UC Davis

Post traumatic osteoarthritis (PTOA) is the result of a joint trauma or injury and accounts for 12.5% of all OA cases. Following joint trauma, inflammatory cytokines are produced and stimulate the secretion of catabolic enzymes that degrade articular cartilage. The degraded articular cartilage further stimulates catabolic enzyme and inflammatory cytokine expression. These lead to the development of the inflammatory cycle associated with PTOA. We hypothesized that inhibiting inflammation and treating the damaged cartilage may prevent the progression of PTOA and restore functionality to osteoarthritic cartilage using nanoparticle-peptide therapeutics.

The first study developed polymeric core-shell nanoparticles composed of N-isopropyl acrylamide (NIPAm), N, N′-bis (acryloyl) cystamine (BAC), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), and acrylic acid (AAc) (poly(NIPAm-co-AMPS-BAC-AAc)) by exploiting the thermoresponsive behavior of NIPAm. NIPAm has a lower critical solution temperature (LCST) of 32°C. Below the LCST, NIPAm is hydrophilic and causes the particle to swell, and above the LCST is hydrophobic and causes the particle to constrict. Poly(NIPAm-co-AMPS-BAC-AAc) shells were crosslinked around the hydrophobically stabilized non-crosslinked poly(NIPAm) (pNIPAm) cores above the LCST of NIPAm. Removal of the cores via diffusion resulted in thermoresponsive, degradable nanoparticles with low density, termed hollow, cores. The hollow nanoparticles (hNPs) encapsulated more of the anti-inflammatory MK2 inhibiting (MK2i) peptide than solid nanoparticles, and loaded roughly 2.5 times more MK2i below the LCST of NIPAm than above it. These hNPs loaded with MK2i inhibited IL-6 production in stimulated bovine chondrocytes in vitro. The hNPs were also retained within the joint space of rats for 7 days. These results show the ability of MK2i loaded hNPs to inhibit inflammation and show that the drug-loaded hNPS have potential as a therapeutic to treat PTOA.

We also showed the ability of hNPs to be conjugated with the hyaluronic acid (HA) binding peptide GAHWQFNALTVRGSG (GAH). AAc served as the carboxylate anchor within the shell of the hNP and allowed for GAH conjugation. hNPs conjugated with roughly 19 GAH, termed 19 GAH-hNP, bound to HA in solution and resulted in a 94.0% increase in dynamic viscosity compared to the HA solution treated with unconjugated hNPs. Bovine cartilage explants were treated with trypsin to remove aggrecan and served as an ex vivo model for OA. The aggrecan-depleted (AD) explants treated with 0.10 mg of unconjugated hNPs had a muted effect on restoring compressive strength and suppression of CS release, likely due to reduced HA interactions. While the AD explants treated with 0.10 mg of 19 GAH-hNP restored the compressive strength to healthy levels 6 days after a single treatment and inhibited degradation of the ECM of cartilage. Based on these results, treatment with 19 GAH-hNP may be able to prevent the development of PTOA.

Finally, the MK2i loaded hNP (hNP+MK2i) and hNPs were tested in a small animal in vivo study. Previous research developed a physiologically relevant non-invasive ACL rupture (NIACLR) model using a tibial compression to tear the ACL. We attempted to utilize this model to test the efficacy of our hNP+MK2i therapeutic to inhibit the progression of PTOA in vivo. However, only 9 of the 28 rats that underwent tibial compression resulted in a complete ACL tear, and we were not able to quantify the efficacy of our hNP+MK2i therapeutic.

These data presented here suggests the use of nanoparticle-peptide therapeutics may be a translatable platform to inhibit the progression of PTOA.