British Journal of Anaesthesia 1992; 68: 293-302
REVIEW ARTICLE
PHARMACOLOGY, MECHANISMS OF ACTION AND USES OF SELECTIVE PHOSPHODIESTERASE INHIBITORS J. R. SKOYLES AND K. M. SHERRY
HISTORICAL PERSPECTIVE
Digitalis glycosides and adrenoceptor agonists are the most commonly used inotropic drugs and have been evaluated extensively in the management of both acute and chronic heart failure. Both groups of drugs have disadvantages. Digoxin acts by blocking the sodium-potassium pump at the sarcolemma, so diverting the removal of intracellular sodium to a sodium-calcium exchange pump. This leads to an increase in intracellular calcium. There is now uncertainty that digoxin maintains beneficial effects in heart failure [58] as judged by mortality studies. Adrenoceptor agonists increase cyclic AMP (cAMP) by activating adenylate-cyclase, the enzyme that catalyses the formation of adenylate cAMP from adenosine triphosphate (ATP). However, tolerance to adrenoceptor agonists may develop by reduction in the beta receptor pool and by uncoupling the beta receptors from adenylate cyclase. In addition, both groups of drugs are potentially detrimental in terms of balancing myocardial oxygen supply and demand and may be arrhythmogenic. There is no doubt that inotropic drugs are associated with tachycardia which may compromise myocardial blood flow, increase the severity of ischaemia and worsen contractile function. For some years there has been an active search for non-glycoside, non-sympathomimetic positive inotropic drugs for use in heart failure. The selective phosphodiesterase (PDE) inhibitors are a heteroKBY WORDS Pharmacology: phosphodiesterase inhibitors.
genous group of compounds that appear to have a competitive inhibitory action on the isoenzymes of phosphodiesterase, reducing the hydrolysis and so increasing the intracellular concentrations of cAMP in the myocardium and vascular smooth muscle. The overall effect of these drugs is to combine positive inotropy with vasodilatation (table I). In 1984, the i.v. form of amrinone, the first clinically used PDE inhibitor, was approved by the Food and Drug Administration in the U.S.A. Since then there has been continuous interest in the newer forms of selective inhibitors. There are several structurally different PDE inhibitors undergoing clinical evaluation, of which the most well known are the bipyridines, amrinone and milrinone, and the imidazolones, piroximone and enoximone (fig. 1). In addition, there are other related and non-related compounds exhibiting selective PDE inhibitory effects with varying efficacy (table II). This review discusses the mechanisms of the specific PDE inhibitors, their role in current clinical practice and TABLE I. Expected hacmodynamic effects of i.v. selective PDE inhibition in heart failure [1, 4, 12, 16, 34, 69, 81, 86]
Heart rate Mean arterial pressure Mean pulmonary artery pressure Pulmonary wedge pressure Systemic vascular resistance Cardiac index
0-10% 0-10% 10-20% 10-40% 20-30% 20-50%
increase decrease decrease decrease decrease increase
CH,-S HN-^NH 0 Enoximone
Milrinone
NH
Piroximone Amrinone FIG. 1. PDE inhibitors currently undergoing clinical evaluation. J. SKOYLES, F.C.ANAES.; K. M. SHERRY, F.C.ANAES.; Department
of Anaesthetics, Cardiothoracic Unit, Northern General Hospital, Herries Road, Sheffield S5 7AU. Accepted for Publication: September 17, 1991.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
Anaesthetists are often involved in the treatment of heart failure in both the intensive care unit and during cardiac anaesthesia. Regardless of its aetiology, heart failure is associated with a reduction in cardiac output, increase in filling pressures and dilatation or hypertrophy of the myocardium. Secondary to this is stimulation of the renin-angiotensin axis, sympathetic nervous system and atrial naturetic peptides, leading to increased systemic vascular resistance and fluid retention. The management of heart failure is directed to the treatment of die underlying cause and manipulation of the circulation by diuretics, vasodilators and drugs with inotropic activity. This article reviews the selective phosphodiesterase inhibitors which have both inotropic and vasodilator activity.
BRITISH JOURNAL OF ANAESTHESIA
294 TABLE II. Drugs inhibiting phosphodiesterase
Group
Selectivity
Imidazalone derivatives Enoximone [3, 18] Piroximone [4] Bipyridine derivatives Amrinone [1] Milrinone [2] Pyridazinones Pimobendan Imadazodan Opium alkaloids Papaverine Alkylxanthincs Theophylline Iso buytl methyl xanthine Caffeine
PDEIII PDEIII PDE III PDE I-V PDE I-V
CALCIUM METABOLISM AND CYCLIC AMP IN THE MYOCARDIUM
In general, the cardiotonic effect of any drug ultimately results from an increase in the concentration of intracellular free calcium available to interact with the contractile proteins, an increased sensitivity of the myofilaments to calcium, or both. Intracellular calcium concentration is increased in part by the influx of extracellular calcium through slow calcium channels during each action potential;
Na+
Na +
Na+
Adrenoceptor
ATP
Phosphodiesterase * III
Sarcoplasmic tubules
e Milrinone Enoximone Piroximone
FIG. 2. Excitation-contraction coupling in the mammalian heart. Gi/Gs = Guanosine regulatory subunits; ATP = adenosine triphosphate; AC = adenylate cyclase; P = phosphorylation reactions. (Adapted from Feldman, Copelas and Gwathmey [31], with permission of the American Heart Association.)
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
possible future use. In this discussion, PDE inhibition refers to the newer generation of selective PDE inhibitors, as opposed to the less selective agents such as the methylxanthines.
this small increase triggers the release of a larger amount of activator calcium from intracellular stores, mainly from the sarcoplasmic reticulum [57]. At the myofilaments, calcium binds to a regulatory protein, troponin C, releasing its inhibitory effect on the formation of cross bridges between actin and myosin and so initiating myocardial contraction [26]. Myocardial force is related to both the intracellular concentration of calcium and the affinity of troponin C for calcium [45]. The termination of contraction is by the active uptake of calcium from the cytoplasm into the sarcoplasmic reticulum. This includes a calcium-ATPase pump (fig. 2). cAMP is an intracellular secondary messenger. It is produced from ATP through the action of the effector enzyme adenylate cyclase. Activation of adenylate cyclase may be direct, or secondary to membrane receptor activation [67]. Drugs such as forskolin stimulate adenylate cyclase directly, but are not being developed further because of poor oral bioavailability. Indirectly, beta receptors in the sarcolemma activate adenylate cyclase via guanine nucleotide regulatory proteins (Gs and Gi), which transduce either stimulatory or inhibitory signals through the subreceptor complex [35]. These receptor-guanosine—adenylate cyclase units are termed RGC complexes [30]. cAMP hydrolysis is effected by the enzyme phosphodiesterase. cAMP promotes protein phosphorylation through activation of intracellular protein kinases. These phosphorylated proteins influence calcium in three main ways: (i) cAMP-dependent kinase phosphorylation promotes the sarcolemmal calcium influx through slow
PHOSPHODIESTERASE INHIBITORS
295
TABLE III. Cyclic nucleotide phosphodiesterase isoenzymes, classified according to primary protein andcDNA sequence [6]. K,, = Michaelis constant; cGMP = cyclicguanosine monophosphate; cAMP = cyclic adenosine monophosphale. {Adapted from Nicholson, Challis and Shahid [61], with permission)
Isoenzyme family
Substrate characteristic CaI+ and calmodulin-dependent cGMP stimulated cGMP inhibits the cAMP hydrolytic action. Low Km for cAMP and cGMP
IV
cAMP specific. Low Km only forcAMP
V
cGMP specific. Isoenzymes with high and low Km for cGMP
Vinpocetine No selective inhibitor Milrinone, enoximone piroximone, amrinone
Pimobendan, imadazodan Rolipram, denbufylline Zaprinast, dipyridamole
calcium channels by increasing the number opening and time they are open in response to membrane depolarization. Evidence of this mechanism is supported by sensitivity of the selective PDE inhibitor effects to extracellular Ga2+ concentration [27] and their competitive inhibition by calcium channel blockers [23]. In addition, there are specific interactions between selective PDE inhibitors and sarcolemma calcium channel activators, suggesting a common mode of action [33]. (ii) cAMP-dependent protein kinases control the phosphorylation of specific substrate proteins at the sarcoplasmic reticulum [25, 45, 66, 74], triggering the faster release of stored calcium from the sarcoplasmic reticulum [50] and so increasing its availability at the myofilaments [25, 26]. (iii) cAMP modulation is involved in myocardial relaxation, which is an active process and partially independent of myocardial contraction. Enhancing the rate of myocardial relaxation is termed positive lusitropy. cAMP affects the phosphorylation of phospholamban, a protein which regulates the ATPdependent calcium pump on the sarcoplasmic reticulum. Phosphorylated phospholamban increases the rate of uptake of calcium into storage and its removal from the myofilaments [45, 75], so enhancing the rate of myocardial relaxation [42, 45, 56, 63, 75, 85]. The phosphatidylinositol cycle is an alternative mammalian myocardial intracellular regulatory system, and has been proposed as a mechanism for modulating calcium movement in myocytes. The system involves activation of the enzyme phospholipase C through a regulatory G protein. Inositol 1,4,5,triphosphate and diacylglycerol are formed as the secondary messengers which act to affect myofibrillar calcium sensitivity [8, 9, 24]. However, the importance of this system remains unresolved. It appears unlikely that the selective PDE agents act through this pathway [33]. PDE ISOENZYMES: CLASSIFICATION AND SELECTIVE INHIBITION
Phosphodiesterase activity affects tissue function by increasing the rate of breakdown of cyclic nucleo-
Functional effect of inhibition Smooth muscle relaxation Unknown Positive inotrope; vascular and airway smooth muscle relaxation Platelet aggregation inhibition Cat+ sensitization Airway, smooth muscle relaxation; inhibition of inflammatory mediators Platelet aggregation inhibition
tides. Inhibition of these enzymes increases intracellular concentrations of cAMP and cGMP. The existence of families of phosphodiesterase isoenzymes is well known. The multiple forms of isoenzymes of PDE that have been identified in mammalian tissues may be classified according to their specificity and interaction with the cyclic guanosine and adenosine nucleotide substrates (cGMP and cAMP). All these isoenzymes may be inhibited, the physiological consequences of which depend on the concentration of PDE within certain tissues and on the selectivity of the particular PDE inhibitor for that isoenzyme fraction. However, this is a field with a complicated enzyme nomenclature and interspecies variability. Table III outlines one major classification according to primary protein and cDNA sequence information [6]. Previous classifications have been based on other analytical techniques, such as diethylaminoethyl ether cellulose elution. The selective PDE inhibitors, combining inotropic and vasodilator activity, act on the isoenzyme fraction present in cardiac and vascular smooth muscle. This PDE fraction is referred to as PDE III (previously termed Peak IIIc, Type IV, F3 or PHI), and contains the most extensively studied PDE isoenzymes. It is characterized by a low Michaelis constant (Km) for cAMP and cGMP where the hydrolytic activity for cAMP is inhibited by cGMP. It is inhibited also by some therapeutic cardiotonic agents including amrinone, milrinone, enoximone and piroximone. The intracellular distribution of PDE III varies between species. For example, in human hearts it is found in both paniculate and soluble subcellular fractions, whereas in canine myocardium it isolates to the paniculate fraction and in the guineapig to the soluble fraction. Such differences have been used to explain discrepancies between species in their positive inotropic response to selective PDE III inhibitors [81]. It is well known that other drugs in regular clinical use act via some degree of PDE inhibition. When Farah and Alousi first described the synthetic positive inotropic vasodilator, amrinone [28], it was recognized that it shared certain features with the
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
I II III
Isoenzyme selective inhibitors
BRITISH JOURNAL OF ANAESTHESIA
296
SELECTIVE PDE INHIBITORS
Effects on myocardial cAMP modulation The simplest unifying hypothesis behind the action of the selective PDE inhibitors is that they competitively inhibit the hydrolysis of cAMP, leading to an increase in its intracellular concentration. Logically, there should be a correlation between cAMP concentration and the degree of contractile response [69, 70]. However, there are some discrepancies in this relationship from studies which show a time lag between peak concentration of cAMP and contraction [44]. For example, after application of small concentrations of milrinone to isolated dog ventricular tissue, the early increase in tension correlated with the increase in cAMP concentrations. However, cAMP concentrations then declined and returned to control values before peak myocardial tension had occurred. After application of large concentrations of milrinone, the peak increase in cAMP concentration occurred later than tension development [33]. The demonstration of a correlation between the increase in cAMP concentration induced by beta receptor stimulation and inotropy is also equivocal [61]. Explanations for these findings have been proposed. There is some evidence that cAMP and isoenzymes are compartmentalized within cells and that compartments have different sensitivities to PDE inhibition. This would explain the biphasic dose—response relationships of cAMP to milrinone
in tissues and the poor correlation between dose and contractile response in animal models [29]. Some drugs with PDE activity have direct effects on myofibrils, by sensitizing them to Ca2+, and the increase in cAMP concentration is less important. The rate of dissociation of Ca2+ from its binding sites on troponin C is increased by phosphorylation. The force of contraction is increased independently of changing calcium concentration. There is evidence that this action is seen with other agents with PDE inhibitor actions (pimobendan and imadazodan) but it is unlikely to occur with the selective PDE agents currently available for clinical use [50, 66]. In conclusion, the positive inotropic effects of selective PDE inhibitors result mainly from inhibition of cardiac PDE III, leading to an increase in myocardial cAMP content. However, with some PDE inhibitors an additional mode of action is possible. Effects on vascular cAMP modulation and vasodilatation Changes in concentrations of Ca2+ and cyclic nucleotides also play central roles in contractionrelaxation cycles of vascular smooth muscle. Block of sarcolemmal calcium channels, or potassium channel opening and membrane hyperpolarization leads to vasodilatation. Increased concentrations of cAMP result from PDE inhibition and cause phosphorylation of myosin light-chain kinase. This leads to a reduction in its affinity for the Ca2+-calmodulin complex, dephosphorylation of myosin light chains and vasodilatation. cGMP is also a mediator in smooth muscle relaxation, via cGMP-dependent protein kinases [51]. Although, as outlined above, PDE III isoenzymes are considered high affinity cAMP PDE, they also hydrolyse cGMP [61]. This action may be important in the regulation of vascular tone and the mechanism underlying the profound vasodilatation that is a principal feature of the cardiotonic selective PDE inhibitors. The exact contribution of cAMP compared with cGMP with regard to this group of drugs is not completely established. Effects on the circulation
PDE inhibitors have been shown to be direct acting vasodilators in isolated limb studies in resting patients with heart failure, mainly by reducing systemic vascular resistance in skeletal muscle [14, 15,48,49]. Therapy with PDE inhibitors also improves skeletal muscle exercise performance in patients with heart failure [71], but it is not clear if this is caused primarily by the vasodilator or by the inotropic properties of the drug, as the ability of skeletal muscle to vasodilate during exercise is impaired in these patients [48]. Strictly speaking, any agent with a true positive inotropic action should always increase the myocardial oxygen consumption associated with an increase in contractility. In clinical studies, PDE inhibitors improve cardiac performance in the failing heart with unchanged or reduced myocardial oxygen extraction [5, 53]. This is in part because the systemic vasodilatation leads to a reduction in left ventricular systolic wall stress
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
classical methylxanthine PDE inhibitor, theophylline. However, this drug and its congenors are capable of inhibiting all fractions of PDE isoenzymes, whereas amrinone was found to be selective for certain PDE subfractions. In addition, although inhibition of cyclic nucleotide PDE is often used to explain all the actions of theophylline, this is probably not the sole mechanism at therapeutic concentrations. The intracellular accumulation of cGMP may be partly responsible for mediating smooth muscle relaxation by theophyllines. However, methylxanthines act also as competitive antagonists of adenosine receptors (Pj purinergic) [62]. This could explain some effects of theophylline on the central nervous and cardiac conduction systems, and may underlie the efficacy of theophylline in antagonizing bronchoconstriction [74]. Theophylline has complex effects on the circulation, through a combination of brainstem vasomotor stimulation, smooth muscle relaxation and diuresis. It also has unfortunately narrow therapeutic margins. Although PDE III isoenzymes exist in airway smooth muscle, bronchodilatation is not a predominant clinical feature of the current cardiotonic selective PDE III inhibitors. Some PDE inhibitors attenuate the release of inflammatory mediators, particularly those affecting PDE IV (table III). It may be that PDE inhibitors, affecting these isoenzyme systems and combining bronchodilatation, inhibition of platelet aggregation and modification of airway inflammation, could be developed for future treatment and prophylaxis of asthma.
PHOSPHODIESTERASE INHIBITORS
297
TABLE IV. Pharmacokinctics of i.v. milrinone and enoximont. CHF = Chrome heart failure; GFR = glomerular filtration rate
Dose (bolus/infusion) Milrinone 50 ng kg-1 slow bolus 0.375-0.5 ug kg-' min"1 infusion Enoximone 0.5 mg kg"1 slow bolus 5-20 ug kg"1 min"1 infusion
Extraction
Terminal half-life
Modifications in renal failure
Renal 80% unchanged
1 h in health, 2.3 h in CHF
Reduce dose with GFR
REVIEW ARTICLE
PHARMACOLOGY, MECHANISMS OF ACTION AND USES OF SELECTIVE PHOSPHODIESTERASE INHIBITORS J. R. SKOYLES AND K. M. SHERRY
HISTORICAL PERSPECTIVE
Digitalis glycosides and adrenoceptor agonists are the most commonly used inotropic drugs and have been evaluated extensively in the management of both acute and chronic heart failure. Both groups of drugs have disadvantages. Digoxin acts by blocking the sodium-potassium pump at the sarcolemma, so diverting the removal of intracellular sodium to a sodium-calcium exchange pump. This leads to an increase in intracellular calcium. There is now uncertainty that digoxin maintains beneficial effects in heart failure [58] as judged by mortality studies. Adrenoceptor agonists increase cyclic AMP (cAMP) by activating adenylate-cyclase, the enzyme that catalyses the formation of adenylate cAMP from adenosine triphosphate (ATP). However, tolerance to adrenoceptor agonists may develop by reduction in the beta receptor pool and by uncoupling the beta receptors from adenylate cyclase. In addition, both groups of drugs are potentially detrimental in terms of balancing myocardial oxygen supply and demand and may be arrhythmogenic. There is no doubt that inotropic drugs are associated with tachycardia which may compromise myocardial blood flow, increase the severity of ischaemia and worsen contractile function. For some years there has been an active search for non-glycoside, non-sympathomimetic positive inotropic drugs for use in heart failure. The selective phosphodiesterase (PDE) inhibitors are a heteroKBY WORDS Pharmacology: phosphodiesterase inhibitors.
genous group of compounds that appear to have a competitive inhibitory action on the isoenzymes of phosphodiesterase, reducing the hydrolysis and so increasing the intracellular concentrations of cAMP in the myocardium and vascular smooth muscle. The overall effect of these drugs is to combine positive inotropy with vasodilatation (table I). In 1984, the i.v. form of amrinone, the first clinically used PDE inhibitor, was approved by the Food and Drug Administration in the U.S.A. Since then there has been continuous interest in the newer forms of selective inhibitors. There are several structurally different PDE inhibitors undergoing clinical evaluation, of which the most well known are the bipyridines, amrinone and milrinone, and the imidazolones, piroximone and enoximone (fig. 1). In addition, there are other related and non-related compounds exhibiting selective PDE inhibitory effects with varying efficacy (table II). This review discusses the mechanisms of the specific PDE inhibitors, their role in current clinical practice and TABLE I. Expected hacmodynamic effects of i.v. selective PDE inhibition in heart failure [1, 4, 12, 16, 34, 69, 81, 86]
Heart rate Mean arterial pressure Mean pulmonary artery pressure Pulmonary wedge pressure Systemic vascular resistance Cardiac index
0-10% 0-10% 10-20% 10-40% 20-30% 20-50%
increase decrease decrease decrease decrease increase
CH,-S HN-^NH 0 Enoximone
Milrinone
NH
Piroximone Amrinone FIG. 1. PDE inhibitors currently undergoing clinical evaluation. J. SKOYLES, F.C.ANAES.; K. M. SHERRY, F.C.ANAES.; Department
of Anaesthetics, Cardiothoracic Unit, Northern General Hospital, Herries Road, Sheffield S5 7AU. Accepted for Publication: September 17, 1991.
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
Anaesthetists are often involved in the treatment of heart failure in both the intensive care unit and during cardiac anaesthesia. Regardless of its aetiology, heart failure is associated with a reduction in cardiac output, increase in filling pressures and dilatation or hypertrophy of the myocardium. Secondary to this is stimulation of the renin-angiotensin axis, sympathetic nervous system and atrial naturetic peptides, leading to increased systemic vascular resistance and fluid retention. The management of heart failure is directed to the treatment of die underlying cause and manipulation of the circulation by diuretics, vasodilators and drugs with inotropic activity. This article reviews the selective phosphodiesterase inhibitors which have both inotropic and vasodilator activity.
BRITISH JOURNAL OF ANAESTHESIA
294 TABLE II. Drugs inhibiting phosphodiesterase
Group
Selectivity
Imidazalone derivatives Enoximone [3, 18] Piroximone [4] Bipyridine derivatives Amrinone [1] Milrinone [2] Pyridazinones Pimobendan Imadazodan Opium alkaloids Papaverine Alkylxanthincs Theophylline Iso buytl methyl xanthine Caffeine
PDEIII PDEIII PDE III PDE I-V PDE I-V
CALCIUM METABOLISM AND CYCLIC AMP IN THE MYOCARDIUM
In general, the cardiotonic effect of any drug ultimately results from an increase in the concentration of intracellular free calcium available to interact with the contractile proteins, an increased sensitivity of the myofilaments to calcium, or both. Intracellular calcium concentration is increased in part by the influx of extracellular calcium through slow calcium channels during each action potential;
Na+
Na +
Na+
Adrenoceptor
ATP
Phosphodiesterase * III
Sarcoplasmic tubules
e Milrinone Enoximone Piroximone
FIG. 2. Excitation-contraction coupling in the mammalian heart. Gi/Gs = Guanosine regulatory subunits; ATP = adenosine triphosphate; AC = adenylate cyclase; P = phosphorylation reactions. (Adapted from Feldman, Copelas and Gwathmey [31], with permission of the American Heart Association.)
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
possible future use. In this discussion, PDE inhibition refers to the newer generation of selective PDE inhibitors, as opposed to the less selective agents such as the methylxanthines.
this small increase triggers the release of a larger amount of activator calcium from intracellular stores, mainly from the sarcoplasmic reticulum [57]. At the myofilaments, calcium binds to a regulatory protein, troponin C, releasing its inhibitory effect on the formation of cross bridges between actin and myosin and so initiating myocardial contraction [26]. Myocardial force is related to both the intracellular concentration of calcium and the affinity of troponin C for calcium [45]. The termination of contraction is by the active uptake of calcium from the cytoplasm into the sarcoplasmic reticulum. This includes a calcium-ATPase pump (fig. 2). cAMP is an intracellular secondary messenger. It is produced from ATP through the action of the effector enzyme adenylate cyclase. Activation of adenylate cyclase may be direct, or secondary to membrane receptor activation [67]. Drugs such as forskolin stimulate adenylate cyclase directly, but are not being developed further because of poor oral bioavailability. Indirectly, beta receptors in the sarcolemma activate adenylate cyclase via guanine nucleotide regulatory proteins (Gs and Gi), which transduce either stimulatory or inhibitory signals through the subreceptor complex [35]. These receptor-guanosine—adenylate cyclase units are termed RGC complexes [30]. cAMP hydrolysis is effected by the enzyme phosphodiesterase. cAMP promotes protein phosphorylation through activation of intracellular protein kinases. These phosphorylated proteins influence calcium in three main ways: (i) cAMP-dependent kinase phosphorylation promotes the sarcolemmal calcium influx through slow
PHOSPHODIESTERASE INHIBITORS
295
TABLE III. Cyclic nucleotide phosphodiesterase isoenzymes, classified according to primary protein andcDNA sequence [6]. K,, = Michaelis constant; cGMP = cyclicguanosine monophosphate; cAMP = cyclic adenosine monophosphale. {Adapted from Nicholson, Challis and Shahid [61], with permission)
Isoenzyme family
Substrate characteristic CaI+ and calmodulin-dependent cGMP stimulated cGMP inhibits the cAMP hydrolytic action. Low Km for cAMP and cGMP
IV
cAMP specific. Low Km only forcAMP
V
cGMP specific. Isoenzymes with high and low Km for cGMP
Vinpocetine No selective inhibitor Milrinone, enoximone piroximone, amrinone
Pimobendan, imadazodan Rolipram, denbufylline Zaprinast, dipyridamole
calcium channels by increasing the number opening and time they are open in response to membrane depolarization. Evidence of this mechanism is supported by sensitivity of the selective PDE inhibitor effects to extracellular Ga2+ concentration [27] and their competitive inhibition by calcium channel blockers [23]. In addition, there are specific interactions between selective PDE inhibitors and sarcolemma calcium channel activators, suggesting a common mode of action [33]. (ii) cAMP-dependent protein kinases control the phosphorylation of specific substrate proteins at the sarcoplasmic reticulum [25, 45, 66, 74], triggering the faster release of stored calcium from the sarcoplasmic reticulum [50] and so increasing its availability at the myofilaments [25, 26]. (iii) cAMP modulation is involved in myocardial relaxation, which is an active process and partially independent of myocardial contraction. Enhancing the rate of myocardial relaxation is termed positive lusitropy. cAMP affects the phosphorylation of phospholamban, a protein which regulates the ATPdependent calcium pump on the sarcoplasmic reticulum. Phosphorylated phospholamban increases the rate of uptake of calcium into storage and its removal from the myofilaments [45, 75], so enhancing the rate of myocardial relaxation [42, 45, 56, 63, 75, 85]. The phosphatidylinositol cycle is an alternative mammalian myocardial intracellular regulatory system, and has been proposed as a mechanism for modulating calcium movement in myocytes. The system involves activation of the enzyme phospholipase C through a regulatory G protein. Inositol 1,4,5,triphosphate and diacylglycerol are formed as the secondary messengers which act to affect myofibrillar calcium sensitivity [8, 9, 24]. However, the importance of this system remains unresolved. It appears unlikely that the selective PDE agents act through this pathway [33]. PDE ISOENZYMES: CLASSIFICATION AND SELECTIVE INHIBITION
Phosphodiesterase activity affects tissue function by increasing the rate of breakdown of cyclic nucleo-
Functional effect of inhibition Smooth muscle relaxation Unknown Positive inotrope; vascular and airway smooth muscle relaxation Platelet aggregation inhibition Cat+ sensitization Airway, smooth muscle relaxation; inhibition of inflammatory mediators Platelet aggregation inhibition
tides. Inhibition of these enzymes increases intracellular concentrations of cAMP and cGMP. The existence of families of phosphodiesterase isoenzymes is well known. The multiple forms of isoenzymes of PDE that have been identified in mammalian tissues may be classified according to their specificity and interaction with the cyclic guanosine and adenosine nucleotide substrates (cGMP and cAMP). All these isoenzymes may be inhibited, the physiological consequences of which depend on the concentration of PDE within certain tissues and on the selectivity of the particular PDE inhibitor for that isoenzyme fraction. However, this is a field with a complicated enzyme nomenclature and interspecies variability. Table III outlines one major classification according to primary protein and cDNA sequence information [6]. Previous classifications have been based on other analytical techniques, such as diethylaminoethyl ether cellulose elution. The selective PDE inhibitors, combining inotropic and vasodilator activity, act on the isoenzyme fraction present in cardiac and vascular smooth muscle. This PDE fraction is referred to as PDE III (previously termed Peak IIIc, Type IV, F3 or PHI), and contains the most extensively studied PDE isoenzymes. It is characterized by a low Michaelis constant (Km) for cAMP and cGMP where the hydrolytic activity for cAMP is inhibited by cGMP. It is inhibited also by some therapeutic cardiotonic agents including amrinone, milrinone, enoximone and piroximone. The intracellular distribution of PDE III varies between species. For example, in human hearts it is found in both paniculate and soluble subcellular fractions, whereas in canine myocardium it isolates to the paniculate fraction and in the guineapig to the soluble fraction. Such differences have been used to explain discrepancies between species in their positive inotropic response to selective PDE III inhibitors [81]. It is well known that other drugs in regular clinical use act via some degree of PDE inhibition. When Farah and Alousi first described the synthetic positive inotropic vasodilator, amrinone [28], it was recognized that it shared certain features with the
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
I II III
Isoenzyme selective inhibitors
BRITISH JOURNAL OF ANAESTHESIA
296
SELECTIVE PDE INHIBITORS
Effects on myocardial cAMP modulation The simplest unifying hypothesis behind the action of the selective PDE inhibitors is that they competitively inhibit the hydrolysis of cAMP, leading to an increase in its intracellular concentration. Logically, there should be a correlation between cAMP concentration and the degree of contractile response [69, 70]. However, there are some discrepancies in this relationship from studies which show a time lag between peak concentration of cAMP and contraction [44]. For example, after application of small concentrations of milrinone to isolated dog ventricular tissue, the early increase in tension correlated with the increase in cAMP concentrations. However, cAMP concentrations then declined and returned to control values before peak myocardial tension had occurred. After application of large concentrations of milrinone, the peak increase in cAMP concentration occurred later than tension development [33]. The demonstration of a correlation between the increase in cAMP concentration induced by beta receptor stimulation and inotropy is also equivocal [61]. Explanations for these findings have been proposed. There is some evidence that cAMP and isoenzymes are compartmentalized within cells and that compartments have different sensitivities to PDE inhibition. This would explain the biphasic dose—response relationships of cAMP to milrinone
in tissues and the poor correlation between dose and contractile response in animal models [29]. Some drugs with PDE activity have direct effects on myofibrils, by sensitizing them to Ca2+, and the increase in cAMP concentration is less important. The rate of dissociation of Ca2+ from its binding sites on troponin C is increased by phosphorylation. The force of contraction is increased independently of changing calcium concentration. There is evidence that this action is seen with other agents with PDE inhibitor actions (pimobendan and imadazodan) but it is unlikely to occur with the selective PDE agents currently available for clinical use [50, 66]. In conclusion, the positive inotropic effects of selective PDE inhibitors result mainly from inhibition of cardiac PDE III, leading to an increase in myocardial cAMP content. However, with some PDE inhibitors an additional mode of action is possible. Effects on vascular cAMP modulation and vasodilatation Changes in concentrations of Ca2+ and cyclic nucleotides also play central roles in contractionrelaxation cycles of vascular smooth muscle. Block of sarcolemmal calcium channels, or potassium channel opening and membrane hyperpolarization leads to vasodilatation. Increased concentrations of cAMP result from PDE inhibition and cause phosphorylation of myosin light-chain kinase. This leads to a reduction in its affinity for the Ca2+-calmodulin complex, dephosphorylation of myosin light chains and vasodilatation. cGMP is also a mediator in smooth muscle relaxation, via cGMP-dependent protein kinases [51]. Although, as outlined above, PDE III isoenzymes are considered high affinity cAMP PDE, they also hydrolyse cGMP [61]. This action may be important in the regulation of vascular tone and the mechanism underlying the profound vasodilatation that is a principal feature of the cardiotonic selective PDE inhibitors. The exact contribution of cAMP compared with cGMP with regard to this group of drugs is not completely established. Effects on the circulation
PDE inhibitors have been shown to be direct acting vasodilators in isolated limb studies in resting patients with heart failure, mainly by reducing systemic vascular resistance in skeletal muscle [14, 15,48,49]. Therapy with PDE inhibitors also improves skeletal muscle exercise performance in patients with heart failure [71], but it is not clear if this is caused primarily by the vasodilator or by the inotropic properties of the drug, as the ability of skeletal muscle to vasodilate during exercise is impaired in these patients [48]. Strictly speaking, any agent with a true positive inotropic action should always increase the myocardial oxygen consumption associated with an increase in contractility. In clinical studies, PDE inhibitors improve cardiac performance in the failing heart with unchanged or reduced myocardial oxygen extraction [5, 53]. This is in part because the systemic vasodilatation leads to a reduction in left ventricular systolic wall stress
Downloaded from http://bja.oxfordjournals.org/ at University of Birmingham on September 1, 2015
classical methylxanthine PDE inhibitor, theophylline. However, this drug and its congenors are capable of inhibiting all fractions of PDE isoenzymes, whereas amrinone was found to be selective for certain PDE subfractions. In addition, although inhibition of cyclic nucleotide PDE is often used to explain all the actions of theophylline, this is probably not the sole mechanism at therapeutic concentrations. The intracellular accumulation of cGMP may be partly responsible for mediating smooth muscle relaxation by theophyllines. However, methylxanthines act also as competitive antagonists of adenosine receptors (Pj purinergic) [62]. This could explain some effects of theophylline on the central nervous and cardiac conduction systems, and may underlie the efficacy of theophylline in antagonizing bronchoconstriction [74]. Theophylline has complex effects on the circulation, through a combination of brainstem vasomotor stimulation, smooth muscle relaxation and diuresis. It also has unfortunately narrow therapeutic margins. Although PDE III isoenzymes exist in airway smooth muscle, bronchodilatation is not a predominant clinical feature of the current cardiotonic selective PDE III inhibitors. Some PDE inhibitors attenuate the release of inflammatory mediators, particularly those affecting PDE IV (table III). It may be that PDE inhibitors, affecting these isoenzyme systems and combining bronchodilatation, inhibition of platelet aggregation and modification of airway inflammation, could be developed for future treatment and prophylaxis of asthma.
PHOSPHODIESTERASE INHIBITORS
297
TABLE IV. Pharmacokinctics of i.v. milrinone and enoximont. CHF = Chrome heart failure; GFR = glomerular filtration rate
Dose (bolus/infusion) Milrinone 50 ng kg-1 slow bolus 0.375-0.5 ug kg-' min"1 infusion Enoximone 0.5 mg kg"1 slow bolus 5-20 ug kg"1 min"1 infusion
Extraction
Terminal half-life
Modifications in renal failure
Renal 80% unchanged
1 h in health, 2.3 h in CHF
Reduce dose with GFR
