Not so symple: End of the road for renal denervation procedures?

Since the advent of minimally invasive renal denervation procedures about a decade ago, the nephrology community had been eagerly awaiting conclusion of the SYMPLICITY 3 trial. As many of you might already know, Medtronic, the device’s company issued a press release on January 9^th about the procedure failing to meet the primary efficacy endpoint. To a lot of us, this result has been surprising, and disappointing to a certain extent.

1952 paper from Mayo: Renal denervation/sympathectomy is not a new concept and has been around for almost a century. 

To understand the context, let’s review the background of renal denervation’s role in treatment of resistant hypertension briefly. Per NHANES data from the period 2003-2008, the prevalence of resistant hypertension is 8.9 ± 0.6% of the US hypertensive population, and continues to increase.  I am guessing the prevalence could theoretically be a little lower if the new, more relaxed, hypertension treatment goals of JNC 8 are considered. This was discussed by Matt earlier. 

Renal denervation is supposed to work on the premise that the sympathetic nervous system is hyperstimulated in patients with treatment resistant high blood pressure, and the kidneys modulate that to a large extent. Afferent signaling from the kidneys increases central sympathetic drive, while efferent signals to the kidneys increase renin release and sodium retention, while reducing renal blood flow. Hence, if you could cut off this two way traffic between the kidneys and the sympathetic nervous system, you should be able to bring the blood pressure down. Here is a picture that explains this. And below is a video of how the procedure is performed: 

Renal sympathetic afferent and efferent fibers run circumferentialy in the wall of the renal artery, and ablation reduces both pathways  

We have seen a succession of studies done to validate this hypothesis. Earlier, Matt had talked about the SYMPLICITY-1 trial, which was a non-randomized pilot study. Then we had SYMPLICITY-2 trial in 2010 which was a larger randomized study with 106 patients. This was conducted in Europe, Australia, and New Zealand. This showed significant reduction in office based blood pressures at 6 months, with no serious side effects. And so, with much fanfare and hype, the SYMPLICITY-3 trial was initiated in August 2011. While the study has not been published, the authors chose to communicate the procedure’s lack of efficacy (in meeting the primary end point) via a press release. There was no mention about the status of secondary end points (which included ambulatory BP changes) in the statement.

WHICH BRINGS US TO THE QUESTION…

Why did the trial fail to meet the primary efficacy end point, in spite of the prior data that looked promising? It appears to be the best designed trial till date to study this issue. SYMPLICITY-3 was bigger (535 patients) and better designed (control patients underwent a sham procedure) than its predecessors. I wonder if that itself could have been the reason for us seeing the lack of any benefit. That in fact, the prior trials were outliers given smaller size of the studies, or lack of randomization. Or, could it have something to do with anatomical and functional regrowth of sympathetic nerves that can happen after they are ablated (as can happen post transplant)?  Let us know your thoughts about what you think could have been the reasons!  

IMPLICATIONS FOR CKD PATIENTS

Something that I have always wondered about (and which has nothing to do with this press release) is the safety and efficacy of this procedure in CKD patients. It is plausible that if you are tinkering with the renin angiotensin system, you could adversely affect renal function. Just like you could be leery of giving ACE inhibitors to an advanced CKD-4 patient with hypertension, would you consider not doing this procedure as well? Could we see hyperkalemia develop? In the absence of adequate data, I would say that I don’t know, but we did see a study that tried to answer this question. However, it was a small study (15 patients, and even those didn’t complete follow up), and a short follow up period.   

  

IF ALL YOU HAVE IS A HAMMER, EVERYTHING LOOKS LIKE A NAIL?

So is this development the beginning of the end of renal denervation procedure? Maybe; or maybe not. Medtronic has already suspended further enrollment in other renal denervation trials in the US (SYMPLICITY-4), Japan, and India. However, the device will still remain available for “discretionary use” by physicians (good luck convincing insurance companies to pay for a procedure that is about as good as sham!). But, we still have other potential indications under the sun which could be a breath of life for the device/procedure. These include heart failure, metabolic syndrome, and obstructive sleep apnea.   

Finally, we also have another device that works on the principle of baroreceptor activation that is being actively studied for treatment of resistant hypertension.

Veeraish Chauhan, MD

Electrolyte Channels and Aldosteronism

Over the past few years, it has become apparent that hyperaldosteronism is far commoner than was once suspected and screening of unselected patients with hypertension reveals that about 5-10% of patients have primary hyperaldosteronism. In patients with resistant hypertension, that percentage increases to 15-20%. About 30% of hyperaldosteronism is caused by aldosterone producing adrenal adenomas (APA). Most of the rest is related to bilateral adrenal hyperplasia with less than 5% of cases being familial. The secretion of aldosterone in adrenal cells is dependent on the intracellular calcium concentration and increases in response to higher plasma calcium. Entry of calcium into the cells is in turn dependent on voltage-gated membrane calcium channels (which allow calcium influx when the cells are depolarized) and a calcium ATPase which removes calcium from the cells. Under normal circumstances, adrenal cells are hyperpolarized thus keeping these calcium channels closed. Cell polarization is maintained by a combination of the action of the Na-K-ATPase (which exchanges 3 intracellular Na for 2 extracellular K) and membrane K channels lead to K loss from the cells.

Angiotensin II inhibits the Na-K-ATPase leading to cell depolarization, calcium influx into cells and aldosterone secretion. Similar effects are seen when cells are treated with oubain, a specific Na-K-ATPase inhibitor that also leads to hyperaldosteronism.

In 2011 in a seminal paper in Science, Choi et al reported finding somatic mutations in KCNJ5, a membrane potassium channel in patients with APA. These were identified by sequencing tissue from the tumors and comparing with the surrounding tissue. Subsequently, it has been found that about 30-40% of patients with APA have somatic mutations in KCNJ5. These mutations are believed to reduce the ion selectivity of the channels, allowing Na to move into the cell and reduce the resting membrane potential. A number of families have been identified with KCNJ5 mutations resulting in bilateral hyperplasia - now called Familial Hyperaldosteronism type III.

Recently, a paper was published in Nature Genetics which attempted to determine if there were other somatic mutations in patients with APA. In this study, they took KCNJ5-normal patients and sequenced the exons of the tumors and the surrounding tissue. There were very few mutations identified but 5/9 patients had mutations in ATP1A1, a component of the Na-K-ATPase or ATP2B3, a component of the calcium ATPase that removes calcium from adrenal cells. Follow-up targeted sequencing of 300 patients with APA revealed that about 7% had mutations in one of these two genes. Patients with these mutations had higher aldosterone levels, lower minimum potassium levels and higher systolic BP, all indicators of more severe disease. Notably, no families have been identified with these mutations. In vitro studies revealed that cells with these mutations have very low membrane potentials and it is speculated that if this was a germline mutation, it would likely not be compatible with life. This is a fascinating insight into how very small changes in electrolyte channels can have far-reaching consequences and shows a great progression from exome sequencing to the bench and to clinical investigation.

The images in this post are taken from the recent paper in Nature Genetics. One would wonder if somatic mutations explain some of the missing heritability that were are seeing in genetic studies of common diseases. See this previous post by Lisa on the genetic causes of hypertension.

Key to angiotensin formation solved? Preeclampsia linked to oxidative stress

Researchers from the UK published a new and exciting article exploring one of the fundamental mechanisms of blood pressure regulation by the renin-angiotensin-system (RAS). This research was, per the authors, 20 years in the making and was fittingly published this week in Nature. The RAS is a multi-enzymatic system with multiple layers of control (click here for a review of the RAS from Nate). The first step of this process is the cleavage of angiotensinogen by the enzyme renin to form angiotensin I (Ang I). Precisely how renin acts upon angiotensinogen has not, until now, been completely elucidated.

The researchers were able to almost completely resolve the structure of angiotensinogen (a large 452 AA protein) by x-ray crystallography to the resolution of 2.1A. This showed that the angiotensin cleavage site was inaccessibly buried in the c-terminal tail of this large protein. They went on to show that when angiotensinogen is oxidized this region changes shape to permit ready access of this site to renin which, in turn, cleaves off a 10 amino acid portion termed Ang I. The supplemental data shows a nice 3D movie of this interaction. The oxidized form of angiotensingen is able to bind to the pro-renin receptor in tissues and has a 4-fold higher catalytic activity for Ang I formation than the reduced form. They authors hypothesize that under conditions of oxidative stress, angiotensinogen is found more often in the oxidized form leading to hypertension.

Lastly, as proof of principle, they showed that patients with preeclampsia had a higher amount of the oxidized form of angioensinogen in their plasma. Typically the ratio of reduced to oxidized angiotensinogen is 40:60 in normal individuals. In preeclampia this ratio is 30:70. They speculate that oxidative stress may be sufficient to cause hypertension associated with preeclampsia. Interestingly, a mutation in the angiotensinogen gene has been reported to cause increased catalytic activity this protein.

In conclusion, this report opens up many new investigative avenues in the field of hypertension research. Not only does this report hint at a possible role for anti-oxidants in hypertension therapy, but a new target for drug modulation of this complex system now exists. However, more research will be needed to confirm how changes in angiotensingen structure leads to alterations in blood pressure homeostasis.

Paging the doctor

I recently saw a young hypertensive patient who had suffered a motor vehicle accident complicated by a transected aorta that was successfully repaired several years back. He has had hypertension since then and was referred to us for evaluation and management of resistant hypertension. The workup is ongoing so we don't yet have the (secondary) diagnosis, if any, however the differential included some interesting entities, such as post-op coarctation and an interesting phenomenon known as Page kidney.

Page kidney, first described by Dr Irvine Page in 1939, is the phenomenon of hyperreninemic hypertension that results from extrinsic compression of the renal parenchyma. Page first described the phenomenon in animal models, showing that wrapping an animal kidney in cellophane led to hypertension (see original JAMA article). Clinically, the phenomenon is seen after trauma or after a nephrologic intervention such as a kidney biopsy. In either case, a subcapsular hematoma leads to compression of the affected kidney parenchyma and reduces renal blood flow, leading to renin release. A tumor or cyst can cause the same phenomenon. Time course from traumatic incident to hypertension is not universally consistent, and the hypertension can be immediate or subacute in onset. The hypertension often resolves after nephrectomy, though some case reports of successful evacuation of the hematoma or drainage of the compressing cyst have been reported.

So next time you biopsy, be sure to watch for tachycardia and hypotension as a sign for post-biopsy bleeding as usual, but don't dismiss new hypertension as a reassuring sign that there is no bleed!

See Page Kidney featured in NEJM's Images in Clinical Medicine here.

A great combination

Since the retraction of the COOPERATE trial by the Lancet late last year (following Regina Kunz's famous 'letter of concern'), combination RAAS blockade has suffered something of a fall from grace. This was dealt a second blow by the results of the ONTARGET trial, where combination treatment in diabetics with vascular disease was associated with an increased rate of AKI and adverse events, without additional benefit. However, it must be remembered that patients in that trial did not have proteinuria, and the study endpoint was a composite outcome of death from cardiovascular causes, not progression of renal disease. As nephrologists, we are commonly faced with the problem of patients with CKD and heavy proteinuria not controlled by an ACE or ARB alone, and whom we know are at high risk for progressive renal disease. Is there evidence for combination therapy in this context?


In the latest AJKD, this trial by Bianchi et al. makes for provocative reading, although I feel the results must be interpreted with caution. They randomized 128 patients with a clinical diagnosis of idiopathic chronic GN to either intensive therapy (ACE + ARB + spironolactone + high dose statin) or conventional therapy (ACE + low dose statin). The endpoints were change in GFR, proteinuria and adverse events at 3 years. The intensive arm experienced a far greater reduction in proteinuria and stable GFR over time compared to the conventional arm (who lost ~ 7ml/min over the study period) at the expense of more hyperkalemia and discontinued therapy.


I was surprised at how little hyperkalemia developed in the intensive arm (9 vs 3 events). I would have to admit to being pretty nervous of triple RAAS blockade for fear of this complication. That said, if patients can tolerate the combination, the outcomes based on this small study look promising.

Thanks to Dr. Rafael Santamaria for bringing this trial to my attention.

A trap for angiotensin type 1 receptors

The renin-angiotensin system is powerful regulator of blood pressure homeostasis. An article published in the March 24 issue of JASN highlights an emerging area of research into the modulation of AT1 receptors on different tissues.

The majority of well-known angiotensin II actions are mediated via AT1 receptor stimulation, and angiotensin converting enzyme inhibitors (ACEi) and AT1 receptor blockers (ARBs) have been widely used as antihypertensive agents with cardiovascular protective effects. Elucidating factors that regulate AT1 receptor expression levels in different tissue compartments will hopefully lead to novel agents to treat hypertension and its associated end-organ damage (ESRD, CHF and Stroke). Recent studies have demonstrated the existence of several proteins interacting with AT1 receptors that may modulate AT1 receptor expression level, sensitivity and internalization. A recent review article highlights this emerging field. Atrap (AT1 receptor-associated protein) is the best characterized protein and the focus of the March 24 article by Oppermann et al.

It was previously reported that Atrap (a 19kD protein) interacts specifically with the carboxyl-terminal domain of the A1a receptor and catalyzes its internalization in cultured cells. Prior in vivo studies over-expressing the Atrap protein in the heart, aortae and femoral artery in mice demonstrated a protective effect in these tissues in response to angiotensin II infusion. Overall, the prevailing literature suggested an inhibitory or protective effect of Atrap on AT1 receptor function. This group knocked out the Atrap gene in mice to test this hypothesis in vivo.

They reported that Atrap KO mice have a higher resting blood pressure (by 10 mmHg) as measured by radiotelemety. Suggesting that Atrap is important in regulating basal blood pressure and that its absence leads to systemic hypertension. Interestingly, they found that Atrap is highly expressed in the kidney (then testis=adrenal>heart>lung=liver=aorta=brain). Specifically, they showed that Atrap is highly expressed in the proximal tubule of the kidney. Previous investigations have focused on the distal tubule as potential sites that affect blood pressure as most of the known genetic mutations in sodium transport causing hyper/hypotension are linked to the distal nephron (Bartter, Little, Gitelman syndromes, etc). This group reports that Atrap KO mice have more AT1 receptors in the proximal tubule which allows for increased angII-dependent NHE-3 activity causing volume expansion and hypertension.

This highlights the importance of investigating novel pathways regulating the renin-angiotensin system. Modulation of this system with the use of ACEi, ARBs and now direct renin inhibitors have continued to be the mainstay of therapy for patients with heart failure, diabetes, hypertension and kidney disease. Activation of Atrap may be more specific and more physiological for inhibition of At1 receptor signaling. New drug discovery modulating Atrap protein expression could lead to novel therapy. For now, much more research is needed.

SGLT Inhibitors

A promising new class of oral hypoglycemic agents for type 2 diabetes, the SGLT inhibitors, are on the horizon. As they work on the kidney, and potentially have renal side effects, it may be helpful to review them.

These agents work by inhibiting the SGLT2 glucose transporter found in the S1 segment of the proximal tubule, responsible for 90% of glucose reabsorption in the kidney. They work by causing dose-dependant renal glycosuria, lowering both plasma glucose levels and insulinemia in animal models. Agents have the suffix -gliflozin, and examples include dapagliflozin and sergliflozin. Here's a recent review.

Interestingly, humans with loss-of-function mutations in SGLT2 live into adulthood, but experience systemic hypotension with markedly elevated renin levels, hypernatremia and metabolic alkalosis. This seems to be driven by renal sodium wasting occurring in association with glycosuria. These phenomena could also possibly occur as side effects of these drugs. Whether this side effect is a blessing (through improved BP and volume control) or a curse (through an adverse interaction with ACE inhibitors) remains to be seen. But with over 20 prototypic agents in the drug development pipeline, we'll definitely be hearing more about them.

Basic Review: The Renin-Angiotensin-Aldosterone Axis

One of the coolest aspects of the renin-angiotensin-aldosterone system (RAAS) is that it involves multiple organ systems: the liver, lung, adrenal gland, kidney, and vasculature are all prominently involved. It never hurts to review basic physiologic principles, right? Listed are the three main components of the RAAS and their main mechanisms of action.

1. Renin is a peptide hormone secreted from the juxtaglomerular cells of the afferent arteriole in response to 3 main stimuli: (a) renal hypoperfusion, (b) decreased distal chloride delivery to the macula densa, and (c) increased sympathetic activity. Renin antagonists such as aliskiren are presently being tested as antihypertensive agenst with thus far promising results.

2. Angiotensinogen--which is synthesized and secreted from the liver--is cleaved by renin in the systemic circulation to form angiotensin I.

Angiotensin I is cleaved to form angiotensin II by angiotensin converting enzyme (ACE), which is found predominantly within lung endothelium. ACE-inhibitors, as their name implies, targets the ACE enzyme and is one of the most potent anti-hypertensives (and GFR-preserving) therapies available.

Angiotensin II has the following physiologic effects, which it carries out via binding to AT1 and AT2 receptors. Drugs which block the ability of angiotensin II to bind to its receptors ("angiotensin receptors blockers", or ARBs) make up another highly successful and renoprotective antihypertensive therapy. Angiotensin II binding to its receptors have the following major effects:

a) angiotensin II acts as a systemic vasoconstrictor.
b) angiotensin II causes renal efferent arterial vasoconstriction. Acutely, efferent vasoconstriction should increase GFR; however, over time the increased glomerular pressure leads to glomerular damage and, ultimately, renal injury.
c) angiotensin II increases secretion of aldosterone from the zona glomerulosa of adrenal cortex.

3. Aldosterone: in cortical collecting duct cells, aldosterone diffuses into the cell and interacts with the mineralocorticoid receptor, which upon binding translocates to the nucleus and increases expression of ENac. The end result of aldosterone action is sodium reabsorption and potassium & hydrogen secretion. In addition to angiotensin II, hyperkalemia can also stimulate aldosterone secretion. The drug spironolactone interferes with aldosterone interacting with its receptors, and can be effective in the treatment of hypertension.

Fibromuscular Dysplasia

Fibromuscular dysplasia (FMD) accounts for a significant fraction of renal artery stenosis. The distinction between fibromuscular dysplasia and atherosclerotic renal artery stenosis is important as the former tends to respond much more readily to interventional therapy than the latter. More specifically, FMD tends to respond favorably to balloon angioplasty.

There is some suggestion that FMD has a genetic component to it, though no gene has yet been identified. The diagnosis is usually made by angiography showing a "beads on a string" pattern in the affected artery. FMD can affect arteries other than just the renal arteries (e.g. the carotid arteries), but the disease is most famous for its effect on the renal arteries where it may cause secondary hypertension due to activation of the renin-angiotensin system.

Mineralocorticoid-Blocking Activity of Oral Contraceptives

I didn't learn this until recently, but some commonly used oral contraceptives--for example Yasmin-28 (the progestin-derivative drospirenone + ethinyl estradiol)--has some mineralocorticoid blocking effect, similar to that seen for spironolactone. It may therefore be associated with hyperkalemia, particularly in patients already on medications that predispose to higher serum K's. The package insert actually recommends monitoring K levels during the first month in patients receiving Yasmin who are also on ACE-I/ARBs, K-sparing diuretics, NSAIDs, or K-supplements, but this recommendation may be overlooked.

Page Kidney

In 1939, Irwin Page performed an experiment in a dog model in which one kidney was wrapped in cellophane. The resultant external compression on the kidney resulted in elevated renin-angiotensin-aldosterone levels and severe hypertension. Over time, a loss of renal function occurs due to some degree of renal ischemia and constrictive perinephritis. Because there are high levels of circulating aldosterone, renal K-wasting and hypokalemia can also be present.

Now, "Page Kidney" refers to any condition in which external compression of the kidney leads to activation of the renin-angiotensin-aldosterone system resulting in hypertension. The most common cause of a Page kidney is a subcapsular hematoma, which can result from blunt trauma or iatrogenesis, such as a renal biopsy or ESWL for kidney stones. Rarely, large cysts or tumors can also cause Page kidney.

Dual ACE/ARB therapy: ONTARGET

The ONTARGET trial was on the major trials looking at the potential benefits of dual ACE-inhibitor (ACE-I)/angiotensin receptor blocker (ARB) therapy.  Although it was not intended to specifically examine the effects of dual blockade of the renin-angiotensin system on progression of CKD as a primary outcome, a 2008 Lancet article analyzes this trial from a renal perspective.  

ONTARGET was a randomized, double-blind comparison of three different groups:  the ACE-inhibitor ramipril alone (10mg po qd), the ARB telmisartan alone (80mg po qd), or a combination group which received both ramipril and telmisartan.

I won't go into the gory details (a good summary in AJKD by Jeffrey Berns can be found here), but the upshot is that although combination therapy improved proteinuria, it resulted in overall worsened renal outcomes, including a requirement for dialysis or a doubling of serum creatinine.  

There are several caveats to this trial--for example, most of the patients in the trial did not have CKD to begin with, so it is not the ideal population to answer the question of whether or not dual blockade therapy is a good strategy for preventing the progression of existing CKD--but it does suggest that we should not uniformly use the dual-blockade strategy blindly, despite its seemingly sound physiologic rationale.  It also suggests that further trials designed to look at the effects of dual ACE/ARB therapy should look at hard outcomes (such as death, need for dialysis, doubling of serum creatinine, etc) rather than simply using the surrogate marker of proteinuria as a primary outcome. 

Review: The Juxtaglomerular Apparatus

The juxtaglomerular apparatus (JGA) is located between the afferent arteriole and the returning distal convoluted tubule of the same nephron. It is responsible for regulating both intrarenal (tubuloglomerular feedback) and extrarenal (renin-angiotensin-aldosterone) mechanisms necessary to maintain both renal and entire body volume status.

The three components of the JGA are the following:

(1) the juxtaglomerular cells of the afferent arteriole, synthesize and store renin, which is secreted in response to specific stimuli (e.g., low blood flow, decreased NaCl delivery). The juxtaglomerular cells could be considered the "effector arm" of the renin-angiotensin-aldosterone axis.

(2) the macula densa, a region of the distal convoluted tubule characterized by tubular epithelial cells which are more densely-packed than in other regions of the nephron (and thereby leading to its characteristic appearance on light microscopy). The macula densa can be considered the "sensory arm" of the renin-angiotensin-aldosterone axis in that these are the cells which sense decreased Na Cl delivery which determines downstream function. They are also involved in the mechanism of tubuloglomerular feedback.

(3) mesangial cells, which form connections via actin and microtubules which allow for selective vasoconstriction/vasodilation of the renal afferent and efferent arterioles with mesangial cell contraction.

ACE DD Polymorphism

The human angiotensin coverting enzyme (ACE) gene is located on chromosome 17q23 and is comprised of 26 exons and 25 introns. Importantly, there is an insertion/deletion polymorphism in which a 278-bp fragment is either present or absent within intron 16. The deletion "D allele" has been shown to result in increased expression of the ACE gene, and interestingly the DD genotype is associated with an increased risk of hypertension, diabetic renal disease, and cardiovascular complications. The DD genotype turns out to be much more common in black populations than in white populations, and perhaps can account for some of the increased susceptibility towards renal disease observed in blacks compared to whites.

Glucocorticoid-Remediable Aldosteronism

Glucocorticoid-Remediable Aldosteronism (GRA) is an inherited autosomal dominant disorder which causes early-onset (often childhood) high blood pressure, often occuring in individuals with Celtic ancestry.

It typically presents with standard symptoms of aldosteronism (hypertension, hypokalemia, and metabolic alkalosis) with a high aldosterone, low renin state.

The mechanism is pretty cool: the genes for both aldosterone synthetase and 11b-hydroxylase are adjacent genes on the same chromosome which are 95% homologous. Sometimes a crossover event occurs yielding a chimeric gene in which aldosterone synthesis is controlled by ACTH!

The treatment, therefore, is prednisone, to suppress ACTH (and therefore aldosterone).

Aliskiren

Are renin inhibitors the next big weapon in the fight against hypertension?

As we all know, ACE-I's & ARB's have so many renoprotective effects that there is substantial evidence towards using these medications as first-line agents for blood pressure management in those with kidney disease, diabetes, or proteinuria. In addition to blockade of angiotensin converting enzyme (ACE) or the angiotensin receptor (ARB), a new class of direct renin inhibitors of which aliskiren is the current prototype are now being tested. While the evidence here is still accumulating, the available data suggests that it gives similar decreases in BP when compared to ACE or ARB, and the results of the recent AVOID trial demonstrate that in patients already treated with ARB, addition of aliskiren compared to placebo led to significant reductions in the degree of proteinuria. It makes sense that inhibition of renin (the protein structure of which is illustrated above) would be an effective pharmacologic target of the renin-angiotensin system given that this, not angiotensin converting enzyme-catalyzing of angiotensin I to angiotensin II, is the rate-limiting step in the pathway.