Loratadine, an antihistamine, blocks antigen-and ionophore-induced leukotriene release from human lung in vitro

Protective effect of desloratadine against oxidative stress in human erythrocytes in vitro

Desloratadine (DCL) is a non-sedating antihistamine approved for the treatment of allergic rhinitis or chronic idiopathic urticaria. The objective of this study was to evaluate the potential protective effect of DCL against oxidative stress in human erythrocytes in vitro. Human erythrocytes were oxidized by a water-soluble radical generators-2,2' azobis (2-amidinopropane) hydrochloride (AAPH; 20, 50mM) or tert-butyl hydroperoxide (TBHP; 0.5mM) and the protective effects of DCL (2, 5, 7, 10 and 26μM) on selected oxidative stress markers were investigated. Erythrocytes were divided into aliquots. The first aliquot was incubated for 2h at 37°C with AAPH or TBHP. The other test aliquots were preincubated with selected concentrations of DCL for 30min and followed by AAPH or TBHP incubation for 2h. Malondialdehyde (MDA) content, catalase (CAT) and superoxide dismutase (SOD) activities, as well as hemolysis percentage (H) were measured in all erythrocyte samples. The influence of solvent (0.5% ethanol) on the parameters studied was also checked. Pretreatment with DCL (7, 10, 26μM) could prevent TBHP-induced increase in MDA formation in a concentration-dependent manner. DCL has no influence on CAT activity and it significantly enhanced SOD activity compared to AAPH treatment samples at 7, 10, 26μM. DCL (26μM) also reduced the hemolytic effect on erythrocytes when compared to the erythrocytes exposed to oxidants only. These results suggest a beneficial effect of DCL as an antioxidant, which might be an additional explanation of its therapeutic action.

[Rational symptomatic therapy for chronic pruritus]

Chronic pruritus is a difficult-to-treat problem which accompanies many dermatologic and systemic diseases. Frequently, therapy is ineffective and prolongs the pruritus. Thus significant costs arise for the health system, which are clearly lowered by a rational therapy concept. Extensive knowledge about both topical and systemic therapy of pruritus has been acquired in recent years. Currently, new therapeutic approaches are available, which intervene in different pruritogenic mechanisms. Substances can block the cutaneous pruritus induction (e.g. antihistamines, leukotriene antagonists, cannabinoid agonists) or mediate central effects such as the inhibition of pruritus transmission via the spinal cord (e.g. naltrexone, gabapentin) or the suppression of the cerebral itch perception (e.g. antidepressants, neuroleptics). Usually combining several therapeutic principles is necessary in order to suppress chronic pruritus with lasting effect. The most effective approach is to gradually combine therapies, always considering both the medical and financial impacts.

Cannabinoids, loratadine and allopurinol as novel additions to the antipsoriatic ammunition

As the current antipsoriatic medications are commonly associated with deleterious side-effects, a determined search for safer agents, which could be used alone or in combination with current antipsoriatic drugs, would be very imperative. Psoriasis is believed to be characterized by a type 1 cytokine pattern; interferon-gamma, interleukin (IL)-2 and tumour necrosis factor (TNF)-alpha are predominantly expressed in this disorder. Nitric oxide, reactive oxygen species, histamine, leukotriene B4, and decreased [corrected] keratinocyte cyclic adenosine monophosphate/cyclic guanosine monophosphate (cAMP/cGMP) ratio are supposed to play roles in the pathogenesis of this disorder. Based on the immunopathogenesis of psoriasis, this paper introduces three novel, potential treatments for this clinical conundrum: (i) cannabinoids, which exert inhibitory effects on antigen processing and macrophage/T-cell interaction and also on the release of IL-2, TNF-alpha and nitric oxide from immune cells; (ii) loratadine, which is an antihistamine capable of increasing [corrected] the cAMP/cGMP ratio and the production of leukotriene B4; and (iii) allopurinol, which scavenges free radicals, inhibits the production of TNF-alpha, and downregulates the expression of intercellular adhesion molecule-1 and P2X7 receptors on monocytes/macrophages, which are involved in antigen presentation and production of the inflammatory response, respectively. Importantly, allopurinol, especially in combination with cyclosporin, has been shown to be effective against experimental autoimmune uveitis, which, like psoriasis, is a cell-mediated autoimmune disorder.

Histamine: A mediator of inflammation

Histamine and its receptors, including the recently discovered receptors (H(3) and H(4)), novel sources of histamine, and the place of histamine in mediator networks continue to be areas of great interest. The 4 major subtypes of histamine receptors, H(1) to H(4), differ in their location, second messengers, and histamine-binding characteristics. In addition, it would appear that different histamine receptor agonists and antagonists bind to different portions of the receptor complex. A fifth receptor subtype, the intracellular H(IC), has only been defined by its location within cell types that are not traditionally associated with histamine. In airway tissue, most cells express at least 1 subtype of histamine receptor; however, blockade of these receptors does not completely abolish the inflammatory response. In addition, some H(1)-antihistamines might also exert anti-inflammatory effects by pathways independent of H(1)-receptor binding. Studies of selected second-generation H(1)-antihistamines have shown that these agents inhibit the release of certain cytokines from basophils, acting at a point downstream from the calcium signaling pathway. It has not yet been determined whether this action represents a class effect.

New insights into the second generation antihistamines

Second generation antihistamines are recognised as being highly effective treatments for allergy-based disease and are among the most frequently prescribed and safest drugs in the world. However, consideration of the therapeutic index or the benefit/risk ratio of the H1 receptor antagonists is of paramount importance when prescribing this class of compounds as they are used to treat non-life threatening conditions. There are many second generation antihistamines available and at first examination these appear to be comparable in terms of safety and efficacy. However, the newer antihistamines in fact represent a heterogeneous group of compounds, having markedly differing chemical structures, adverse effects, half-life, tissue distribution and metabolism, spectrum of antihistaminic properties, and varying degrees of anti-inflammatory effects. With regard to the latter, there is growing awareness that some of these compounds might represent useful adjunct medications in asthma therapy. In terms of safety issues, the current second generation grouping includes compounds with proven cardiotoxic effects and others with the potential for adverse drug interactions. Moreover, some of the second generation H1 antagonists have given cause for concern regarding their potential to cause a degree of somnolence in some individuals. It can be argued, therefore, that the present second generation grouping is too large and indistinct since this was based primarily on the concept of separating the first generation sedating compounds from nonsedating H1 antagonists. Although it is too early to talk about a third generation grouping of antihistamines, future membership of such a classification could be based on a low volume of distribution coupled with a lack of sedating effects, drug interactions and cardiotoxicity.

Beyond the histamine receptor: effect of antihistamines on mast cells

Many antihistamines exhibit inhibition of mediator release from mast cells and basophils, in in vitro studies in addition to H1 antagonism. The underlying mechanism is unclear but is unrelated to H1-receptor antagonism. Clinical studies of antihistamins in antigen challenge and seasonal allergy demonstrate reduction of mast cell mediators in nasal lavage. It is not known what mechanism(s) underly these observations, although the concentrations required in in vitro studies suggests that a direct effect on mast cells is unlikely. Furthermore, the therapeutic contribution of this effect is difficult to assess because of concomitant clinically significant H1 antagonism. This and other potential anti-allergic effects may enhance the therapeutic benefit of antihistamines and long-term studies are underway to explore this possibility.

In vivo and ex vivo inhibitory effects of loratadine on histamine release in patients with allergic rhinitis

BACKGROUND

The aim of this study was to evaluate the in vivo and ex vivo effects of the H1-antagonist loratadine on histamine release.

METHODS

The study was designed as a double-blind, crossover trial. Ten patients with allergic rhinitis due to Dermatophagoides pteronyssinus were treated with loratadine (10 mg daily p.o.) and with placebo for 1 week, with a 2-week interval between the two treatments. Nasal lavages with saline solution were done before and after challenge with the relevant allergen at the end of treatments with loratadine and placebo. Venous blood was taken after treatments, and basophil histamine release induced by anti-IgE (10 microg/ml), N-formyl-methionyl-leucyl-phenylalanine (fMLP, 1 microM), and Ca2+ ionophore A23187 (1 microM) was evaluated by an automated fluorometric method.

RESULTS

Treatment with loratadine attenuated early antigen-induced nasal obstruction, rhinorrhea, and itching. Nasal symptoms were accompanied by a significant histamine release in the nasal lavages collected 5 min after stimulation when the patients received placebo (median 4 ng/ml, range 1-28; P < 0.05). After treatment with loratadine, histamine release in the 5-min postchallenge lavages was almost abrogated (median 0.5 ng/ml, range 0-3; P < 0.01 vs placebo). Median anti-IgE-induced histamine release from basophils was 41.9% (range 27.8-79.2) after placebo and 30.0% (range 1.7-73.3, P < 0.05) after loratadine. Active treatment exerted an inhibitory effect also on basophil histamine release induced by fMLP and Ca2+ ionophore A23187.

CONCLUSIONS

Treatment for 1 week with loratadine reduces allergen-induced nasal symptoms and inhibits in vivo and ex vivo histamine release in patients with allergic rhinitis.

Changes in skin-test reactivity do not correlate with clinical efficacy of H1-blockers in seasonal allergic rhinitis

New-generation H1-blockers may possess antiallergic properties, and their effect may differ, depending on the target organ. A double-blind, placebo-controlled, parallel-group study was carried out during the pollen season to compare the clinical effect on nasal and conjunctival symptoms of astemizole (10 mg o.d.) and loratadine (10 mg o.d.) with their effect on skin-test reactivity to allergen and histamine. Thirty-eight patients (12-56 years of age) were studied. Nasal and ocular symptoms were recorded daily from days 4 to 7. Skin prick tests with serial concentrations of allergens and one concentration of histamine were carried out before and at the end of the 7-day treatment period. Parallel-line bioassay, analysis of variance, and covariance were used to analyze skin test data. Loratadine and astemizole significantly decreased symptoms from baseline (P < 0.004 and P < 0.006). Skin-test reactivity to allergen and histamine was more profoundly decreased by astemizole than loratadine. The histamine covariant was more important in the allergen effect of astemizole than in that of loratadine. Two H1-blockers having the same clinical effect on nasal and ocular symptoms during the pollen season have totally different effects on skin-test reactivity. Skin-test reactivity to allergen or histamine is not predictive of the clinical efficacy of H1-blockers during seasonal allergic rhinitis.

The effect of cetirizine and loratadine on codeine-induced histamine release in human skin in vivo assessed by cutaneous microdialysis

OBJECTIVE AND DESIGN

To determine whether or not cetirizine and loratadine inhibit codeine- induced histamine release in human skin in vivo, we conducted a placebo-controlled double-blind trial in which histamine release was assessed by dermal microdialysis.

SUBJECTS

A group of ten normal volunteers were studied, each subject visiting the laboratory on three occasions with intervals of at least 2 weeks between visits.

TREATMENT

Cetirizine, loratadine (both 10 mg) or placebo was given orally 4 h before provocation of weal and flare responses in the skin by intradermal injection of 25 microliters of 3 or 10 mg/ml codeine 1 mm from the centre of individual 216 microns diameter microdialysis fibres inserted in the dermis.

METHODS

Dialysate was collected at 2 min intervals for 4 min before and 20 min after codeine injection and histamine assayed spectrofluorometrically. Weal and flare responses to codeine were assessed in the opposite arm.

RESULTS

Histamine concentrations in the microdialysis fibre outflow with 3 and 10 mg/ml codeine were maximal at 2-4 min when 910 +/- 156 and 1194 +/- 304 nM respectively were found in the placebo group. Cetirizine and loratadine did not modify either the kinetics or total histamine release while significantly (p < 0.01) inhibiting weal and flare responses.

CONCLUSIONS

Neither cetirizine nor loratadine inhibited codeine-induced histamine release or modified the time course of its release in human skin in vivo when given in clinically used doses which are sufficient to significantly reduce weal and flare responses.

Antihistamines and production of granulocyte-macrophage colony-stimulating factor and interleukin-8 by human bronchial epithelial cells in vitro: evaluation of the effects of loratadine and cetirizine

1. In this study, we compared the effects of two antihistamine drugs on the production of granulocyte-macrophage colony-stimulating factor and interleukin-8 by human bronchial epithelial cells in vitro. 2. Cytokine production was assessed by the use of an enzyme-linked immunosorbent assay. 3. Epithelial cells spontaneously released both cytokines and tumor necrosis factor alone induced a significant increase in this production but loratadine and cetirizine had no effect at the various concentrations studied. 4. The antihistamines have no effect and this suggests that histamine plays no role in cytokine production under these conditions.

Antiallergic activity of loratadine: inhibition of leukotriene C4 release from human leucocytes

The H1 antagonist loratadine has the capacity to inhibit histamine release from human basophils. The aim of this study was to investigate whether loratadine can also inhibit leukotriene C4 (LTC4) release from human leucocytes. Basophil-enriched mononuclear cell suspensions were prepared by centrifugation of peripheral venous blood (n = 10) on discontinuous Percoll gradients. Leucocytes were stimulated with anti-IgE, N-formylmethionyl-leucyl-phenylalanine (FMLP) and Ca2+ ionophore A23187; immunoreactive (i) LTC4 release in the cell supernatant was measured by a competitive radioimmunoassay and histamine release was evaluated by an automated fluorometric technique. Loratadine, in the concentration range of 1-50 microM, exerted a dose-dependent inhibitory effect on IgE-mediated and IgE-independent histamine and iLTC4 release. The concentrations inhibiting 50% of histamine release were 30 microM (anti-IgE), 27 microM (FMLP) and 19 microM (Ca2+ ionophore A23187). The concentrations inhibiting 50% of iLTC4 release were 2.3 microM (anti-IgE). 11 microM (FMLP) and 1.7 microM (Ca2+ ionophore A23187). The inhibitory activity on iLTC4 release was optimal after preincubation for 2 h at 37 degrees C, and was no longer evident when leucocytes were stimulated 2 h after cell washing. Increased extracellular Ca2+ concentrations reduced the inhibitory activity of loratadine. These results indicate that loratadine has the capacity to inhibit the release of preformed and newly generated mediators from human basophil-enriched mononuclear cell suspensions.

Pharmacologic heterogeneity of human lung and colon cells: effect of terfenadine and cetirizine

H1-blockers may have antiallergic properties which cause the blocking of eicosanoid release, and the effect of these drugs may differ according to the phenotype of mast cells. This study examined the ability of terfenadine and cetirizine to inhibit the release of arachidonic acid-derived mediators from human lung and colon cells. Dispersed cells were challenged with anti-IgE in the presence or absence of 10 microM of terfenadine or cetirizine, and the release of prostaglandin (PG)D2 and leukotriene (LT)C4/D4 was assessed by enzyme immunoassay (EIA). Terfenadine caused significant inhibition of both PGD2 and LTC4/D4 (49 +/- 9 and 29 +/- 19%, respectively) from human lung cells but had a less marked effect on PGD2 release from human colon cells (21 +/- 9% for PGD2 and 18 +/- 9% for LTC4/D4). In contrast, although cetirizine caused significant inhibition of both mediators measured in lung cells (38 +/- 16% for PGD2 and 34 +/- 19% for LTC4), it did not cause any significant inhibition of either mediator from human colon cells. These findings suggest that H1-antagonists may have additional properties, and the differential effects of cetirizine on lung and colon tissue may indicate differences in mast cell phenotype.

Inhibitory activity of loratadine and descarboethoxyloratadine on expression of ICAM-1 and HLA-DR by nasal epithelial cells

Nasal epithelial cells represent the first barrier against noxious agents and allergens. In allergic rhinitis, these cells are activated and histamine may be involved in this activation. Loratadine and one of its active metabolites, descarboethoxyloratadine, were studied for their ability to reduce the activation of nasal epithelial cells by histamine. Nasal turbinates or polyps were removed during surgery from 19 subjects, and nasal epithelial cells were recovered after enzymatic digestion. The in vitro activation of epithelial cells with histamine using an optimal dose (1 microM) and an optimal time (24 h) of incubation was studied, and the effect of loratadine or descarboethoxyloratadine (10 microM) was investigated. The expression of membrane markers (intercellular adhesion molecule-1 (ICAM-1) and a human leukocyte class II antigen (HLA-DR) was assessed by immunocytochemical analysis using an alkaline-antialkaline phosphatase (APAAP) system. The spontaneous expression of both markers was not significantly different in cells recovered from nasal turbinates or polyps, and there was a highly significant increase in the numbers of cells expressing ICAM-1 and HLA-DR following incubation with histamine. Loratadine or descarboethoxyloratadine significantly blocked these effects. This study shows a new possible antiallergic effect of H1-blockers and suggests that their effects on epithelial cells may be relevant in vivo.

Loratadine. A reappraisal of its pharmacological properties and therapeutic use in allergic disorders

Loratadine is a long-acting antihistamine agent, exhibiting partial selectivity for peripheral histamine H1-receptors. To date, loratadine has been evaluated in allergic rhinitis, urticaria and, to a limited extent, in asthma. In several large controlled comparative clinical studies, loratadine was superior to placebo, faster acting than astemizole and as effective as azatadine, cetirizine, chlorpheniramine (chlorphenamine), clemastine, hydroxyzine, mequitazine and terfenadine in patients with allergic rhinitis and chronic urticaria. The clinical effectiveness of loratadine in asthma is at present unclear. Loratadine is well tolerated. At dosages of 10 mg daily, commonly reported adverse events were somnolence, fatigue and headache. Sedation occurred less frequently with loratadine than with azatadine, cetirizine, chlorpheniramine, clemastine and mequitazine. Serious ventricular arrhythmias, as reported with some other second generation histamine H1-receptor antagonists, have not been observed with loratadine to date. Thus, loratadine, with its attributes of once daily administration, fast onset of action and essentially nonsedating properties, would appear to be an appropriate first-line agent for the treatment of allergic rhinitis or urticaria.

In vitro inhibition, by loratadine and descarboxyethoxyloratadine, of histamine release from human basophils, and of histamine release and intracellular calcium fluxes in rat basophilic leukemia cells (RBL-2H3)

The effect of the H1-antihistamine drug loratadine and its active metabolite descarboxyethoxyloratadine upon histamine release was examined on anti-immunoglobulin E (IgE) triggered human basophils and 2,4-dinitrophenyl (DNP) triggered rat basophilic leukemia (RBL-2H3) cells. In both experimental systems, dose-dependent inhibition of histamine release was observed at descarboxyethoxyloratadine and loratadine doses above 2 and 7 microM, respectively. In the RBL-2H3 experimental system, inhibition by loratadine increased when the concentration of extracellular Ca2+ was reduced from 1.8 to 0.45 mM. We further investigated the effect of loratadine and descarboxyethoxyloratadine on the increase in cytosolic calcium concentration (Ca2+)i, an early step in biochemical events leading to exocytosis. The effect of these two drugs upon (Ca2+)i changes was measured using the fluorescent probe fura-2 loaded into RBL-2H3 cells passively sensitized with DNP-specific IgE. Both drugs inhibited, in a dose-dependent manner (2.5-25 microM), the (Ca2+)i rise induced by DNP-BSA challenge in sensitized RBL cells, a process observed in both the presence and absence of extracellular Ca2+. Loratadine also inhibited the Mn2+ influx into these cells, thus reflecting the Ca2+ influx. These results suggest that loratadine and descarboxyethoxyloratadine impair the increase in (Ca2+)i following cell activation by decreasing both the influx of extracellular Ca2+ and the release of Ca2+ from intracellular stores.

Effects of loratadine on cytosolic Ca2+ levels and leukotriene release: novel mechanisms of action independent of the anti-histamine activity

Loratadine, a non-sedating anti-histamine drug, displays in vitro potential anti-allergic properties not related to its interaction with the histamine H1 receptor. In a search for the mechanisms of these actions, we have found that loratadine induces an elevation of cytosolic calcium ion, [Ca2+]i, in rat peritoneal macrophages or human platelets. The mechanism of this elevation resides in the ability of loratadine to discharge intracellular Ca2+ stores, similarly to thapsigargin. This in turn brings about the inhibition of [Ca2+]i rise induced by physiological activators (platelet activating factor and ADP), as well as by thapsigargin. One of the active metabolites of loratadine, descarbo-ethoxy-loratadine, and another anti-histamine, namely terfenadine, exhibit the same effects. In addition, loratadine partially inhibits antigen-induced leukotriene release from human bronchi, but is unable to inhibit the concomitant contraction. We conclude that loratadine can interfere with the mechanisms controlling Ca2+ release, thus inhibiting the cell activation elicited by various agonists through [Ca2+]i elevation. This might be the mechanism underlying its anti-allergic actions in vitro. Furthermore, loratadine might represent an interesting tool in the study of Ca2+ homeostasis.

Inhibition of IgE- and non-IgE-mediated histamine release from human basophil leukocytes in vitro by a histamine H1-antagonist, desethoxycarbonyl-loratadine

Loratadine, a new nonsedating histamine H1-antagonist, has been shown to inhibit immunologic release of inflammatory mediators in addition to its H1-receptor blocking properties. After oral administration, the agent is metabolized primarily to desethoxycarbonyl-loratadine (DCL). The basic piperidine, DCL, is readily soluble in water, whereas the nonbasic urethane, loratadine, is insufficiently soluble in water for some in vitro investigations. Therefore we used the metabolite, DCL, to study its influence on in vitro leukocyte histamine release (LHR) in 24 allergic and 22 nonallergic subjects. IgE-mediated and calcium ionophore A23187-induced LHR were inhibited by DCL in a dose-dependent fashion (values of drug concentration to induce 30% inhibition after stimulation with inhalant antigen, anti-IgE, concanavalin A, and calcium ionophore A23187 were 6, 8, 5, and 11 mumol/L, respectively). Higher concentrations of DCL caused mediator release in all subjects (n = 45, 30 mumol/L DC: 11% +/- 2% LHR, 100 mumol/L DCL: 35% +/- 1% LHR), abolishing any inhibitory effect of the drug. Rapid onset of inhibition by 10 mumol/L DCL was found in kinetic studies (n = 10). The inhibition of anti-IgE-induced histamine secretion was synergistically increased by simultaneous preincubation of DCL with the potent histamine H2-agonist, FRA-19. Additional data indicate that the inhibition of LHR by DCL might involve biochemical events that occur after cellular Ca++ influx because LHR induced by N-formyl-methionyl-leucyl-phenylalanine or the phorbol ester, 12-O-tetradecanoyl phorbol-12-acetate, was not significantly affected by DCL.

Loratadine. A review of recent findings in pharmacology, pharmacokinetics, efficacy, and safety, with a look at its use in combination with pseudoephedrine

Antihistamines are considered first-line therapy for the relief of symptoms from allergic rhinitis and chronic urticaria. The newer, second-generation, nonsedating antihistamines reduce the central nervous system and anticholinergic side effects commonly found with previous drugs. The availability of H1-receptor antagonists that produce therapeutic effects without causing unwanted CNS effects fulfills an important practical need, since these drugs are clearly preferable in patients who drive or operate heavy machinery, or who are involved in activities requiring full alertness. Physicians and patients alike are pleased with the efficacy and safety the second-generation antihistamines bring to the treatment of allergy symptoms. Loratadine is an especially effective second-generation H1-receptor antagonist and is comparable to many of the other second-generation antihistamines. Loratadine may be particularly advantageous because of its low dose and the convenience of once-daily dosing. A more subtle advantage, loratadine's antiallergic properties, may warrant its use for specific treatment situations as future research clarifies the nature of the inflammatory response and the mechanisms of action antiallergic antagonists have in this regard.

Modulation of eicosanoid and histamine release from human dispersed lung cells by terfenadine

Terfenadine is an H1-blocker that may have antiallergic properties. A study was carried out to examine the ability of terfenadine to inhibit the release of histamine and arachidonic-acid-derived mediators from human lung cells. Cells were dispersed from fresh human lung tissue obtained from four accident victims whose hearts were donated for transplantation and four lung cancer resections. Cells were dispersed by enzymatic digestion with type XIV protease and chymopapain, and this resulted in a cell population containing approximately 5% mast cells. The remaining cells were mainly macrophages. The cells were challenged with anti-IgE at a 1/1000 dilution. Cells were challenged without terfenadine and after a preincubation of 0.1, 1, and 10 mumol terfenadine. The release of PGD2 and LTC4/D4 was assessed with an EIA. Histamine was assayed by RIA with a monoclonal antibody against acylated histamine. A release of both eicosanoids and histamine was observed in all experiments. An inhibition of eicosanoids was observed at both 1 and 10 mumol terfenadine (median percentage of inhibition of PGD2: 38.00 +/- 15.65 and 56.00 +/- 13.12; median percentage of inhibition of LTC4/D4: 37.5 +/- 19.80 and 52.5 +/- 26.8). On the other hand, histamine release was not blocked by terfenadine. Terfenadine inhibits, in a dose-dependent manner, the release of eicosanoids after challenge of dispersed lung cells by anti-IgE, and this effect may have some clinical relevance.

Pharmacology of non-sedating H1 antihistamines

A review of the preclinical pharmacology of non-sedating H1 antihistamines includes comparative data on potency, duration of action, side effect liability, especially CNS depressant and anticholinergic activity and new approaches that may further enhance the efficacy of these newer compounds. Data discussed mainly involve studies in which multiple comparative agents were used and usually include astemizole, cetirizine, loratadine and terfenadine. New research approaches include the combination of PAF inhibition with antihistamine activity and the inhibitory effects these compounds may have on cellular influx in allergic diseases.

Loratadine: a nonsedating antihistamine with once-daily dosing

Loratadine is an addition to the class of nonsedating antihistamines which includes terfenadine, astemizole, and acrivastine. Loratadine is chemically related to the tricyclic antidepressants. Animal data have shown that insignificant amounts of loratadine enter the brain, and it has a threefold greater affinity for peripheral as compared with central histamine 1-receptors. Loratadine has one main metabolite, descarbethoxyloratadine, which is four times more active than the parent drug. Loratadine reaches peak plasma concentration in 1-2 hours; the metabolite does so in 3-4 hours. Their respective elimination half-lives are about 10 and 20 hours. Onset of action is within 1 hour and duration is at least 24 hours. Once-daily dosing is recommended. Generally, loratadine is as efficacious as existing antihistamines in relieving symptoms of allergic rhinitis, urticaria, and in suppressing wheal formation. Reports of sedation and other adverse reactions are no more frequent than found with placebo. Tachyphylaxis has not been noted in humans, and there is minimal potential for drug interactions based on animal data. Loratadine and terfenadine have comparable therapeutic applications. Both have shown minimal adverse effects, but loratadine has a quicker onset and longer duration of action. These two agents are useful for acute allergic reactions. In contrast, astemizole has an onset of action of several days and is most useful for prophylactic treatment of seasonal allergies.