Brett C Geiger
Osteoarthritis is a debilitating disease of the joint which manifests as a loss of articular cartilage, causing serious pain and impeding mobility. There are a variety of causes of the disease – however, one particular etiology, post-traumatic osteoarthritis (PTOA), is of particular interest for clinical treatment because it has a biologically and temporally well-defined onset – a traumatic joint injury. As PTOA does not induce loss of cartilage until years after the traumatic injury, there is potential for immediate intervention with a disease modifying osteoarthritis drug to delay or even halt the progression of the disease. However, no such disease modifying drug currently exists and the current standard of care is merely palliative. There are several classes of molecules that show promise in vitro and in animal studies, such as anti-inflammatory small molecules, cytokine receptor antagonists, anabolic growth factors, and inhibitors of matrix metalloproteinases. Unfortunately, they have not been useful as therapeutics in humans because even when injected directly into the joint, they are unable to penetrate and reside in cartilage for long enough to have disease modifying effect. The high clearance rate of molecules in the joint space and small pore size of cartilage tissue (<15 nm) pose considerable delivery challenges for any drug of interest. However, the high sulfated proteoglycan content in cartilage provides an opportunity to harness electrostatic interactions between the negatively charged tissue and a positively charged nanocarrier. We hypothesized that such interactions, if strong enough, could bind the nanocarrier to the cartilage faster than it could be cleared from the joint. Yet the interactions must also be weak enough to allow for dissociation and diffusion throughout the tissue. Thus, tight control of positive charge and small (<15 nm) size were crucial design criteria for these nanocarriers. We chose to conjugate cationic dendrimers to an anabolic growth factor, insulin like growth factor 1 (IGF1) to test this hypothesis. The terminal groups of dendrimers provide a high degree of positive charge, which can be tightly controlled by conjugation with different amounts of PEG. To find an optimal degree of positive charge for binding and penetration of cartilage, a small library of partially PEGylated dendrimers was constructed. This library was screened with cartilage binding and toxicity assays, and the top two formulations were selected for further study. One formulation was about 3 times as charged as the other. The top two dendrimer – IGF1 formulations were tested for penetration of both ex vivo bovine cartilage biopsies and in vivo rat joint cartilage. Both formulations showed vastly superior binding and penetration of cartilage compared to IGF1 alone. As expected, the more charged formulation was more effective than the less charged formulation at binding cartilage, but took longer to diffuse through the depth of the tissue. This was followed by a pharmacokinetic study in which IGF1 alone or one of the top two dendrimer-IGF1 formulations were injected into rat joints (n=8 per group) and monitored by an in vivo imaging system over 28 days. Half-lives were calculated for each formulation based on single phase exponential decay fits of the data. IGF-1 alone had a joint half-life of 0.41 days, the less charged dendrimer-IGF1 had a joint half-life of 1.08 days, and the more charged dendrimer-IGF1 had a joint half-life of 4.21 days, nearly 10 times that of unconjugated IGF1. We then sought to determine if the sustained delivery and cartilage penetration capability of these IGF1 nanocarriers could rescue cartilage in a surgically induced rat model of osteoarthritis. Rat joints were destabilized by ACL transection and partial meniscectomy. 48 hours after surgery, dendrimer-IGF1 conjugates, IGF1 alone, or no treatment was injected. 4 weeks later, the rats were sacrificed and joint histology was scored for various parameters relevant to osteoarthritis. While IGF-1 alone was largely ineffective, the sustained, targeted drug delivery achieved by the dendrimer-IGF1 conjugate led to significantly reduced cartilage degeneration and osteophyte (bone spur) burden at 30 days post-surgery, relative to the untreated animals.