The Search for Uremic Toxins
Kidney dialysis has not changed much since it was first introduced to a broad public in the 1960s as a miraculous life-saving system to clean the blood when natural kidney function fails. Although dialysis continues to save lives, it does only about 10 percent of what a functioning kidney can do to remove toxic wastes (called “uremic toxins”) from the blood stream. A patient on dialysis faces an exhausting and time-consuming process multiple times a week, with the prospect of serious health problems, from heart and bone disease to anemia, and a significantly shortened life expectancy once dialysis begins.
The problem is that after dialysis, patients continue to suffer from a previously non-existent, life-threatening disease that has been called “residual syndrome.” Scientists now believe that this syndrome that strikes down dialysis patients is probably caused by as-yet unidentified toxic molecules that remain in the blood stream when they are not removed by dialysis. It is known that dialysis removes urea from the patient’s blood, to alleviate symptoms after kidney failure.
What if dialysis could identify and target the remaining toxins, among the hundreds of “waste” molecules left in the bloodstream after urea has been removed, so patients could live longer, healthier lives after treatment. That has been the focus of a decade of investigations by Timothy Meyer, MD (professor, Nephrology), and new studies with his colleague, Tammy Sirich, MD (instructor, Nephrology). Both specialize in kidney research, along with the care and treatment of patients with kidney disease at Stanford and its affiliate Palo Alto Veterans Affairs (VA) Hospital.
Meyer and Sirich are determined to change the way that dialysis works.
“We think that dialysis patients still feel sick because many different substances could be removed by dialysis—but we have not yet identified which of those left in the bloodstream after treatment are the harmful ones,” explains Meyer. “It is shocking that with all the technology at our disposal, we have not yet been able to identify exactly which chemicals are the ones that cause illness when the kidneys fail.”
“This is a chemistry problem with a solution that can change the face of dialysis,” Meyer says. “We were both chemistry majors in college, which predisposes you to go into nephrology to study the waste chemicals that the kidney cleans from the body.” Their background in chemistry has led them to the current investigations.
It is Meyer and Sirich’s goal to find new ways to establish the chemical identity of the specific molecules that make patients sick. Scientists have characterized over 200 molecules (called “solutes”) that appear in high concentrations in the blood after kidney failure occurs, and there could be thousands more. It is known that certain classes of solutes are removed less well by dialysis than urea, including those that are protein-bound, relatively large ones, sequestered compounds, and substances removed by the normal kidney at rates higher than urea. But until recently, there were no good analytical tools to determine which of these were the uremic toxins—that caused patient symptoms and illness.
Meyer’s research has focused on elucidating the cellular and pathophysiologic mechanisms responsible for the progression of kidney disease. His work includes studies of which molecules are toxic, how these are produced by the body, and how their production could be decreased or their removal could be increased.
“This is like searching for a needle in a haystack, to solve a major clinical problem in the field of kidney disease”
Meyer and Sirich now want to identify the toxic substances causing harm in the bloodstream—to provide a more rational basis for prescribing dialysis to patients before they become seriously ill. Ultimately it could lead to improved treatment of patients with kidney failure.
The mass spectrometer is what first brought Meyer and Sirich together in their search for uremic toxins. Sometimes called the smallest scale in the world, the mass spectrometer is an analytical chemistry device with software and detection tools that can measure the size and volume of atoms and molecules. It can identify the specific chemicals in a sample.
“We were both interested in identifying uremic toxins, and we were both interested in using mass spectrometry to characterize the toxic solutes in the blood that were poisoning our patients,” Sirich recalls.
Meyer had just acquired a mass spectrometer for his research lab when Sirich joined his team as a research fellow and chose “the search for uremic toxins” as her research focus. Together, they began to unravel the candidates for “most toxic solute” in the waste chemicals they found in samples from patients who were on dialysis, as compared with the compounds found in patients with healthy kidney function. They learned what mass spectrometry could do to identify the mass and abundance of the compounds they found. With a grant from the National Institutes of Health (NIH) in 2008 they began to study patient samples in a large dialysis cohort, and they have since received additional funding from the NIH and the Department of Veterans Affairs to continue their work in the field.
“We use mass spectrometry to examine the biochemical garbage that is left after dialysis is done, and our goal is to sort out which streams of garbage—which solutes left in the bloodstream after dialysis—are causing so many symptoms for patients,” Meyer says. They use sophisticated metabolic studies to identify and characterize small molecules in the blood, and then establish which ones appear in the highest concentrations in patients with kidney failure and disease symptoms.
Meyer and Sirich have characterized new solutes in the blood of patients after kidney failure, with their mass spectrometry studies of patient samples. The investigators have also employed untargeted mass spectrometry to identify ones that are protein-bound and that are most efficiently cleared by the kidney, and they believe that further analysis of those could indicate a route to the identification of other harmful substances.
Their studies to date have focused largely on two specific protein-bound molecules that may turn out to be uremic toxins in dialysis patients. Indoxyl sulfate and p-cresyl sulfate may contribute to cardiovascular disease in kidney failure; and indoxyl sulfate may also contribute to progression of kidney disease. These are among the large number of waste substances produced by colon microbes; and because they are made by microbes in an isolated compartment, they may prove simpler to suppress than other kidney waste.
A clinical trial that derives from this work, Dietary Maneuvers to Reduce Production of Colon-Derived Uremic Solutes, directed by Meyer, is now recruiting patients to evaluate whether dietary fiber supplements can reduce production of chemicals produced by colon bacteria that build up in the body in patients on dialysis.
Further studies and expanded clinical trials are the next steps in the search for uremic toxins. Although it is now possible to reduce the levels of some solutes by modifying the dialysis procedure or by limiting production, clinical trials must determine if these changes will clinically benefit dialysis patients.
“This is like searching for a needle in a haystack, to solve a major clinical problem in the field of kidney disease,” explains Sirich. “But our studies could impact all the patients that we see every day at Stanford and the VA, and more than 350,000 kidney patients who are on dialysis in the US and beyond.”