Investigation of the Impact of Hydrolytically Cleavable Groups on the Stability of Poly(ethylene glycol) Based Hydrogels Cross-Linked via the Inverse Electron Demand Diels-Alder (iEDDA) Reaction

Eight-armed poly(ethylene glycol) (PEG) hydrogels cross-linked via inverse electron demand Diels-Alder reaction between norbornene and tetrazine groups are promising materials for long-term protein delivery. While a controlled release over 265 days is achieved for 15% w/v hydrogels in the previous study, the material shows high stability over 500 days despite having cleavable ester linkages between the PEG macromonomers and their functionalities. In this study, the hydrolyzable ester linkers in the PEG-norbornene precursor structure are exchanged to reduce the degradation time. To this end, 3,6-epoxy-1,2,3,6-tetrahydrophthalimide, phenyl carbamate, carbonate ester, and phenyl carbonate ester are introduced as degradable functional groups. Oscillatory shear experiments reveal that they are not affected the in situ gelation. All hydrogel types have gel points of less than 20 s even at a low polymer concentration of 5% w/v. Hydrogels with varying polymer concentrations have similar mesh sizes, all of which fell in the range of 4-12 nm. The inclusion of phenyl carbonate ester accelerates degradation considerably, with complete dissolution of 15% w/v hydrogels after 302 days of incubation in phosphate buffer (pH 7.4). Controlled release of 150 kDa fluorescein isothiocyanate-dextran over a period of at least 150 days is achieved with 15% w/v hydrogels.

Injectable and biocompatible alginate-derived porous hydrogels cross-linked by IEDDA click chemistry for reduction-responsive drug release application

In this work, novel injectable and reduction-responsive hydrogels were successfully prepared via inverse electron demand Diels-Alder reaction between alginate-norbornene and a water-soluble PEG based disulfide cross-linker. The reduction-responsive cross-linker was designed to contain a PEG chain within two disulfide linkages, and two terminal tetrazine groups. The resulting hydrogels possessed high swelling ratios, porous morphology, excellent drug loading efficiency (~92%), and suitable mechanical properties. The drug release experiments demonstrated that the hydrogels released more than 90% of the encapsulated doxorubicin (DOX) in the presence of 10 mM glutathione while a minimal DOX release (<25%) was measured in physiological buffer (PBS, pH = 7.4) after 11 d. The cross-linker and hydrogels did not exhibit any apparent cytotoxicity to fibroblast cells. In contrast, DOX-loaded hydrogels induced anti-tumor activity against cancer cells. The injectable and reduction-responsive hydrogels hold great potential as a biomaterial for stimuli responsive drug delivery applications.

Proximity-Induced Bioorthogonal Chemistry Using Inverse Electron Demand Diels-Alder Reaction

Bioorthogonal chemistry techniques enable the selective and targeted manipulation of living systems. In order to yield universally applicable techniques, it is of great importance for bioorthogonal reactions to take place rapidly, selectively, and with the formation of only benign side products. One of the reactions that match these criteria well is the inverse electron demand Diels-Alder reaction (DA_inv) between tetrazines and strained dienophiles. However, even this prime technique comes with the disadvantage of its reactants having limited stability under physiological conditions. In our protocol, an unreactive and therefore stable DA_inv diene/dienophile pair reacts rapidly using DNA hybridization as secondary rate-accelerating process. Due to the fluorogenicity of the presented tetrazine rhodamine conjugate, this method enables the selective screening and evaluation of reactant pairs for proximity-mediated bioorthogonal chemistry.

STED Imaging of Golgi Dynamics with Cer-SiR: A Two-Component, Photostable, High-Density Lipid Probe for Live Cells

Long time-lapse super-resolution imaging in live cells requires a labeling strategy that combines a bright, photostable fluorophore with a high-density localization probe. Lipids are ideal high-density localization probes, as they are >100 times more abundant than most membrane-bound proteins and simultaneously demark the boundaries of cellular organelles. Here, we describe Cer-SiR, a two-component, high-density lipid probe that is exceptionally photostable. Cer-SiR is generated in cells via a bioorthogonal reaction of two components: a ceramide lipid tagged with trans-cyclooctene (Cer-TCO) and a reactive, photostable Si-rhodamine dye (SiR-Tz). These components assemble within the Golgi apparatus of live cells to form Cer-SiR. Cer-SiR is benign to cellular function, localizes within the Golgi at a high density, and is sufficiently photostable to enable visualization of Golgi structure and dynamics by 3D confocal or long time-lapse STED microscopy.