The ultimate goal of intracellular drug delivery is the spatial and temporal control of payload release upon cell entry without decomposition during transit. Thus, the interest in selective activation of bioactive agents for advanced therapeutics and diagnostics, aiming to efficiently suppress background interferences and attenuate systemic toxicity, is constantly growing. Over the last years, a new tool for controlled payload transport and release was developed to overcome this barrier. Self-immolative linkers (SILs) are a class of spacers that disintegrate upon a certain trigger to release the conjugated biomolecule in a traceless manner. Typically, the desired drug is conjugated to the proximal end of the spacer, whereas the distal end, which typically initiates the decomposition, is protected, e.g. by being bound to a carrier. Upon deprotection and self-immolation, the conjugated cargo is finally released.

Schematic illustration of the concept of self-immolative linkers. PG = protecting group.

One powerful system relies on disulfide-based self-immolative linkers. The overall red/ox potential in the human blood is oxidative, making disulfide linkages stable during circulation. In contrast, the intracellular milieu of mammalian cells is characterized by an overall reductive potential, thus allowing to revert the disulfide bond formation. Furthermore, cytoplasm typically shows millimolar (1-10 mM) concentrations of the reductant tripeptide glutathione (GSH) in contrast to 2-10 µM in blood and other body fluids.

Chemical structure of the tripeptide Glutathione.

Notably, GSH has been recognized as a biomarker of tumor development. Elevated levels are reported inside tumor mass (~100-fold) and cancer cells (~sevenfold) compared to those of normal tissues and cells, respectively. This drastic discrepancy in GSH concentration renders it possible to develop GSH-responsive materials which can be selectively activated in pathological tissues. Those specifications allow for sufficient stability of the linker in the extracellular milieu but spontaneous self-immolative reaction within the cytosol upon GSH-mediated disulfide cleavage.

SILs can be mainly classified into two groups depending on the self-immolative fragmentation process either based on the quinone methide cascade (e.g. 1,6-elimination) or an intramolecular cyclization reaction. In addition, the linker’s chemical composition (e.g. disulfide ethoxycarbonyl (SSE) vs. disulfide benzyloxycarbonyl (SSB)) results in chemically tunable kinetics of the self-immolative cleavage due to different response rate towards GSH, showing higher rates for SSB-based DSILs compared to SSE-based ones. Thus, the choice of the linker allows for fine-tuning of the cleavage speed and payload release.

Disulfide-based self-immolative linkers either based on an intramolecular cyclization cascade or an 1,6-elimination.

Please find many different disulfide-based self-immolative linkers within our section Related Products (see below). If you cannot find your desired derivative, please contact us for a custom synthesis!

Interested in further information on linker technologies? -> Please feel free to download our new brochure on Linkerology!

References:

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• Engineering Intracellular Delivery Nanocarriers and Nanoreactors from Oxidation-Responsive Polymersomes via Synchronized Bilayer Cross-Linking and Permeabilizing Inside Live Cells; Z. Deng, Y. Qian, Y. Yu, G. Liu, J. Hu, G. Zhang, S. Liu; J. Am. Chem. Soc. 2016; 138: 10452-10466. https://doi.org/10.1021/jacs.6b04115.