ENGINEERING L-TYROSINE BASED POLYCARBONATE COPOLYMERS AS DEGRADABLE DEVICES FOR LONG-TERM, LOCALIZED DELIVERY OF A HYDROPHOBIC MACROLIDE IMMUNOSUPPRESSANT TO PREVENT TRANSPLANT REJECTION
Advances in surgical techniques and immunology have enabled transplantation of vascularized composite tissue allografts (VCAs, e.g., upper extremity, face, genitourinary tissues) to treat complex soft tissue injuries and extremity amputations. However, more than 85% of transplant recipients develop at least 1 episode of rejection within the first year, and over 50% experience multiple such episodes. Standard treatments involving oral or intravenous delivery of immunosuppressants result in systemic levels that are far higher than needed in the graft tissue. High systemic immunosuppressant levels increase the likelihood of widespread side effects including organ toxicity and increased risk of illness. Furthermore, the effectiveness of oral administration is dependent on patient compliance with medications requiring twice-daily dosing.
This work presents the development of polymeric local drug delivery systems to address these limitations. A biodegradable, bioresorbable tyrosine-derived polycarbonate was used a carrier matrix loaded with standard immunosuppressants. The devices were implanted at the interface between host and donor tissue during transplant surgery where they provide long-term, localized immunosuppression and reduce the incidence of rejection episodes. Engineering steps were taken to establish reproducible fabrication protocols, stabilize devices during sterilization and storage, and control the early release profile. In vitro release testing elucidated a three-stage release profile that is impacted by changes in drug distribution and polymer matrix during degradation. Release from the devices was consistent over 6 months following the transition to the late-stage release profile, providing the consistency required for continual localized immunosuppression. In vivo testing in small animal models elucidated similar release profiles correlating with in vitro testing and directed dosing in non-human primate (NHP) VCA models. Implantation of the devices in a NHP VCA model prevented rejection episodes for over 60 days with minimal exogenous immunosuppression, reducing the injected drug required over this time period by up to 83%. This is the first example of effective localized immunosuppression in a model as rigorous as NHPs.
A second generation of localized delivery devices were developed to address diffusional limitations to associated with point-source devices. Immunosuppressant-loaded tyrosine-derived polycarbonate nanospheres (TyroSpheres) were incorporated into water-soluble, biodegradable pullulan films. In vitro release testing was carried out on a variety of loading levels, focusing on the effect of drug loading in TyroSpheres and TyroSphere loading in films. In vivo the films rapidly hydrated and dissolved upon contact with the tissue bed, depositing TyroSpheres from which the immunosuppressant is released. Later explantation of the site revealed no residual material in the tissue bed.
Finally, hierarchical scaffolds are explored using a novel airbrushing method for incorporation of microfiber mats throughout 3D printed structures. This hierarchical scaffold exhibited greater cell growth in vitro and improved cellular penetration in vivo. Furthermore, they were functionalized by decellularized ECM laid down by living cells, pro-angiogenic peptide sequences through click-chemistry reaction, and cell affecting drugs through incorporation in the airbrushing solution. This scaffold fabrication approach provides a tool for increasing the bioactivity of 3D printed scaffold for locally engineering the tissue environment.