Gene Therapy for the Enhancement of Fracture Healing - Engineered Bone: Tissue Engineering Intelligence Unit - page 141

years post surgery without any evidence of bone loss. 129 The optimal dosage range of hPTH
1-34 must be identified; however, hPTH 1-34 is a reproducibly effective and a safe anabolic
agent for fracture repair. One important limitation to this study, which must be considered, is
that these beagles were young, metabolically intact adults. This makes it difficult to predict if
such a therapy would be effective in an osteoporotic elderly human. Fortunately, the dose
response for hPTH 1-34 peptide and new bone formation has been well documented in the
literature. 171,173,174 The most common effective dose for rats was 80.0 µ g/kg, administered
subcutaneously three times per week. 168 Lower doses (30.0 µ g/kg-60.0 µ g/kg) have also pro-
duced consistently positive results. 173,174 A low dose of hPTH 1-34 peptide has be used in
human clinical trials as a protein therapy, and an equivalent dose (2.5 µ g/kg) consistent with
these has been used in osteoporotic rats. 175,176 This low dose was able to stimulate bone forma-
tion partially in the osteoporotic rats and restore bone that was surgically removed. 174 Studies
including high doses of hPTH 1-34 (400 µ g/kg-1000 µ g/kg) have shown strong anabolic
response but some of the new bone is woven rather than lamellar and marrow fibrosis (undesir-
able side effect). 168 More recently, studies in other animal species, including monkeys has shown
that hPTH 1-34 induced a rapid and significant increase in bone mass at the spine and hip and
increased biomechanical strength. 177 Two doses of hPTH 1-34 were administered (1 and 5 µ g/
kg) as a daily cutaneous injection for one year. A dose response was observed with no safety
risks. Current and ongoing clinical trials indicate that hPTH 1-34 is rapidly, significantly and
reproducibly anabolic for both cortical and trabecular bone. 168,175,176 Higher doses will persist
years after plasmid DNA delivery to the wound site and shown to be safe and effective.
It is well established that hPTH 1-34 anabolic effects are enhanced when the peptide is
administered intermittently. 168,174 Continuous administration increases bone formation in in-
tact rats, however, resorption is increased to roughly the same level as formation so that overall
bone density/amount is unchanged. 172,178 Intermittent treatment stimulates new bone forma-
tion, total amount and density. 171 There is no major evidence for gene therapy success in
wound healing. For a gene therapy application, the administration of hPTH 1-34 from a
gene-activated matrix to the wound site is the most important factor. Local gene delivery is the
best strategy to achieve dose-dependent, reproducible and safe tissue regeneration. Given the
evidence from rat, canine and nonhuman primate studies, it is feasible that hPTH 1-34 would
be effective as a gene therapy for bone healing. A viral vector for hPTH 1-34 delivery to the
wound site would be advantageous due to its ability to promote long term expression. For an
osteoporotic patient, long term expression of an osteoinductive factor delivered to the site is
desirable, however, not necessarily required for the healing of fractures in younger patients.
The specifics of each case, risk factors contributed by the patient and nature of the defect all
play a role in the success of a gene therapy technique.
Spinal Fusion
Spinal fusion is a relatively common surgical procedure. Internal fixation devices currently
used keep the fusion site stable for the time that is usually necessary for the bone to achieve
fusion. However, solid bone fusion failure can reach 45%. 179 Autogenous grafts are severely
limited in volume and harvest from the iliac crest has shown a high incidence of morbidity at
the donor site. 159 Allograft bone is an alternative; however it has also shown antigenic potential
and disease transmission. 159 Methods of sterilization or “bio-cleansing” for allograft materials
are under investigation but current knowledge is sufficiently lacking in this area. Therefore, the
focus has shifted towards enhancing growth factor activity to stimulate osteogenesis for suc-
cessful spinal fusion.
Osteogenic proteins such as the BMPs represent an attractive alternative to autografting.
Delivering BMP to the site of the wound can contribute to the recruitment of host
osteoprogenitor cells and stimulate bone cell formation locally to promote fusion. BMPs are
secreted signaling molecules. They form dimers after secretion and bind to a complex of trans-
membrane receptors. This event triggers a phosphorylation cascade that turns on bone specific
genes in the nucleus via the Smad proteins (Fig. 5). Examples of genes that are upregulated