ADC PAYLOADS - Progress in Development of Camptothecin-Based ADC Therapeutics

INTRODUCTION

Antibody-drug conjugates (ADCs) first arose as an area of interest in oncology drug development in the early 1980s, when researchers started to conjugate cytotoxic drugs (such as DNA-alkylating agents and Vinca alkaloids) via linkers to polyclonal antibodies targeting antigens found on cancer cells. Early ADC clinical trials didn’t show any benefit, with only one patient show­ing a partial response out of 10 different Phase I studies. Gem­tuzumab ozogamicin (GO), an ADC targeting CD33 and bearing a calicheamicin payload, was the first ADC to show benefits for patients with relapsed acute myelogenous leukemia (AML). GO received initial approval from the US FDA in 2000, although with a black box warning due to the increased risk of veno-occlusive disease. In 2010, GO was withdrawn from the US market after a subsequent confirmatory trial failed to verify clinical benefit and demonstrated safety concerns. GO was later re-approved in 2017 with a different dosing schedule to improve the safety pro­file. ADC research continued steadily in the 2000s-2010s, focus­ing mainly on microtubule inhibitors (MTIs) as ADC payloads (maytansinoids and auristatins). In 2011, brentuximab vedotin was approved for patients with non-Hodgkin lymphoma, followed by the first approval of an ADC to treat solid tumors, trastuzumab emtansine, in 2013. Other classes of payloads have been inves­tigated, including more potent DNA-damaging agents such as pyrrolobenzodiazepine (PBD) dimers and duocarmycins. Between 2017 and 2022, eight other ADCs were approved by the U.S. FDA, most bearing MTI payloads. Notably, the exceptional clinical successes of trastuzumab deruxtecan (approved in 2019) has re­cently sparked a renewed focus on ADCs. Currently, there are more than 280 ADCs at different stages of clinical development, including datopotamab deruxtecan and patritumab deruxtecan, which are currently under regulatory review. Collectively, these therapies represent a generational advance in therapies for a wide variety of liquid and solid tumors, many of which are con­sidered difficult to treat.

CHALLENGES IN ADCS

Despite groundbreaking progress in research, ADCs present several challenges that can limit their applications in cancer treat­ment. In particular, ADCs with more established payload classes, such as maytansinoids, auristatins, and PBD dimers, are associ­ated with high clinical failure rates. In fact, more than 150 ADCs have been discontinued in clinical trials due to limited efficacy at tolerated doses. Given the significant unmet needs in cancer treatment, improving efficacy while reducing treatment-related toxicities is an increasingly important issue.

In recent years, researchers have worked to identify new ap­proaches in ADC therapeutic structures to circumvent these limitations. One focus has been the use of camptothecin payloads. Camptothecin ADCs, and in particular trastuzumab deruxtecan, have shown significant benefits for patients across several types of cancer. In 2024 the FDA granted accelerated approval to trastuzumab deruxtecan for patients with unresectable or metastatic HER2-positive (IHC3+) solid tumors, regardless of type or location.¹ This represents the first tumor-agnostic approval of an ADC. While ADCs bearing campothecin pay­loads can deliver significant therapeutic benefit for patients, they are also associ­ated with potentially severe payload-re­lated toxicities, including nausea, neutropenia, anemia, gastrointestinal tox­icities, and interstitial lung disease (among others, and payload dependent).

Emerging clinical data suggest the ef­ficacy of ADC-based treatments, especially for solid tumors, is likely driven by a com­plex combination of targeted payload de­livery, free payload exposure, and tumor sensitivity to the molecule’s components. ADC features (including conjugation and drug-linker designs) and target expression may influence sites and rates of ADC dis­position, and thus tumor, tissue, and sys­temic exposure to the payload. In addition, treatment-related toxicities and maximum tolerated doses (MTDs) in patients are pri­marily related to the ADC payloads. Based on these insights, researchers are now de­veloping new strategies to maximize clini­cal efficacy and reduce the risk of toxicities. A key area of intensive research involves optimizing payload properties along with the overall properties of ADCs, including the target, antibody, linker, and payload it­self.²

RECENT ADVANCES IN CAMPTOTHECIN-BASED ADC RESEARCH

Camptothecin is a natural product topoisomerase I inhibitor that acts by bind­ing to DNA-topoisomerase I complexes to inhibit DNA cutting, relaxing, and rean­nealing processes, which are essential for cell survival and reproduction. Camp­tothecin was first isolated in 1966, but clin­ical progress involving synthetic camptothecin small molecules has been hampered by challenges such as low sol­ubility, rapid in vivo clearance, limited oral bioavailability, and severe hematologic and gastrointestinal toxicities. Additionally, camptothecins are susceptible to a re­versible hydrolysis of the camptothecin lac­tone form at physiological pH to a less active carboxylate form^.3,4 Thus, when a camptothecin small molecule is injected intravenously, it rapidly establishes an equilibrium in plasma favoring the less ac­tive form. Despite these challenges, the FDA has approved two camptothecin small molecules, topotecan and irinote­can, for the treatment of colon, ovarian, cervical, and small cell lung cancers, and the Korean Ministry of Food and Drug Safety (MFDS) has approved another, be­lotecan, for the treatment of ovarian can­cers.

ADCs address many of the major challenges associated with camptothecin small molecules. When conjugated to an antibody, the solubility of camptothecin small molecules can be enhanced, and their exposure in patients can be improved due to the prolonged half-life inherent to antibody therapeutics. Furthermore, through trafficking of the ADC into endo­somal and lysosomal compartments after cellular uptake, the camptothecin payload is exposed to a low-pH environment, which shifts the equilibrium toward the more active lactone form.^5,6

In a promising advance in ADC de­velopment using camptothecin as a pay­load, researchers at the life sciences company Zymeworks prepared and as­sessed a panel of camptothecin analogs. Leveraging insights gained from over 60 years of camptothecin SAR data, they pro­duced a library of approximately 100 compounds featuring different substituents at the C-7 and C-10 positions of the camptothecin core scaffold.⁷ They then advanced research to screen these novel camptothecin small mole­cules in a cytotoxicity assay against a panel of cell lines. The selected analogs spanned a range of potency and hydrophilicity profiles and were evaluated as ADC payloads using different linker strategies. Ultimately, the ZD06519 payload was selected based on a favorable in vitro ADME/DMPK profile, and in vivo efficacy in multiple in vivo CDX and PDX models and superior tolerability observed in rats and non-human pri­mates (NHPs) when conjugated to different antibodies. Importantly, ZD06519 also has a unique structure and properties compared to other camptothecin payloads currently in development. Findings from this re­search effort were published recently in Molecular Cancer Therapeutics.⁷ The design of the novel camptothecin analog supported a range of ad­vantageous properties for ADCs, including moderate payload potency, low hydrophobicity, strong bystander activity, robust plasma stability, and high monomeric ADC content.

DELIVERING BETTER ADCS TO PATIENTS

The recent progress in research on novel camptothecin payloads highlights the potential for developing a new generation of ADCs that can overcome some of the limitations of first-generation therapies. As drug developers explore innovative technologies and strategies, interest in ADCs is poised to continually expand. Lessons learned from developing novel ADC payloads, including camptothecin analogs, suggest multiple opportunities exist to improve ADC design, despite the complexity of these molecules. To advance these efforts, researchers must assess the antibody, linker, and payload together, using a range of technologies to carefully select and refine each ADC design feature. By better understanding the role and potential of each component and how they can work together in an integrated design, we have the opportunity to bring forward a new generation of ADCs to benefit patients.

REFERENCES

1. Tarantino P, Carmagnani Pestana R, Corti C, Modi S, Bardia A, Tolaney SM, Cortes J, Soria JC, Curigliano G. Antibody–drug conjugates: Smart chemotherapy delivery across tumor histologies. CA: Cancer J Clin. 2022;72(2):165-182. doi:10.3322/caac.21705.
2. Colombo R, Tarantino P, Rich JR, LoRusso PM, de Vries EGE. The Journey of Antibody-Drug Conjugates: Lessons Learned from 40 Years of Development. Cancer Discov. 2024;14(11):2089-2108.
3. Wall ME, Wani MC, Cook CE, Palmer KH, McPhail AT, Sim GA. Plant Antitumor Agents. I. The Isolation and Structure of Camptothecin, a Novel Alkaloidal Leukemia and Tumor Inhibitor from Camptotheca acuminata. J Am Chem Soc. 1966;88(16):3888-3890. doi:10.1021/ja00968a057.
4. Wani MC, Nicholas AW, Wall ME. ChemInform Abstract: Plant Antitumor Agents. Part 23. Synthesis and Antileukemic Activity of Camptothecin Analogues. ChemInform. 1987;18(15). doi:10.1002/chin.198715233.
5. Casey JR, Grinstein S, Orlowski J. Sensors and regulators of intracellular pH. Nat Rev Mol Cell Biol 2010;11(1):50-61. doi:10.1038/nrm2820.
6. Lau UY, Benoit LT, Stevens NS, Emmerton KK, Zaval M, Cochran JH, Senter PD. Lactone Stabilization is Not a Necessary Fea­ture for Antibody Conjugates of Camptothecins. Mol Pharm. 2018;15(9):4063-4072.
7. Petersen ME, Brant MG, Lasalle M, Das S, Duan R, Wong J, Ding T, Wu KJ, Siddappa D, Fang D, Zhang W, Wu AML, Hirkala-Schaefer T, Garnett GAE, Fung V, Yang L, Hernandez Rojas A, Lawn SO, Barnscher SD, Rich JR, Colombo R. Design and Eval­uation of ZD06519, a Novel Camptothecin Payload for Antibody Drug Conjugates. Mol Cancer Ther 2024; 23(5): 606–618.

Dr. Paul Moore is Chief Scientific Officer at Zymeworks, Inc. He has more than 25 years of US-based experience in biologics drug discovery and development in biotechnology research. His career efforts have led to the discovery and development of a range of FDA-approved and clinical-stage biologics for patients with difficult-to-treat cancers and autoimmune conditions. He earned a PhD in Molecular Genetics from the University of Glasgow. He has an extensive research record co-authoring more than 75 peer-reviewed manuscripts and is a named co-inventor on over 50 issued US patents.

Dr. Raffaele Colombo is Associate Director, Medicinal Chemistry at Zymeworks Inc. He has more than 14 years of experience in studying the design and synthesis of new payloads and drug-linkers for antibodies, small molecules, and nanoparticles. He has a strong research background co-authoring 23 publications and is a named co-inventor on 10 patents. He earned a PhD in Organic and Medicinal Chemistry from Università degli Studi di Milano.

Dr. Jamie Rich is Senior Director of Technology, ADC Therapeutic Development at Zymeworks Inc. He has more than 17 years of experience in ADC and protein chemistry research and drug development. He earned a PhD in Organic Chemistry from the University of Alberta.