COLUMBUS, Ohio – Researchers at Ohio State University have successfully tested micro-sized gelatin particles that may one day deliver therapeutic genes to treat a type of kidney disease.
The biodegradable gelatin particles are so small that at least 10 would fit on the period at the end of this sentence. Researchers believe such particles could carry therapeutic genes to the glomerulus, a tiny cluster of capillaries within the kidney that filters toxins from the blood. This filtration system becomes blocked when patients develop glomerular disease.
The researchers injected these extremely small, biodegradable particles into arteries that led to an animal subject's kidneys. This study was designed to only test the gelatin delivery system, so the particles didn't carry any genetic material.
"Gene therapy is a new approach to treating kidney disease," said N. Stanley Nahman Jr., the study's senior author and a professor of nephrology at Ohio State. "Ours is a preliminary study – we have at least five to 10 years to go before this kind of treatment might become a reality for patients.
"But gelatin appears to be a good choice for delivering potentially therapeutic genetic material to the glomerulus."
Glomerular disease is the most common cause of kidney failure in the United States. In 1997, more than half of the 304,000 people on dialysis had some form of glomerular disease, according to a report by the U.S. Renal Data System.
The study appears in a recent issue of the journal Biomedical Microdevices.
The researchers found that it took about 10 hours for the gelatin particles, or particles, to dissolve enough to pass through the glomerulus of a piglet. Ten hours is a short enough period for a gene to be released and to begin replicating in the tissue, Nahman said.
The scientists injected perhaps a million biodegradable gelatin particles into the renal artery of a piglet. (Each kidney has about a million glomeruli.)
The particles were tagged with radioactive material prior to being injected into the animals. After injection, the researchers used a gamma camera to take pictures of the particles while they were inside the piglet's body. This camera, which detects radioactivity inside the body, was placed over the kidneys.
The researchers measured the distribution of particles based on the level of radioactivity observed in the piglet's kidney at various points in time.
They took two sets of pictures of the particles between 30 minutes and six hours following injection, and again at 24 hours after the injection.
"The level of radioactivity declined rather rapidly in the kidney," said Nahman, who is also an investigator with Ohio State's Heart and Lung Research Institute. "This suggests that the gelatin dissolved enough to pass through the glomerulus, and it did so in a short amount of time. So it should be useful in transferring genes to tissues."
The gelatin material that the researchers used in this study is similar in composition to that of gelatin capsules found in over-the-counter medications, Nahman said. But size is one issue; getting the genetic material to stick to a biodegradable particle is another issue.
"Gelatin used in medications is mixed with the medication," Nahman said. "We can't mix the genetic material in a gelatin microparticle, simply because the particle is so small. We're currently working on ways to get the genetic material to stick to these particles."
While the gelatin particles quickly dissolved, they were still quite large – about 64 micrometers in diameter. Capillaries range in diameter from five to 10 micrometers. However, the gelatin particles did eventually make it to the glomeruli.
The particle needs to be large enough to stay inside the capillary long enough for the genetic material to be released, but small enough to make it to the capillary. Previous work by these researchers showed that a 16-micrometer particle is the right size to lodge in the glomerulus of a rat.
"A particle of this size can get into the glomerulus, but it can't get out," Nahman said. "But the particle doesn't stay its original size for long. We saw dissolution as soon as 30 minutes after the injection of the particles in our study.
"We wouldn't want to use particles as large as the gelatin particles used in this study in a real gene therapy treatment program," he said. "For one, the particles may get stuck in a blood vessel near the capillary and never make it to the glomerulus. But as the research continues, the gelatin particles should become much smaller.
Using biodegradable material may also reduce the risk of ischemia – poor circulation due to a blockage in the blood vessel – in the glomerulus.
"Overall, though, the gelatin particles are superior simply because they pass through the body and won't cause long-term blockage of blood flow. The results mean a great deal for organs other than the kidney, too."
While the architecture of the glomerulus allows the filter to withstand some degree of blood vessel blockage – it is a mass of interconnected capillaries – other organs, such as the heart, don't have the same sort of network.
"Circulation in the glomerulus is redundant, so the chances of tissue in this tiny filter dying due to lack of blood and oxygen are slim," Nahman said. "That's not the case with most other organs. So having a substance that will begin to dissolve quickly and pass through the organ is desirable in gene therapy."
###The research was supported by grants from the National Institutes of Health; the William H. Davis Foundation of Ohio State; the National Kidney Foundation of Ohio; the American Heart Association and Battelle Memorial Institute.
Nahman co-authored the paper with Ohio State researchers Udayan Bhatt, Amy Johnson, Christopher Hickey, Jonathan Ellison, Abigail Mansfield, Scott Shie and Na Shen, all with the department of internal medicine; William Tod Drost and John Bonagura, of the department of veterinary clinical sciences; Thomas Sferra, of the department of pediatrics; Pablo Gamboa, George Hinkle and Aaron Haynam, of the department of radiology; Valerie Bergdall of the department of veterinary preventive medicine; and Lisa Brannon-Peppas, of the University of Texas at Austin.
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