Background
A major barrier for Vascularized Composite Allotransplants (VCA) to be more frequently advised is the necessity of long-term, systemic immunosuppression, elevating the risks of kidney and liver toxicity, cancer and opportunistic infections in patients To minimize these risks, we developed a hydrogel drug delivery system, which can be injected directly into the graft and locally releases immunosuppressive drugs in response to inflammation We now aimed to demonstrate that this hydrogel indeed responds to inflammation in vivo with drug release and provides adequate immunosuppression in experimental VCA.
Methods
To show on-demand drug release, hydrogel loaded with tacrolimus was injected in hind-limbs of naïve Lewis rats After 7 d rats were challenged with lipopolysaccharide (LPS) sc in the same limb to induce inflammation Tacrolimus blood levels were then monitored To assess long-term graft survival using the hydrogel and study the immunological response in this context, hydrogel loaded with tacrolimus together with infrared-fluorescent dye was injected in Brown Norway hind-limbs transplanted to Lewis recipients Local and systemic release of tacrolimus as well as fluorescent dye were followed and circulating immune cells were characterized.
Results
We observed initial burst release (over 65 ng/uL) lasting 48 h after injection of the hydrogel, followed by significant decrease and stabilization of tacrolimus to therapeutically relevant systemic levels for over 30 d LPS injection on day 7 led to increased tacrolimus levels, while animals not receiving LPS did not show increase of tacrolimus release Transplanted hind limbs treated with tacrolimus and fluorescent dye loaded hydrogel showed signs of graft rejection around post-operative day 83 Local levels of dye and tacrolimus in the graft were higher and more stable than systemically FACS analyses of circulating graft T lymphocytes showed significantly higher levels of chimerism in animals treated with the hydrogel than in controls with daily tacrolimus treatment.
Conclusions
We could confirm that our hydrogel releases tacrolimus in response to inflammation Drug and dye release correlated well, allowing a non-invasive monitoring in future studies, in which repeated injections will be used to promote indefinite graft survival furnishing high intragraft and sub-therapeutic systemic levels of the drug.