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Universal adenosine receptor antagonism is neither necessary nor desirable with xanthine antiasthmatics
Medical Hypotheses8:
515-526, 1982
UNIVERSAL ADENOSINE RECEPTOR ANTAGONISM IS NEITHER NECESSARY NOR DESIRABLE WITH XANTHINE ANTIASTHMATICS. C.G.A. Persson. Department of Clinical Pharmacology, University Hospital, Lund and Research and Development Department, AB Draco, Box 1707, Lund, Sweden. ABSTRACT The great diversity of pharmacological effects of xanthines may well reflect different cellular mechanisms of action. Major attention is presently devoted to adenosine receptor antagonism that, in contrast to phosphodiesterase inhibition, is clearly produced by therapeutic concentrations of theophylline. The ubiquitous adenosine effects together with the universal and potent blocking action of methylxanthines have led investigators to believe that most pharmacological actions including antiasthmatic effects of theophylline reflect adenosine antagonism. The present hypothesis proposes that universal adenosine receptor antagonism is neither necessary nor desirable with xanthine antiasthmatics. Supporting the hypothesis a xanthine derivative that seems to be devoid of functional effects at important adenosine receptor sites has been shown to be a potent bronchodilator drug that lacks theophylline-like diuretic and CNS-stimulant behavioural effects. Key words: Xanthines,
Adenosine,
Asthma therapy
INTRODUCTION "One of the commonest and best-reputed remedies of asthma, one that is almost sure to have been tried in any case that may come under our observation, and one that in many cases is more efficacious than any other, is strong coffee". These are words by H Salter in 1859 (1). Despite the recommendation by Salter coffee or caffeine do not seem to have been much in use and for various reasons the acceptance of the slightly more potent xanthine theophylline as an asthma drug was a slow process (2). At present theophylline is subject to much renewed interest; world wide it may be the most frequently used single agent among bronchodilator
515
drugs. Many lung effects are produced by xanthines (theophylline) as judged from various experimental in vitro and in vivo data: xanthines relax large and small airway smooth muscle irrespective of what mediator has produced constriction (2,3,4), they increase the rate of mucociliary clearance (5,6), they inhibit release of mediators at anaphylactic reactions (7) and they may prevent a mediator-induced pulmonary edema (8). In patients with reversible obstructive lung disease xanthines (theophylline) seem to be equally effective to sympathomimetic drugs (9). Within sympathomimetics some significant drug developments have taken place since the early use of the 'adrenal substance' (and ephedrine). Isoprenaline, that is almost devoid of a-receptor stimulant properties was introduced about 1940 (10). Ahlquist (11) later named this drug a p-receptor agonist. More recently bronchoselective B2-receptor agonists were produced (12,13,14). Since theophylline produces a great variety of extrapulmonary effects, some of which are feared side effects, it is obvious that significant drug development is desirable within xanthines. Many attempts to achieve this goal have been made. Lately these efforts have been intensified. Phosphodiesterase
inhibition
The finding by Butcher and Sutherland (15) (1962) that theophylline is an inhibitor of phosphodiesterase, the catabolic enzyme for cyclic AMP was thought to explain many pharmacological actions of theophylline, in particular the bronchodilator effects. This discovery and thought initiated the synthesis of a large variety of phosphodiesterase inhibiting compounds with the aim of producing new therapeutic agents i.a. for treatment of asthma. The failure of these drugs (16,17,18,19) to produce an acceptable clinical response is one indication that the importance of phosphodiesterase inhibition as an asthma drug mechanism can be questioned. Furthermore, there is a discrepancy between the therapeutic concentrations of theophylline and those needed to get pronounced inhibition of phosphodiesterases (20). Also at marked airway smooth muscle relaxant concentrations the theophylline-induced effects are difficult to reconcile with a cyclic AMP-mediated relaxation (21,22). In summary, phosphodiesterase inhibition may, despite its attractiveness, be an irrelevant mechanism for xanthine antiasthmatics. Adenosine
receptor antagonism
Of the major xanthine-induced cellular actions it seems that only-adenosine receptor antagonism is clearly produced within the therapeutic range of concentrations of theophylline (and caffeine)(20). Many tissues may normally be
516
under some tonical_ influence of endogenous adenosine, and various insults such as hypoxia, ischemia and nervous overstimulation are potent stimuli for adenosine formation (23). Through activation of specific external cellular receptors (24) adenosine generally has inhibitory effects. It relaxes vascular smooth muscle (except in the kidney) (25) and its depressant effects may lead to a decreased metabolic demand. For example in the heart adenosine increases coronary flow and decreases rate and force of contraction (26,27), and in many areas of the brain adenosine increases blood flow and depresses nerve cell firing (26,28,29,30,31,32). The potent blocking effect of methylxanthines together with the ubiquitous adenosineeffects have led investigators to believe that many pharmacological effects of theophylline, including the anti-asthmatic effects, reflect adenosine receptor antagonism (20,25,31,32,33,34,35). Two different external adenosine receptors have been described (24). At the present state of knowledge it seems that blocking xanthines do not differentiate between these (24,32). However, this issue has not been adequat.ely addressed as yet. HYPOTHESIS The present hypothesis proposes that a universal antagonism at adenosine receptors is neither necessary nor desirable with xanthine antiasthmatics. Much of the evidence in support of this hypothesis is obtained from recent structure-activity studies with xanthine-derivatives and from the growing experiences in animals and man with a selected xanthine derivative. Antiasthmatic
effects
First, the above hypothesis apparently is at variance with the suggestions that adenosine antagonism is an important mechanism behind the antiasthmatic effects of xanthines (33, 35). They were based on findings that adenosine enhanced antigen-induced histamine release from mast cells, which was prevented by theophylline in relevant concentrations (33,36). Adenosine was also reported to produce a contracting effect on tracheal smooth muscle. Again theophylline and adenosine interacted as antagonists (37). However, both the enhancing effect on mast cell release of histamine and the contractile effect on tracheal smooth muscle were slight (33,37). Indeed, it is conceivable that adenosine in contrast is associated with bronchodilator effects. Thus in basophils adenosine has been shown to inhibit antigen-induced histamine-secretion (38) and in isolated tracheal smooth muscle the consistent and pronounced effect of adenosine seems to be relaxation (39,40). It may, therefore, be speculated that the bronchorelaxant effects of adenosine can have a modulatory role during hypoxia or other stresses
517
when the lung tissue levels of adenosine are increased severalfold (41,42). Another speculation is that adenosine may produce an undesirable (in asthma) stimulation of afferent nervous activity in the lung. This possibility arises by extrapolating to lung the way adenosine is thought to act in a human skin blister preparation (60). Further studies are warranted to elucidate the role of adenosine as an autacoid in the lung. It has now been shown that bronchodilation by xanthines is unrelated to functional antagonism (40,43) by these drugs at important adenosine receptor sites. Findinqs with one selected compound 3-propyl-xanthine (enprofylline) may illustrate the therapeutic implications of "nonblocking" xanthines (44,45). Enprofylline is 4 to 5 times more potent than theophylline as a bronchodilator in vitro and in vivo in animals (44) and in vitro (45) and in vivo in man (46), and it shares with theophylline anti-edema and anti-anaphylactic effects in the lung (guinea-pigs) (cf 44). Enprofylline is a phosphodiesterase inhibitor (cf 44) but the relevance of this mechanism can be questioned as discussed above. More importantly, enprofylline has been shown to practically lack antagonistic properties at inhibitory adenosine receptors in smooth muscle (trachea) and nervous tissue (myenteric plexus)(45). A functional lack of antagonism at adenosine receptors can be expected to manifest itself in the profile of action. A "non-blocking" xanthine would differ from theophylline for example concerning diuretic (25,45) and CNS-stimulant effects (29, 45). Among other areas where differences can be expected are some cardiovascular and metabolic effects (cf 23) (not dealt with here). Diuretic and CNS-stimulant
effects
The well-known diuretic (natriuretic) effect of theophylline seems to reflect antagonism of the renal actions of endogenous adenosine. One important aspect of adenosine is that it decreases renal blood flow and may participate in local regulation of urinary volume and sodium excretion (25, 47,48). The tubuloqlomerular feedback control of filtration rate in rats is antagonized by theophylline possibly reflecting antagonism of adenosine (47). Theophylline 5-20 mg/kg given to rats dose-dependently produced natriuretic effects. The same doses of enprofylline were without effect (45) (at the largest dose enprofylline rather decreased sodium excretion). This finding has been confirmed in man (unpublished work) and supports the possibility that the kidneys are under tonical influence of antidiuretic effects of adenosine.
518
Probably the best known effects of methylxanthines are their stimulant action on the CNS. Slight CNS-stimulation as produced by intake of xanthine-containing beverages may be a desirable effect on alertness but sleep disturbances, tremor, and restlessness are also early signs of the CNS-stimulant action of theophylline and caffeine (20,49). By increasing doses the stimulant effect will gradually progress to extreme agitation. This is followed by a maniacal behaviour, and generalized massive convulsions (20, 50,51,52). The progressive nature of the CNS-stimulant behavioural effects agrees with the notion that the same mechanism is involved in both the slight and the massive manifestations. Rapidly cumulating experimental evidence favours the view that the CNS-stimulant activity of theophylline (and caffeine) reflects its ability to antagonize nerve cell inhibitory effects of adenosine (20,29,30,32). Furthermore, administration of adenosine to animals has been shown to depress activity and give some protection against seizures (32,59). The possibility that adenosine antagonism may explain both the slight and the ma,ssive manifestations of CNS-stimulation is supported by animal data on the effects of theophylline and enprofylline. Theophylline over a wide range of concentrations (30-900 m) produced a concentration-dependent and surmountable antagonism at nervous adenosine receptors, while the same concentrations of enprofylline were almost devoid of adenosine receptor antagonism (43,45). This was studied in a myenteric plexus preparation that is a readily accessible nervous tissue that has revealed many drug mechanisms of relevance for effects in the central nervous system (53). In vivo theophylline (6-24 mg/kg) dose-dependently increased locomotor activity in mice. Enprofylline (2-48 mg/kg) was without any effect in this behavioural test (43,45). Furthermore, enprofylline did not produce theophylline-like seizure activity when examined in different animal species (44,52,61). Quantitative studies in man have so far suggested that enprofylline does not produce the tremor of hand that is recorded after intake of theophylline (54). It may be argued that at high (convulsant) dosages theophylline also inhibits phosphodiesterases. However, the effects of theophylline on brain levels of cyclic nucleotides have been shown to be slight and the direction of the effect may vary (55,56). Indeed, the cyclic nucleotide changes occurring in the brain during seizures are likely to be a manifestation of the excited state rather than its cause (55,57,58).
519
Mechanisms
of action
The question arises as to what cellular mechanisms are behind the bronchodilator effects of xanthines? Firstly one should look for mechanisms that operate in different cell types: The lung actions of xanthines may well reflect effects on bronchial smooth muscle cells (2), mastcells or other mediator containing cells (7,62), cells regulating mucociliary transport (6), cells regulating permeability in the microcirculation (8,63) etc. Another prerequisite would be that the mechanism(s) should operate in the presence of any type of asthma mediator, i.e. xanthines seem to relax bronchi equally well whether these have been constricted by amines, arachidonic acid metabolites, peptides or acetylcholine (unpublished work cf. 2,3,4). At the present state of knowledge it is difficult to exclude the contribution of any of the suggested mechanisms: phosphodiesterase inhibition, adenosine receptor antagonism (both dealt with above), inhibition (64) or stimulation (65) of prostaglandine synthesis, prostaglandine antagonism (66), effects on catecholamine metabolism (67), reduction of membrane permeability to calcium (68) and promoting the binding of calcium (21,66). One aspect is that lung selective xanthines such as enprofylline may well be of significant aid in future studies designed to elucidate the mechanisms of action of xanthines in asthma. CONCLUSION Xanthines seem to be a heterogenous group of compounds exhibiting many diverse pharmacodynamic patterns. Based on the hypothesis that phosphodiesterase inhibition is the important cellular basis for effects of theophylline many attempts have been made to produce new and improved therapeutic agents. This has not been a very fruitful approach, however, and it is conceivable that phosphodiesterase inhibition is of little value to explain pharmacological effects of theophylline. The present hypothesis is based on the possibility that the kind of adenosine receptor antagonism that can explain, partly or fully, many effects of theophylline (and caffeine) cannot explain their antiasthmatic actions. Experiences with a new xanthine derivative, enprofylline, that functionally seems to lack universal adenosine receptor antagonism has supported, and illustrated some therapeutic implications of this hypothesis. Further experiences in animals and man with compounds such as enprofylline, together with results emerging from the rapidly growing field of research on the biological role of adenosine and related endogenous compounds, will have to show to what extent the present hypothesis can develop into a more firmly based theory.
520
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26. Berne, RM, Foley DH, Watkinson WP, Miller WL, Winn HR, Rubio R: Role of adenosine as a mediator of metabolic vasodilation in the heart and brain: A brief overview and update. In: Physiological and regulatory functions of adenosine and adenine nucleotides. Baer HP, Drummond GI eds. Raven, New York 117-126, 1979. 27. Schrader J, Gerlach E, Baumann G: Sites and mode of action of adenosine in the heart II. Ventricular myocardium. In: Physiological and regulatory functions of adenosine and adenine nucleotides. Baer HP, Drummond GI eds. Raven, New York, 1979. 28. Wahl M, Kuschinsky W: The dilatatory action of adenosine on pial arteries of cats and its inhibition by theophylline. Pfliigers Arch. 362: 55-59, 1976. 29.
Phillis JW, Edstrijm JP, Kostopoulos GK, Kirkpatrick JR: Effects of adenosine and adenine nucleotides on synaptic neurotransmission in the cerebral cortex. Can. J. Physiol. Pharmac. 57: 1289-1312, 1979.
30. Dunwiddie TV, Hoffer BJ, Fredholm BB: Alkylxanthines elevate hippocampal excitability. Evidence for a role of endogenous adenosine. N.S. Arch. Pharmacol. 316: 326-330, 1981. 31. Perkins MN, Stone TW: Aminophylline and theophylline derivatives as antagonists of neuronal depression by adenosine: a microiontophoretic study. Arch. int. Pharmacodyn. 246: 205-214, 1980. 32. Daly JW, Bruns RF, Snyder SH: Adenosine receptors in the central nervous system: Relationship to the central actions of methylxanthines. Life Sci. 28: 2083-2097, 1981. 33. Marquardt DL, Parker CW, Sullivan TJ: Potentiation of mast cell mediator release by adenosine. J. Immunol. 120: 871-878, 1978. 34. Fredholm BB, Hedquist P: Modulation of neurotransmission by purine nucleotides and nucleosides. Biochem. Pharmacol. 29: 1635-1643, 1980. 35. Fredholm BB: Are the actions of methyl-xanthines to antagonism of adenosine? Trends in Pharmacol. 1: 129-132, 1980.
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36. Fredholm BB, Sydbom A: Are the antiallergic actions of theophylline due to antagonism at the adenosine receptor? Agents and Actions 10: 145-147, 1980.
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37. Fredholm BB, Brodin K, Strandberg K: On the mechanism of relaxation of tracheal smooth muscle by theophylline and other cyclic nucleotide phosphodiesterase inhibitors. Acta Pharmacol. Tox. 45: 336-344, 1979. 38. Marone G, Findlay SR, Lichtenstein LM: Adenosine receptor on human basophils: Modulation of histamine release. J. Immunol. 123: 1473-1477, 1979. 39. Farmer JB, Farrar DG: Pharmacological studies with adenine, adenosine and some phosphorylated derivatives on guinea-pig tracheal muscle. J. Pharm. Pharmac. 28: 748-752, 1976. 40. Karlsson J-A, Persson CGA: Adenosine-receptor antagonism and bronchodilator activity by methylsubstituted xanthines. Br. J. Pharmacol. 1981 (In press). 41. Mentzer RM, Rubio R, Berne RM: Release of adenosine by hypoxic canine lung tissue and its possible role in pulmonary circulation. Am. J. Physiol. 229: 1125-1631, 1975. 42. Fredholm BB: Release of adenosine from rat lung by antigen and compound 48/80. Acta Physiol. Stand. 111: 507-508, 1981. 43. Persson CGA, Karlsson J-A, Erjefalt I: Differentiation between bronchodilation and adenosine antagonism among xanthine derivatives. Submitted manuscript. 44. Persson CGA, Kjellin G: Enprofylline, a principally antiasthmatic xanthine. Acta Pharmacol. Tox. 49: 313-316, 1981.
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45. Persson CGA, Karlsson J-A, Erjefalt I: Adenosine antagonism, a less desirable characteristic of xanthine asthma drugs? Acta Pharmacol. Tox. 1981 49: 317-320, 1981. 46. Lunell E, Svedmyr N, Andersson K-E, Persson CGA: Effects of enprofylline, a xanthine lacking adenosine antagonism, in patients with reversible obstructive lung disease. Eur. J. Clin. Pharmacol. 1982 (In press). 47.
Schneermann J, Osswald H, Hermle M: Inhibitory effect of methylxanthines on feedback control of glomerular filtration rate in the rat kidney. Pfliigers Arch. 369: 39-48, 1977.
48. Tagawa H, Vander AJ: Effects of adenosine compounds on renal function and renin secretion in dogs. Circ. Res. 26: 327-338, 1970.
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Eichler 0: Toxicitat in: Kaffee und Coffein. ed. Springer Berlin 337-350, 1976.
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50. Zwillig CW, Sutton FD, Neff TA, Cohen WM, Matthay HA, Weinberger MM: Theophylline-induced seizures in adults. Ann. Internal Med. 82: 784-787, 1975. 51. Gleason MM, Gosselin HE, Hodge HC. Clinical toxicology of commercial products. Ed 3, Williams Wilkins Co 13-16, 1969. 52. Persson CGA, Erjefalt I: Convulsive effects of theophylline in conscious and anaesthetized guinea-pigs. Acta Pharmacol. Tox. 48: 32-38, 1981. 53. North HA: Pharmacology of the forgotten nervous Trends in Pharmacol. Sci. 1: 439-442, 1980.
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54. Lunell E, Svedmyr N, Andersson K-E, Persson CGA: Difference in tremorogenic but not bronchodilator effects between theophylline and enprofylline, two xanthines with partly different cellular modes of action. Submitted manuscript. 55. Sattin A: Increase in the content of adenosine 3',5'-monophosphate in mouse forebrain during seizures and prevention of the increase by methylxanthines. J. Neurothem. 18: 1087-1096, 1971. 56. Gross PA, Ferrendelli JA: Effects of reserpine, propranolol and aminophylline on seizure activity and CNS cyclic nucleotides. Ann. Neurol. 6: 296-301, 1979. 57. Phillis JW: The role of cyclic nucleotides Can. J. Neurol. 4: 151-195, 1977.
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58. Folbergrova J: Cyclic GMP and cyclic AMP in the cerebral cortex of mice during seizures induced by 3-mercaptopropionic acid: Effects of anticonvulsant agents. Neurosci. Letters 16: 291-296, 1980. 59.
Maitre M, Ciesielski L, Lehman A, Kempf E, Mandel P: Protective effects of adenosine and nicotinamid against audiogenic seizure. Biochem. Pharm. 23: 2807-2816, 1974.
60. Bleehen T, Keele CA: Observations on the algogenic actions of adenosine compounds on the human blister preparation. Pain 3: 367-377, 1977. 61. Persson CGA, Erjefalt I: Seizure activity in animals given enprofylline and theophylline, two xanthines with partly different mechanisms of action. Submitted manuscript.
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62. Andersson P: Antigen-induced bronchial anaphylaxis in actively sensitized guinea-pigs. Anti-anaphylactic effects of sodium cromoglyeate and aminophylline. Br. J. Pharmacol. 69: 467-472, 1980. 63. Persson CGA, Ekman M, Erjefalt I: Vascular antipermeability effects of S-receptor agonists and theophylline in the lung. Acta Pharmacol. Tox. 44: 216-220, 1979. 64. Vinegar R, Truax JF, Selph JL, Welch RM, White HL: Potentiation of the antiinflammatory and analgesic activity of aspirin by caffeine in the rat. Proc. Sot. Exp. Biol. Med. 151: 556-560, 1976. 65. Okabe E: Preventive effects of theophylline on anaphylactic shock in rats. Japan J. Pharmacol. 30: 367-376, 1980. 66. Horrobin DF, Manku MS, Franks DJ, Harmet P: Methylxanthine phosphodiesterase inhibitors behave as prostaglandin antagonists in a perfused rat mesenteric artery preparation. Prostaglandins 13: 33-40, 1977. 67. Snider SR, Waldeck B: Increased synthesis of adrenomedullary catecholamines induced by caffeine and theophylline. N.S. Arch. Pharmacol. 281: 257-260, 1974. 68. Syson AJ, Huddart H: The effect of caffeine on excitation-contraction coupling in skeletal and smooth muscle. J. Exp. Biol. 63: 789-798, 1976.
526
515-526, 1982
UNIVERSAL ADENOSINE RECEPTOR ANTAGONISM IS NEITHER NECESSARY NOR DESIRABLE WITH XANTHINE ANTIASTHMATICS. C.G.A. Persson. Department of Clinical Pharmacology, University Hospital, Lund and Research and Development Department, AB Draco, Box 1707, Lund, Sweden. ABSTRACT The great diversity of pharmacological effects of xanthines may well reflect different cellular mechanisms of action. Major attention is presently devoted to adenosine receptor antagonism that, in contrast to phosphodiesterase inhibition, is clearly produced by therapeutic concentrations of theophylline. The ubiquitous adenosine effects together with the universal and potent blocking action of methylxanthines have led investigators to believe that most pharmacological actions including antiasthmatic effects of theophylline reflect adenosine antagonism. The present hypothesis proposes that universal adenosine receptor antagonism is neither necessary nor desirable with xanthine antiasthmatics. Supporting the hypothesis a xanthine derivative that seems to be devoid of functional effects at important adenosine receptor sites has been shown to be a potent bronchodilator drug that lacks theophylline-like diuretic and CNS-stimulant behavioural effects. Key words: Xanthines,
Adenosine,
Asthma therapy
INTRODUCTION "One of the commonest and best-reputed remedies of asthma, one that is almost sure to have been tried in any case that may come under our observation, and one that in many cases is more efficacious than any other, is strong coffee". These are words by H Salter in 1859 (1). Despite the recommendation by Salter coffee or caffeine do not seem to have been much in use and for various reasons the acceptance of the slightly more potent xanthine theophylline as an asthma drug was a slow process (2). At present theophylline is subject to much renewed interest; world wide it may be the most frequently used single agent among bronchodilator
515
drugs. Many lung effects are produced by xanthines (theophylline) as judged from various experimental in vitro and in vivo data: xanthines relax large and small airway smooth muscle irrespective of what mediator has produced constriction (2,3,4), they increase the rate of mucociliary clearance (5,6), they inhibit release of mediators at anaphylactic reactions (7) and they may prevent a mediator-induced pulmonary edema (8). In patients with reversible obstructive lung disease xanthines (theophylline) seem to be equally effective to sympathomimetic drugs (9). Within sympathomimetics some significant drug developments have taken place since the early use of the 'adrenal substance' (and ephedrine). Isoprenaline, that is almost devoid of a-receptor stimulant properties was introduced about 1940 (10). Ahlquist (11) later named this drug a p-receptor agonist. More recently bronchoselective B2-receptor agonists were produced (12,13,14). Since theophylline produces a great variety of extrapulmonary effects, some of which are feared side effects, it is obvious that significant drug development is desirable within xanthines. Many attempts to achieve this goal have been made. Lately these efforts have been intensified. Phosphodiesterase
inhibition
The finding by Butcher and Sutherland (15) (1962) that theophylline is an inhibitor of phosphodiesterase, the catabolic enzyme for cyclic AMP was thought to explain many pharmacological actions of theophylline, in particular the bronchodilator effects. This discovery and thought initiated the synthesis of a large variety of phosphodiesterase inhibiting compounds with the aim of producing new therapeutic agents i.a. for treatment of asthma. The failure of these drugs (16,17,18,19) to produce an acceptable clinical response is one indication that the importance of phosphodiesterase inhibition as an asthma drug mechanism can be questioned. Furthermore, there is a discrepancy between the therapeutic concentrations of theophylline and those needed to get pronounced inhibition of phosphodiesterases (20). Also at marked airway smooth muscle relaxant concentrations the theophylline-induced effects are difficult to reconcile with a cyclic AMP-mediated relaxation (21,22). In summary, phosphodiesterase inhibition may, despite its attractiveness, be an irrelevant mechanism for xanthine antiasthmatics. Adenosine
receptor antagonism
Of the major xanthine-induced cellular actions it seems that only-adenosine receptor antagonism is clearly produced within the therapeutic range of concentrations of theophylline (and caffeine)(20). Many tissues may normally be
516
under some tonical_ influence of endogenous adenosine, and various insults such as hypoxia, ischemia and nervous overstimulation are potent stimuli for adenosine formation (23). Through activation of specific external cellular receptors (24) adenosine generally has inhibitory effects. It relaxes vascular smooth muscle (except in the kidney) (25) and its depressant effects may lead to a decreased metabolic demand. For example in the heart adenosine increases coronary flow and decreases rate and force of contraction (26,27), and in many areas of the brain adenosine increases blood flow and depresses nerve cell firing (26,28,29,30,31,32). The potent blocking effect of methylxanthines together with the ubiquitous adenosineeffects have led investigators to believe that many pharmacological effects of theophylline, including the anti-asthmatic effects, reflect adenosine receptor antagonism (20,25,31,32,33,34,35). Two different external adenosine receptors have been described (24). At the present state of knowledge it seems that blocking xanthines do not differentiate between these (24,32). However, this issue has not been adequat.ely addressed as yet. HYPOTHESIS The present hypothesis proposes that a universal antagonism at adenosine receptors is neither necessary nor desirable with xanthine antiasthmatics. Much of the evidence in support of this hypothesis is obtained from recent structure-activity studies with xanthine-derivatives and from the growing experiences in animals and man with a selected xanthine derivative. Antiasthmatic
effects
First, the above hypothesis apparently is at variance with the suggestions that adenosine antagonism is an important mechanism behind the antiasthmatic effects of xanthines (33, 35). They were based on findings that adenosine enhanced antigen-induced histamine release from mast cells, which was prevented by theophylline in relevant concentrations (33,36). Adenosine was also reported to produce a contracting effect on tracheal smooth muscle. Again theophylline and adenosine interacted as antagonists (37). However, both the enhancing effect on mast cell release of histamine and the contractile effect on tracheal smooth muscle were slight (33,37). Indeed, it is conceivable that adenosine in contrast is associated with bronchodilator effects. Thus in basophils adenosine has been shown to inhibit antigen-induced histamine-secretion (38) and in isolated tracheal smooth muscle the consistent and pronounced effect of adenosine seems to be relaxation (39,40). It may, therefore, be speculated that the bronchorelaxant effects of adenosine can have a modulatory role during hypoxia or other stresses
517
when the lung tissue levels of adenosine are increased severalfold (41,42). Another speculation is that adenosine may produce an undesirable (in asthma) stimulation of afferent nervous activity in the lung. This possibility arises by extrapolating to lung the way adenosine is thought to act in a human skin blister preparation (60). Further studies are warranted to elucidate the role of adenosine as an autacoid in the lung. It has now been shown that bronchodilation by xanthines is unrelated to functional antagonism (40,43) by these drugs at important adenosine receptor sites. Findinqs with one selected compound 3-propyl-xanthine (enprofylline) may illustrate the therapeutic implications of "nonblocking" xanthines (44,45). Enprofylline is 4 to 5 times more potent than theophylline as a bronchodilator in vitro and in vivo in animals (44) and in vitro (45) and in vivo in man (46), and it shares with theophylline anti-edema and anti-anaphylactic effects in the lung (guinea-pigs) (cf 44). Enprofylline is a phosphodiesterase inhibitor (cf 44) but the relevance of this mechanism can be questioned as discussed above. More importantly, enprofylline has been shown to practically lack antagonistic properties at inhibitory adenosine receptors in smooth muscle (trachea) and nervous tissue (myenteric plexus)(45). A functional lack of antagonism at adenosine receptors can be expected to manifest itself in the profile of action. A "non-blocking" xanthine would differ from theophylline for example concerning diuretic (25,45) and CNS-stimulant effects (29, 45). Among other areas where differences can be expected are some cardiovascular and metabolic effects (cf 23) (not dealt with here). Diuretic and CNS-stimulant
effects
The well-known diuretic (natriuretic) effect of theophylline seems to reflect antagonism of the renal actions of endogenous adenosine. One important aspect of adenosine is that it decreases renal blood flow and may participate in local regulation of urinary volume and sodium excretion (25, 47,48). The tubuloqlomerular feedback control of filtration rate in rats is antagonized by theophylline possibly reflecting antagonism of adenosine (47). Theophylline 5-20 mg/kg given to rats dose-dependently produced natriuretic effects. The same doses of enprofylline were without effect (45) (at the largest dose enprofylline rather decreased sodium excretion). This finding has been confirmed in man (unpublished work) and supports the possibility that the kidneys are under tonical influence of antidiuretic effects of adenosine.
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Probably the best known effects of methylxanthines are their stimulant action on the CNS. Slight CNS-stimulation as produced by intake of xanthine-containing beverages may be a desirable effect on alertness but sleep disturbances, tremor, and restlessness are also early signs of the CNS-stimulant action of theophylline and caffeine (20,49). By increasing doses the stimulant effect will gradually progress to extreme agitation. This is followed by a maniacal behaviour, and generalized massive convulsions (20, 50,51,52). The progressive nature of the CNS-stimulant behavioural effects agrees with the notion that the same mechanism is involved in both the slight and the massive manifestations. Rapidly cumulating experimental evidence favours the view that the CNS-stimulant activity of theophylline (and caffeine) reflects its ability to antagonize nerve cell inhibitory effects of adenosine (20,29,30,32). Furthermore, administration of adenosine to animals has been shown to depress activity and give some protection against seizures (32,59). The possibility that adenosine antagonism may explain both the slight and the ma,ssive manifestations of CNS-stimulation is supported by animal data on the effects of theophylline and enprofylline. Theophylline over a wide range of concentrations (30-900 m) produced a concentration-dependent and surmountable antagonism at nervous adenosine receptors, while the same concentrations of enprofylline were almost devoid of adenosine receptor antagonism (43,45). This was studied in a myenteric plexus preparation that is a readily accessible nervous tissue that has revealed many drug mechanisms of relevance for effects in the central nervous system (53). In vivo theophylline (6-24 mg/kg) dose-dependently increased locomotor activity in mice. Enprofylline (2-48 mg/kg) was without any effect in this behavioural test (43,45). Furthermore, enprofylline did not produce theophylline-like seizure activity when examined in different animal species (44,52,61). Quantitative studies in man have so far suggested that enprofylline does not produce the tremor of hand that is recorded after intake of theophylline (54). It may be argued that at high (convulsant) dosages theophylline also inhibits phosphodiesterases. However, the effects of theophylline on brain levels of cyclic nucleotides have been shown to be slight and the direction of the effect may vary (55,56). Indeed, the cyclic nucleotide changes occurring in the brain during seizures are likely to be a manifestation of the excited state rather than its cause (55,57,58).
519
Mechanisms
of action
The question arises as to what cellular mechanisms are behind the bronchodilator effects of xanthines? Firstly one should look for mechanisms that operate in different cell types: The lung actions of xanthines may well reflect effects on bronchial smooth muscle cells (2), mastcells or other mediator containing cells (7,62), cells regulating mucociliary transport (6), cells regulating permeability in the microcirculation (8,63) etc. Another prerequisite would be that the mechanism(s) should operate in the presence of any type of asthma mediator, i.e. xanthines seem to relax bronchi equally well whether these have been constricted by amines, arachidonic acid metabolites, peptides or acetylcholine (unpublished work cf. 2,3,4). At the present state of knowledge it is difficult to exclude the contribution of any of the suggested mechanisms: phosphodiesterase inhibition, adenosine receptor antagonism (both dealt with above), inhibition (64) or stimulation (65) of prostaglandine synthesis, prostaglandine antagonism (66), effects on catecholamine metabolism (67), reduction of membrane permeability to calcium (68) and promoting the binding of calcium (21,66). One aspect is that lung selective xanthines such as enprofylline may well be of significant aid in future studies designed to elucidate the mechanisms of action of xanthines in asthma. CONCLUSION Xanthines seem to be a heterogenous group of compounds exhibiting many diverse pharmacodynamic patterns. Based on the hypothesis that phosphodiesterase inhibition is the important cellular basis for effects of theophylline many attempts have been made to produce new and improved therapeutic agents. This has not been a very fruitful approach, however, and it is conceivable that phosphodiesterase inhibition is of little value to explain pharmacological effects of theophylline. The present hypothesis is based on the possibility that the kind of adenosine receptor antagonism that can explain, partly or fully, many effects of theophylline (and caffeine) cannot explain their antiasthmatic actions. Experiences with a new xanthine derivative, enprofylline, that functionally seems to lack universal adenosine receptor antagonism has supported, and illustrated some therapeutic implications of this hypothesis. Further experiences in animals and man with compounds such as enprofylline, together with results emerging from the rapidly growing field of research on the biological role of adenosine and related endogenous compounds, will have to show to what extent the present hypothesis can develop into a more firmly based theory.
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