The Rise of Antibody–Drug Conjugates and Related Modalities: Innovations and Partnerships Lead the Way

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Antibody–drug conjugates (ADCs) are straightforward as a concept. When a cytotoxic small-molecule drug is attached to an antibody raised against a specific molecular receptor, it theoretically creates a highly targeted and effective therapy. And the ability to target cells precisely has obvious applications for indications such as cancer.

However, in practice, developing and manufacturing ADCs has proven difficult, said Janice M. Reichert, chief operating officer of The Antibody Society, which is an international nonprofit organization that supports antibody-related research and development (R&D). In an email to BPI, she stated that “ADCs are complicated to develop because of the additional factors that must be considered, such as the nature of the linker and the payload and the location and the amount of the payload conjugated to the drug. As with standard antibody therapeutics, a suitable target antigen and the right patient population also need to be identified.” She added, “Clinical development of ADCs can be challenging […] due to unexpected toxicities and the development of resistance to therapy.”

Despite those obstacles, industry interest in ADCs remains strong (1). A recent US Food and Drug Administration (FDA) analysis found that 77 novel antibody–drug combinations were undergoing testing at the time of the report in 2021, spanning 113 clinical trials that involved at least 40 different targets. Current research suggests there are more than 100 ADCs undergoing human studies (2).

Clinical Interest
The surge in activity reflects both the biopharmaceutical industry’s willingness to solve problems associated with first-generation ADCs and the increasing availability of new conjugation methods, linkers, and delivery technologies. Said Reichert, “Development of ADCs has accelerated recently because research over the past decade has resulted in a substantial expansion of available options for antibody design, type of linker, and the active payload. The number of validated target antigens has also substantially increased.” To support her idea that the market it poised to expand, Reichert also pointed to a surge in recent regulatory approvals and the increase in candidate ADCs reaching late-stage trials. “The FDA has granted approvals to eight ADCs in the past four years. Another one, trastuzumab duocarmazine, is undergoing FDA review, and nearly 20 ADCs are in late-stage clinical development,” she explained. “With the success of this class of antibody therapeutic and the additional companies now involved in ADC development, the early-stage pipeline is likely to expand well into the future, ultimately leading to even more ADC options for cancer patients.”

German scientist Paul Ehrlich proposed his “magic bullet” idea in the early 20th century, suggesting that future medicine would selectively target diseased cells or disease-causing organisms and leave patient cells unscathed (3). Although this established the concept of therapeutic specificity, it was not until the development of monoclonal antibodies (MAbs) in the mid-1970s that scientists could target medicines in the way Ehrlich envisioned. ADCs are the latest iteration of his idea (4).

As the name suggests, ADCs are composed of an antibody and a drug that is usually a small-molecule active pharmaceutical ingredient (API). The antibody is designed to bind a specific residue on the surface of a target cell, delivering the API. To date, only a handful of ADCs have been brought to market. The FDA has approved only 13 ADCs as of August 2023, the first of which was Pfizer’s acute myeloid leukemia (AML) treatment, Mylotarg (gemtuzumab ozogamicin) in 2000 (5). In part, market growth has been slowed by postapproval issues.

For example, Pfizer voluntarily withdrew the Mylotarg product from the US market in 2010 after, according to the FDA, “confirmatory trials failed to verify clinical benefit and demonstrated safety concerns, including a high number of early deaths” (6, 7). The product since has been approved for use at a lower dose and on a different schedule (8). It can be prescribed to be used on its own or in combination with chemotherapy.

Ocular toxicity is another issue for some ADCs, as evidenced by gynecologic malignancy treatments tisotumab and mirvetuximab. Ocular toxicity occurs most commonly with ADCs that have payloads based on monomethyl auristatin F (MMAF) or maytansinoid DM4 (9). Other ADCs have also raised safety concerns, with several linked to an increased risk of pneumonitis (10). Some developers have struggled to produce and handle cytotoxic payloads at scale (11).

But despite those problems, development efforts continue. According to the FDA, technical and manufacturing innovation in the ADC sector has increased markedly in recent years (1). The organization’s 2021 document Recent Advances in the Antibody–Drug Conjugate Clinical Pipeline concluded that “updates in ADCs within the clinical pipeline revealed the emergence of several novel formats to address the drawbacks of conventional ADCs including conditionally active biologic technology for antibodies, new toxic payloads, stable linkers and site-specific conjugation platforms. More unique developments are likely underway toward the aim of optimizing ADC design for clinical applications.”

Linking Technologies
Linkers play a key role in enhancing ADC efficacy and safety. They connect and stabilize a cytotoxic payload so it can be released when the antibody portion of a conjugate binds to its target. Linker technologies can be divided into two categories: cleavable and noncleavable (12). The former category, as the name suggests, contains a chemical trigger that can be cleaved on binding. Most approved ADCs, including Besponsa (inotuzumab ozogamicin) and Adcetris (brentuximab vedotin) use cleavable linkers. Noncleavable linkers are part of the payload such that release is triggered on internalization and digestion by cellular enzymes. Such linkers are used in products such as Kadcyla (ado-trastuzumab emtansine), the rationale being that a noncleavable linker reduces both the risk of off-target toxicity and a bystander effect. A bystander effect occurs when cells react unpredictably to events that happen in nearby cells.

Most noncleavable linkers are composed of either maleimidocaproyl (MC) or 4-maleimidomethyl cyclohexane-1-carboxylate (MCC). However, efforts are underway to combine polyethylene glycol (PEG) with alkynes and piperazine to create hydrophilic noncleavable linkers (12).

Conjugation Advances
In most first-generation ADCs, the drug payload is attached to the antibody at lysine or cysteine residues using a partial reduction-based conjugation process. Antibodies have multiple such residues, and therefore multiple potential points of attachment. That generates a heterogeneous mixture of product, “creating significant challenges for process consistency and product characterization,” according to author Qun Zhou in 2017 (13). To address that problem, researchers developed methods to fix payloads to specific points — known as site-specific conjugation — using a range of antibody modification methods.

Those methods usually are based on the introduction of specific amino acids, unnatural amino acids, short peptide tags, or glycans into the antibody. They constitute a breakthrough, according to Geoff Hale, chief executive officer of antibody technology at mAbsolve. He told BPI that “most ADCs to date have been made by nonspecific conjugation of drug to antibody at multiple potential sites. This leads to heterogeneity and complexity for manufacturing and quality control. More recently, ADCs are being developed with site-specific modification, so are more homogenous and reproducible. I expect this trend to increase in future.” But according to Hale, the ADC sector’s engineering has not been limited to conjugation sites according to Hale. He cited his own company’s work in modifying antibodies to improve their safety (14). “We have discovered mutations that completely eliminate binding to Fc-gamma receptors. This is important for ADCs as Fc-receptor binding can lead to nonspecific uptake and unwanted toxicity. So our technology could improve their safety and effectiveness.”

Collaborating To Advance
A surge in research collaborations among companies is furthering the advancement of ADC science and driving the industry toward technical solutions. For example, in January 2023, Japanese contractor Ajinomoto licensed its proprietary site-specific bioconjugation and linker technologies to Exelixis, a oncology-focused company that had candidate ADCs under development (17). Ajinomoto has designed its AJICAP technology to enable scientists to conjugate therapeutic antibodies at any stage of development to drug payloads with no need for engineering or cell-line development. The company says that its “stable/hydrophilic linkers” generate ADCs with an enhanced therapeutic window. Other collaborations have focused on redesigning antibodies to improve effectiveness. Novartis, for example, uses cell-engineering technology to modify antibody-producer cells so that they cannot synthesize fucose (18). Antibodies lacking that sugar have enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) activity.

Other ADC partnerships focus on conjugation science. Kodiak Sciences is working on a phase 3 candidate ADC called KSI-301 (tarcocimab tedromer) (23). It was developed using the company’s antibody–biopolymer conjugate (ABC) platform, which was designed to create bispecific products that bind multiple disease-relevant targets.

Finding New Targets
For decades, specificity has been a defining characteristic of ADCs. Products such as Adcetris and Kadcyla are marketed on their ability to hit specific molecular targets, thereby limiting the risk of side effects. But in recent years the industry has broadened its approach to understanding diseases with an increasing interest in developing ADCs that can address multiple targets. Bispecific ADCs are designed to target either two separate antigens or different epitopes of a single target (15, 16). For example, Sutro’s candidate MI231 targets separate antigens — specifically epidermal growth factor receptor (EGFR) and mucin 1 (MUC-1), a protein that helps cancer cells evade cell death by preventing the activation of the intrinsic apoptotic pathway (15). In contrast, Zymeworks’s ZW-49 binds both nonoverlapping epitopes of the human epidermal growth factor receptor 2 (HER2). Scientists reason that ADCs with two targets have enhanced tumor targeting with reduced toxicity to other tissues. Some also suggest that bispecific ADCs can overcome resistances caused by decreased single-target expression.

Biotechnology company Antiverse is developing an artificial-intelligence (AI) based therapeutic-antibody discovery system (19). The technology uses machine learning to model antibody–antigen interactions and design de novo antibodies. The company has identified eight antibodies that target human G-protein–coupled receptors (GPCRs), which are known to play a role in many diseases such as diabetes, cardiovascular disorders, and psychiatric disorders. Elsewhere, oncology developer Corbus Pharmaceuticals teamed with Megalith Pharmaceuticals to develop and commercialize CRB-701, a clinical stage ADC designed to target nectin-4–expressing tumors, including urothelial cancer (20).

Claudin protein is a tight-junction molecule that regulates the permeability of the epithelial layer. Claudin 18.2 is a highly tissue-specific protein that is expressed in gastric epithelial cells. It also is highly expressed mainly in primary malignant tumors such as gastric, breast, colon, liver, and pancreatic cancers. Elevation Oncology’s candidate ADC EO-3021 targets claudin 18.2–expressing cancer cells. The drug is composed of a human MAb conjugated to the cytotoxic agent monomethyl through a cleavable linker. It is undergoing a phase 1 trial in China through Elevation’s partner there, CSPC Pharmaceutical Group. The company plans to initiate a phase 1 trial in the United States during the second half of 2023. Similarly, RemeGen is working on RC118, an ADC that also targets solid tumors in patients with a positive claudin 18.2 expression. It was granted orphan-drug designation by the FDA in 2022 (21). In April 2023, TORL Biotherapeutics raised US$158 million to develop its pipeline of ADCs including clinical candidates TORL-1-23, which targets claudin 6, and TORL-2-307, which targets claudin 18.2 (22). In March 2023, Singapore regulators approved Pfizer’s Apexxnar (20-valent pneumococcal conjugate vaccine), which protects against the bacteria responsible for invasive pneumococcal disease. The vaccine, sold in the United States under the Prevnar brand name, contains bacterial polysaccharides conjugated to a carrier protein that helps the human immune system to recognize them.

Ian Wilkinson, chief scientific officer at mAbsolve, said that such partnerships and innovations in payload, linker, and conjugation R&D are positive signs for the ADC sector. “For both ADCs and bispecifics there are hundreds of antibodies in the pipeline, so I expect we will start to see a steady increase in approvals for both these categories over the coming years.” He added, “Someone tends to break the mold early on, and then it takes 10 years of R&D for everyone else to start catching up.”

References
1 Dean A, Zhang B. Recent Advances in the Antibody-Drug Conjugate Clinical Pipeline. US Food and Drug Administration: Silver Spring, MD, May 2021; https://www.fda.gov/science-research/fda-science-forum/recent-advances-antibody-drug-conjugate-clinical-pipeline.

2 Shastry M, et al. Rise of Antibody-Drug Conjugates: The Present and Future. Am. Soc. Clin. Oncol. Educ. Book 43, 2023; https://ascopubs.org/doi/full/10.1200/EDBK_390094.

3 Strebhardt K, Ullrich A. Paul Ehrlich’s Magic Bullet Concept: 100 Years of Progress. Nat. Rev. Cancer 8, 2008: 473–480; https://www.nature.com/articles/nrc2394.

4 Köhler G, Milstein C. Continuous Cultures of Fused Cells Secreting Antibody of Predefined Specificity. Nature 256, 1975: 495–497; https://pubmed.ncbi.nlm.nih.gov/1172191.

5 Temple R. Approval Letter. US Food and Drug Administration: Silver Spring, MD, May 2020; https://www.accessdata.fda.gov/drugsatfda_docs/nda/2000/21174_MYLOTARG_APPROV.PDF.

6 Pfizer Voluntarily Withdraws Cancer Treatment Mylotarg From U.S. Market. US Food and Drug Administration: Silver Spring, MD, June 2010; https://www.prnewswire.com/news-releases/fda-pfizer-voluntarily-withdraws-cancer-treatment-mylotarg-from-us-market-96819064.html.

7 FDA Approves Mylotarg for Treatment of Acute Myeloid Leukemia. US Food and Drug Administration: Silver Spring, MD, September 2017; https://www.fda.gov/news-events/press-announcements/fda-approves-mylotarg-treatment-acute-myeloid-leukemia.

8 Selby C, Yacko LR, Glode AE. Gemtuzumab Ozogamicin: Back Again. J. Adv. Pract. Oncol. 10(1) 2019: 68–82; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6605703.

9 Richardson D. Ocular Toxicity and Mitigation Strategies for Antibody Drug Conjugates in Gynecologic Oncology. Gynecol. Oncol. Rep. 46, 2023; https://www.sciencedirect.com/science/article/pii/S2352578923000176.

10 Zhu Z, et al. Incidence of Antibody–Drug Conjugates-Related Pneumonitis in Patients with Solid Tumors: A Systematic Review and Meta-Analysis. Crit. Rev. Oncol./Hematol. 184, 2023; https://www.sciencedirect.com/science/article/abs/pii/S1040842823000483.

11 Dunny E, O’Connor I, Bones J. Containment Challenges in HPAPI Manufacture for ADC Generation. Drug Discov. Today 22(6) 2017: 947–951; https://www.sciencedirect.com/science/article/abs/pii/S1359644617301058.

12 Su Z, et al. Antibody–Drug Conjugates: Recent Advances in Linker Chemistry. Acta Pharm. Sin. B. 11(12) 2021: 3889–3907; https://www.sciencedirect.com/science/article/pii/S2211383521001143.

13 Zhou Q. Site-Specific Antibody Conjugation for ADC and Beyond. Biomedicines 5(4) 2017; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5744088.

14 Wilkinson I, et al. Fc-Engineered Antibodies with Immune Effector Functions Completely Abolished PLoS ONE 16(12) 2021; https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0260954.

15 Knuehl C, et al. Abstract 5284: M1231 Is a Bispecific Anti-MUC1xEGFR Antibody-Drug Conjugate Designed To Treat Solid Tumors with MUC1 and EGFR Co-Expression. Cancer Res. 82(12), 2022; https://aacrjournals.org/cancerres/article/82/12_Supplement/5284/703051/Abstract-5284-M1231-is-a-bispecific-anti-MUC1xEGFR.

16 Andreev J, et al. Bispecific Antibodies and Antibody-Drug Conjugates (ADCs) Bridging HER2 and Prolactin Receptor Improve Efficacy of HER2 ADCs. Mol. Cancer Ther. 16(4) 2017: 681–693; https://pubmed.ncbi.nlm.nih.gov/28108597.

17 Ajinomoto and Exelixis Enter Into a License Agreement To Discover and Develop Novel Antibody-Drug Conjugates for the Treatment of Cancer. Ajinomoto Bio-Pharma Services: San Diego, CA, May 2023; https://www.prnewswire.com/news-releases/ajinomoto-and-exelixis-enter-into-a-license-agreement-to-discover-and-develop-novel-antibody-drug-conjugates-for-the-treatment-of-cancer-301717126.html.

18 Another Clinical Asset Using ProBioGen’s GlymaxX® Technology Begins Phase III. ProBioGen: Berlin, Germany, January 2023; https://www.b3cnewswire.com/202301262440/another-clinical-asset-using-probiogen-s-glymaxx-technology-begins-phase-iii.html.

19 Antiverse Raises Funding and Demonstrates Therapeutic Antibody Identification with AI-driven Drug Discovery Platform. Antiverse Ltd: Cardiff, United Kingdom, March 2023; https://www.businesswire.com/news/home/20230301005550/en/Antiverse-Raises-Funding-and-Demonstrates-Therapeutic-Antibody-Identification-With-AI-driven-Drug-Discovery-Platform.

20 Corbus Pharmaceuticals Expands Oncology Pipeline with the Addition of a Clinical Stage Nectin-4 Targeting Antibody Drug Conjugate (ADC). Corbus Pharmaceuticals Holdings: Norwood, MA, February 2023; https://www.corbuspharma.com/press-releases/detail/378/corbus-pharmaceuticals-expands-oncology-pipeline-with-the.

21 RemeGen’s RC118 for Injection Targeting Claudin 18.2 in Patients with Gastric and Pancreatic Cancers Granted Two Orphan Drug Designations by US FDA. RemeGen Co.: Yantai, China, December 2022; https://www.prnewswire.com/news-releases/remegens-rc118-for-injection-targeting-claudin-18-2-in-patients-with-gastric-and-pancreatic-cancers-granted-two-orphan-drug-designations-by-us-fda-301701054.html.

22 Million Series B Financing To Advance Development of Novel Oncology Biologics. TORL Biotherapeutics LLC: Los Angeles, CA, April 2023; https://tdg.ucla.edu/torl-biotherapeutics-launches-158-million-series-b-financing-advance-development-novel-oncology.

23 Kodiak Sciences Announces Upcoming Presentation of KSI-301 (Tarcocimab Tedromer) Clinical Data and Antibody Biopolymer Conjugate Development Programs at the Angiogenesis, Exudation and Degeneration 2023 Virtual Meeting. Kodiak Sciences Inc: Palo Alto, CA, February 2023; https://www.prnewswire.com/news-releases/kodiak-sciences-announces-upcoming-presentation-of-ksi-301-tarcocimab-tedromer-clinical-data-and-antibody-biopolymer-conjugate-development-programs-at-the-angiogenesis-exudation-and-degeneration-2023-virtual-meeting-301742857.html.

24 Singapore’s Health Sciences Authority Approves Pfizer’s Pneumococcal 20-Valent Conjugate Vaccine for Adults 18 Years or Older. MediaOutreach: Singapore, March 2023; https://vietnamnews.vn/media-outreach/1499537/singapore-s-health-sciences-authority-approves-pfizer-s-pneumococcal-20-valent-conjugate-vaccine-for-adults-18-years-or-older.html.

Further Reading
Kaplon H, et al. Antibodies To Watch in 2023. mAbs 15(1) 2022; https://doi.org/10.1080/19420862.2022.2153410.

Barnscher SD. The Clinical Landscape of ADCs in 2023: Diverse Technologies, Narrow Target. Clinical Leader March 2023; https://www.clinicalleader.com/doc/the-clinical-landscape-of-adcs-in-diverse-technologies-narrow-target-0001.

Ellis-Tate L. ADCs Coming Of Age: Deals, Targets and Catalysts. In Vivo April 2023; https://invivo.pharmaintelligence.informa.com/IV147692/ADCs-Coming-Of-Age-Deals-Targets-And-Catalysts.

Corresponding author Gareth Macdonald is a freelance contributor to BioProcess International and BioProcess Insider ([email protected]).