properties. However, bilastine has more potent binding affinity to the H1 receptor, and its action tends to last longer. In well-controlled studies using objective indices, bilastine does not affect psychomotor or driving performance even at twice the usual dose (20 mg). Upon selecting antihistamines for allergic rhinitis, various situations should be taken into our consideration. This review summarizes that the non-brain-penetrating antihistamines should be chosen for the first-line therapy of mild allergic rhinitis.

Report: A comparative study of loratidine physiochemical properties from different brands.

Loratidine is a piperidine derivative resemble to azatadine long acting non sedating commonly used for the treatment of allergic condition like watery or itchy eyes, runny nose, chronic urticaria or throat itching. In the present study different brands of loratidine were evaluated for the weight variation, hardness, friability, disintegration time and dissolution. Dissolution release study performed by using paddle method (USP 2) in 900 ml of 0.1N HCl at 50 rpm. The physicochemical of loratidine did not give any variation. By this study we conclude that all parameter for physicochemical properties like weight variation, hardness of tablets, friability, their disintegration time and the dissolution release study for all the brands of loratidine that are available in Karachi meet the British pharmacopoeia (BP) and United State pharmacopoeia (USP) specification for quality control analysis.Weight variation, hardness and friability value requirement was complied by all brands .Disintegration time for all brands was less than 15 minutes complying the BP/USP recommendation. All brands showed more than 80% drug release within 45 minutes. The present findings suggest that almost all the brands of loratidine meet the BP/USP specification for QC analysis.

A Novel Surfactant-free Lipid-based Formulation for Improving Oral Bioavailability of Loratadine Using Colloidal Silicon Dioxide as Emulsifier and Solid Carrier.

BACKGROUND: The purpose of this study was to develop an innovative surfactant-free lipidbased formulation (LF) for improving oral bioavailability of loratadine based on using solid particles colloidal silicon dioxide (CSD) as emulsifier and solid carrier. METHODS: Loratadine was dissolved in oil solution with the aid of co-solvent and LF formulations were prepared by a simple adsorption and milling technique. The LF Powder was evaluated in terms of angle of repose and X-ray powder diffraction. After dispersing and emulsifying in water, the particle size and morphology were also characterized. In vitro dissolution and pharmacokinetic behavior in vivo were also studied. RESULTS: Orthogonal design indicated that the amount of CSD in formulations had a major and significant influence on emulsification. The optimal formulation showed LF with good flowability and without crystallization or deposition of loratadine in it. CONCLUSION: After dispersing in water, an emulsion with the mean droplet size of 1.2µm was obtained. Although the dissolution of drug from LF was slower in vitro in acidic aqueous solution, pharmacokinetic studies in vivo showed that the bioavailability of loratadine increased 2.49-fold by CF compared to a commercial tablet.

Improving mechanical properties of desloratadine via multicomponent crystal formation.

We report the first multicomponent crystal of desloratadine, an important anti-histamine drug, with a pharmaceutically acceptable coformer of benzoic acid. The single crystal structure analysis revealed that this novel multicomponent crystal is categorized as salt due to the proton transfer from benzoic acid to the desloratadine molecule. By forming the salt multicomponent crystal, we demonstrated that the tabletability and plasticity of the multicomponent crystal was improved from the parent drug. In addition, neither capping nor lamination tendency was observed in the desloratadine-benzoic acid multicomponent crystal. The existence of a layered structure and slip planes are proposed to be associated with this improvement. The desloratadine-benzoate in this case shows an improved solubility in water and HCl 0.1N media and a better dissolution profile in water. However, the dissolution rate in HCl 0.1N media was found to be essentially indifference.

Original research paper. Characterization and taste masking evaluation of microparticles with cetirizine dihydrochloride and methacrylate-based copolymer obtained by spray drying.

Taste of a pharmaceutical formulation is an important parameter for the effectiveness of pharmacotherapy. Cetirizine dihydrochloride (CET) is a second-generation antihistamine that is commonly administered in allergy treatment. CET is characterized by extremely bitter taste and it is a great challenge to successfully mask its taste; therefore the goal of this work was to formulate and characterize the microparticles obtained by the spray drying method with CET and poly(butyl methacrylate-co-(2-dimethylaminoethyl) methacrylate-co-methyl methacrylate 1:2:1 copolymer (Eudragit E PO) as a barrier coating. Assessment of taste masking by the electronic tongue has revealed that designed formulations created an effective taste masking barrier. Taste masking effect was also confirmed by the in vivo model and the in vitro release profile of CET. Obtained data have shown that microparticles with a drug/polymer ratio (0.5:1) are promising CET carriers with efficient taste masking potential and might be further used in designing orodispersible dosage forms with CET.

PREPARATION AND CHARACTERIZATION OF ORALLY DISINTEGRATING LORATADINE TABLETS MANUFACTURED WITH CO-PROCESSED MIXTURES.

The aim of this study was to develop orally disintegrated tablets (ODT) with loratadine using Parteck ODT and Ludiflash--new commercially available tableting excipients based on co-processed mannitol. ODT containing loratadine were prepared with 3% addition of various superdisintegrants (AcDiSol, Kollidon CL-F and Kollidon CL-SF) by direct compression method. Obtained tablets were characterized for friability, pore structure, and wetting and disintegration time measured by four independents methods. In order to identify possible interactions between loratadine and the excipients, differential scanning calorimetry was used. The results showed that all formulated ODT were characterized by appropriate mechanical properties (friability < 1%), the uniform content of the drug substance and pleasant mouth feeling. Disintegration time below 30 s was observed in formulations with crospovidones as disintegrant.

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.

Determination of Some Non-sedating Antihistamines via Their Native Fluorescence and Derivation of Some Quantitative Fluorescence Intensity - Structure Relationships.

A validated simple, novel, and rapid spectrofluorimetric method was developed for the determination of some non-sedating antihistamines (NSAs); namely cetirizine (CTZ), ebastine (EBS), fexofenadine (FXD), and loratadine (LOR). The method is based on measuring the native fluorescence of the cited drugs after protonation in acidic media and studying their quantitative fluorescence intensity - structure relationships. There was a linear relationship between the relative fluorescence intensity and the concentration of the investigated drug. Under the optimal conditions, the linear ranges of calibration curves for the determination of the studied NSAs were 0.10-2.0, 0.20-6.0, and 0.02-1.0 [Formula: see text] for (CTZ, FXD), (EBS), and (LOR); respectively. The factors affecting the protonation of the studied drugs were carefully studied and optimized. The method was validated according to ICH guidelines. The suggested method is applicable for the determination of the four investigated drugs in bulk and pharmaceutical dosage forms with excellent recoveries (97.67-103.80%). Quantitative relationships were found between the relative fluorescence intensities of the protonated drugs and their physicochemical parameters namely: the pKa, log P, connectivity indexes (χ(v)) and their squares. Regression equations (76) were obtained and not previously reported. Six of these equations were highly significant and used for the prediction of RFI of the studied NSAs.

Novel levocetirizine HCl tablets with enhanced palatability: synergistic effect of combining taste modifiers and effervescence technique.

OBJECTIVES: Levocetirizine HCl, a second-generation piperazine derivative and H1-selective antihistaminic agent, possesses few side effects. The first objective of the study was to compare and evaluate the taste-masking effect of different ratios of 2-hydroxypropyl-ß-cyclodextrin and mannitol on levocetirizine HCl using an inclusion complex and solid dispersion, respectively. The second objective was to study the possibility of preparing and evaluating effervescent tablets from the best-chosen taste-masked blends for the purpose of their use either as orodispersible tablets or as water-soluble effervescent tablets, according to patients' will. MATERIALS AND METHODS: Prepared taste-masked blends were prepared and subjected to palatability, Fourier-transform infrared spectroscopy, and differential scanning calorimetry studies. Tablets containing different percentages of effervescent mixtures were prepared by direct compression on the selected taste-modified blends. Evaluation tests were conducted, including flowability and compressibility on the precompressed blends and hardness, friability, wetting time, effervescent time, in vitro, in vivo disintegration time, and in vitro dissolution study on the compressed tablets. Formulated tablets were evaluated and compared to marketed orodispersible tablets for mouth feel and palatability. RESULTS: All prepared tablets showed convenient physical and palatability properties compared to the selected brand. The in vitro drug-release study revealed fast release of levocetirizine HCl within 5 minutes from all prepared tablets. CONCLUSION: Levocetirizine HCl effervescent tablets are likely to increase patient compliance with drug administration. Moreover, the use of these effervescent tablets in an orodispersible dosage form can improve oral drug bioavailability and act as an attractive pediatric dosage form.

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.

Pharmaceutical equivalent studies of some commercially available brands of Loratadine hydrochloride tablets.

BACKGROUND: This study was undertaken with the objective of evaluating the pharmaceutical and chemical equivalence of some commercially available loratadine tablets, and offers a possible explanation for the therapeutic failure of the drug products. METHOD: The equivalence of eight brands of loratadine hydrochloride tablets labelled A to H was assessed and compared with the Innovator brand labelled I. Visual observation and uniformity of weight tests were carried out on the tablets, mechanical properties were assessed using friability and crushing strength tests as parameters. Release properties of the tablets were assessed by disintegration and dissolution tests. Assay was based on non-aqueous titration procedure using crystal violet solution indicator. RESULT: All the brands studied were white in colour with different shapes and lustre, and complied with the official specification for uniformity of tablet weight. Friability tests showed that only brand G lost more than 1% of its initial weight, while brands A and E failed the crushing strength test. Brand C did not undergo complete disintegration within 15 minutes, while brands A, B, F and G had less than 70% of the active drug content still in solution after 45 minutes. Two of the brands had active drug content between officially specified range of 98.5% and 101.5% for loratadine tablets. CONCLUSION: There was a large variation in the pharmaceutical properties of the commercially available loratadine hydrochloride tablets that were selected for this study. Six of the brands evaluated exhibited poor pharmaceutical properties. Generally, only two of the brands were pharmaceutically equivalent with the innovator brand.

Statistical optimization of controlled release microspheres containing cetirizine hydrochloride as a model for water soluble drugs.

The purpose was to improve the encapsulation efficiency of cetirizine hydrochloride (CTZ) microspheres as a model for water soluble drugs and control its release by applying response surface methodology. A 3(3) Box-Behnken design was used to determine the effect of drug/polymer ratio (X1), surfactant concentration (X2) and stirring speed (X3), on the mean particle size (Y1), percentage encapsulation efficiency (Y2) and cumulative percent drug released for 12 h (Y3). Emulsion solvent evaporation (ESE) technique was applied utilizing Eudragit RS100 as coating polymer and span 80 as surfactant. All formulations were evaluated for micromeritic properties and morphologically characterized by scanning electron microscopy (SEM). The relative bioavailability of the optimized microspheres was compared with CTZ marketed product after oral administration on healthy human volunteers using a double blind, randomized, cross-over design. The results revealed that the mean particle sizes of the microspheres ranged from 62 to 348 µm and the efficiency of entrapment ranged from 36.3% to 70.1%. The optimized CTZ microspheres exhibited a slow and controlled release over 12 h. The pharmacokinetic data of optimized CTZ microspheres showed prolonged tmax, decreased Cmax and AUC0-∞ value of 3309 ± 211 ng h/ml indicating improved relative bioavailability by 169.4% compared with marketed tablets.

Determination of stress-induced degradation products of cetirizine dihydrochloride by a stability-indicating RP-HPLC method.

BACKGROUND: A new, simple and accurate stability-indicating reverse phase high performance liquid chromatography method was developed and validated during the early stage of drug development of an oral lyophilizate dosage form of cetirizine dihydrochloride. METHODS: For RP-HPLC analysis it was used an Eclipse XDB C8 column 150 mm × 4.6 mm, 5 µm (Agilent columns, Barcelona, Spain) as the stationary phase with a mobile phase consisted of a mixture of 0.2 M K2HPO4 pH 7.00 and acetonitrile (65:35, v/v) at a flow rate of 1 mL min (-1). Detection was performed at 230 nm using diode array detector. The method was validated in accordance with ICH guidelines with respect to linearity, accuracy, precision, specificity, limit of detection and quantification. RESULTS: The method results in excellent separation between the drug substance and its stress-induced degradation products. The peak purity factor is >950 for the drug substance after all types of stress, which confirms the complete separation of the drug substance peak from its stress induced degradation products. Regression analysis showed r(2) > 0.999 for cetirizine dihydrochloride in the concentration range of 650 µg mL (-1) to 350 µg mL(-1) for drug substance assay and a r(2) > 0.999 in the concentration range of 0.25 µg mL(-1) to 5 µg mL(-1) for degradation products. The method presents a limit of detection of 0.056 µg mL (-1) and a limit of quantification of 0.25 µg mL(-1). The obtained results for precision and accuracy for drug substance and degradation products are within the specifications established for the validation of the method. CONCLUSIONS: The proposed stability-indicating method developed in the early phase of drug development proved to be a simple, sensitive, accurate, precise, reproducible and therefore useful for the following stages of the cetirizine dihydrochloride oral lyophilizate dosage form development.

Loratadine and analogues: discovery and preliminary structure-activity relationship of inhibitors of the amino acid transporter B(0)AT2.

B(0)AT2, encoded by the SLC6A15 gene, is a transporter for neutral amino acids that has recently been implicated in mood and metabolic disorders. It is predominantly expressed in the brain, but little is otherwise known about its function. To identify inhibitors for this transporter, we screened a library of 3133 different bioactive compounds. Loratadine, a clinically used histamine H1 receptor antagonist, was identified as a selective inhibitor of B(0)AT2 with an IC50 of 4 µM while being less active or inactive against several other members of the SLC6 family. Reversible inhibition of B(0)AT2 was confirmed by electrophysiology. A series of loratadine analogues were synthesized to gain insight into the structure-activity relationships. Our studies provide the first chemical tool for B(0)AT2.