A stimuli-responsive hydrogel that targets myeloid differentiation primary response 88 signaling in the foreign body response.

Myeloid differentiation primary response protein 88 (MyD88) is an intracellular signaling molecule that is a central mediator of inflammation. MyD88 mediates inflammation in the foreign body response (FBR), a complex immune reaction triggered by implanted biomaterials that leads to fibrous encapsulation. Inhibiting MyD88 offers a promising approach to combat the FBR. A stimuli-responsive therapeutic poly(ethylene glycol) (PEG) hydrogel was developed to release a small molecule inhibitor of MyD88 (T6167923) when cleaved by matrix metalloproteinase 12 (MMP-12). MMP-12, also known as macrophage elastase, is produced primarily by macrophages, the immune cells that are responsible for the FBR. The therapeutic hydrogel was designed by modifying T6167923 with a free thiol (T6167923(SH)) enabling its immobilization into a hydrogel via a PEG spacer to increase its solubility once released and with a peptide linker PLGLEEA known to be sensitive to MMP-12. The bioactivity of free T6167923(SH) in solution was first confirmed in murine macrophages in vitro, leading to a dose-dependent inhibition of NF-κB activation and pro-inflammatory cytokine (IL-6 and TNF-α) production. Once tethered and released from the hydrogel, the released inhibitor is linked to the PEG spacer and the amino acid sequence PLG (i.e., Inhibitor-PEG-PLG). As MyD88 inhibition occurs intracellularly, cell uptake of the inhibitor was confirmed but the efficacy decreased with increasing length of PEG. Thus, a small PEG12 linker was chosen to design the therapeutic hydrogel. MMP-12 diffusion through the hydrogel network was confirmed, which successfully cleaved the MMP-sensitive sequence enabling release of a payload. The therapeutic hydrogel was subcutaneously implanted in mice, leading to a 50% decrease in the inflammatory cell layer thickness relative to the non-therapeutic PEG hydrogel control at day 7. By day 28, the thicknesses of the inflammatory cell layer and the fibrous capsule were reduced by 74% and 61%, respectively, relative to hydrogel implants without tethered inhibitor. In summary, this study demonstrates a promising novel therapeutic hydrogel that is capable of attenuating inflammation when triggered by macrophage elastase.