The modified drug, which would be injected directly into arthritic joints, could last for several weeks rather than just the few hours the unmodified drug would last, the researchers said.
In their study, the researchers modified a drug called interleukin-1 receptor antagonist (IL1RA). They found that the drug, which is a protein, could be improved by attaching a second protein that clumps together at normal body temperatures. The combined drug likewise would assemble into clumps in the body to serve as "drug depots" that gradually release active drug particles, the researchers said.
"Although the conventional drug is being used for autoimmune diseases, no one yet knows how much of it would be needed to achieve a therapeutic effect for osteoarthritis," said Lori Setton, associate professor of biomedical engineering and surgery. "Current estimates suggest it would require perhaps two injections per week of the unmodified drug.
"With this advance, we believe treatments could go from twice a week to perhaps twice a month, and that would be a huge clinical gain," she said.
By remaining at the site of disease, the drug also might cause fewer negative side effects than the unmodified drug, the researchers added.
The team reported an initial proof-of-concept study on Saturday, Oct. 14, at the Biomedical Engineering Society annual meeting, in Chicago. The team also has reported related findings online in the Journal of Controlled Release.
The work was supported by a Coulter Foundation Translational Research Partnership award to the biomedical engineering department at Duke's Pratt School of Engineering, and by the National Institutes of Health and a United Negro College Fund -Merck graduate dissertation fellowship.
Osteoarthritis is a degenerative joint disease that affects an estimated 21 million adults in the United States. It is the nation's most prevalent musculoskeletal disorder and the leading cause of disability, the researchers said, adding that as the average age of the population continues to rise, the incidence of osteoarthritis will increase.
Osteoarthritis had been attributed primarily to the gradual wear and tear of joint surfaces. More recently, however, scientists have discovered that inflammation sparked by the immune system also plays an important role in the worsening of the disease.
Specifically, scientists have linked a key molecule involved in inflammation, called interleukin-1 (IL-1), to osteoarthritis. IL-1 heightens the activity of enzymes that damage joints by breaking down their collagen "framework," Setton explained. She credits team member Virginia Kraus, professor of rheumatology and immunology at Duke University Medical Center, for focusing on IL-1 as a major mediator of osteoarthritis in the joint.
Given the immune system's suspected role in osteoarthritis, scientists have suspected that the interleukin-1 receptor antagonist drug, which is known to block IL-1 activity, might have value in diminishing the severity of osteoarthritis, said Mohammed Shamji, a neurosurgery resident pursuing his Ph.D. in Setton's laboratory.
However, trials of IL1RA for osteoarthritis have had limited success, primarily because the drug tends to break down quickly, according to the researchers.
"Physicians can inject large amounts of this drug systemically, but it's cleared very rapidly and there is no evidence that it reaches the joint space," said Helawe Betre, who performed these studies for his doctoral work at Duke and now works at Zimmer Orthobiologics. "We set out to develop a modified version of the drug that when injected directly might stay in the joint long enough to be effective."
To build in such durability, Betre turned to a class of proteins called elastin-like polypeptides (ELPs). Once ELPs in solution reach a certain threshold temperature, they assemble into protein aggregates. Study collaborator Ashutosh Chilkoti, a professor of biomedical engineering at the Pratt School, previously had investigated the use of ELPs that clump at temperatures higher than normal body temperature for treating cancers.
ELPs aren't recognized by the body's immune system as foreign substances, and thus they have some unique advantages for biomedical applications, according to the researchers. Also, ELPs can be joined directly to genes that control the production of various proteins in cells, with the combination forming "fusion proteins." As the proteins degrade, they yield simple amino acids, which are the building blocks of all proteins.
By experimenting with composition, molecular weight and concentration of various ELPs, Betre developed a protein for use in joints that would precipitate out of solution and clump at normal body temperature.
When he injected the sticky ELP into the knees of rats, the protein proved to have a 25-fold longer half-life in the joint than a similar soluble protein, Betre reported in the journal article. Half-life is the time required for half the quantity of the protein to be broken down and eliminated from the joint space.
"At 12 hours, the soluble protein was already at a concentration in the joint below 10 percent of the injected dose," Setton said.
In contrast, the sticky ELP took about two weeks to fall to the 10 percent level in the animals' joints -- a "really substantial increase," she said.
Moreover, rats injected with the clumping ELP contained lower blood levels of the protein, suggesting that a protein drug paired to the ELP might also have the benefit of fewer side effects, Shamji said. This would be important, he said, because IL1RA c an weaken the immune system and make patients more susceptible to infection.
Such vulnerability to infection is a particular problem for patients treated for chronic diseases such as osteoarthritis, and this vulnerability suggests the need for a local treatment, Shamji added.
Based on the initial investigation, the team paired the ELP with the IL1RA drug in a second study, to test whether the fusion protein would retain the ability to suppress IL-1 activity and fight inflammation.
The researchers treated human white blood cells with either the original or the modified drug. White blood cells are responsible for mounting the body's immune response and normally react to IL-1 by growing in numbers and releasing molecules that further activate the immune system. Active IL1RA therefore serves to dampen the immune cells' rate of proliferation, Shamji said.
White blood cells treated with the modified drug increased in number at a slower than normal pace, evidence of the drug's activity, Shamji reported at the Biomedical Engineering Society meeting. But its potency dropped by a factor of about 20 compared to the pure drug.
Further studies, however, suggested the fusion protein might do better in an arthritic joint, Shamji said. The researchers were surprised to find that the same inflammatory enzymes that destroy joint collagen also chew ELP from the original drug protein, thereby restoring its activity.
"With the ELP tag gone, you get back your originally active drug," Shamji said.
"That finding was pretty unexpected," Setton added. "We hadn't intentionally designed it to have that feature at all."
The researchers now plan to work with collaborators to test the modified drug's ability to interfere with the progression of disease in the joints of animals.