Pharmacological assessment of the duration of action of glycopyrrolate vs tiotropium and ipratropium in guinea-pig and human airways

Our study was aimed at investigating the duration of the bronchodilator action of the antimuscarinc drug glycopyrrolate compared to tiotropium and ipratropium. In the guinea-pig isolated trachea, the time (t1/2) necessary for a contractile response to carbachol (0.3 μM) to return to 50% recovery after washout of the antagonist was studied. The offset of the antagonist effect of glycopyrrolate, tiotropium and ipratropium (10 nM each) was t1/2=4.0±0.5, >4.5 and 0.5±0.1 h, respectively. At 4.5 h from the washout of the antagonist, the recovery of the response to carbachol was 50±8, 10±4 and 70±7%, respectively. In the human isolated bronchus, the offset of the bronchodilator effects of glycopyrrolate (3 nM), tiotropium (1 nM) and ipratropium (10 nM) was t1/2=3.7±0.2; >6 and 3.0±0.2 h, respectively. At 6.0 h from the washout of the antagonist, the recovery of the response to carbachol (1 μM) was 101±10, 27±3 and 110±10%, respectively. In anaesthetized guinea-pigs, acetylcholine-induced bronchoconstriction was markedly reduced by intratracheal instillation of glycopyrrolate (3 nmol kg−1; 88.1±4% inhibition), tiotropium (1.3 nmol kg−1; 86.2±5% inhibition) or ipratropium (1.45 nmol kg−1; 88.1±10% inhibition). These inhibitory effects assessed 3 or 24 h after antagonist administration were reduced to 69.9±5 and 29.7±6%; 28.3±5 and 14.2±5% for glycopyrrolate and ipratropium, respectively, whereas they remained stable (83.5±4; 70.6±6) for tiotropium. The residual inhibitory effect of glycopyrrolate was also assessed at 16 h from administration, and proved to be as low as that found at 24 h (31.2±10 vs 29.7±6%, respectively). In conclusion, glycopyrrolate-induced bronchodilation has a longer duration than that of ipratropium, but less than that of tiotropium. The efficacy of a possible glycopyrrolate-based therapy for asthma or chronic obstructive pulmonary disease given once-a-day is not guaranteed by the present investigation. Keywords: Antimuscarinic drugs, bronchodilation, glycopyrrolate, tiotropium, ipratropium, human bronchus, COPD Introduction Chronic obstructive pulmonary disease (COPD) is a syndrome characterized by progressive airflow limitation caused by chronic inflammation of the airways and lung parenchyma, which is caused predominantly by chronic cigarette smoking. The current treatment of COPD is palliative, as there is no available therapy capable of halting the decline in lung function and the parenchymal destruction associated with the disease (Krishna et al., 2004; Belvisi et al., 2004 for a review). Bronchodilators are the mainstay therapy in COPD, as they increase expiratory flow by decreasing airway smooth muscle tone, thus leading to enhanced lung emptying. In asthma and COPD, bronchoconstriction and mucus secretion are increased and the airways become hyperresponsive to contractile agents. In COPD, these changes are caused mostly by increased parasympathetic nerve activity. Acetylcholine (ACh) released from parasympathetic nerves activates postjunctional muscarinic M3 receptors present on airway smooth muscle and submucosal glands to produce bronchoconstriction and mucus secretion, respectively. ACh also feeds back onto prejunctional muscarinic M2 receptors to inhibit further ACh release (Haddad & Rousell, 1998; Barnes, 2004b). Anticholinergics are the most effective class of bronchodilators in COPD, more effective than β2 receptor agonists which, in contrast, are very potent in asthma where bronchoconstriction is sustained by a variety of mediators (Gross & Skorodin, 1984; Rennard et al., 1996; Barnes 2004a). Anticholinergics that are used more frequently in COPD are quaternary ammonium salts like ipratropium bromide (Gross, 1988), oxitropium bromide (Skorodin et al., 1986) and tiotropium bromide (Disse et al., 1993). The introduction of tiotropium bromide has represented a breakthrough in the pharmacological management of the COPD. Tiotropium bromide possesses higher affinity for muscarinic receptors than ipratropium and, more importantly, it dissociates very slowly from M3 muscarinic receptors (Disse et al., 1993; Haddad et al., 1994), a behavior thought to be responsible for the prolonged action of tiotropium. Accordingly, high potency and long-lasting duration of action of tiotropium were demonstrated in functional studies conducted in guinea-pig and human isolated airways (Takahashi et al., 1994) and in anaesthetized dogs (Disse et al., 1993). As a consequence, the long duration of action of tiotropium allows for once-a-day administration of this drug in human therapy (Littner et al., 2000; Gross, 2004). Another well-known quaternary ammonium compound endowed with potent antimuscarinic activity is glycopyrrolate. Glycopyrrolate is currently used as gastric antisecretory agent, to reduce salivary secretions, and in preanaesthetic medication (Mirakhur & Dundee, 1981; Muir & von Gunten, 2000; Tscheng, 2002). In addition, pilot clinical studies have shown that inhaled glycopyrrolate displays bronchodilator activity in asthmatic and COPD patients, an effect apparently lasting for (at least) 8–12 h (Walker et al., 1987; Schroeckenstein et al., 1988; Tzelepis et al., 1996). However, limited preclinical information is available concerning the duration of action of glycopyrrolate as a bronchodilator, thus it is difficult to estimate whether a future glycopyrrolate-based therapy for COPD could be administered once-daily, or instead would require more frequent administrations. Indeed, in the sole preclinical evaluation of the duration of antibronchospastic action of glycopyrrolate, performed in guinea-pig and human isolated airways, the inhibitory effects produced by this antagonist were studied for only 100–120 min (Haddad et al., 1999). Thus, in the present investigation we have studied the duration of action of the bronchodilator activity of glycopyrrolate in comparison with tiotropium and ipratropium up to 4.5–6.0 h from its administration in the isolated guinea-pig trachea and human bronchus, and up to 24 h in guinea-pig in vivo.