Combination therapy with ACE inhibitors and angiotensin II receptor blockers to halt progression of chronic renal disease: Pathophysiology and indications

Kidney International, Vol. 67 (2005), pp. 799–812

PERSPECTIVES IN RENAL MEDICINE

Combination therapy with ACE inhibitors and angiotensin II receptor blockers to halt progression of chronic renal disease: Pathophysiology and indications GUNTER WOLF and EBERHARD RITZ Department of Medicine, Division of Nephrology, Osteology, and Rheumatology, University of Hamburg, Hamburg, Germany; and Department of Internal Medicine, Renal Unit, Ruperto Carola University, Heidelberg, Germany

however, significantly lower in the majority of this studies in the combination treatment arms compared to the respective monotherapies. In a recent prospective study Japanese authors avoided this confounder and demonstrated that combination therapy reduced hard end-points (end stage renal failure or doubling of serum creatinine concentration) by 50% compared to the respective monotherapies. This effect could not be explained by a more pronounced reduction of blood pressure in the combination therapy group. Although these results are encouraging, administration of combination therapy should be reserved currently to special high risk groups. Further studies are necessary to confirm these promising results. It is possible that combination therapy may increase the risk of hyperkalemia, particularly when with coadministered with medications such as nonsteroidal anti-inflammatory drugs (NSAIDs) or spironolactone. In our opinion patients with proteinuria >1 g/day despite optimal blood pressure control under RAS-blocking monotherapy are a high-risk group which will presumably benefit from combination therapy.

Combination therapy with ACE inhibitors and angiotensin II receptor blockers to halt progression of chronic renal disease: Pathophysiology and indications. It is no a secret that we are confronted by an alarmingly increasing number of patients with progressive renal disease. There is ample evidence for the notion that angiotensin II (Ang II) is a major culprit in progression. The vasopeptide Ang II turned out to have also multiple nonhemodynamic pathophysiologic actions on the kidney, including proinflammatory and profibrogenic effects. Diverse complex Ang II generating systems have been identified, including specifically local tissue-specific renin-angiotensin systems (RAS). For example, proximal tubular cells have all components required for a functional RAS capable of synthesizing Ang II. On the other hand, Ang II is not the only effector of the RAS and other peptides generated by the RAS influence renal function and structure as well. Moreover, the discoveries that Ang II can be generated by enzymes other than angiotensin-converting enzyme (ACE) and that Ang II and other RAS derived peptides bind to various receptors with different functional consequences have further added to the complexity of this system. Several major clinical trials have clearly shown that ACE inhibitor treatment slows the progression of renal diseases, including in diabetic nephropathy. Well-controlled studies demonstrated that this effect is in part independent of blood pressure control. More recently, with Ang II type 1 receptor (AT 1 ) receptor antagonists a similarly protective effect on renal function was seen in patients with type 2 diabetes. Neither ACE inhibitor treatment nor AT 1 receptor blockade completely abrogate progression of renal disease. A recently introduced novel therapeutic approach is combination treatment comprising both ACE inhibitor and AT 1 receptor antagonists. The rationale for this approach is based on several considerations. Small-scale clinical studies, mainly of crossover design, documented that combination therapy is more potent in reducing proteinuria in patients with different chronic renal diseases. Blood pressure as an important confounder was,

One of the major challenges in nephrology is the exponentially growing number of patients developing terminal renal failure. It is now well-appreciated that the majority of these patients have a chronic decline of renal function over years before dialysis is required. This nonspecific process continues even when the initial insult is no longer present and has been called progression of chronic renal disease [1]. Regardless of the primary cause, the common underlying histologic lesions comprise tubulointerstitial fibrosis, tubular atrophy, and glomerulosclerosis [1, 2]. Clinical studies have provided ample evidence that hypertension and the degree proteinuria are the most important factors influencing progression [3]. The role of these mechanisms in progression of chronic nephropathy has been reviewed elsewhere [3]. Angiotensin II (Ang II) mediates both hemodynamic changes and renal growth suggesting that these processes are intimately connected. Both are thought to contribute to progression [4]. As summarized in Table 1, the nonhemodynamic actions of Ang II include stimulation of tubular transport, facilitation of proteinuria by modifying

Key words: renin-angiotensin system, proteinuria, AT1 and AT2 receptors, chymase, hypertension, end-stage renal failure. Received for publication November 4, 2003 and in revised form December 23, 2003, February 20, 2004, and April 28, 2004 Accepted for publication October 1, 2004
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2005 by the International Society of Nephrology 799

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Table 1. Effects of angiotensin II (Ang II) implicated in the progression of renal disease Vasoconstriction Expression of further vasoconstrictors (endothelins) and inhibition of vasodilators (nitric oxide, atrial natriuretic peptides) Mediation of glomerular hyperfiltration/glomerular hypertension Increased proteinuria by hemodynamic and nonhemodynamic effects (as modification of glomerular basement membrane characteristics, suppression of nephrin) Increased tubular uptake of proteins Stimulation or inhibition of tubular transporters (depending on the dose) Induction of profibrogenic cytokines [transforming growth factor-b (TGF-b)] Stimulation of glomerular and tubular growth (proliferation, hypertrophy) Induction of proinflammatory cytokines and chemokines [interleukin 6, interleukin 8, osteopontin, regulated upon activation, normal T-cell expressed and secreted (RANTES), and monocyte chemoattractant protein-1 (MCP-1)] Generation of reactive oxygen species, activation of nuclear factor-kappaB (NF-jB) Stimulation of fibroblasts proliferation Facilitation of lymphocyte proliferation Up-regulation of adherence molecules on endothelial cells Up-regulation of specific lipoprotein receptors

structure and function of the glomerular ultrafiltration barrier, induction of proinflammatory as well as profibrogenic cytokines such as transforming growth factor-b (TGF-b) (for review see [4]). Therefore, antagonizing these effects of Ang II is a key component of therapeutic strategies to halt progression. The efficacy of angiotensin-converting enzyme (ACE) inhibitors to lower blood pressure has been known since the middle 1970s [5]. Back then a normal kidney function was often considered a prerequisite for the treatment with ACE inhibitors. This naive ¨ view was changed forever by the seminal studies of Anderson, Rennke, and Brenner [6]. They documented that an ACE inhibitor reduced proteinuria and limited glomerular injury in the rat renal ablation model. The renoprotective action of ACE inhibitors was subsequently shown in human studies as well, initially in type 1 diabetes [7], but subsequently also in nondiabetic renal disease [8]. There is now limited evidence from experimental [9] as well as from clinical studies [3] that ACE inhibitor treatment may even cause regression of renal disease. The development of specific competitive nonpeptide Ang II receptor antagonists of the imidazole-5-acetic acid compound group provided an investigative breakthrough [10]. In 1990, losartan, the prototype Ang II type 1 receptor (AT 1 ) receptor antagonist, was the first orally active nonpeptide Ang II receptor antagonist to be used in humans [10]. On the other hand, the development of substances such as PD123177 which had little, if any, effect on the AT 1 receptor but specifically blocked the AT 2 receptor confirmed the long-known heterogeneity of Ang II receptors [10]. The AT 1 receptor has been cloned from vascular smooth muscle cells and adrenal glands (for review see [4]). More recently, the AT 2 receptor has been cloned as well and there is even preliminary evidence for further Ang II receptor subtypes [4]. In some [11], but not all [1, 13], experimental models of renal damage AT 1 receptor antagonists prevented progression of renal injury to a similar degree as ACE inhibitors. Initial studies in pa-

tients with essential hypertension suggested that losartan has virtually no side-effects and similar antihypertensive potency as ACE inhibitors [14]. Short-term clinical trials with a crossover design have shown identical antiproteinuric effects of AT 1 blockers and ACE inhibitors in proteinuric patients with various chronic renal diseases [15]. At least five large clinical studies provide now evidence that AT 1 receptor antagonists slow the rate of progression in patients with diabetic type 2 nephropathy [16–20]. Yet, neither monotherapy with an ACE inhibitor nor with an AT 1 receptor antagonist is able to completely prevent end-stage renal disease (ESRD). Recently the concept has been introduced that combination therapy (i.e., coadministration of an ACE inhibitor and an AT 1 receptor antagonist) is superior to the respective monotherapies. Although such combination may at first sight seem redundant, a sound pathophysiologic rationale can be offered to explain the superiority of combination therapy. In the following, we discuss the pathophysiologic background and the clinical data. We then provide arguments why combination therapy with licensed doses of ACE inhibitors and AT 1 receptor antagonists may become the standard management to delay progression in patients who have failed to respond satisfactorily to the respective monotherapies. THE RENIN-ANGIOTENSIN SYSTEM (RAS) IS MUCH MORE COMPLEX THAN WE THOUGHT IN THE PAST Traditionally, the RAS has been considered as an endocrine system. In this view, the glycoprotein angiotensinogen, produced in the liver, is cleaved by rennin, which is released from renal juxtaglomerular cells. Thus, angiotensin I is generated, which in turn is converted by the high ACE activity of the lungs into the active substance Ang II. Recently, is has become apparent that the system is much more complex. Genetic polymorphisms for various

Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

components of the RAS have been described [21]. The results are controversial, presumably in part explained by the different ethnic backgrounds of the study populations, and the controversies are far from being settled. Nevertheless, overexpression of the ACE gene in mice with three copies of the gene and experimentally induced diabetes aggravates nephropathy indicating that the overall activity of ACE, systemically and perhaps even more important locally, could be important at least under pathophysiologic conditions [22]. Accumulating evidence suggests that local angiotensin-generating systems exist, in various tissues, among others in the kidney, which operate independently of their systemic counterpart (reviewed in [4]). Such local tissue specific RASs have also been found in the vascular wall, brain, adrenal gland, testis, and ovary. In the kidney, a complete local RAS, including all its components, has been localized in proximal tubular cells. Micropuncture experiments demonstrated that proximal tubular fluid contains Ang II concentrations in the nanomolar range compared with picomolar concentrations in the systemic circulation [23]. Moreover, using microdialysis techniques Ang II concentrations in the nanomolar range were measured in the renal interstitial fluid. These studies suggest that tubular cells generate Ang II [24, 25]. It remains, however, controversial whether Ang II is actually produced in the intracellular compartment. It is possible that angiotensinogen is secreted by tubular cells and that Ang II is subsequently formed in the extracellular space by the enzymatic machinery of the ACE containing brush border membrane in the extracellular space. In line with this hypothesis, tubular cells are known to secrete angiotensinogen into the urine [26]. Urinary angiotensinogen, but not urinary Ang II, appears to reflect activation of the intrarenal RAS [26]. A positive autocrine feedback system exists in proximal tubular cells in that Ang II further stimulates angiotensinogen synthesis [27, 28]. Angiotensinogen delivered into the tubular fluid might be processed by more distal cells. Indeed, recent evidence suggests that Ang II modulates the activity of the sodium channel in distal tubules [29]. Local production of Ang II at these sites is obviously independent from the regulation of the systemic RAS [28]. Renal injury activates the local RAS directly and indirectly [4]. For example, 1,25dihydroxyvitamin D 3 is a negative regulator of the RAS and suppresses renin expression [30]. A reduction in 1,25dihydroxyvitamin D 3 , as occurs early in renal failure, will allow the local RAS to escape from feedback inhibition stimulating renin transcription and leading to a local increase in ANG II [30]. Proteinuria is a potent stimulator of the RAS in proximal tubular cells [31]. It is obvious that the local tubular RAS may interact with the systemic RAS at different levels [33]. For example, renin or angiotensinogen could be taken up from the circulation thus affecting the local generation of Ang II. Vascular en-

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dothelium could convert Ang I to Ang II, although it does not express renin. Studies in perfused hindlimbs of rats convincingly demonstrated that Ang II, locally formed in the vasculature, contributes to elevated blood pressure in renal failure despite suppressed plasma Ang II [32]. Furthermore, renin is taken up from the circulation into endothelial cells and contributes to local formation of Ang I demonstrating interaction between local and systemic components of the RAS [32]. Ang II itself could be taken up into the renal interstitium from the circulation via AT 1 receptors [33]. DOES THE ACTIVITY OF THE SYSTEMIC AND THE LOCAL RAS CHANGE IN PARALLEL? Particularly important is the observation that systemic administration of an ACE inhibitor results in almost complete inhibition of systemic Ang II formation, but affects intrarenal Ang II production little or not at all [34]. Furthermore, intrarenal Ang II is regionally compartmentalized. For instance, overall concentrations are higher in the medulla compared with the cortex [35]. Intact Ang II is present in intracellular compartments (endosomes). It is derived from receptor-mediated endocytosis subsequent to binding of Ang II to its putative receptor [35]. This could be an important mechanism because it has been demonstrated that Ang II may penetrate into adjacent cells by diffusion and may be even translocate into the nucleus where it directly regulates gene transcription [36]. Specific Ang II receptors have been localized on the nuclear membrane [4]. IS ACE THE ONLY ENZYME INVOLVED IN THE GENERATION OF ANG II? The traditional view held that ACE is the only enzyme capable of Ang II formation. This paradigm has recently been challenged [37]. Cardiovascular tissues express Ang II–generating enzymes other than ACE [37, 38]. One important example is the serine protease chymase. ACE inhibitors do not inhibit chymase activity, but today this enzyme can be inhibited by chymasespecific inhibitors. This allows us to differentiate between chymase-dependent and ACE-dependent Ang II formation [37]. It has been shown that more than 80% of Ang II formation in the heart and more than 60% in vessels may be due to chymase [37]. Specific chymase inhibitors attenuated cardiac fibrosis and Ang II–mediated TGF-b expression in animal models [38]. Since chymase is chiefly synthesized in mast cells, an accumulation of these cells in atherosclerotic plaques accounts for ACE independent formation of Ang II in atherosclerotic tissue [39]. Is chymase present in the kidney? A recent study demonstrated expression of chymase in the kidney of diabetic patients [40]. Up-regulation of chymase, in contrast

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Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

to ACE expression, was closely correlated with the presence of hypertension and extracellular matrix deposition [40]. This study provides direct evidence that chymase represents an alternative pathway for intrarenal Ang II formation at least in the diabetic kidney. In this context it is of interest that renal Ang II generation is not attenuated in ACE knockout (−/−) compared to the ACE+/+ wildtype mice [41]. Renal chymase activity was increased in ACE−/− animals suggesting that that this enzyme could easily compensate for the lack of ACE in mediating Ang II generation [41]. In addition to ACE and chymase, other enzymes, including chymostatin-sensitive angiotensin IIgenerating enzyme (CAGE) have been postulated to play a, presumably minor, role in Ang II generation [42]. It has been calculated that renal Ang II formation from enzymes other than ACE and chymase contributes <2% of total tissue Aang II [42], but the proportion might be drastically altered in diseased tissue. Durvasula et al [43] demonstrated in cultured podocytes that mechanical strain increases ANG II synthesis. This interesting mechanism may link elevated pressure gradient across the capillary wall that occurs in remnant nephrons after loss of renal tissue, with alteration in function and structure of podocytes. Importantly, preincubation with an ACE inhibitor did not block the stretch-induced increase in Ang II indicating that generation occurs by other enzymes. Another fascinating aspect of ACE has recently been discovered. Kohlstedt et al [44] provided convincing evidence for a novel function for ACE as a key signal transduction molecule. Using endothelial cells, Kohlstedt et al demonstrated that the ACE inhibitors ramiprilat and perindoprilat increase CK2-mediated phosphorylation of serine1270 . Furthermore, ACE inhibitor treatment also increased the activity of N-terminal kinase (JNK) in endothelial cells [44]. These provocative findings indicate that ACE inhibitors may mediate cellular function by “outside-in” signaling directly through ACE, an effect totally independent of the generation of Ang II. ARE ALL DETRIMENTAL ACTIONS OF ANG II MEDIATED BY THE AT 1 RECEPTOR? Two main Ang II receptors (AT 1 and AT 2 ) exist which are differentially expressed within in the kidney [45]. Both receptors have been cloned. They have the configuration of a seven-transmembrane receptor, but share only around 30% homology on the protein level. Pharmacologic evidence suggests that there may be additional binding sites for Ang II, at least in certain species, but those putative receptors have not yet been identified on a molecular level [45]. AT 1 receptors are coupled to heterotrimeric G proteins and engage various signal transduction pathways, including activation of phospholipases, inhibition of adenylate cyclase, stimulation

of tyrosine phosphorylation, and other second messenger pathways [45]. Whether the AT 2 receptor associates with G proteins is controversial [46]. A number of studies have indicated that activation of AT 2 receptors leads to an increase in intracellular protein phosphatase activity [46]. Depending on the cell type, either tyrosine or serine/threonine phosphatases are stimulated. Both AT 1 and AT 2 receptors are developmentally regulated, but the AT 2 receptor is much more abundant in the fetal kidney. Throughout nephrogenesis, the AT 2 receptor is mainly expressed in undifferentiated mesenchyme. It is down-regulated after birth. AT 1 receptors are initially localized in the nephrogenic cortex and developing glomeruli, proximal tubules, and vessels. During maturation of the kidney, AT 1 receptor expression becomes more abundant. AT 1 receptor expression is induced by a variety of stimuli, including hypercholesteremia and changes in osmolarity, but is down-regulated by high Ang II concentrations [45]. In contrast, AT 2 receptors are induced by tissue injury but are not down-regulated by high Ang II [46]. Specifically, AT 2 receptors are reexpressed in the kidney during renal injury and remodeling of nephrons [46]. Thus, in pathophysiologic situations when tissue injury has occurred and when the local RAS is activated Ang II may preferentially bind to up-regulated AT 2 receptors. Almost all classic Ang II–associated functions such as vasoconstriction, aldosterone release, stimulation of tubular transport, proinflammatory effects as well as profibrogenic and growth stimulatory actions are mediated by the AT 1 receptors [4]. The function of the AT 2 receptor is less clear, but is has been proposed that activation of this receptor antagonizes those functions which are mediated by the AT 1 receptor, providing negative feedback control. Along this line, it has been shown that activation of the AT 2 receptors decreases blood pressure through release of nitric oxide, inhibits growth and induces differentiation, and even mediates apoptosis [46, 47]. Recent evidence, however, indicates that cell hypertrophy and proinflammatory effects are also transduced by AT 2 receptor activation under certain conditions [48– 51]. IS STIMULATION OF THE AT 2 RECEPTOR BENEFICIAL OR INJURIOUS? Treatment with an ACE inhibitor or an AT 1 receptor antagonist has strikingly different effects on the activation of the AT 2 receptor. ACE inhibition leads, at least initially, to a decrease in Ang II formation so that less Ang II binds to AT 1 as well as AT 2 receptors. In contrast, AT 1 receptor blockade does not reduce, and actually increases, the concentration of Ang II (Fig. 1). This excess of Ang II will then bind to and activate AT 2 receptors. In the past it had been assumed that activation of AT 2 receptors

Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

AT1 antagonist ANG IV

AT4

ANG II

AT1

AT2

ACE inhibitor ANG II

AT1

AT2

Bradykinin

Fig. 1. Differences between angiotensin II type 1 (AT 1 ) receptor antagonists and angiotensin-converting enzyme (ACE) inhibitors. Since AT 1 receptor blockers increase the local angiotensin II (Ang II) concentration (interruption of negative feedback at the juxtaglomerular apparatus), Ang II could either bind to AT 2 receptors or, after conversion to Ang IV, interact with AT 4 receptors. Obviously, binding of excess Ang II to AT 2 and AT 4 receptors is not antagonized by AT 1 blockers. In contrast, inhibition of ACE reduces the Ang II concentrations resulting in less agonistic stimulation of both for AT 1 and AT 2 receptors. Furthermore, ACE inhibitor increase bradykinin levels through inhibition of its breakdown by the kinase activity of ACE.

is exclusively beneficial [10]. Nevertheless, evidence has recently accumulated which clearly suggests that AT 2 receptors may mediate adverse effects. The activated AT 2 receptor stimulates proinflammatory pathways by induction of nuclear factor-kappaB (NF-jB) [49, 50]. Further evidence for potentially adverse effects of AT 2 receptor stimulation is provided by in vivo studies demonstrating that AT 2 receptor blockers exhibit anti-inflammatory effects in models of renal injury and are associated with attenuation of renal damage [51]. A further difference between the effects of ACE inhibition and AT 1 receptor blockade concerns the concentration of bradykinin. ACE inhibition attenuates the breakdown of bradykinin, a vasodilatory peptide (Fig. 1). Experimental data argue against a major role of bradykinin in the renoprotective action of ACE inhibitors. Using specific bradykinin receptor blockers, it has been shown in animal models of chronic renal disease that the protective effects of ACE inhibitors are due to reduced Ang II and not increased bradykinin concentrations [52, 53]. A further difference between ACE inhibitors and AT 1 receptor blockers concerns the plasma concentration of the tetrapeptide acetyl-seryl-aspartyl-lysyl-proline a natural substrate for ACE which increases after ACE inhibition. It has recently been shown that it interferes with the development of cardiac fibrosis [54] and possibly also renal fibrosis. To introduce further complexity, a specific extracellular receptor for renin with no homology to any known membrane protein has been identified on mesangial and vascular smooth muscle cells [55]. Binding of renin to this receptor leads to activation of mitogen-activated protein (MAP) kinases and also increased intracellular efficiency of angiotensinogen cleavage to Ang I [55]. This finding raises the possibility that very high local concentrations

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of Ang II may be build up in the vicinity of this receptor. The role of renin receptors in patients with renal disease is currently unknown.

METABOLISM OF ANG II AND GENERATION OF ALTERNATIVE PEPTIDES So far we have only discussed generation and action of Ang II. How about breakdown of Ang II? Ang II is metabolized by peptidases, such as aminopeptidase A (APA) [56], into Ang III and further into Ang IV. Ang IV binds to a specific receptor named AT 4 [57]. This receptor is widely expressed in the kidney including endothelial cells, and proximal and convoluted tubules [57]. Recent evidence suggests that this receptor may be identical with the enzyme insulin-regulated aminopeptidase [58]. Ang IV stimulates plasminogen activator inhibitor-1 (PAI-1) expression in proximal tubular and endothelial cells through AT 4 receptors [57, 59]. Since PAI-1 reduces extracellular matrix turnover, Ang IV may contribute to renal fibrosis independently of activation of AT 1 and AT 2 receptors. Moreover, Ang IV generating enzymes are up-regulated in conditions of renal injury associated with high Ang II concentrations [56], presumably shifting more Ang II into the degradation pathway to Ang IV. Other effects of Ang IV are release of nitric oxide and phosphorylation of focal adhesion kinase [60, 61]. Another bioactive peptide derived from the RAS is Ang 1-7. Interestingly, Ang 1-7 has properties opposing the effects of Ang II [62]. For example, Ang 1-7 has antihypertensive effects by stimulating the release of vasodilator prostaglandins and nitric oxide [62]. Furthermore, Ang 1-7 inhibits proliferation of cells and opposes the growth stimulatory effects of Ang II [62]. Some of these effects may be explained by Ang 1-7–induced down-regulation of AT 1 receptors [63]. Recent evidence suggests that ANG 1-7 is the endogenous ligand for a G protein–coupled receptor encoded by the mas protooncogene [63]. Ang 1-7 is formed by a two step process involving Ang 1-9 as an intermediate from Ang I, bypassing the requisite production of Ang II (Fig. 2). It has also recently been shown that Ang 1-7 blocks interconversion of Ang I to Ang II mediated by tissue ACE.

IS ACE THE ONLY CARBOXYPEPTIDASE ACTING ON ANG I? A novel enzyme with similarities to ACE has recently been identified and called ACE2 [65]. ACE2 is expressed predominantly in vascular endothelial cells, including those of the kidney [66]. ACE and ACE2 have different actions. Whereas the “classic” ACE converts Ang I to the octapeptide Ang II, ACE2 cleaves one amino acid less off from Ang I so that Ang 1-9 is formed [65]. Ang 1-9 itself

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Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

Angiotensin I

ACE1

ACE2

Angiotensin II

Angiotensin 1-9

Aminopeptidase A

Angiotensin III

ACE1

Angiotensin 1-7

Angiotensin IV Fig. 2. Overview of angiotensin-converting enzyme (ACE) and ACE2 functions. The “classic” ACE1 leads to the formation of the octapeptide angiotensin II that is further metabolized by aminopeptidase A into angiotensin III and finally angiotensin IV. The recently discovered ACE2 enzyme cleaves one amino acid less, forming angiotensin 1-9. This intermediate without known function is subsequently converted by the “classic” ACE inhibition into angiotensin 1-7. The effects of angiotensin 1-7 are opposite to those of angiotensin II such as vasodilatation and also down-regulates angiotensin II type 1 (AT 1 ) receptors. It has been hypothesized that it is an endogenous antagonist of angiotensin II. Some data suggest that ACE2 is influenced by ACE inhibitors. ACE inhibition causes less formation of less angiotensin II but possibly also less conversion of angiotensin 1-9 to angiotensin 1-7. The result would be less antagonism to the activity of angiotensin II by reduced availability of its endogenous antagonist.

has no known biologic activity. In a second step it can be further converted to Ang 1-7, however, by the “classic” ACE. Thus, it has been be suggested that up-regulation of ACE2 counteracts the function of Ang II, because more Ang I is shifted into the Ang 1-7 forming pathway which mediates biologic effects opposite to those of Ang II [65]. Interestingly, ACE2 knockout mice exhibit severely impaired cardiac function and up-regulation of hypoxia-inducible genes, but have normal blood pressure [65]. Although ACE2 is not directly influenced by an ACE inhibitor, these drugs may indirectly diminish the production of Ang 1-7, because the second conversion step of this pathway (i.e., from Ang 1-9 to Ang 1-7, depends on the “classic” ACE (Fig. 2). In the kidney, ACE2 is preferentially expressed in renal tubules and this expression is reduced in diabetic rats [66]. Such altered expression could further increase tubular Ang II because more substrate is provided for ACE [66].

WHAT IS THE CLINICAL EVIDENCE FOR A RENOPROTECTIVE ACTION OF ACE INHIBITORS? The first trial documenting the renoprotective action of ACE inhibitors compared captopril with placebo in type 1 diabetic patients with advanced nephropathy [7].

Captopril treatment reduced the combined endpoints of death, dialysis, and transplantation by 50%. Importantly, the reduction of the risk was statistically significant by multivariate analysis even when the minor blood pressure difference between the groups was taken into account [7]. In the “Angiotensin-Converting Enzyme Inhibition in Progressive Renal Insufficiency” (AIPRI) trial patients with renal insufficiency caused by various kidney diseases received 10 mg/day benazepril or placebo [67]. In the benazepril group the risk of reaching a renal end point (i.e., doubling of serum creatinine or dialysis) was reduced by more than 50%. Diastolic blood pressure was somewhat lower in treated patients than in the placebo group so that it remained undecided whether the renoprotective effect was caused by better blood pressure control. In the “Ramipril Efficacy in Nephropathy Study” (REIN) patients mainly with chronic nondiabetic renal disease were categorized according to baseline proteinuria, and assigned either to ramipril or placebo on top of conventional antihypertensive therapy targeted to achieve diastolic blood pressure values <90 mm Hg [68, 69]. In patients with proteinuria of 1 to 3 g/day, progression to ESRD was significantly less frequent in the ramipril arm and so was progression to nephrotic proteinuria. Importantly, blood pressure was titrated to identical values in the ramipril and placebo groups [69]. In patients with proteinuria >3.5 g/day the REIN study was stopped at the second interim analysis, since the difference in decline in glomerular filtration rate (GFR) between the ramipril and placebo groups was highly significant. Ramipril treatment reduced the combined risk of doubling of serum creatinine or end-stage renal failure by half [69]. The REIN core and follow-up studies offer convincing evidence that the tendency of GFR to decline in chronic renal disease can be effectively halted [8, 70]. A recent meta-analysis indicated that antihypertensive regimens which include ACE inhibitors are superior to regimens without ACE inhibitors in delaying the progression of disease across a spectrum of renal diseases [71]. It was concluded that this beneficial effect of ACE inhibition is independent of the decrease in blood pressure and urinary protein excretion [71]. In a small prospective crossover trial Lods et al [72] investigated the effect of an ACE inhibitor and an AT 1 receptor antagonist on the activities of metalloproteinases (MMPs) in patients with glomerulonephritis [72]. MMPs are enzymes involved in the turnover of extracellular matrix proteins and their activity is increased in different models of renal inflammation. ACE inhibitor treatment, but not irbesartan reduced MMP activity in patients with glomerulonephritis. Although it is currently unclear whether this is beneficial for the kidney in the long run, this study documents differences with potential pathophysiologic relevance between ACE inhibitors and AT 1 receptor antagonists [72].

Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

ARE AT 1 RECEPTOR BLOCKERS RENOPROTECTIVE? Several studies provided evidence that AT 1 receptor blockers slow the progression of renal disease in patients with type 2 diabetes [16–20]. Parving et al [16] administered the AT 1 antagonist irbesartan to hypertensive patients with type 2 diabetes who had early renal damage as manifested by microalbuminuria [16]. Irbesartan treatment significantly reduced proteinuria [16]. This and other trials demonstrated that AT 1 receptor blockers reduce microalbuminuria in patients independent of blood pressure control and are even effective in reducing urinary albumin excretion in normotensive diabetics [19, 20]. Reduction of proteinuria presumably reflects protection against protein-induced renal injury that will hopefully translate into preservation of kidney function in the longer term. Two other studies used the AT 1 antagonists irbesartan and losartan in patients with type 2 diabetes and advanced diabetic nephropathy [17, 18]. The AT 1 antagonists reduced protein excretion, and the rate of decline in GFR causing later onset of dialysisdependent renal failure compared to the control medication [17, 18]. In a cohort of patients that comprised 13% type 2 diabetics the Losartan Intervention for End Point Reduction in Hypertension (LIFE) showed that losartan prevented cardiovascular morbidity and death more than did atenolol [73]. However, these studies do not answer the question whether an AT 1 antagonist is as good as, or better, or even worse than an ACE inhibitor in delaying progression of renal disease. They also do not answer the question whether AT 1 receptor blockade will be effective in halting progression in nondiabetic renal disease. Although it has been argued that ACE inhibitors are as renoprotective as AT 1 receptor blockers, no data from clinical trials with head-to-head comparison between ACE-inhibitors and AT 1 antagonists and hard end points are currently available. IS THERE CLINICAL EVIDENCE FOR SUPERIOR BENEFIT WITH COMBINATION THERAPY? An overview of clinical trials investigating dual blockade of the RAS is shown in Table 2 [74–91]. Zoccali et al [74] were among the first to investigate a potential effect of an AT 1 receptor antagonist as add-on therapy in 11 patients treated with an ACE inhibitor. The patients had various renal diseases, including diabetic nephropathy [74]. In this pilot study, losartan caused a 30% reduction of proteinuria in patients who were already on ACE inhibitors [74]. Although not significant, addition of losartan to ongoing ACE inhibitor therapy also caused a marked decrease in blood pressure (from 105 ± 7 mm Hg to 98 ± 15 mm Hg mean arterial pressure). This report

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exemplifies the problems with which riddled almost all clinical studies investigating combination therapy in patients with chronic renal disease because blood pressure was lower in the combination therapy arm. Nevertheless, these smaller studies contributed important data in regard of side-effects, investigated combination therapy of various ACE inhibitors with different AT 1 receptor antagonists, and looked at patients with heterogeneous causes of chronic renal disease. All studies with one exception [90] looked exclusively at the potential reduction in proteinuria, but failed to evaluate renal function as an outcome parameter. This is partly explained by the short observation period (between 2 weeks and 6 months) in these studies (Table 2). We do not deny the relevance of proteinuria. For example, a mass screening study in more than 100,000 patients in Japan showed convincingly that proteinuria was a strong independent predictor for the development of end-stage renal failure [92]. How does proteinuria mediate progression of renal disease? This issue has recently been elucidated in some detail [93–98]. Proteinuria stimulates protein uptake by proximal tubular cells through megalin- and cubilin-dependent transport processes [95]. Increased protein traffic causes tubular cell dysfunction through various mechanisms, including complementdependent pathways and generation of oxidative stress, among others catalyzed by iron-containing proteins. Excessive endocytosis of proteins induces local production of proinflammatory and profibrogenic cytokines [94, 96]. The effects of Ang II and proteinuria on tubular cytokine production are additive and presumably reinforce each other. In the context of the present discussion it is of particular interest that proteinuria activates the local tubular RAS [31, 97]. In the amphibian kidney is has been demonstrated that the role of proteinuria in the genesis of tubulointerstitial injury is totally independent of glomerular damage [98]. The model allows direct exposure of “open” nephrons with a stoma connecting the tubule with the peritoneal cavity to proteins after intraperitoneal injection. This maneuver caused progressive focal accumulation of fibrous tissue around protein-storing open, but not around protein-free closed, tubules [98]. Nevertheless, although proteinuria is a marker for progression and even a plausibly pathophysiologically relevant culprit, it is only a surrogate marker and not a hard end point. Dual blockade of the RAS led in the majority of the smaller studies summarized in Table 2 to a significant reduction in systemic blood pressure. Since hypertension is a very important factor in progression, and since proteinuria is also closely associated with the severity of hypertension [99], it had remained unclear whether the observed reduction of proteinuria was caused by the more pronounced reduction in blood pressure with combination therapy compared to the respective monotherapies. In other words, it had not been safely excluded that

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Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

Table 2. Clinical studies with combination therapy Authors (year)

Reference

Number of patients and disease

Zocalli et al (1997) Russo et al (1999) Ruilope et al (2000) Mogensen et al (2000) Russo et al (2001) Agarval et al (200l, 2002)

[74]

11 patients with various renal diseases 8 patients with IgA nephropathy 108 patients with chronic renal insufficiency 199 patients with type 2 diabetes 10 normotensive patients with IgA nephropathy 16 patients (12 with diabetes and 4 with glomerulonephritis 45 patients with various proteinuric nephropathies 18 patients with type 2 diabetes

[75] [76] [77] [78] [79, 80]

Maximal dose

Follow-up

50 mg add-on losartan

2 weeks

30% reduction in proteinuria

10 mg enalapril/50 mg losartan 10 mg benazepril/160 mg valsartan 20 mg lisinopril/16 mg candesartan 20 mg enalapril/100 mg losartan 40 mg lisinopril/50 mg losartan

1 month

30% to 40% reduction in proteinuria

1 month

Reduction in blood pressure and proteinuria Reduction in blood pressure and small effect on proteinuria 40% reduction in proteinuria

3 months 2 months 1 month

Luno et al (2001)

[81]

Rossing et al (2002) Berger et al (2002) Laverman et al (2002) Jacobsen et al (2002) Kincaid-Smith et al (2002) Ferrari et al (2002) Jacobsen et al (2003) Campbell et al (2003)

[82]

[89]

24 patients with nondiabetic chronic nephropathies

20 mg lisinopril/16 mg candesartan 8 mg add-on candesartan 8 mg add-on candesartan 40 mg lisinopril/100 mg losartan 300 mg add-on irbesartan 8 mg add-on candesartan 20 mg fosinopril/150 mg irbesartan 20 mg benazepril/80 mg valsartan 10 mg benazepril/80 mg valsartan

Nakao et al (2003)

[90]

263 patients with nondiabetic renal disease

3 mg trandolapril/100 mg losartan

36 months

Iodice et al (2003)

[91]

10 patients with nonproliferative glomerulonephropathy

20 mg ramipril/300 mg irbesartan

2 months

[83] [84] [85] [86] [87] [88]

12 patients with various chronic glomerulopathies 9 patients with various chronic glomerulonephropathies 21 patients with type 1 diabetes 60 patients with various proteinuric nephropathies 10 patients with glomerulonephritis 20 patients with type 1 diabetes

Results

an equal effect might have been reached by the respective monotherapies had lowering of blood pressure been equivalent. Equal blood pressure control was achieved in a recent prospective randomized crossover study comparing combination therapy with benazepril and valsartan with the respective monotherapies [89]. Despite comparable blood pressure values, combination therapy decreased proteinuria more than ACE inhibitor or AT 1 receptor blocker alone [89]. A further problem is raised by the fact that often only the licensed maximal doses had been used [100]. The validity of this point is illustrated by the experimental studies of Ots et al [101] and Peters, Noble, and Border [102] which failed to show an additive effect on glomerulosclerosis when maximal doses of losartan and enalapril were combined. In pharmacologic studies it is not permitted to draw the conclusion that two drugs have additive effects unless maximal doses (i.e., at the plateau of the dose response relationship) had been used. This point is all the more important, since, as described above, the intrarenal

6 months 2 months 2 months 1.5 months 2 months 3 months 6 weeks 2 months 2 months

No effect on proteinuria and reduction in transforming growth factor-b (TGF-b) excretion 30% reduction in proteinuria independent of blood pressure 25% reduction in proteinuria and blood pressure reduction 30% reduction in proteinuria and blood pressure reduction 10% reduction in proteinuria 16% reduction in proteinuria and blood pressure reduction 16% reduction in proteinuria and blood pressure reduction 40% reduction in proteinuria independent of blood pressure 15% reduction in proteinuria and blood pressure reduction 14% reduction in proteinuria and no significant effect on blood pressure and creatinine clearance Significant reduction in end points, significant reduction in proteinuria, and no difference in blood pressure No additional effect by combination therapy on proteinuria

RAS is compartmentalized. It has been shown that doses of ACE inhibitors higher than the maximally blood pressure lowering dose are necessary to affect local Ang II formation. Maximal doses of enalapril or losartan that failed to cause further reduction of blood pressure nevertheless further reduced TGF-b expression and renal fibrosis in an experimental model of a mesangioproliferative glomerulonephritis [101, 102]. Yet, even at maximally effective doses of enalapril or losartan failed to normalize TGF-b synthesis so that it is unlikely that even complete inhibition of the RAS, both systemic and local, would ever completely halt progression. This point has been also illustrated by clinical observations. In a well-controlled study Ruggenenti et al [103] assessed forced up titration of the ACE inhibitor lisinopril in patients with chronic nephropathies. They found that increasing lisinopril to maximally tolerated doses ameliorated lipid abnormalities but failed to reduce proteinuria further compared to conventional doses [103]. This finding calls for additional interventions aiming at different targets as suggested by Hostetter [104].

Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

Taking into consideration that Ang II is pivotal for expression of TGF-b, the above findings suggest that maximal antifibrotic benefit from RAS blockade requires higher doses than are needed to normalize blood pressure. Titrating ACE inhibitors or AT 1 receptor blockers solely based on their blood pressure lowering effect according to currently usual practice is clearly inadequate to satisfactorily block the local RAS. Despite maximal blood pressure lowering, a high enalapril dose which had caused maximal lowering of blood pressure failed to reduce, let alone abrogate, intrarenal Ang II concentration [24]. In the case of AT 1 receptor blockade one could speculate that the reduction in blood pressure induced by AT 1 receptor blockers is due not only to an inhibition of AT 1 but also to an activation of AT 2 as a result of high ambient levels of Ang II. Thus, a significant reduction in blood pressure could occur even without a total blockade of AT 1 receptors. In a small study with 16 patients, Agarwal [80] found that add-on therapy with losartan on a background of maximal ACE inhibitor treatment leads to a significant lowering of urinary TGF-b excretion. This effect was independent of changes in proteinuria and blood pressure values as determined by 24-hour ambulatory measurements [80]. Since urinary TGF-b excretion presumably reflects renal synthesis, combination therapy appears to be significantly more potent in suppressing renal production of this important profibrogenic cytokine. The situation has completely changed with one recent study assessing the effect of combination therapy on a hard end point (i.e., end-stage renal failure or doubling of serum creatinine concentration). In a randomized controlled trial the doses of the ACE inhibitor trandolapril and the AT 1 receptor blocker losartan were carefully adjusted to reach absolutely identical blood pressure values in all treatment arms (i.e., trandolapril, losartan, or their combination) [90]. Nakao et al [90] showed that a combination of trandolapril and losartan reduced progression more effectively than the respective monotherapies. The risk of reaching the combined end point was reduced in the combination therapy group by approximately 50% compared to the respective monotherapies. There was no difference between the trandolapril and losartan monotherapies in reaching the end points [90]. Although office measured blood pressures were not different among the various, 24-hour blood pressure measurements were not performed. Thus, its seems possible, but unlikely, that a better control of night time blood pressure or a different distribution of dippers and non-dippers in the various groups may explain the superiority of dual blockade. Since office blood pressure measurements were performed 15 minutes before administration of the drugs, it remains unclear whether the blood pressure values were different at the times of peak actions of the drugs [90]. Although the theoretical argument whether the ef-

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fects of ACE inhibition and AT 1 receptor blockade are additive is not definitely answered, by this study, it nevertheless clearly proves that despite similar blood pressure control the combination of the maximally licensed doses of ACE inhibitor and AT 1 receptor blocker provides further benefit not only on proteinuria, but also on hard end points. Combination therapy caused more pronounced lowering in proteinuria and this observation points to a potential mechanism of action. ARE THERE POTENTIAL MECHANISMS TO EXPLAIN THE SUPERIORITY OF COMBINATION THERAPY? Azizi et al [105] studied combination therapy in normotensive volunteers with mild sodium depletion, and proposed the hypothesis that the combination abrogates the potentially adverse consequences of persistently high Ang II concentrations during ACE inhibition. Other potential mechanisms to explain the superiority of combination therapy are summarized in Table 3. Nishiyama, Seth, and Navar [24] convincingly demonstrated that the components of the tissue specific RAS are compartmentalized. In animal experiments tissue ACE, including ACE in the kidney, is not inhibited by the plasma concentrations of ACE inhibitors that are achieved with currently used doses [24]. Consequently, renal Ang II formation presumably continues despite blockade of the systemic RAS by ACE inhibitors in doses which achieve maximal lowering of blood pressure. Furthermore, alternative enzymes which are not inhibited by ACE inhibitors such as chymase [40] and other serine proteases might still lead to significant local Ang II concentrations within the kidney even when one would achieve complete inhibition of the local ACE [37]. Experimental studies have demonstrated that a compensatory Ang II release during ACE inhibition occurs exclusively from within the kidney, indicating that intrarenal non-ACE–dependent pathways of Ang II formation are stimulated [106]. Since AT 1 receptor blockers increase the concentration of Ang II, it is conceivable that, at least at low concentrations of Ang II receptor blockers, high Ang II concentrations may overcome inhibition of the AT 1 receptor. Furthermore, the increased availability of Ang II will increase the generation of Ang IV that binds to a separate receptor and mediates, at least in experimental settings, profibrogenic effects that are not antagonized by AT 1 receptor blockers [59, 60]. We have shown that aminopeptidases in the kidney which are responsible for Ang IV formation are up-regulated in situations with high local Ang II and/or renal injury [56]. In animals fed a low-salt diet to activate the RAS, combination therapy with ACE inhibitor and AT 1 receptor blocker reduced renal tissue Ang II concentrations more than higher doses of either agent alone [107].

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Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

Table 3. Potential mechanisms why combination therapy could be superior to the respective monotherapies Angiotensin-converting enzyme (ACE) inhibitors Shortcomings Ongoing angiotensin II generation by ACE independent enzymes (chymase) Failure to block the compartmentalized intrarenal ACE Inhibition of angiotensin 1-7 formation that partly antagonizes effects of angiotensin II Advantages Feedback inhibition of renin release by angiotensin II relatively maintained Less angiotensin II available to stimulate angiotensin II type 2 (AT 2 ) receptors if one assumes that stimulation causes negative effects Direct protective effect of ACE inhibitors independent of renin-angiotensin system? AT 1 receptor blocker Shortcomings High angiotensin II concentrations because of interruption of negative feedback High renin (interruption of negative feedback) that may bind to specific renin receptors Stimulation by Angiotensin II of AT 2 receptors with activation of pathophysiologic pathways [apoptosis, nuclear factor-kappaB (NF-jB) activation, induction of chemokines] Metabolism of angiotensin II through local peptidases into other peptides, including angiotensin IV, which binds to specific receptors not blocked by AT 1 receptor antagonists Since angiotensin II binds to AT 2 receptors, which may transduce vasodilatory effects, in the presence of AT 1 receptor antagonists, titration of the drug according to blood pressure may lead to incomplete AT 1 receptor blockade Advantages Complete blockade of AT 1 receptor-mediated effects Vasodilatation by stimulation of AT 2 receptors No aldosterone escape

A further possibility is binding of Ang II to unoccupied AT 2 receptors during blockade of the AT 1 receptor. This may not be only beneficial as had been widely assumed [108]. It is true that in AT 1A receptor knockout mice less renal fibrosis is seen when the animals are exposed to renal injury [109]. Nevertheless, since apoptosis could be mediated through the AT 2 receptors, it is hard to imagine that an increase in programmed cell death of renal cells should be protective [47]. Moreover, experimental studies show that the NF-jB signal transduction pathways are activated through the AT 2 receptor. Infusion of Ang II in rats leads to glomerular regulated upon activation, normal T-cell expressed and secreted (RANTES) expression and influx of macrophages/monocytes [48–50]. These effects are attenuated in the presence of an AT 2 receptor antagonist. AT 2 receptor blockade was also beneficial in a renal ablation model [51]. Using double-transgenic rats harboring human renin and angiotensinogen genes, Mervaala et al [110] showed an additional effect of combination therapy on sodium handling. In these rats the pressure natriuresis curve is shifted to the right, indicating that a higher blood pressure is necessary for sodium excretion. ACE inhibition increased GFR and renal blood flow, but had no significant effects on fractional sodium or water reabsorption. In contrast, AT 1 receptor blockade shifted the curves for fractional sodium and water reabsorption to the left, but had only small effects on renal blood flow and GFR [110]. A combination of ACE inhibition and AT 1 receptor blockade had an additive effect and shifted the pressure-natriuresis curves leftward more than did either drug alone [110]. Aldosterone exerts directs effects on renal cells independent of Ang II that may contribute to progres-

sion of renal disease [111]. It is therefore important that plasma aldosterone increases after long-term ACE inhibitor treatment. This phenomenon of “aldosterone escape” is presumably caused by non ACE–dependent Aang II formation with subsequent induction of aldosterone synthesis [112]. In this context it is of interest that a certain proportion of proteinuric patients with type 2 diabetes who had been treated with an ACE inhibitor had a secondary increase in aldosterone (aldosterone escape). Additional administration of spironolactone reduced proteinuria in these patients [113]. Whether further addition of spironolactone or eplerenone will provide further benefit in patients in whom proteinuria is not satisfactorily lowered by combination therapy and whether this treatment is safe will require further studies. Finally, it is important to pay attention to some pharmacokinetic mechanisms. The normal dosing schedule used in patients is twice daily administration of ACE inhibitors with intermediate half life (such as enalapril) or of AT 1 receptor antagonists (such as losartan). It is questionable whether this regimen guarantees that plateau effects are maintained around the clock. In renal insufficiency the pharmacokinetics are altered, however, and the half life is considerably longer. IS COMBINATION THERAPY RISKY? Clinical observations suggest a link between administration of ACE inhibitors and worsening of anemia, in patients with chronic renal insufficiency [114– 116]. Ang II stimulates erythropoesis through different mechanisms. Ang II induces proliferation of erythroid progenitors by reducing the peptide N-acetyl-serylaspartyl-lysyl-proline (acetyl-SDKP), a possible stem cell

Wolf and Ritz: Combination therapy to halt progression of chronic renal disease

suppressor (for review see [115]). Through AT 2 receptors Ang II down-regulates SM-20, a dioxygenase responsible for prolyl hydroxylation of hypoxia-inducible factor 1 (HIF-1) [117]. HIF-1 is an important transcription factor for erythropoetin. Hydroxylation of HIF-1 targets this factor into a degradation pathway. Thus, hydroxylation of HIF-1a results presumably in accelerated degradation in the absence of Ang II with suppression of erythropoetin gene transcription. Iodice et al [91] observed a significant decrease of hemoglobin concentrations in patients treated with the combination of 20 mg ramipril/300 mg irbesartan per day compared to irbesartan alone. However, this effect was likely caused by the relatively high dose of the ACE inhibitor because 20 mg ramipril/day induced a similar degree of anemia [91]. Adverse effects of a combination therapy were addressed in the study by Ruilope et al [76]. Serum potassium concentration was significantly higher in patients on combination therapy compared to patients on an AT 1 receptor antagonist alone [76]. This study, unfortunately, did not contain an ACE inhibitor monotherapy arm. Other trials, however, found a similar frequency of hyperkalemia in patients on combination therapy compared with ACE inhibitor monotherapy [90]. It is known that the increase in serum potassium is less marked with AT 1 receptor antagonists than with ACE inhibitors [118]. Therefore, hyperkalemia in the presence of combination therapy appears to be mainly determined by the ACE inhibitor, but the situation may be different in patients with renal failure. Nevertheless, the incidence of hyperkalemia in the COOPERATE study was 7.9% in the combination therapy group, 9.3% in the trandolapril groups, and 4.4% in the losartan group [90]. Therefore, patients should be regularly monitored for hyperkalemia and coadministration of medication that could increase potassium such as nonsteroidal anti-inflammatory drugs (NSAIDs) or spironolactone should be avoided. Further observational studies are necessary to define the rate of hyperkalemia outside the well-monitored prospective COOPERATE study with a selected population of patients. CONCLUSION According to Sackett et al [119] evidence-based medicine is “the conscientious, explicit and judicious use of current best evidence in making decisions about the care of the individual patient. It means integrating individual clinical expertise with the best available external clinical evidence from systematic research.” Is the current evidence enough to treat every patient with chronic renal insufficiency with combination therapy? We don’t think so. Although intriguing on the basis of pathophysiologic considerations, only one large trial has so far documented a convincing effect of the combination therapy on hard end points [90]. Certainly, more studies in different

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ethnic populations and a different spectrum of renal diseases are necessary to confirm these findings from Japan. This is particularly true for the COOPERATE study that excluded diabetic patients and it is currently unclear whether similar effects of combination therapy would be equally effective in patients with diabetic nephropathy. AT 1 receptor antagonists are currently expensive drugs. Not to the least because of financial considerations, administration of combination therapy should be reserved currently to special high-risk groups. A recent meta-analysis revealed that in patients with urine protein excretion greater than 1 g/day the risk of progression of renal disease increased steeply with the systolic blood pressure [120]. In patients on monotherapy despite optimal blood pressure control in whom proteinuria does not decrease to values <1 g/day, the risk of progression is very high. We think it is plausible to assume that such patients would derive particular benefit from combination therapy. The beneficial effect of combination therapy has to be weighed against the risk of hyperkalemia and further studies are necessary to obtain data regarding the incidence of hyperkalemia under combination therapy, particularly in patients such as diabetics. Before one recommends more widespread use of combination therapy, however, we have to wait for the results of additional clinical trials. We admit that the demonstration of the superiority of combination therapy is intellectually exciting and pathophysiologically plausible. If further evidence is generated, it may some day become the treatment of choice. Reprint requests to Gunter Wolf, M.D., Medizinische Klinik III, Universit¨atsklinikum Jena, Erlanger Allee 101, D-07747 Jena. E-mail: [email protected]

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