Patients with impaired vision because of a damaged cornea could soon regain their sight without need of a human donor transplant. Instead, such patients could be aided by an artificial but biosynthetic implant. One such implant has now been tested in patients over two years, and the results are as good as, or even better than, those achieved with donor corneas.
The transparent tissue that covers the surface of the eyes, the cornea, can be damaged by injury, infection, or inflammation, causing the eye to lose much of its ability to refract light and focus images on the retina. Such damage has caused loss of vision in millions of people around the world. The best treatment for cornea damage remains a transplant, but donor corneas are in chronically short supply.
Plastic replacements have been available for decades, but their implantation is still plagued by side effects such as infection and glaucoma. “They remain a last resort option for patients where all other options have failed, including donor transplants,” says Joachim Storsberg at the Fraunhofer Institute for Applied Polymer Research in Potsdam, Germany. Storsberg is developing plastic implants but was not involved with the current work.
Several other research groups are working on artificial corneas made from materials that encourage cell growth and are less likely to be rejected. But this is the first time the long-term effectiveness of such an implant has been tested in humans.
May Griffith of Linköping University in Sweden and Ottawa Hospital Research Institute, along with colleagues, developed the implant for patients with damage to only the top layers of the cornea. A partner company, Fibrogen, engineered yeast cells to manufacture the human protein collagen. The team then chemically cross-linked this collagen and let it harden in a mold in the shape of corneas, which they then implanted in place of the damaged cornea layers of 10 patients.
Although the implants do not contain any live cells, they mimic the flexible scaffold material that makes up the bulk of the stroma, the thickest layer of the cornea, which is essentially a natural hydrogel consisting mostly of collagen.
“Although donor corneas remain the gold standard, Griffith’s approach looks like it’s a close second, and very promising, at least if you don’t have persistent infections that destroy the regenerating tissue,” says Storsberg.
Griffith’s team reports this week in the journal Science Translational Medicine that two years after implantation, cells had repopulated the implants, and the outermost epithelial cell layer, which protects the eye from infection, had grown back over the implant in all patients. Vision in all ten patients improved to levels comparable to that of patients who have received donor corneas–but only when those ten patients also wore contact lens. “This is because stitches on the implants introduced bumps that impaired vision and need to be smoothed by contact lenses,” says Griffith. But using different suturing methods or replacing the stitches by gluing the implants to the eye with tissue adhesives could solve this problem, she argues, adding that the team has had encouraging results testing such alternatives in preliminary follow-up work.
The team also observed regenerating nerves in all the corneas, and in nine out of 10 patients, the nerves grew all the way to the center of the implant, a result Griffith is particularly excited about. “The nerves are really important for the long-term health of the rest of the cornea–but regeneration does not happen reliably even in donor corneas,” she says. Her long-term expectation is that the implant will slowly degrade and be completely replaced by the natural scaffold regenerated by the cells that have repopulated the cornea.
Christopher Ta, an associate professor at Stanford University who’s working on another kind of hydrogel substitute for donor corneas, is also optimistic about Griffith’s work, which he says “has the potential to revolutionize the field of cornea transplantation. It is possible to see widespread use of this type of engineered cornea in the next five years.”