Antibody-Drug Conjugates: New Strategies of ADC Payloads

Driven by increasingly mature technology, ADC drugs have gone through three iterations (Fig. 1). Although ADCs have gone through three iterations, current payloads still have clinical limitations, such as severe side effects and drug resistance. The development of novel ADC payloads is a complex and dynamic process, encompassing a diverse range of agents such as RNA inhibitors, Bcl-xL inhibitors, NAMPT inhibitors, and the proteasome inhibitor carbimycin, among others. Moreover, the emergence of immune ADCs featuring immunomodulators as payloads has sparked considerable interest, given their pivotal contribution to the field of tumor immunotherapy.

The disparity between the novel ADC payload and its traditional counterpart is noteworthy. 1) The traditional ADC payload predominantly favors payloads with heightened efficacy and toxicity, exemplified by PBD. In contrast, the new payload leans towards smaller molecules with moderate toxicity and efficacy, exemplified by Dxd. 2) While the traditional ADC payload is primarily comprised of microtubule inhibitors and DNA damage agents, the new ADC payload exhibits a more diverse array of target types, offering a myriad of choices and encompassing distinct mechanisms of action, setting it apart from the conventional payload. For example, use immune agonists as payloads for ADCs. 3) The structures of traditional ADC small molecule payloads are generally complex and difficult to synthesize. New ADC payloads tend to select small molecules with simple structures and low molecular weights, which greatly reduces the difficulty of synthesis. 4) Traditional ADC payloads usually use a single type of small molecule compound to be directly connected to the antibody through a linker, while new ADCs can combine different types of payloads to achieve a 1+1>2 effect. In addition to traditional small molecule compounds, new types of payloads such as PROTACs and light-activated molecules have also been selected as payloads.

New Strategies for ADC Payloads

ADC with PROTAC Molecules as Payload

A major challenge in ADC development is dose-limiting toxicity (DLT), which demonstrates the difficulty in balancing the efficacy of drug therapy with off-target toxicity. The actual dose of drug delivered by the ADC to the tumor is very small, which means that the drug molecule must be extremely cytotoxic, although this may cause toxic side effects. PROTACs consist of ligand targeting protein of interest (POI), E3 ligase ligand, and PROTAC linker. PROTACs bind POI and E3 ligases more tightly together, label POI with ubiquitination, and then be degraded by the proteasome. PROTAC is catalytic and therefore can effectively degrade target proteins at lower doses. PROTACs may be ideal payloads for ADCs. PROTAC antibody conjugates formed by coupling antibodies and PROTAC molecules can specifically degrade target proteins in specific cells, achieving the selectivity of PROTAC technology at the cellular or tissue level. Antibody-PROTAC conjugates combine the catalytic properties of PROTAC with the tissue specificity of ADC, thereby overcoming the traditional limitations of targeting degraders and ADCs, and have great potential in targeting new targets.

ADC with NIR-PIT Drug as Payload

Near-infrared photoimmunotherapy (NIR-PIT) drugs typically consist of tumor-specific monoclonal antibodies that target tumors and light-activated chemicals linked by a linker. They are essentially ADC drugs. NIR-PIT drugs can form a new targeted anti-tumor platform that irradiates infrared light to the tumor site through the device. This platform enables antibody-mediated targeted delivery to achieve a high degree of tumor specificity, while using infrared light to activate the biophysical mechanism of the drug to accurately induce rapid death of cancer cells without harming surrounding normal tissue. In addition, NIR-PIT drugs can also be used as a supplement to PD-1 monoclonal antibodies, PD-L1 monoclonal antibodies or CTLA-4 monoclonal antibodies to enhance their tumor immune response. The NIR-PIT drug is not a cytotoxic substance, but a water-soluble phthalocyanine derivative.

Dual Payload ADC

As resistance to ADC therapies emerges, combining one antibody with two or more different cytotoxic payloads provides options for the development of next-generation ADCs. To demonstrate the advantages of dual-payload drug ADCs, Levengood et al. prepared a class of ADCs containing two different tubulin polymerization inhibitors in 2017. The team conjugated MMAE and MMAF, which have different physical and chemical properties, to CD30 antibodies.

PDC Payload

Peptide drug conjugates (PDCs) are a new type of targeted therapy consisting of a linker, a homing peptide, and a cytotoxic payload. Compared with ADC drugs, PDC drugs have the advantages of small molecular weight, strong tumor penetration, low immunogenicity, large-scale solid-phase synthesis, low production cost, and better pharmacokinetic properties. They are the next generation of targeted anti-tumor drugs after small molecule targeted drugs, monoclonal antibodies and ADCs. The application of PDC as a therapeutic drug also has certain limitations, such as low oral bioavailability, short half-life, incomplete cleavage of the payload, resulting in significantly reduced biological activity compared with the prototype drug, and the targeting may be lower than that of ADC. The mechanism of action of PDC depends on linkers and homing peptides. Peptides can affect the efficiency of PDC internalization. The ideal PDC peptide should have strong target binding affinity, high stability, low immunogenicity, high internalization rate and long plasma half-life. Linker selection is one of the key factors in PDC design. Linkers need to balance stability and release efficiency. Cleavable linkers have higher release efficiency, while non-cleavable linkers are more stable. The toxicity and physicochemical properties of the payload can directly affect the drug's ability to kill tumors, thereby affecting its efficacy. The ideal payload should have high cytotoxicity (usually a low half-inhibitory concentration), low immunogenicity, high stability, appropriate hydrophobicity, and good solubility.

PDC payloads can be divided into chemical drugs, protein drugs and peptide drugs. Among them, chemical drugs and protein drugs are more common. Chemical drugs such as DM1, MMAE, KSP inhibitors, camptothecin, doxorubicin, paclitaxel, methotrexate, daunorubicin and gemcitabine are commonly used. Protein drugs mainly include interferon and tumor necrosis factor. Radioactive isotopes are also commonly used as payloads in PDCs. When used for cancer diagnosis, PDCs can be labeled with positron-emitting radioactive isotopes (fluorine 18 (¹⁸F), copper 64 (⁶⁴Cu), and gallium 68 (⁶⁸Ga)) to generate PET imaging. Radioisotope-labeled PDCs can also be used therapeutically. Peptide receptor radionuclide therapy (PRRT) targets and tissue-specific radiation based on receptors overexpressed by tumor cells. The most commonly used radioactive isotopes are indium-111 (¹¹¹In), yttrium-90 (⁹⁰Y) and yellow ruthenium-177 (¹⁷⁷Lu).

Although the specificity and cytotoxicity of the new generation ADC payloads are significantly improved compared to the early stages, current payloads still have certain limitations. First, they often have limited solid tumor permeability and toxicity, which limits their application in solid tumors. Second, some tumors are insensitive to current ADC drugs. Third, the complexity of payload pharmacokinetics, tumor targeting, and insufficient release will affect the efficacy of ADC drugs. Fourth, like other anti-tumor drugs, ADC drugs can also develop drug resistance. Finally, early payload selections tended to be toxic but effective drugs, but their higher toxicity limited the payload's system application. Although anti-human antibody ADCs have shown efficacy in treatment, the combination with highly toxic payloads has been less promising. Therefore, other types of payloads such as RNA inhibitors, immune agonists, and pro-apoptotic Bcl-xL inhibitors are under development.

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