Linkers Having a Crucial Role in Antibody-Drug Conjugates

Abstract

Antibody-drug conjugates (ADCs) comprised of a desirable monoclonal antibody, an active cytotoxic drug and an appropriate linker are considered to be an innovative therapeutic approach for targeted treatment of various types of tumors and cancers, enhancing the therapeutic parameter of the cytotoxic drug and reducing the possibility of systemic cytotoxicity. An appropriate linker between the antibody and the cytotoxic drug provides a specific bridge, and thus helps the antibody to selectively deliver the cytotoxic drug to tumor cells and accurately releases the cytotoxic drug at tumor sites. In addition to conjugation, the linkers maintain ADCs' stability during the preparation and storage stages of the ADCs and during the systemic circulation period. The design of linkers for ADCs is a challenge in terms of extracellular stability and intracellular release, and intracellular circumstances, such as the acid environment, the reducing environment and cathepsin, are considered as the catalysts to activate the triggers for initiating the cleavage of ADCs. This review discusses the linkers used in the clinical and marketing stages for ADCs and details the fracture modes of the linkers for the further development of ADCs.

Keywords: antibody–drug conjugates; attachment site; cytotoxic drug; linker; monoclonal antibody; tumor.

Figure 1

Schematic for the structure of an antibody–drug conjugate (ADC). Adapted from reference [6].

Figure 2

The structural formula of calicheamicin, maytansine, monomethyl auristatin F (MMAF), monomethyl auristatin E (MMAE), doxorubicin.

Figure 3

Schematic representation of the mechanism of drug delivery mediated by ADCs. Reproduced with permission from reference [41]. Solid line arrows indicate specific tumor cell killing through receptor-mediated endocytosis. Dash line arrows elicit the specific tumor cell killing through extracellular drug release.

Figure 4

The structural formula of huC242-SMCC-DM1 and cantuzumab mertansine. Adapted from reference [47].

Figure 5

The structural formula of cAC10-L1-MMAF and cAC10-L4-MMAF. Adapted from reference [48,49].

Figure 6

The structural formula of the cAC10-L4-MMAFand the supposed cleavage mechanism after internalization into the lysosome. Adapted from reference [43,50].

Figure 7

The structural formula of BR96-doxorubicin. Adapted from reference [52].

Figure 8

The structural formula of BR96-doxorubicin. Adapted from reference [56].

Figure 9

The structural formula of huC242-SPDB-DM4 and the supposed cleavage mechanism after internalization into the lysosome. Adapted from reference [49,67].

Figure 10

The structural formula ofIMGN901. Adapted from reference [72].

Figure 11

The structural formula of MAb-Val-Cit-MMAE (SGN-35) and the supposed cleavage mechanism after internalization into the lysosome. Adapted from reference [49,76].

Figure 12

The structural formula of MAb-glu-CIB and the supposed cleavage mechanism after internalization into the lysosome. Adapted from reference [83].

Figure 13

The structural formula of MAb-glu-DOX. Adapted from reference [79].

Figure 14

The amines of lysines on the antibody react with N-hydroxysuccinimide (NHS) esters forming the amides and react with Traut’s reagent forming the amidines.

Figure 15

The disulfides of cysteines on the antibody were reduced by DL-Dithiothreitol (DTT) or Tris(2-carboxyethyl)phosphine (TCEP) and the thiols react with maleimide, halogenoalkane, disulfide or thiol compounds.

Figure 16

The hydrolysis of the succinimide-thioether ring results in a “ring-opened” linker. Adapted from reference [95].

Figure 17

The commonly employed unnatural amino acids in the antibodies: para-acetyl Phe, para-azido Phe and propynyl-Tyr.

Figure 18

A glutamine side chain is ligated to a lysine side chain by transglutaminase.