Yale researchers dig for new kidney biomarkers

“In acute kidney injury, creatinine was the first test and we quickly figured out you can find AKI by noting the change in serum creatinine,” said Dr. Parikh, director of the Yale Program of Applied Translational Research. “That started being used in the 1950s, and here in the 2000s we’re still talking about the same test. So no wonder the therapeutics [in AKI] are lagging behind.”

“In acute kidney injury, we need to reduce the dependence on serum creatinine,” Dr. Parikh said. “It doesn’t increase very quickly. It’s very nonspecific. If we had specific biomarkers of structural injury from when the first nephron got injured, we’d pick up on these episodes of subtle injury and try to make therapeutic progress.”

Responding to a call from the American Society of Nephrology for better AKI biomarkers, Dr. Parikh and colleagues formed Translational Research Investigating Biomarker Endpoints-Acute Kidney Injury in 2005. The TRIBE-AKI consortium of nine medical centers has enrolled more than 3,000 patients who are being actively followed for outcomes related to AKI and major cardiac surgery.

To test various biomarkers of interest, Dr. Parikh and his co-investigators selected patients undergoing coronary artery bypass graft or valve surgery deemed to be at high risk for AKI (serum creatinine greater than 2 mg/dL or other clinical or surgical criteria). One 10-mL preoperative blood sample was taken, as was a 10-mL preop urine sample. After surgery, 10-mL samples of blood were collected each of the first five days following the procedure. On the day of surgery, 10-mL samples of urine were collected four times in the first 24 hours. On each of the following four days, one 10-mL sample of urine was collected.

The researchers took care to ensure specimens were handled the same in all steps of the testing process to avoid introducing bias in comparing the biomarkers’ performance, Dr. Parikh said. Elements that, if done differently, could skew results included: the time from blood draw to spin/freeze; the number of thaw-freeze cycles; the duration of storage; the type of blood-collection tube; time from thawing to assay; and the addition of protease inhibitors.

In a slide showing how the urinary biomarkers interleukin-18 (IL-18) and kidney injury molecule-1 (KIM-1) performed, Dr. Parikh highlighted that they demonstrated signs of kidney injury well before serum creatinine. IL-18 became elevated on day one while KIM-1 went up on day two, but it was not until day three that serum creatinine rose in the patients who developed AKI (Parikh CR, et al. J Am Soc Nephrol. 2011;22[9]:1748–1757).

“You can see that in the first 24 to 48 hours, it [serum creatinine] is not elevated. That’s almost two days’ lead time, which can be very helpful if you wanted to design a therapeutic study,” said Dr. Parikh, a professor of medicine (nephrology) at Yale University School of Medicine and a professor in the Clinical Epidemiology Research Center at the VA Connecticut Healthcare System.

Patients with elevated IL-18 had nearly a seven-fold higher odds of developing AKI than patients who did not, after adjusting for age, gender, race, type of surgery, preoperative eGFR, diabetes, hypertension, and cardiopulmonary bypass duration. Patients with elevated KIM-1 had nearly five times higher risk of AKI, as did those whose plasma neutrophil gelatinase-associated lipocalin (NGAL) rose. The liver-type fatty acid-binding protein (L-FABP) found in urine was less predictive of AKI, showing a 1.8 adjusted odds ratio.

Dr. Parikh also published data showing how tightly correlated these biomarkers were with serum creatinine, but noted the same objection as Dr. El-Khoury did in his talk.

“If you compare the new test to the existing test, the new test cannot look better because the existing test is treated as the gold standard,” he said. “So, with most of these biomarkers there’s a 0.75 to 0.8 correlation, but there is no way to get better because the false-positives could actually represent an improvement in the clinical test.”

To overcome that hurdle, Dr. Parikh and his colleagues followed a subset of patients they classified as having subclinical acute kidney injury—ones who were negative for AKI by creatinine but positive by one or more of the newer biomarkers. How well did these patients do in the years following their cardiac surgery and apparent kidney injuries? Researchers followed them through a combination of phone calls and search of vital records, medical records, and government databases.

The TRIBE-AKI researchers found a consistent pattern. Patients with no AKI—who were negative by the newer biomarkers as well as creatinine—had a mortality rate of 40 per 1,000 patient years. But the patients who were positive by any one of the biomarkers consistently had a death rate twice as high, 80 per 1,000 patient years.

“So we know that any AKI puts a patient at a long-term risk of higher mortality,” Dr. Parikh said.

The researchers further stratified the newer biomarker results by tertile depending on how elevated they were. The death rates among the highest tertile group were greater than 100 per 1,000 patient years for each of the new biomarkers, except urine L-FABP, illustrating how pharmaceutical investigators, clinicians, and health systems might focus their efforts to aid this particular group of patients in better surviving their kidney injuries.
Dr. Parikh drew the AACC audience’s attention to another pattern in the results. For each of the biomarkers examined, the mortality rate—ranging from about 45 to 60 deaths per 1,000 patient years—for the lowest tertile of patients positive for AKI by creatinine was strikingly similar to that for the patients in the subclinical AKI category (negative by creatinine but positive by the new biomarker).

“Both of these have a very similar risk as far as long-term mortality is concerned,” he said. “Clearly, the new biomarkers would add a lot of additional information if they were available.”

This pattern of catching subclinical AKI that increases patients’ odds of death also held true for another biomarker Dr. Parikh and his team studied, not a novel biomarker but an old standby—urine albumin.

A questioner from the AACC audience made note of the fact, asking, “The urine albumin results looked pretty good. . . . Why not just use albumin?”

Dr. Parikh responded: “Absolutely . . . we don’t need to get fancy. And if albumin does the job, then people can offer it and clinicians can make use of that. It is very available, so that is great.”

The urine albumin option is especially important because, Dr. Parikh says, the chances seem low that one of the other biomarkers will be commercialized anytime soon despite the promise unveiled in the research.

“I’ve come to realize that the diagnostic companies are small companies, not like the pharmaceutical companies,” he tells CAP TODAY. “And the regulatory burden is so high that if they really like a biomarker, they have to pour a lot into it before they’ll see the profits. I think it’s a gap in our translational development. In order for more biomarkers to become available, they need to anticipate that financial return. At the same time, you can’t charge $100 for each new test because you’re going to break the bank of the country’s health care budget. I go back and forth between my optimism that comes out of our studies and then the cynicism arises when I think about how to get it [a novel AKI biomarker test] to each and every patient.”

The biggest gap in AKI biomarker development—and, consequently, therapeutic development—has been the lack of long-term patient follow-up, Dr. Parikh says. That is a space the TRIBE-AKI consortium was made to plug, to help explain why patients who undergo AKI have a higher death rate in the years that follow.

Because serum creatinine is relatively insensitive, it provides a “false sense of security” about patients who experience acute kidney injuries, he adds. Dr. Parikh cautions, however, against the view that creatinine, despite its flaws in AKI and CKD diagnosis, will or should disappear entirely from clinical use.

“Replacing serum creatinine is a tall order because we have over 100 years’ worth of knowledge around creatinine,” he says. “Even if that happens, I believe it will be several decades away. I think the simplest thing which can happen, in small increments, is to add other biomarkers that provide supplementary information that is not easily available from serum creatinine. And once we start using those and get comfortable with those, I think we’ll have a lot more information to help manage patients with kidney disease.”
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Kevin B. O’Reilly is CAP TODAY senior editor.