- Engineered PlyCB for the Treatment of Orthopaedic Injuries in the Wounded Warrior
- Use of Photodynamic Therapy Treatment to Promote Long Bone Fracture Healing
- Enhanced Healing of Segmental Bone Defects by Modulation of the Mechanical Environment
Over 65% of all injuries suffered by casualties in the current conflicts in Iraq and Afghanistan are orthopaedic in nature. Some of the major challenges in treating upper and lower extremity injuries include infection of the bone (osteomyelitis) and the integration between the healing bone and orthopaedic implants (osseointegration). Many strains of bacteria are highly resistant to antibiotic drugs, and infections are becoming increasingly difficult to treat. Moreover, currently available orthopaedic implant materials are incompatible with osteoblasts and often do not integrate with the healing bone, leading to implant failure.
To address these challenges, Dr. Daniel Nelson, a recipient of two Fiscal Year 2009 Peer Reviewed Orthopaedic Research Program (PRORP) Hypothesis Development Awards, is utilizing a bacteriophage-derived lysin called PlyCB as a platform. Bacteriophages are viruses that prey on bacteria and produce enzymes called lysins to break down their hosts' cellular wall. Previously, Dr. Nelson purified PlyCB from the streptococcal bacteriophage C1, which self-assembles into a highly stable octamer. Some of PlyCB's unique features include its high resistance to heat and chemicals, its ability to bind specifically to streptococcus bacteria, as well as its high affinity for hydroxyapatite (HA) and tricalcium phosphate (PTC), materials that are often used to coat orthopaedic implants. Additionally, PlyCB can be genetically engineered to display any number of bioactive peptides or proteins without losing these properties.
With PRORP support, Dr. Nelson developed customized PlyCB to tackle both osteomyelitis and osseointegration. To address osteomyelitis, Dr. Nelson aimed to develop gold nanoparticles that specifically home to the surface of bacteria and produce intense local heat when exposed to infrared radiation, killing only the target cells while sparing healthy cells. In proof-of-principle experiments, Dr. Nelson constructed PlyCB/gold nanoparticle complexes, which successfully achieved streptococcal-specific binding and thermal ablation in vitro. Additionally, Dr. Nelson produced PlyCB fused with a pneumococcal binding domain, demonstrating that PlyCB's binding specificity may be altered as needed. To address challenges in osseointegration, the investigator sought to develop synthetic bone scaffolds that can actively recruit osteoblasts to the implant surface to promote bone differentiation and osseointegration. In support, Dr. Nelson evaluated whether PlyCB can be engineered to display bioactive peptides (e.g., RGD) that recruit osteoblasts while bound to HA and TPC. Several versions of PlyCB-RGDs were engineered, all of which retained their stability and high affinity for HA and TPC. Importantly, Dr. Nelson demonstrated direct binding between PlyCB-RGD and osteoblasts in vitro.
While the development of PlyCB for therapeutic applications is still in its initial stages, these results showcase PlyCB's versatility and potential for addressing important issues in treating upper and lower extremity injuries. The success of these studies may lead to significant advancements in treating antibiotic-resistant infections and improve the outcome of those receiving orthopaedic implants.
Dr. Akens, of the Sunnybrook Research Institute in Toronto, Canada, knows that when high-impact trauma is the cause of a long bone fracture, nothing is straightforward. This type of extremity combat injury is usually associated with complications such as lacerated soft tissue or an open wound, making the wound prone to infections that negatively impact bone healing. Even with the currently available treatments to enhance bone healing, these fractures can take up to a year to fully heal. Dr. Akens' strategy is to improve healing in complex skeletal injuries by using a drug-light therapy method called photodynamic therapy (PDT). In PDT, a photosensitizing drug is locally or intravenously administered and later activated at the site of the fracture with a laser light. One previous study with PDT showed that this approach could rapidly improve vertebral bone strength, stiffness, and architecture, while another study showed that PDT could reduce bacterial growth within bone in a preclinical model of infected bone.
With funding from a 2009 Hypothesis Development Award, Dr. Akens has been able to test the treatment in fractured long bones in rats using the photosensitizer Visudyne, which is FDA-approved for treating macular degeneration in the eye. Dr. Akens demonstrated an increase in bone and callus formation after PDT treatment, with best results when PDT is applied during the secondary stage of fracture healing. She hopes that if ongoing studies confirm the benefits of PDT treatment at the secondary stage of fracture healing, PDT could be applied to trauma patients expected to encounter impaired fracture healing.
Akens MK, Wright D, Yousef S, et al. 2012. The effect of photodynamic therapy (PDT) on long bone fracture healing. Orthopaedic Research Society Annual Meeting, San Francisco, California.
Won E and Akens MK. 2010. Effects of photodynamic therapy on the structural integrity of vertebral bone. Spine 35(3):272-277.
Military service members deployed to Iraq and Afghanistan frequently suffer from blast-related injuries, including the loss of large sections of bone (segmental bone defects) and severe soft tissue damage. Segmental bone defects often fail to heal due to the size of the lost bone piece and the extent of damage to surrounding soft tissue. Failure of these large segmental bone defects to heal can result in severe deformity and/or amputation. Although mechanical stabilization through nailing and plating increases the rate of bone fracture healing, the effect of mechanical stabilization on large segmental bone defects is not well understood. Dr. Christopher Evans, with funding from a fiscal year 2009 Peer Reviewed Orthopaedic Research Program Idea Development Award, is investigating the effect of mechanical manipulation on the healing of large segmental bone defects in an animal model. Dr. Evans' research team has developed an external fixator that fits onto the thigh bones of rats with large segmental bone defects. The stiffness/stabilization level of the novel fixator can be modulated by a connection element that is secured to the rat femur with titanium screws. In a preliminary study, the degree of stiffness of the fixators was either maintained at a constant level or increased through the different stages of bone healing. Interestingly, it was demonstrated that stiffness modulation significantly accelerated large segmental bone healing, suggesting that troops suffering from blast-related segmental bone defects may benefit from this technique.