Abstract
Physiological barriers inside of tumor tissue often result in poor interstitial penetration and heterogeneous intratumoral distribution of nanoparticle-based drug delivery systems (DDS). Novel, matrix metalloproteinase (MMP)-sensitive peptide-crosslinked nanogels (pNGs) as multistage DDS are reported with a beneficial size reduction property to promote the process of deep tissue penetration. Methods: The presented pNGs are based on a dendritic polyglycerol (dPG) scaffold crosslinked by a modified MMP-sensitive fluorogenic peptide. The crosslinker integrates degradability in response to proteases present in the tumor microenvironment. Surfactant-free, inverse nanoprecipitation is employed to prepare the nanogels using strain-promoted click chemistry. The size and crosslinking density of the pNGs are controlled by the functionalization degree of dPG with cyclooctyne groups and by the peptide crosslinker fraction. The intrinsic reporter moiety of the crosslinker was used to study the influence of pNG compositions on the degradation profile. The therapeutic drug Doxorubicin was conjugated through a pH-sensitive linkage to dPG to form a multistage DDS. The penetration behavior of the pNGs was studied using agarose matrix and multicellular tumor spheroids (MCTS). Results: Nanogel sizes were controlled in the range of 150-650 nm with narrow size distributions and varying degrees of crosslinking. The pNGs showed stability in PBS and cell media but were readily degraded in the presence of MMP-7. The crosslinking density influenced the degradation kinetic mediated by MMP-7 or cells. Stable conjugation of DOX at physiological pH and controlled drug release at acidic pH were observed. The digestions of nanogels lead to a size reduction to polymer-drug fragments which efficiently penetrated into agarose gels. Moreover, the degradable multistage pNGs demonstrated deeper penetration into MCTS as compared to their non-degradable counterparts. Thus, degradable pNGs were able to deliver their cargo and efficiently reduce the cell viability in MCTS. Conclusion: The triggered size reduction of the pNGs by enzymatic degradation can facilitate the infiltration of the nanocarrier into dense tissue, and thereby promote the delivery of its cargo.