A Precisely Controlled Long-Acting Immunosuppression Platform Enables Prolonged Survival of Vascularized Composite Allografts

Abstract

Vascularized composite allotransplantation (VCA) restores complex multi-tissue defects but remains limited by the need for lifelong immunosuppression. Daily oral tacrolimus, the primary maintenance regimen, poses adherence challenges and produces large peak-trough fluctuations within a narrow therapeutic window, resulting in toxicity and periods of under-immunosuppression. Although injectable depots could improve adherence, existing systems frequently generate prolonged supratherapeutic exposure. Most rely primarily on depot matrix properties to govern release kinetics, which alone are insufficient, and in certain systems uncontrolled solvent efflux during phase inversion can exacerbate early burst release. To address these limitations, we developed PRECISE (Programmable, Retrievable, Embedded Controlled ImmunoSuppression Encapsulator), a graft-injectable, in situ-forming depot engineered to precisely regulate systemic tacrolimus exposure. PRECISE integrates a polymeric matrix with rationally selected small-molecule excipients that interact with tacrolimus at the molecular level and modulate solvent dynamics during phase inversion, enabling release control beyond polymer tuning alone. Notably, rapamycin, under select dosing conditions, can also interact with tacrolimus and regulate solvent dynamics, enabling controlled co-delivery of a complementary immunosuppressive combination. This integration of drug-excipient and drug-drug interactions introduces a previously unexplored mechanism for controlling systemic exposure in in situ-forming depots. In a rat hindlimb transplant model, monthly intragraft administration of PRECISE maintained systemic tacrolimus and rapamycin concentrations within the therapeutic window for over 300 days, prolonged graft survival, expanded regulatory T cells, and induced donor-specific hyporesponsiveness. Translational validation in fully MHC-mismatched (SLA Class I and II) pigs, one of the most stringent large-animal transplant models, demonstrated sustained therapeutic drug levels, preserved graft vascular integrity, and graft survival through 90 days. Collectively, these findings establish PRECISE as the first long-acting immunosuppressive platform to maintain systemic drug concentrations precisely within the therapeutic window while enabling controlled dual-drug immunosuppression and the first long-acting immunosuppressive system validated in a fully MHC-mismatched large-animal VCA model, with broad applicability to solid-organ transplantation.