Device-based local delivery of siRNA against mammalian target of rapamycin (mTOR) in a murine subcutaneous implant model to inhibit fibrous encapsulation

Abstract

Fibrous encapsulation of surgically implant devices is associated with elevated proliferation and activation of fibroblasts in tissues surrounding these implants, frequently causing foreign body complications. Here we test the hypothesis that inhibition of the expression of mammalian target of rapamycin (mTOR) in fibroblasts can mitigate the soft tissue implant foreign body response by suppressing fibrotic responses around implants. In this study, mTOR was knocked down using small interfering RNA conjugated with branched cationic polyethylenimine (bPEI) in fibroblastic lineage cells in serum-based cell culture as shown by both gene and protein analysis. This mTOR knockdown led to an inhibition in fibroblast proliferation by 70% and simultaneous down-regulation in the expression of type I collagen in fibroblasts in vitro. These siRNA/bPEI complexes were released from poly(ethylene glycol) (PEG)-based hydrogel coatings surrounding model polymer implants in a subcutaneous rodent model in vivo. No significant reduction in fibrous capsule thickness and mTOR expression in the foreign body capsules was observed. Observed siRNA inefficacy in this in vivo implant model was attributed to siRNA dosing limitations in the gel delivery system, and lack of targeting ability of the siRNA complex specifically to fibroblasts. While in vitro data supported mTOR knock-down in fibroblast cultures, in vivo siRNA delivery must be further improved to produce clinically relevant effects on fibrotic encapsulation around implants.

Extracellular matrix-based biomaterial scaffolds and the host response

“…Common surgical implant models to study the FBR include the subcutaneous pouch (either through blunt dissection or minimally invasive needle injection) [8, 13, 19, 22, 160, 161], abdominal wall defect [162, 163], intraperitoneal space [154, 164, 165] and dorsal skinfold chamber [166–168] models. Previously, the cage implant system was a popular method for studying implant/host tissue responses [169–171].…”

Bioactive factor delivery strategies from engineered polymer hydrogels for therapeutic medicine

“…In addition, the encapsulation of polymer particles may increase their biocompatibility by “hiding” them within the polymer networks and also prevents rapid clearance of the particles at the site of interest in vivo [279]. Hybrid hydrogels have been used to modulate the release of small molecule drugs [280,281], genetic material [282–284], proteins [285–287] and vaccines [288]. These systems also allow for delivery of multiple different bioactive agents with independent release rates [289].…”

“…Compared to the release of free VEGF from the hydrogels, the encapsulation of VEGF into nanoparticles significantly reduced the burst release, due to the larger size of the VEGF nanoparticles than that of the soluble VEGF. PEG gels were also used to release siRNA/PEI nanocomplexes in a sustained manner, and the release was modulated by the degree of hydrogel crosslinking [282]. In another work, calcium phosphate–DNA nanoparticles were released with a slower rate than naked DNA from calcium ALG hydrogels [35].…”

The Application of Nanotechnology for Implant Drug Release

“…Additionally, release rates and duration of silencing could be altered by choosing different nanoparticle formulations. Takahashi et al used a similar strategy, encapsulating in PEG based hydrogels, polyethylenimine formulated siRNA nanoparticles targeted towards mTOR, a gene involved in fi broblast proliferation and collagen production [ 89 ]. While they observed effective results in vitro no effect was observed in vivo.…”