Antibody

The tumor-targeting antibody in ADC serves as the crucial mediator for specific binding to target antigens on cancer cells. The ideal antibody moiety must exhibit appropriate binding affinity and specificity for the target antigen and facilitate efficient internalization. Additionally, it should demonstrate minimal immunogenicity and maintain an extended plasma half-life, thereby maximizing therapeutic benefit while minimizing potential immune responses (Nat Rev Clin Oncol. 18(6): 327-344, 2021).

Currently, the development of ADCs has increasingly pivoted toward the utilization of fully humanized antibodies, which significantly reduces immunogenicity. Immunoglobulin G1 (IgG1) is a commonly employed subclass for ADCs, a choice driven by its abundance in serum and its ability to induce potent effector functions (Pharmacol Ther. 229: 107917, 2022).

Antigen

The target antigen expressed on tumor cells serves as the navigation beacon for ADCs to identify and bind to malignant cells specifically. To minimize off-target toxicity and broaden the therapeutic window, the suitable antigen should be expressed exclusively or predominantly on target cells, with low or no expression in disease-free tissues. Additionally, for conventional internalizing ADCs, an ideal target antigen would be internalized upon binding with its corresponding ligand, thereby enabling effective intracellular delivery of the cytotoxic payload (Signal Transduct Target Ther. 7(1): 93, 2022).

Recent advances in ADC research have expanded the scope of target antigen selection beyond conventional tumor cell surface markers. Modern ADCs increasingly target components within the tumor microenvironment, such as the neovascular system, subendothelial extracellular matrix, and tumor matrix (Pharmacol Ther. 221:107753, 2021; Nat Rev Drug Discov. 22(8):641-661, 2023; J Hematol Oncol. 14(1): 20, 2021).

Payload

The payload in antibody-drug conjugates (ADCs) is typically a potent chemotherapeutic agent that exerts cytotoxicity following internalization into cancer cells. Such compounds must possess high potency, often exhibiting IC50 values in the nM and pM range, remain stable under physiological conditions, and contain functional groups amenable to conjugation. Traditional payloads primarily encompass potent tubulin inhibitors and DNA-damaging agents, which have been widely employed for their robust cytotoxic profiles (J Hematol Oncol. 16(1): 3, 2023).

In addition to conventional cytotoxins, there is a burgeoning interest in incorporating payloads with novel mechanisms of action into ADC design. For instance, small-molecule immunomodulators have recently been explored for the development of innovative ADC therapeutics. These include toll-like receptor (TLR) agonists and stimulator of interferon genes (STING) agonists, which leverage the host immune response to augment the anti-tumor effects of ADCs (J Med Chem. 64(21): 15716-15726, 2021; Nat Cancer. 2(1): 18-33, 2021; Pharmacol Ther. 236: 108106, 2022).

Mechanism of action

Antibody-drug conjugates (ADCs) selectively bind to target antigens that are specifically expressed on the surface of cancer cells. Upon binding, the ADCs are rapidly internalized into the cell, leading to the formation of early endosomes. These endosomes then mature into late endosomes, which eventually fuse with lysosomes, a process mediated through complex intracellular signaling pathways. Within the lysosome, the cytotoxic payload of the ADC is released through chemical or enzymatic means, leading to the induction of apoptosis in the malignant cell. This represents the primary mechanism of action of ADCs in targeting cancerous tissues (Lancet. 394(10200): 793-804, 2019; Pharmacol Ther. 200: 100-125, 2019).

Furthermore, the efficacy of ADCs may be potentiated through a bystander effect, wherein the released payload, if permeable or transmembrane, can exert toxic effects on neighboring cells that do not express the target antigen. This multifaceted approach leverages both targeted delivery and bystander toxicity to enhance the therapeutic potential of ADCs (Chem Soc Rev. 51(22): 9182-9202, 2022; CA Cancer J Clin. 72(2): 165-182, 2022).

Linker

The linker plays a critical role in bridging the antibody with the payload. An optimal linker must fulfill several key criteria, including avoidance of ADC aggregation, prevention of premature payload release in the plasma, and facilitation of targeted drug release within the desired cellular compartment. Linkers are broadly classified into two categories: cleavable and non-cleavable, each with distinct characteristics that influence their application in ADC design (Acta Pharm Sin B. 11(12): 3889-3907, 2021).

Cleavable linkers are specifically engineered to undergo hydrolysis or enzymatic cleavage in response to tumor-associated factors, such as acidic pH or the presence of abundant proteolytic enzymes within the tumor microenvironment. This ensures a targeted release of the payload, minimizing off-target toxicity. In contrast, non-cleavable linkers are designed for enhanced stability in plasma, rendering them resistant to premature breakdown (Chem Soc Rev. 48(16): 4361-4374, 2019).

Conjugation method

Beyond the selection of antibody, linker, and payload, the approach in which the small molecule moiety (comprising the linker and payload) is tethered to the antibody also significantly influences the effective assembly of ADCs. Typically, the accessible reaction sites for conjugation are provided by the existence of lysine, cysteine, and other engineered reactive residues on the antibody (Protein Cell. 9(1): 33-46, 2018; Nat Rev Drug Discov. 16(5): 315-337, 2017).

These conjugation methods are mainly categorized into two primary approaches: random conjugation and site-specific conjugation. Site-specific conjugation ensures that the payload is consistently connected to predetermined sites on the antibody, thereby reducing heterogeneity. This consistency in conjugation often translates to more predictable pharmacokinetics and enhanced therapeutic efficacy (J Natl Cancer Inst. 111(6): 538-549, 2019; Cancer Cell. 40(11): 1255-1263, 2022).