Linkers: An Assurance for Controlled Delivery of Antibody-Drug Conjugate

Abstract

As one of the major therapeutic options for cancer treatment, chemotherapy has limited selectivity against cancer cells. Consequently, this therapeutic strategy offers a small therapeutic window with potentially high toxicity and thus limited efficacy of doses that can be tolerated by patients. Antibody-drug conjugates (ADCs) are an emerging class of anti-cancer therapeutic drugs that can deliver highly cytotoxic molecules directly to cancer cells. To date, twelve ADCs have received market approval, with several others in clinical stages. ADCs have become a powerful class of therapeutic agents in oncology and hematology. ADCs consist of recombinant monoclonal antibodies that are covalently bound to cytotoxic chemicals via synthetic linkers. The linker has a key role in ADC outcomes because its characteristics substantially impact the therapeutic index efficacy and pharmacokinetics of these drugs. Stable linkers and ADCs can maintain antibody concentration in blood circulation, and they do not release the cytotoxic drug before it reaches its target, thus resulting in minimum off-target effects. The linkers used in ADC development can be classified as cleavable and non-cleavable. The former, in turn, can be grouped into three types: hydrazone, disulfide, or peptide linkers. In this review, we highlight the various linkers used in ADC development and their design strategy, release mechanisms, and future perspectives.

Keywords: FDA; antibody-drug conjugates (ADCs); bioconjugation; chemotherapy; cytotoxic drug; linker; monoclonal antibody; tumor.

Figure 1

Schematic diagram of an antibody-drug conjugate (ADC).

Figure 2

Cytotoxic drugs used in ADC design.

Figure 3

Classification of linkers in ADCs.

Figure 4

Structure of BR96-doxorubicin.

Figure 5

Cleavage mode of hydrazone linker in Mylotarg. Under strong acidic conditions, the metabolite—calicheamicin—is released first by hydrolysis of the hydrazone moiety, followed by the cytosolic reduction of the disulfide bond to afford the free sulfide anion. This then forms a thiophen ring through cyclization.

Figure 6

Cleavage mode of disulfide linker in huC242-SPDB-DM4. The ADC loses antibodies by proteolysis and then undergoes disulfide bond cleavage to form the active drug. The drug is then metabolized with S methyl transferase.

Figure 7

The structure of IMGN901.

Figure 8

Straightforward cleavage mechanism of p-aminobenzyl carbamate (PABC) containing conjugate.

Figure 9

Cleavage mode of Brentuzumab vedotin.

Figure 10

Cleavage mode of Val–Ala-containing Rovalpituzumab tesirine.

Figure 11

The structure of an ADC containing β-glucuronic acid.

Figure 12

The mechanism by which an ADC containing β-glucuronic acid is released.

Figure 13

The mechanism by which an ADC containing β-glucuronic acid is released.

Figure 14

The mechanism by which an ADC containing pyrophosphate is released.

Figure 15

The mechanism by which an ADC containing pyrophosphate is released.

Figure 16

Structure of T-DM1 containing a thioether linker.

Figure 17

The mechanism for the formation of amide attachment sites through N-hydroxysuccinimide (NHS).

Figure 18

Mechanism for the formation of the sulfyhydryl attachment site.

Figure 19

Side-reactions undergone by the succinimide–thioether moiety.

Figure 20

Mechanism for the locking of the thioether conjugation bond via transcyclization.

Figure 21

Chemical structures of some recently approved ADCs.

Figure 21

Chemical structures of some recently approved ADCs.

Figure 22

Mechanism of action of ADCs: ADC binds to a cell-surface antigen that is ideally specific to a cancer cell. Upon binding, the ADC-antigen is internalized into the tumor cell. When the complex is degraded, it releases the cytotoxin, which then binds to its target to cause cancer cell apoptosis.