Controlled heparin-based delivery strategies to promote musculoskeletal tissue regeneration after injury

Abstract

Despite the prevalence of musculoskeletal disorders, one of the leading causes of work- related disability in the United States, effective therapeutic delivery to the affected musculoskeletal tissues such as bone, cartilage, and muscle remains a significant challenge. Thus, the long term goal of this research was to develop a tunable biomaterial-based system for the delivery of regenerative therapeutics to musculoskeletal tissues. This goal was approached through the engineering of a heparin-based microparticle (MP) system with tunable heparin sulfation, heparin content, and hydrolytic degradation, which was subsequently employed for in vitro release of an osteoinductive growth factor and in vivo delivery of a chondroprotective anti-inflammatory protein and a chemokine capable of recruiting pro-regenerative cells to muscle. In aim 1, hydrolytically-degradable, heparin-based MPs were fabricated containing heparin derivatives of varying levels of sulfation, and N-desulfated heparin MPs were found to efficiently load and release an osteoinductive growth factor, bone morphogenetic protein-2 (BMP-2), in vitro. In aim 2, heparin-based MPs were loaded with tumor necrosis factor- stimulated gene-6 (TSG-6), an anti-inflammatory protein known to inhibit plasmin, and delivered via intra-articular injection to rat knees in the context of post-traumatic osteoarthritis. After 21 days, TSG-6 loaded MPs reduced cartilage damage following injury, whereas a 3X higher dose of soluble TSG-6 did not, suggesting that Hep-N can enhance TSG-6 bioactivity in vivo and that Hep-N-based MPs can effectively deliver a chondroprotective protein for cartilage regeneration. Finally, in aim 3, heparin-based MPs were loaded with the chemokine stromal cell-derived factor- 1 (SDF-1) and delivered via local injection to the supraspinatus muscle in combination with systemic delivery of the bone marrow mobilizing agent, VPC01091, following severe rotator cuff injury in rats. Co-delivery of SDF-1 loaded MPs and VPC01091 led to significant modulation of the inflammatory cellular milieu and mesenchymal stem cell population in muscle 3 and 7 days following injury, along with significantly more regenerating muscle fibers compared to either treatment alone. Overall, in this thesis, heparin-based MPs were utilized to deliver therapeutics capable of stimulating endogenous healing processes in two unique contexts of cartilage and muscle degeneration. Given the tunability of the MP system and the ability for heparin to bind and interact with a myriad of proteins, the application of heparin-based MPs extends beyond cartilage and muscle, and may be used for a wide range of applications where controlled release of bioactive positively-charged therapeutics is required.