Eur Respir J, 1995, 8, 637–642 Copyright ERS Journals Ltd 1995 DOI: 10.1183/09031936.95.08040637 European Respiratory Journal Printed in UK - all rights reserved ISSN 0903 - 1936 SERIES 'THEOPHYLLINE AND PHOSPHODIESTERASE INHIBITORS' Edited by M. Aubier and P.J. Barnes Theophylline and selective PDE inhibitors as bronchodilators and smooth muscle relaxants K.F. Rabe, H. Magnussen, G. Dent Theophylline and selective PDE inhibitors as bronchodilators and smooth muscle relaxants. Krankenhaus Grosshansdorf, Zentrum für K.F. Rabe, H. Magnussen, G. Dent. ERS Journals Ltd 1995. Pneumologie und Thoraxchirurgie, LVA ABSTRACT: In addition to its emerging immunomodulatory properties, theophy- Hamburg, Grosshansdorf, Germany. lline is a bronchodilator and also decreases mean pulmonary arterial pressure in vivo. The mechanism of action of this drug remains controversial; adenosine Correspondence: K.F. Rabe Krankenhaus Grosshansdorf antagonism, phosphodiesterase (PDE) inhibition and other actions have been advanced Wöhrendamm 80 to explain its effectiveness in asthma. Cyclic adenosine monophosphate (AMP) and D-22927 Grosshansdorf cyclic guanosine monophosphate (GMP) are involved in the regulation of smooth Germany muscle tone, and the breakdown of these nucleotides is catalysed by multiple PDE isoenzymes. The PDE isoenzymes present in human bronchus and pulmonary artery Keywords: Bronchi have been identified, and the pharmacological actions of inhibitors of these enzy- 3',5'-cyclic-nucleotide phosphodiesterase mes have been investigated. phosphodiesterase inhibitors Human bronchus and pulmonary arteries are relaxed by theophylline and by pulmonary artery selective inhibitors of PDE III, while PDE IV inhibitors also relax precontracted smooth muscle theophylline bronchus and PDE V/I inhibitors relax pulmonary artery. There appears to be some synergy between inhibitors of PDE III and PDE IV in relaxing bronchus, and Received: February 1 1995 a pronounced synergy between PDE III and PDE V inhibitors in relaxing pulmon- Accepted for publication February 1 1995 ary artery. In neither tissue does 8-phenyltheophylline, a xanthine exhibiting adenosine antagonism but not PDE inhibition, cause any significant relaxation, implying that theophylline does not exert its actions through adenosine antagonism. The close correspondence of theophylline concentrations inhibiting bronchus or pulmonary artery PDE and those causing relaxation points towards PDE inhibition as the major mechanism of action of theophylline in smooth muscle relaxation. Eur Respir J., 1995, 8, 637–642. For several years, theophylline (1,3-dimethylxanthine) induced bronchoconstriction [14]. Theophylline also and other xanthine derivatives have been recognized as decreases mean pulmonary arterial pressure [16–18], effective agents for the treatment of reversible and chronic implying a relaxation both of airway and vascular smooth obstructive airways diseases [1, 2]. The introduction of muscle. Various mechanisms have been proposed for slow-release formulations and the development of im- this action, of which inhibition of cyclic nucleotide proved monitoring techniques for the avoidance of toxicity phosphodiesterase (PDE) is the most widely accepted. have led to increased use of theophylline in the clinic, In this article, the functional role of cyclic nucleotides whilst extensive laboratory investigations have increa- and PDE in regulation of smooth muscle tone are sed our understanding of the mechanisms through which summarized, and the actions both of theophylline and of the drug exerts its actions. selective inhibitors of PDE isoenzymes on airway and Evidence is emerging that theophylline suppresses some pulmonary vascular smooth muscle are described. aspects of the inflammatory response underlying asthma [3, 4], but the basis for the drug's therapeutic use in this Role of cyclic nucleotides in regulation of disease, until now, has been its bronchodilatory action. smooth muscle tone Orally or intravenously administered theophylline or aminophylline improve forced expiratory volume in one Several neurotransmitters, hormones and autacoids, β second (FEV1) in patients with chronic obstructive as well as such drugs as -adrenoceptor agonists, activate pulmonary disease (COPD) [2, 5–11] or asthma [1, 12, cell surface receptors which, through stimulation of the 13], whilst intravenous theophylline also protects against enzyme adenylyl cyclase, cause increased production of bronchoconstriction induced by methacholine, histamine cyclic adenosine 3',5'-monophosphate (cAMP). PDE, a or exercise in asthmatic patients [14, 15]. The actions group of enzymes catalysing the breakdown of cAMP of theophylline in asthmatics are clearly dose-related [12, and/or cyclic guanosine 3',5'-monophosphate (cGMP), 14], and, interestingly, the protective effect is more pro- presents a second target for regulation of intracellular nounced against histamine-induced than methacholine- cAMP levels. 638 K.F. RABE, H. MAGNUSSEN, G. DENT In smooth muscle, cAMP activates two cyclic nucleo- cAMP hydrolysis by PDE in homogenates of bronchial tide-dependent protein kinases, protein kinase A ("cAMP- tissue (fig. 2). Theophylline appears to display no se- dependent") and protein kinase G ("cGMP-dependent") lectivity for particular PDE isoenzyme families, and the [19, 20], which phosphorylate a range of proteins in- PDE inhibition curve is essentially monophasic. A simi- volved in the induction and maintenance of contraction. lar pattern of inhibition of cAMP hydrolysis is obser- Thus, intracellular calcium concentration ([Ca2+]i) is ved with zardaverine, a dual inhibitor of the PDE III and lowered, through the increased activity of Ca2+-adenosine PDE IV isoenzymes (table 1) that account for most of triphosphatase (ATPase) and inhibition of Ca2+ channel the cAMP-hydrolysing PDE in bronchus [22]. The poten- opening, whilst large conductance Ca2+-activated K+ cy of theophylline in relaxing bronchial smooth muscle channels are opened and phosphatidylinositol metabo- is independent of the size of the airways [26–28], a lism and myosin light chain kinase activity are reduced property that is not shared by a selective PDE III inhibi- [19]; thereby, suppressing force generation in the smooth tor, siguazodan (see below). muscle cell. a) 0 100 Phosphodiesterases and PDE inhibitor actions in the airways 20 80 Biochemical investigations have identified PDEs of the 40 60 I, II, III, IV and V families in human bronchus and tra- chea [21–24]. PDE I is relatively scarce in bronchus, 60 40 whilst PDE IV and cGMP-specific PDE V are the most abundant isoenzymes (fig. 1). In the trachea, PDE I re- 80 20 presents two distinct enzymes and accounts for a large proportion of the total PDE activity [23]. 100 0 Theophylline relaxes the inherent tone of human bron- 1 10 100 1000 chial rings at similar concentrations to those which inhibit [Theophylline] µM b) a) 40 0 100 Relaxation % control 20 80 PDE activity % control 30 40 60 -1 60 40 20 80 20 ·mg protein 10 -1 100 0 1 10 100 1000 0 [Theophylline] µM I II III IV V Fig. 2. – Inhibition of cyclic AMP PDE ( G ) and relaxation of b) smooth muscle ( ❍ ) of: a) human bronchus; and b) pulmonary 80 artery by theophylline. AMP: adenosine monophosphate; PDE: phosphodiesterase. Table 1. – Isoenzyme selectivity of PDE inhibitors cited 60 in this article Drug Selectivity 40 Cyclic nucleotide hydrolysis pmol·min IBMX Nonselective Theophylline Nonselective 20 Milrinone III Motapizone III Org 9935 III Siguazodan III 0 Org 30029 III/IV I II III IV V Zardaverine III/IV Fig. 1. – Activities of PDE isoenzymes I–V in the cytosolic ( ) Rolipram IV and particulate phases ( ) of homogenates of: a) human bronchus; Zaprinast V/I and b) pulmonary artery PDE: phosphodiesterase. (Data are taken from RABE and co-workers [22, 25]). PDE: phosphodiesterase; IBMX: isobutyl-1-methylxanthine. BRONCHODILATION AND SMOOTH MUSCLE RELAXATION 639 The selective PDE III inhibitors, motapizone and Org a) 9935, and the PDE IV-selective inhibitor, rolipram, give 0 L L biphasic curves for inhibition of PDE activity, apparen- L tly reflecting loss of isoenzyme selectivity at higher L concentrations [21, 22]. Zardaverine and motapizone are also effective relaxants of bronchial smooth muscle inherent tone whilst rolipram relaxes bronchi only slight- L ly at high concentrations (fig. 3), and the PDE V/I inhi- 50 L L bitor, zaprinast, is also a rather weak relaxant [22]. In L L contrast, zaprinast is effective - and rolipram is highly L effective - in relaxing bronchial smooth muscle precon- L tracted with methacholine or histamine [21, 29] although data from another laboratory indicate that rolipram is also ineffective against carbachol-induced contractions 100 in preparations which relax readily in response to the 10-9 10-8 10-7 10-6 10-5 10-4 PDE III inhibitor, siguazodan [23]. In this latter study, [Rolipram] M it was also observed that, while siguazodan causes a b) L % relaxation 0 tenfold potentiation of the relaxation of leukotriene D4 L β (LTD4)-precontracted bronchi to the -adrenoceptor ago- L nist, isoprenaline, this phenomenon occurs neither with inhibitors of PDE IV or V nor in tissues precontracted with carbachol instead of LTD4 [23]. It has also been demonstrated that nonselective (isobutyl-1-methylxan- L thine (IBMX)), PDE III-selective (Org 9935), and PDE 50 L L V/I-selective (zaprinast) PDE inhibitors are more potent L L L against contractions induced by histamine than by metha- L choline (fig. 4) (cf. the effect of theophylline