Antibody Basics: Part 6 - Antibody formats: Antibody-Drug Conjugates (ADCs)

Welcome back to Antibody Basics by Biointron, Part 6. In this episode, we’ll talk about Antibody-Drug Conjugates (ADCs), the drug type taking the biopharmaceutical industry by storm.

What are antibody-drug conjugates (ADCs)?

• Composed of a monoclonal antibody (mAb) covalently bonded to a biologically active cytotoxic drug via a chemical linker.

• The mAb can then bind to a specific target protein or receptor, allowing the delivery of the drug into the target cell. 

• This combination is valuable in cancer, as it allows for a highly specific targeting and potent killing effect.
  

The emergence and evolution of ADCs

• 1900: Paul Ehrlich’s magic bullet concept of targeted chemotherapy: His magic bullet would target a cytotoxin to intended structures in unwanted cells but spare healthy tissues.

• 1980s: Inception of hybridoma technology for generation of mAbs helped realize Ehrlich's vision with the first ADC trials underway.

• 2000s: The first approved ADC was produced:gemtuzumab ozogamicin (a CD33 antibody conjugated to an antitumor antibiotic, calicheamicin).

• Present: Growing understanding of the ADC mechanism of action and technological breakthroughs have heralded the approvals of over a dozen ADCs.

Advantages of ADCs

Targeted Delivery: Combination of the unique targeting capabilities of monoclonal antibodies with the cancer-killing ability of cytotoxic drugs. Increased cell-killing potential of monoclonal antibodies.

Tumor Selectivity: ADCs achieve accurate and efficient elimination of target cells, without harming healthy cells. This discrimination between cancer and normal tissue is advantageous over traditional chemotherapeutic agents.

Stability: ADCs have biodegradable linkers which are either cleavable or non-cleavable. Linker design plays a critical role in modulating stability in the systemic circulation and payload release efficiency in the tumors, which thus affects ADC pharmacokinetic (PK), efficacy and toxicity profiles.

Designing an ADC

Antibody Moiety: The antibody component determines the ADC’s duration in the bloodstream, immunogenicity, immune functions, and target specificity. Most ADCs use IgG1, as it offers advantages like long serum half-life and strong Fc-mediated immune functions, including antibody-dependent cell-mediated cytotoxicity (ADCC). To mitigate potential immunogenic reactions, murine antibodies have largely been replaced with chimeric or humanized antibodies. An ideal target should be expressed at high levels on the surface of tumor cells and not on normal cells, e.g. HER2 and TROP2.

Linkers: Linkers make sure the payload is bound to the antibody while in circulation and later released at the tumor site. Cleavable linkers: releases payload upon reduction, proteolysis, or hydrolysis. •Noncleavable linkers: requires complete lysosomal degradation for payload release. Noncleavable linkers provide stability to the ADC during circulation and may contribute to a better safety profile, but may also hinder the potential for bystander killing. This is when the payload is released into the tumor microenvironment and kills antigen-less cancer cells or cancer-supporting cells.

Cytotoxic Payloads: Payloads are the chemotherapeutic agents which exert cytotoxic effects on the tumor cells targeted by the ADCs. Usually, they are microtubule binding or DNA damage-inducing agents like calicheamycins, auristatins and maytansinoids. Newer payloads include topoisomerase I inhibitors like camptothecin derivatives. Further development of more powerful ADCs have varied payloads like immune-stimulating agents, which are nonchemotherapeutic. Progress is also being made in improving the therapeutic window and decreasing systemic side effects to healthy cells.

All 13 approved ADCs are focused on cancer

• Gemtuzumab ozogamicin (Mylotarg): Approved in 2000, it targets CD33 to treat acute myeloid leukemia. It is the very first ADC to be approved in the world.

• Ado-trastuzumab emtansine (Kadcyla): Approved in 2013, it targets HER2 to treat early or metastatic platinum-resistant breast cancer. It is the top selling ADC with sales revenues of ~USD 2.2B in 2022.

• Sacituzumab govitecan (Trodelvy): Approved in 2020, it targets TROP-2 to treat metastatic triple-negative breast cancer and metastatic urothelial cancer.

• Mirvetuximab soravtansine (Elahere): Approved in 2022, it targets FR𝛂 to treat fallopian tube cancer or primary peritoneal cancer.

Third Generation ADCs

• Site-Specific ADCs: Up until 2019, all ADCs on the market were heterogeneous mixtures, meaning there were variable numbers of cytotoxic drugs on the mAb, in various places. To broaden the therapeutic index of ADCs, regiospecific versions were introduced: Bioconjugation onto natural or non-natural amino acids; Bioconjugation using enzymes; Linker-based bioconjugation.

• Alternative ADC Formats: Most ADCs targeting solid tumors have failed, likely due to the large IgG structure. Smaller conjugate formats: Peptides, Single domain antibody fragments (sdAb or VHH), Single chain variable fragments (scFv), Antigen binding fragments (Fab), Small immunoproteins (SIP).

• New Targets and Associated Release Systems: Since targeting antigens on the surface of cancer cells can be difficult in solid tumors rich in intercellular stroma, we may instead target the tumor microenvironment (stroma or vasculature). Thus, extracellular proteases and other matrix components could be used for extracellular release of cytotoxics.

• New Cytotoxic Agents: Target cancer cells with low Ag expression or resistance to auristatins or maytansinoids with PBD (pyrrolobenzodiazepine) dimers. They are ~50-100 times more effective than conventional cytotoxics used in ADCs (e.g. MMAE or DM1), demonstrating picomolar activity against several human tumor cell lines.