Background
Initial experience with vascularized composite allotransplantation (VCA) has shown medication non-compliance results in increased acute rejections and poor long-term outcomes. Delivering continuous immunosuppression targeted regionally to sites of allorecognition without daily systemic immunosuppression would lead to improved compliance and overall allograft survival without systemic side-effects.
Methods
50mg FK506 was encapsulated in PLGA/PLA microspheres and incorporated into 5mm PLGA discs (Fk disc). Brown-Norway(BN) to Lewis(Lew) functional orthotopic hind-limb transplants performed.6 rats per group. Group1: No treatment following allotransplantation. Group2: 1 Fk disc implanted subcutaneously in the native un-transplanted leg. Group3: 1 Fk disc implanted subcutaneously in the transplanted allograft. End-point 180 d survival or grade3 irreversible rejection. Blood FK506 levels measured at regular intervals. At end-point, blood and allograft tissue Fk506 levels measured. Splenic T-cells alloreactivity measured via mixed lymphocyte reactivity (MLR) assay and compared to that of T-cells from groin draining lymph nodes (DLN)
Results
In groups 2 and 3, systemic blood Fk506 levels were maintained at a constant therapeutic level between 5–15 ng/ml for 146(+/−11.1) days. Following that the levels declined to <5 ng/ml through the end-point. There was significantly increased Fk506 level in groin lymph nodes draining the implanted Fk disc compared to blood in groups 2 and 3. In Group 1, the allograft rejected in 7(+/−1.1) days. In group 2, once the systemic Fk levels declined < 5ng/ml, the allograft rejected in 146 (+/−23) days. However, in group 3, there was 100% allograft survival to >180 days despite sub-therapeutic levels < 5ng/ml. At end-point, in group 3, significant T cell hyporesponsiveness was seen in Fk disc draining groin DLN while there was a robust splenic T-cell proliferation. In addition, significant groin tissue Fk levels were seen despitesubtherapeutic systemic Fk levels.
Conclusion
Implantation of one Fk506 disc leads to sustained therapeutic systemic Fk506 levels without any further systemic FK506 administration. A single Fk506 disc implanted in the transplanted allograft maintains the allograft even at sub therapeutic systemic Fk506 levels via regional concentration of Fk506 in the DLN and consequent blunting of alloresponse restricted to the transplanted hind limb only.