Subcutaneous nanotherapy repurposes the immunosuppressive mechanism of rapamycin to enhance allogeneic islet graft viability

Standard oral rapamycin (that is, Rapamune) administration is plagued by poor bioavailability and broad biodistribution. Thus, this pleotropic mammalian target of rapamycin (mTOR) inhibitor has a narrow therapeutic window and numerous side effects and provides inadequate protection to transplanted cells and tissues. Furthermore, the hydrophobicity of rapamycin limits its use in parenteral formulations. Here, we demonstrate that subcutaneous delivery via poly(ethylene glycol)-b-poly(propylene sulfide) polymersome nanocarriers significantly alters rapamycin's cellular biodistribution to repurpose its mechanism of action for tolerance, instead of immunosuppression, and minimize side effects. While oral rapamycin inhibits T cell proliferation directly, subcutaneously administered rapamycin-loaded polymersomes modulate antigen presenting cells in lieu of T cells, significantly improving maintenance of normoglycemia in a clinically relevant, major histocompatibility complex-mismatched, allogeneic, intraportal (liver) islet transplantation model. These results demonstrate the ability of a rationally designed nanocarrier to re-engineer the immunosuppressive mechanism of a drug by controlling cellular biodistribution. Orally delivered rapamycin is an immunosuppressant that inhibits islet graft rejection in patients treated for type 1 diabetes, but it suffers from poor bioavailability, inconsistent cellular distribution and adverse reactions. Here the authors show that subcutaneous delivery of rapamycin using a polymersome platform allows for control of the drug's biodistribution and activity on specific immune cells, which changes its mechanism of action from immunosuppression to tolerance, reduces side effects and enhances anti-inflammatory efficacy.