By David N. Leff
Some time next year, if all goes well, an elderly person with a broken hip will receive gene therapy along with conventional surgery to heal the fracture. So far, this DNA-based treatment has mended broken bones in rats and in beagle dogs.
Pathologists carried out those animal studies at the University of Michigan's Orthopedic Research Laboratories in Ann Arbor. They reported their localized gene therapy approach on the rodents in 1996, and on the canines this month, in Nature Medicine for July 1999. A co-author of both preclinical trials was molecular geneticist/pathologist Jeffrey Bonadio, vice president of pharmaceutical development at Selective Genetics Inc. in San Diego, and an adjunct professor at the university. (See BioWorld Today, June 11, 1996, p.1.)
The just-reported beagle study in Nature Medicine bears the title: "Localized, direct plasmid gene delivery in vivo prolonged therapy results in reproducible tissue regeneration." Its co-senior authors are Bonadio and biomedical engineer Steven Goldstein, who heads the university's Orthopedic Research Laboratories.
"The dogs for this clinical-simulating study," Goldstein told BioWorld Today, "were purpose-bred beagles. That involved using their tibia or shinbone from which we sawed out a segment of the shaft to create a gap between 1 centimeter and more than 2 centimeters in length."
Into that space, he and his co-authors placed a plasmid implant construct they call GAM - gene-activated matrix. It consists of two ingredients, a recombinant cDNA sequence physically entrapped in a biodegradable, bovine type 1 collagen sponge.
The bone-repairing gene sequence encoded the first 34 amino acids of the 84 residues comprising the human parathyroid hormone. In its full-length version, Goldstein explained, this endocrine molecule resorbs, or sops up, redundant bone calcium. But its short 34 amino acid fragment does just the opposite. "This fragment of the molecule," he said, "has been shown to be anabolic for bone, stimulating bone formation."
Dogs Got Clamps 'Just Like Humans'
With this package in place, he continued, "we closed the incision and placed external titanium fixators to stabilize the gap, very similar to what's used in human fractures. It looks like an erector set," Goldstein added, "holding the bone ends very stable. When the dogs woke up from the anesthetic, they could ambulate with no pain or problem at all. But the big gap in the bone, in which we placed the GAM sponges, is where we stimulated the new bone formation. Control animals, who got dummy GAMs, couldn't heal or bridge that defect."
He pointed out that the space between bone ends, which represents a fracture, "is what we call a 'critical defect.' It defines experimentally the size of a segment of bone that, when removed, the shaft can't heal in the space that's created. That makes a robust test of a therapeutic strategy." Specifically, such an experimental gap measures twice the diameter of the bone under study.
"We had a series of about 20 or 25 beagles," Bonadio recalled, "of which we took four out of the final five to complete necropsy. That fifth dog is still alive, and our plan is to leave it in an in-life phase for as long as possible. We'll try to look at the consequences of what we've done beyond the 53-week time point that we reported in the paper. Since then, add another 10 to 20 weeks at least."
He pointed out, "The results of those studies will be part of our IND [investigational new drug] submission, in December of this year, if all goes well."
Selective Genetics plans on treating osteoporotic hip fracture patients at an orthopedic center, tentatively designated as the Mayo Clinic in Rochester, Minn. "We see it as a Phase I/II trial," Bonadio observed, "Phase I safety is certainly the priority, but it's formally possible at this point that we will be looking at some clinical utility-type outcomes in the study as well."
Strategy Would Complement Standard Therapies
Bonadio made the point that the localized gene therapy would not replace conventional surgical repair. "In the musculo-skeletal arena, we have always thought of GAMs as an adjunctive treatment to the standard of care. That is, gene therapy to be coordinated with an established medical device, the goal being to somehow add to the functionality of that device."
Beyond geriatric hip fractures, Bonadio pointed out, "I think that broadly speaking, what we're looking at are fractures in general that heal with difficulty. There are three common themes that underlie those kinds of breaks. One is aging. It seems that in and of itself, as we age it becomes more and more difficult to heal a fracture. The second factor is ischemia, and the idea here is that you disrupt blood vessels at the same time that you injure the bone, and that creates the ischemic situation that affects the bone-healing process. The third one is infection. GAMs we believe could be appropriate - and that's what the clinical trial is designed to look at, at least in part - in aging and ischemia."
The co-authors' paper in Nature Medicine noted, "In 1995, there were about 300,000 hip fractures in osteoporotic individuals in the U.S., with excess mortality, a considerable loss of functional ambulation and independence, The societal cost was estimated at more than $9 billion. Therefore, the need for improved therapeutic strategies is great."
Goldstein observed: "Our rationale was that in moving toward the clinic, the preclinical dog model needed to get to a physiology that more closely resembled the human bone-fracture repair physiology. And the dog has been a very good model, as a patient, of bone-fracture behavior. So moving from the rat to a larger animal," he concluded, "allowed us to really get efficacy as well as safety issues - to make the step toward the human patient."