Graft-localized programmable immunosuppression enables durable survival in vascularized composite allotransplantation

Introduction: Hand and face transplantation have transformed reconstruction of devastating injuries but require lifelong systemic immunosuppression limited by toxicity, adherence, and narrow therapeutic windows. Unlike solid-organ transplantation, vascularized composite allografts offer a unique opportunity for graft-embedded, sustained drug delivery within accessible transplanted tissue. However, current strategies suffer from burst release and prolonged supratherapeutic exposure, as depot matrix properties alone are insufficient to control release, and solvent-efflux during phase inversion in certain systems can further exacerbate early burst release.
Method: We developed PRECISE (Programmable, Retrievable, Embedded Controlled ImmunoSuppression Encapsulator), a graft-injectable, in situ-forming depot which integrates a polymeric matrix with rationally selected small-molecule excipients that interact with TAC at the molecular level and modulate solvent dynamics during phase inversion, enabling release control beyond polymer tuning alone. Notably, rapamycin (RAPA) can also interact with TAC and regulate solvent dynamics, enabling controlled co-delivery of the drug combination. Efficacy was evaluated in a rodent hindlimb transplant model and a stringent porcine VCA model using fully SLA class I/II–mismatched donor–recipient pairs across outbred strains (Duroc, Landrace, Yorkshire), approximating complete HLA mismatch.
Results: Incorporation of RAPA along with TAC suppressed burst release in vitro (Fig. 1). In rodent VCA (Fig. 1), PRECISE eliminated burst release and maintained TAC levels within the therapeutic range (5-10 ng/mL) for over 300 days with monthly intragraft dosing. This resulted in prolonged graft survival (median >300 days vs. <14 days untreated; p<0.001), a 2-3-fold expansion of regulatory T cells, and induction of donor-specific hyporesponsiveness. Depot retrieval resulted in rapid decline of TAC to sub-therapeutic levels within 48-72 hours, confirming on-demand reversibility. In the porcine gracilis flap model (Fig. 2), PRECISE maintained stable TAC exposure (~4-8 ng/mL) with minimal peak-to-trough fluctuation, preserved vascular perfusion and tissue integrity of highly antigenic skin-containing grafts, and achieved >90-day survival with minimal clinical and histologic rejection. No evidence of systemic toxicity or local adverse tissue response was observed. Preliminary non-human primate studies demonstrate >50-day survival of fully mismatched flaps with graft-embedded delivery and no observed toxicity.

Conclusion: Collectively, these findings establish PRECISE as the first long-acting sustained, controllable, and retrievable immunosuppressive platform to maintain systemic drug concentrations precisely within the therapeutic window while enabling controlled dual-drug delivery. It is also the first long-acting immunosuppressive system validated in a fully MHC-mismatched large-animal VCA model, which is one of the most immunologically stringent models reported to date, with broad applicability to solid-organ transplantation.