Histamine antagonists and asthma

Exercise-induced bronchospasm in the elite athlete

The term exercise-induced bronchospasm (EIB) describes the acute transient airway narrowing that occurs during and most often after exercise in 10 to 50% of elite athletes, depending upon the sport examined. Although multiple factors are unquestionably involved in the EIB response, airway drying caused by a high exercise-ventilation rate is primary in most cases. The severity of this reaction reflects the allergic predisposition of the athlete, the water content of the inspired air, the type and concentration of air pollutants inspired, and the intensity (or ventilation rate) of the exercise. The highest prevalence of EIB is seen in winter-sport populations, where athletes are chronically exposed to cold dry air and/or environmental pollutants found in indoor ice arenas. When airway surface liquid lost during the natural warming and humidification process of respiration is not replenished at a rate equal to the loss, the ensuing osmolarity change stimulates the release of inflammatory mediators and results in bronchospasm; this cascade of events is exacerbated by airway inflammation and airway remodelling. The acute EIB response is characterised by airway smooth muscle contraction, membrane swelling, and/or mucus plug formation. Evidence suggests that histamine, leukotrienes and prostanoids are likely mediators for this response. Although the presence of symptoms and a basic physical examination are marginally effective, objective measures of lung function should be used for accurate and reliable diagnosis of EIB. Diagnosis should include baseline spirometry, followed by an appropriate bronchial provocation test. To date, the best test to confirm EIB may simply be standard pulmonary function testing before and after high-intensity dry air exercise. A 10% post-challenge fall in forced expiratory volume in 1 second is used as diagnostic criteria. The goal of medical intervention is to limit EIB exacerbation and allow the athlete to train and compete symptom free. This is attempted through daily controller medications such as inhaled corticosteroids or by the prophylactic use of medications before exercise. In many cases, EIB is difficult to control. These and other data suggest that EIB in the elite athlete is in contrast with classic asthma.

Dual antagonists of platelet activating factor and histamine. 3. Synthesis, biological activity and conformational implications of substituted N-acyl-bis-arylcycloheptapiperazines

A series of N-acyl-4-(5,6-dihydro-11H-benzo[5,6]cyclohepta[1,2-b]pyridin- 11-ylidene)piperazines is described that are dual antagonists of PAF and histamine. The structural requirements for activity in this series parallel those of their previously reported piperidinylidene counterparts. Whereas their global minimum energy conformations are different for both series of compounds, computer assisted molecular modeling suggests that a common bioactive conformation is possible.

Discovery and preliminary pharmacology of Sch 37370, a dual antagonist of PAF and histamine

From a series of amide analogs of the histamine H1 antagonist, azatadine, a potent, orally active, dual platelet-activating factor (PAF) and histamine antagonist, Sch 37370, namely 1-acetyl-4-(8-chloro-5,6-dihydro-11H-benzo- [5,6]cyclohepta[1,2-b]pyridin-11-ylidine)piperidine, was discovered. Sch 37370 selectively inhibits PAF-induced aggregation of human platelets in vitro (IC50 = 0.6 microM), and in vivo inhibits PAF- and histamine-induced bronchospasm in guinea pigs with ED50 values of 6.0 and 2.4 mg/kg p.o., respectively. Sch 37370 is expected to be more efficacious than single mediator antagonists in allergic diseases, such as asthma.

Effect of the selective PAF antagonist SM-10661 on an asthmatic model. 1. Effect on passive anaphylactic bronchoconstriction in guinea pigs

The effect of SM-10661, a selective antagonist of platelet-activating factor (PAF), on passive anaphylactic bronchoconstriction was examined in guinea pigs. A challenge of ovalbumin to passively sensitized guinea pigs induced bronchoconstriction, which peaked at 4 min. When SM-10661 was administered intravenously 2 min before ovalbumin challenge, bronchoconstriction was inhibited dose-dependently with an ID50 of 68 mg/kg. In guinea pigs pretreated with 15 micrograms/kg mepyramine which is a suboptimal dose, antigen-induced bronchoconstriction peaked at 4-6 min, but was inhibited by SM-10661 with an ID50 of 21 mg/kg. When guinea pigs were pretreated intravenously with 2.5 mg/kg mepyramine, 1 mg/kg indomethacin and 0.01 mg/kg propranolol, the antigen-induced bronchoconstriction peaked at 6 min. SM-10661 inhibited the response with an ID50 of 45 mg/kg. Histamine- and leukotriene D4-induced bronchoconstrictions were unaffected by up to 100 mg/kg SM-10661. Ovalbumin challenge of minced lungs from passively sensitized guinea pigs triggered the release of leukotrienes and histamine. SM-10661 had no effect on the antigen-induced release of peptide leukotrienes or histamine up to 10(-4) M. These results indicate that SM-10661 may be a useful tool to investigate the role of PAF in antigen-induced anaphylactic bronchoconstriction.

Mechanisms of the antiallergic action of N-methylmequitazine (LG 30435)

LG 30435 (N-methylmequitazine) was assayed in passive lung (PLA) and cutaneous (PCA) anaphylaxis in guinea-pigs and rats. At doses from 0.3 to 3 mumol kg-1 i.v., it produced a dose-dependent inhibition of guinea-pig PLA and of rat PCA and PLA, while the parent compound was ineffective or poorly effective up to 3 mumol kg-1. An attempt was made to elucidate the mechanism of LG 30435's action in these anaphylactic models, by means of various antagonists. It was tentatively concluded that different mechanisms are involved in the protective action of LG 30435 in each of the three models: histamine antagonism, possibly accompanied by an inhibition of the effects of peptido-leukotrienes in guinea-pig PLA; histamine antagonism in rat PCA and 5-hydroxytryptamine antagonism in rat PLA, possibly accompanied by a mast-cell stabilizing action in both cases. LG 30435 is devoid of smooth muscle relaxant effects on the airways and its demonstrated anticholinergic and anti-PAF effects do not appear to be involved in its antiallergic action.

Effects of LG 30435 on different platelet activating factor-induced responses

LG 30435 is a new potential anti-asthmatic agent with multiple sites of action. It can inhibit the bronchoconstriction induced by various agonists, including platelet activating factor (PAF). The effects of this compound on PAF-induced biological responses have now been investigated. LG 30435 inhibited both serotonin release from, and aggregation of, rabbit washed platelets evoked by PAF, being four times more potent on release than on aggregation. In contrast, it did not inhibit a number of the biological effects of PAF which are not platelet-mediated, such as contraction of guinea-pig lung parenchymal strips, hypotension in rats and lethality in mice. It may be concluded that, unless platelet PAF receptors are different from those in other tissues, LG 30435 is capable of inhibiting platelet-mediated effects of PAF by interfering with secondary mechanisms activated by this autacoid and not by direct antagonism of PAF at receptor sites.

Pharmacologic analysis of LY188695 (KB-2413), 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)-benzimidazole difumarate, a potent histamine1 receptor antagonist

LY188695 was evaluated both in vitro and in vivo in the guinea pig to determine its pharmacologic profile. The compound antagonized histamine-induced contractions of ileum, aorta, and trachea with pKB values of 9.9, 9.9, and 9.2 respectively. In the lung parenchymal strip, LY188695 caused a rightward shift of the histamine concentration-response curve with a reduction in the maximal response at all antagonist concentrations tested. The reason for this effect is unknown, but it was not due to a nonspecific depressant action of the compound on the parenchyma. Selectivity was shown by its inactivity against leukotriene D4, bradykinin, prostaglandin F2 alpha, acetylcholine, norepinephrine, and serotonin on various guinea pig and rat smooth muscles. Similarly, H2 receptor-mediated relaxation of the rat uterus was unaltered by LY188695. Increases in total pulmonary impedance caused by i.v. histamine to anesthetized guinea pigs were reduced by as little as 3 micrograms/kg given orally 1 hour prior to histamine challenge. In this system, LY188695 was 15 times more potent than chlorpheniramine and 100 times more potent than terfenadine. Similar responses elicited by acetylcholine were not antagonized by LY188695. A duration of action greater than 4 hours was observed in this model. Ovalbumin given i.v. to sensitized guinea pigs increased total pulmonary impedance which was markedly decreased after oral administration of 30 or 100 micrograms/kg LY188695. These results indicate that LY188695 is a very potent antagonist of H1-mediated responses and suggest that this agent might be useful in disease states characterized by an overproduction of histamine.

LG 30435 is a new bronchodilator agent with multiple sites of action

LG 30435, a new quaternary derivative of the H1-histamine antagonist mequitazine, was evaluated against bronchospasm induced by different agonists. This compound inhibited equipotently methacholine- and histamine-induced contractions of isolated guinea-pig trachea. When administered to guinea-pig by the i.v. and/or the aerosol route, LG 30435 inhibited dose dependently the bronchospasm induced by acetylcholine and histamine and to a lesser degree that induced by 5-hydroxytryptamine and LTD4. When compared to that of mequitazine, its potency was higher in each case, up to 500-fold when tested in vitro against methacholine. LG 30435 also counteracted antigen-induced bronchospasm both in passively sensitized guinea-pigs and in actively sensitized rats. This compound had a rapid onset of action and an adequate duration after aerosol administration. It was poorly absorbed both by the oral and the aerosol routes and it did not appear to penetrate the blood-brain barrier. These results show that LG 30435 is a new aerosol bronchodilator agent, which, due to its multiple pharmacodynamic actions and to its suitable pharmacokinetics, is potentially useful in the therapy of obstructive airways disease.

Effects of long-term treatment with low dose cimetidine on allergen-induced airway responses and selected immunological parameters in atopic asthmatics

Twenty asymptomatic atopic asthmatics were treated with either cimetidine 100 mg orally (13 patients) or placebo (7 patients) once a day for 4 weeks. Bronchial challenges were performed with the pertinent allergen immediately before and 2 and 4 weeks after the initiation of treatment and, finally, 4 weeks after the cessation of treatment. Before each challenge blood was drawn for the determination of specific IgE antibody levels (RAST procedure) and total IgE (PRIST), allergen- and anti-IgE-induced basophil histamine release, and mitogen-induced lymphocyte (3H)-thymidine incorporation. Patients treated with cimetidine were found to be significantly (P less than 0.05) less responsive to bronchial allergen challenge during the treatment than before it; patients treated with placebo were more reactive (P less than 0.05) 14 days after the initiation of treatment. The difference in responsiveness to treatment between the placebo and the cimetidine groups was significant 14 days (P less than 0.01) and 4 weeks (P less than 0.05) after the initiation of treatment; no significant difference in allergen responsiveness was recorded between the groups 1 month after cessation of treatment. No clear-cut changes in specific IgE antibody or total IgE levels, histamine release capacity, or mitogen-induced lymphocyte responsiveness were observed in either group, except that lymphocytes from cimetidine-treated patients tended to show an increased ratio of PHA- to PMA-induced thymidine incorporation. Thus, it was found that the treatment of asymptomatic atopic asthmatics with low-dose cimetidine reduced their allergen sensitivity in bronchial provocation tests by a mechanism which remains to be elucidated.