Cartilage Penetrating Nanocarriers to Provide Sustained Delivery of Disease Modifying Drugs in Post-Traumatic Osteoarthritis

Osteoarthritis is a debilitating disease of the joint. There are a variety of causes of osteoarthritis – however, one particular etiology, post-traumatic osteoarthritis (PTOA), is of particular clinical interest because it has a biologically and temporally well-defined onset – a traumatic joint injury. There is potential for immediate post-injury intervention with a drug to mitigate the progression of the disease. However, no such disease modifying osteoarthritis drug currently exists.

There are numerous molecules that show promise as disease modifying drugs in preclinical studies. Unfortunately, even when injected directly into the human 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 drug delivery challenges, preventing drugs from being translated to the clinic.

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 cartilage faster than it could be cleared from the joint. Yet the interactions must remain 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 conjugated cationic dendrimers to an anabolic growth factor, insulin like growth factor 1 (IGF1), to test this hypothesis. The high dendrimer surface charge can be tightly controlled by covalent modification with different amounts of PEG. To optimize dendrimer surface charge, a small library of partially PEGylated dendrimers was constructed and screened with cartilage binding and toxicity assays. The top two formulations were selected for further study - one of which was about 3 times more charged than the other.

The top two dendrimer – IGF1 formulations were tested for penetration of both ex vivo bovine cartilage biopsies of human thickness 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 to cartilage, but diffused throughout the tissue more slowly.

In a pharmacokinetic study, IGF1 alone or one of the top two dendrimer-IGF1 formulations was injected into rat knee joints and monitored by an in vivo imaging system over 28 days. IGF1 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.

A surgical PTOA model was used to determine the therapeutic effect of targeted, sustained drug delivery of dendrimer-IGF1. Rat joints were destabilized by ACL transection and partial meniscectomy. 48 hours after surgery, a dendrimer-IGF1 conjugate, 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.