Using Proenkephalin A 119-159 as a novel biomarker to estimate GFR

What is Acute Kidney Injury (AKI) and what problems come with it?

Acute kidney injury (AKI) is a very prevalent syndrome for patients in the hospital and especially in the Intensive Care Unit (ICU). Up to 50% of patients in the ICU develop AKI which is associated with both morbidity, as well as mortality. Apart from the period of time in the intensive care or in the hospital, AKI is related to a higher chance to develop end-stage renal damage in the following years and its impact on renal function is a huge burden for both the patient, as well as society having in mind it's very expensive to treat. 
Unfortunately, there is no specific treatment for AKI to date. Nevertheless, there are multiple things that physicians can do for patients that are at risk for AKI or in whom AKI is developing. For example, the KDIGO guidelines recommend secondary prevention, including discontinuation of nephrotoxic agents, hemodynamic monitoring, etc that aim to prevent further damage in these patients.
 

How is AKI diagnosed today and do you see any difficulties?

In daily practice, less a decrease in urine output may be a trigger to improve volume status or hemodynamic situation, but is not really viewed as AKI. AKI is mainly diagnosed based on the serum creatinine level. We can measure this biomarker very easily and we do know that there is a relationship between the glomerular filtration rate (GFR), a measure of renal function, and the creatinine value circulating in the blood. However, the problem is that this is not a linear relationship and a change in creatinine level is detected late and the marker is insensitive. This means it will take up to two days before you will detect an increase in creatinine while a huge number of nephrons, the basic functional unit of the kidney where urine is formed and then excreted, are being lost already. And moreover, if the GFR is slowly improving again, creatinine is still peaking, and it will take another two days before the decrease will become apparent. 

This indicates that the timing of AKI diagnostic seems to be quite important. Why?

The most important thing that we can do if a patient is at risk for or is developing AKI is to try to prevent further damage. But the therapeutic window to act is short and steadily decreases over time. Since creatinine is a late biomarker, this attenuates the options to take preventive measures and treat AKI, as irreversible damage may already have occurred.
In addition, a clinician may need to adjust dosing of drug treatment based on renal function, because that drug is nephrotoxic by itself, or the drug is cleared by the kidneys. 

How can early biomarkers improve patient outcomes?

Without a timely biomarker, it will remain difficult to discover new treatment options because the damage is already done. So, we do need to look for new biomarkers with more swift kinetics that allow early intervention. It is clear that creatinine and urine output criteria are late functional biomarkers. To bring the field forward, early and specific biomarkers are urgently needed.

Next to creatinine, are there other more precise measures available and if yes, are they already used in clinical routine?

Currently, the gold standard technique to measure the GFR - and with that assessing and diagnosing AKI - is using Iohexol or Iothalamate. A small amount of these compounds is administered and then serially measured in the bloodstream over several hours.  It is necessary to measure a couple of samples to calculate the GFR. Even though the measurement is reliable and the contrast agents are easy to obtain, it is very labor-intensive and expensive. That is why it is not commonly used in clinical practice. 

Are there more practicable and precise alternatives to the current available standard biomarkers?

Yes, e.g., Enkephalin, an endogenous opioid that is produced in specific places in the body and has several properties that make it a good measure of renal function. It's completely filtered in the glomerulus, there's no protein binding and there's no active tubular secretion or reabsorption, as it is the case for creatinine, which is a problem because, during AKI, you will have an overestimation of the GFR due to active excretion into the urine. Therefore, changes in Enkephalin might be more swift in detecting changes in GFR. It is a functional biomarker, not a tubular injury biomarker.
The problem was that enkephalin was really difficult to measure in the blood, but this got solved by the diagnostic company SphingoTec which can now provide a test for easy and rapid measurement of a surrogate marker, Proenkephalin A 119 -159 (penKid), a stable fragment of enkephalins.
Enkephalin by itself has effects on the kidneys, such as increased diuresis. The biomarker penKid is very well characterized in healthy volunteers, showing, that there is little variation within the normal population allowing to identify a baseline and working with just single measurements is already sufficient. Moreover, it could be also shown, that non-renal factors, like muscle mass, do not influence penKid levels, unlike creatinine.

You mentioned penKid being a functional biomarker, not a tubular injury biomarker, can you elaborate on the differences?

Tubular injury biomarkers are mainly molecules, contained in the tubular cells and if the cells die, they release proteins. And you can measure these in the urine or sometimes also in the plasma. That means it is a measure of damage to tubular cells. At that moment, the function of the kidney does not have to be influenced yet, so GFR can still be good. Another shortcoming in the case of sepsis or other conditions that induce systemic inflammation is that some injury biomarkers can lose their ability to correlate with AKI, especially stage one of AKI. Functional biomarkers are a measure of the renal function itself.   

PenKid seems not to be influenced by non-renal factors as is demonstrated so far. What about age, does penKid also work in children? 

Yes, different levels of penKid are associated with differences in severity of AKI, But interestingly, the baseline level of critically ill children without AKI is ten times higher than in adults. That means, the normal value is really age-dependent, but there is a relationship with the development of AKI in all age groups. 

Do you see additional benefits of penKid, e.g., in renal replacement therapy?

For creatinine, it is known that the level goes down in patients in the ICU on renal replacement therapy (RRT) because the machine also clears creatinine. Therefore, it is difficult to decide if the lower creatinine concentration is actually the consequence of better renal function of the patient or just a consequence of RRT. This is not the case with penKid: levels of this biomarker are stable - even in patients on RRT. So that means that a change in penKid level may reflect a change in renal function even if the patient is on RRT.
In addition, we observed that penKid levels not only increase earlier compared to creatinine, but also the decrease is 1 day earlier. This means that in a patient with an improving renal function, you can predict that creatinine will be lower tomorrow. And this is relevant because a patient with increasing creatinine values might need to go on renal replacement therapy, but if the penKid level is decreasing at the same time, then it's very likely that the next day the creatinine value will also decrease. RRT in that patient might not be necessary because the next day the situation will improve. That is another interesting indication for penKid: it not only indicates the worsening of the patients’ condition, but also the improvement.
Just to demonstrate, why this is important for patient care and outcome: Observational data from Finland show that if a patient is in need of RRT during the stay in the ICU, even if it’s only one day, then that patient is approximately 100 times more likely to develop end-stage renal disease within the next five years. Getting to know earlier that the patient is developing AKI, would support the clinician in taking nephroprotective measures to prevent the disease progression. This could have a big impact on the long-term outcomes of these patients.

PenKid seems to have a lot of advantages compared to current standard biomarkers. How specific and sensitive is it?

In our lab, we measured both, creatinine and penKid, every day in over 600 adult ICU patients. We found that the average increase in penKid, was one day earlier than the diagnosis of AKI itself, allowing us to predict AKI. We found that for a patient with a low penKid level (<50 pmol/L), there is 90% certainty that that patient will not develop AKI in the coming day. A high penKid value (>100 pmol/L) on the other hand, points to a 90% chance that this patient indeed will develop AKI the next day. 

Is the use of penKid values intuitive in the clinical routine? And if not, how could it be made easier for physicians to work with this innovative biomarker?

Of course, penKid is a new biomarker and physicians arenot yet used to it. We are used to looking at creatinine and we are used to having an eGFR, an estimated GFR based on creatinine. That's why we decided to make a model, looking at different items that we measure in different patients to see if we can actually improve the current existing formulas and have a better estimation of the GFR based on penKid. In our study, we had access to over 1300 patients that had indeed measurements with the earlier mentioned gold standard (iohexol/iothalamate), as well as creatinine and penKid values. Here, we included a huge spread of patients that were either in steady-state situations or in non-steady state renal situations. 
We compared our newly modeled penKid-creatinine formula to other available formulas, including the recent 2021 CKD-EPI formula, and demonstrated that adding penKid resulted in an even better performance. Recently, we submitted our new eGFR formula for publication so everybody will be able to use it in the future. Using the penKid concentration to estimate GFR will help clinicians, as they are used to work with GFR. All a clinician needs to know is the age, the creatinine and penKid value, put it in this formula, and then he/she will be able to use the most accurate eGFR that we have found so far.

Could the new GFR formula be applied at the Emergency Department and Intensive Care Unit?

In our study, different patient cohorts were used to build the algorithm. This included steady-state patients, as well as patients in the ICU, cardiac surgery patients, septic shock patients and more. So yes, I think this allows the use of our new GFR formula in the different departments and for all these patient groups. 

Does this include children as well?

We did not test it on children so far. I think it will be different in children because in children without AKI we found much higher penKid levels compared to an adult healthy person, as I previously mentioned. That means we will have to adjust the algorithm in children. This is especially the in children under one year of age, which is of importance for pediatric wards. With an increase in age, the normal range of penKid values decrease and become comparable to those of adults.