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The variations for linkers are endless, as in our catalogue program already hundreds of different derivatives are available for conjugation of small molecules, biomolecules and surfaces to each other.
In the following we demonstrate a few examples with their specific properties.
Tetramer peptidic linker for antibody-drug conjugation designed for lysosomal enzymatic cleavage:
The Ala-Leu-Ala-Leu tetramer will be cleaved by the enzyme catepsin B, which is occurs only in the lysosome. Hence, this linker is stable in plasma. As this peptide sequence is rather hydrophobic and the active molecule Okilactomycin also shows low solubility in aqueous media and plasma, a PEG linker of appropriate length is attached. The length of this solubilizing polymer is specifically designed to develop the required solubility of the whole linker compound. Drug-linker conjugate will be used to couple to solvent accessible amino groups form lysines of an antibody. The glass tubes show solutions in water of antibodies bearing conjugates without (left) and with (right) PEG linker. The addition of a hydrophilic spacer clearly increases solubility of the conjugate. The activation of the carboxylic acid function does not happen through conventional N- hydroxysuccinimidyl ester, but with benzotriazole, as Bt ester are stable in water at pH7 and and ensure quantitative coupling to the antibody. An NHS ester, however, would hydrolyze and coupling yield wold be much lower.
Dye molecules are normally large multi-ring aromatic systems, like BODIPY, therefore, hardly soluble in polar media. Multiple attachment of α-sulfo-β-alanine enhances drastically solubility, enabling superior purification possibilities and high coupling yields.
The choice of appropriate spacer fragment combined with sophisticated linker chemistry ensures success of the application. Recently new 3rd generation Click-Linkers carrying functional groups like cyclooctynes, trans-cyclooctenes, tetrazines, or dienophiles on PEGs, polyamines or short peptides became more popular, as they deploy high conjugation yield combined with excellent solubility properties driving complex applications to success.