Antibody-Drug Conjugates in Breast Cancer: A Transformative Treatment Era Antibody-drug conjugates (ADCs) have fundamentally changed how breast cancer is treated, evolving from experimental concepts to cornerstone therapies. These “smart chemo” agents combine the precision of monoclonal antibodies with the cell-killing power of chemotherapy, delivering toxic drugs directly to cancer cells while sparing healthy tissue. This targeted approach has redefined treatment pathways, altered biomarker interpretation and reshaped clinical thinking around resistance and sequencing in both metastatic and early-stage breast cancer. Four specific ADCs mark the pivotal moment when this class of drugs transitioned from promising to central in breast oncology: trastuzumab emtansine (T-DM1), trastuzumab deruxtecan (T-DXd), sacituzumab govitecan, and datopotamab deruxtecan. Each played a distinct role in advancing the field—T-DM1 validated the ADC concept, T-DXd expanded treatment beyond strict HER2-positivity, sacituzumab govitecan established TROP2 as a viable target, and datopotamab deruxtecan highlighted the growing need for therapeutic selection among multiple effective options. How Antibody-Drug Conjugates Work ADCs consist of three key components: a monoclonal antibody that binds to a specific protein overexpressed on cancer cells, a cytotoxic chemotherapy payload, and a chemical linker that connects the two. Once the antibody binds to its target antigen on a cancer cell, the entire complex is internalized. Inside the cell, the linker breaks down, releasing the chemotherapy drug to destroy the cancer from within. This mechanism allows for potent anticancer effects while minimizing damage to healthy cells—a significant improvement over conventional chemotherapy, which affects both malignant and non-malignant tissues indiscriminately. Trastuzumab Emtansine (T-DM1): Validating the ADC Approach Trastuzumab emtansine, marketed as Kadcyla, was the first ADC to demonstrate clear clinical benefit in breast cancer, proving the technology could work in practice. It combines the HER2-targeting antibody trastuzumab with the microtubule inhibitor emtansine. The pivotal EMILIA trial showed that in patients with previously treated HER2-positive metastatic breast cancer, T-DM1 improved median progression-free survival to 9.6 months compared to 6.4 months with lapatinib plus capecitabine. Median overall survival was too significantly extended at 30.9 months versus 25.1 months in the control group. These results established T-DM1 as a new standard of care and confirmed that ADCs could deliver meaningful therapeutic advances. Trastuzumab Deruxtecan (T-DXd): Expanding Beyond HER2-Positivity Trastuzumab deruxtecan, known commercially as Enhertu, pairs the same HER2-targeting antibody with a potent topoisomerase I inhibitor payload and a linker designed for high drug-to-antibody ratio and membrane permeability. This configuration contributes to its notable bystander effect—where the released drug can kill nearby cancer cells, even those with low or heterogeneous HER2 expression. In the DESTINY-Breast03 trial, T-DXd demonstrated superior progression-free survival compared to T-DM1 in HER2-positive metastatic breast cancer previously treated with taxane, and trastuzumab. Further studies, including DESTINY-Breast04, showed benefit in patients with HER2-low metastatic breast cancer (defined as IHC 1+ or IHC 2+/ISH-), a group previously considered ineligible for HER2-targeted therapies. This expansion of the HER2 therapeutic paradigm marked a major shift in breast cancer classification and treatment eligibility. Sacituzumab Govitecan: Establishing TROP2 as a Target Sacituzumab govitecan (Trodelvy) targets Trop-2, a calcium signal transducer often overexpressed in aggressive breast cancers, particularly triple-negative breast cancer (TNBC). It links an anti-Trop-2 antibody to SN-38, the active metabolite of irinotecan. In the ASCENT trial for metastatic TNBC, sacituzumab govitecan significantly improved progression-free survival (5.6 months vs. 1.7 months) and overall survival (12.1 months vs. 6.7 months) compared to single-agent chemotherapy treatment of physician’s choice. These findings led to FDA approval for metastatic TNBC and later for hormone receptor-positive, HER2-negative breast cancer, broadening its impact across subtypes. The success of sacituzumab govitecan validated TROP2 as a clinically meaningful target and expanded ADC utility beyond HER2-driven diseases. Datopotamab Deruxtecan: Defining the Era of Therapeutic Choice Datopotamab deruxtecan (Datroway or Dato-DXd) is an ADC targeting TROP2 with a distinct linker-payload system designed to optimize stability and drug release. While sharing a target with sacituzumab govitecan, its different molecular properties aim to improve tolerability and efficacy. Clinical trials such as TROPION-Breast01 have evaluated datopotamab deruxtecan in hormone receptor-positive, HER2-negative breast cancer, showing promising activity. Its development reflects the evolving landscape where multiple ADCs may be available for similar indications, necessitating careful consideration of sequencing, toxicity profiles, and patient-specific factors when selecting therapy. Clinical Impact and Future Directions The integration of ADCs into breast cancer treatment has influenced clinical practice in several key ways. Biomarker assessment has become more nuanced, particularly with the recognition of HER2-low disease as a treatable category. Treatment sequencing now requires evaluation of prior therapies, resistance mechanisms, and overlapping toxicities. The success of ADCs has spurred investigation into novel targets, alternative payloads, and improved linker technologies to enhance efficacy and reduce side effects. Common side effects associated with ADCs include nausea, fatigue, alopecia, and hematologic toxicities such as neutropenia and anemia. Specific risks vary by drug—for example, T-DXd carries a risk of interstitial lung disease, requiring monitoring for pulmonary symptoms. Managing these adverse events through dose modifications, prophylactic measures, and vigilant monitoring is essential to maintaining treatment benefit. As of today, over a dozen ADCs have received FDA approval for various cancers, with breast cancer representing one of the most active areas of development. Ongoing research explores ADCs in earlier lines of treatment, combination regimens, and novel targets such as HER3, B7-H3, and folate receptor alpha. The continued advancement of ADC technology promises to further refine personalized treatment strategies and improve outcomes for patients across breast cancer subtypes. Key Takeaways – Antibody-drug conjugates deliver chemotherapy directly to cancer cells using monoclonal antibodies for targeting. – Four ADCs—T-DM1, T-DXd, sacituzumab govitecan, and datopotamab deruxtecan—have been instrumental in advancing breast cancer treatment. – T-DM1 validated the ADC platform in HER2-positive disease. – T-DXd demonstrated efficacy in HER2-low breast cancer, expanding the HER2 therapeutic paradigm. – Sacituzumab govitecan established TROP2 as a target, particularly in triple-negative and hormone receptor-positive cancers. – Datopotamab deruxtecan adds to the growing arsenal of TROP2-directed therapies, increasing options for treatment selection. – ADCs have changed clinical practice by redefining biomarkers, influencing sequencing, and highlighting resistance management. – Side effects vary by agent but commonly include gastrointestinal symptoms, fatigue, and blood-related toxicities. – Ongoing research aims to expand ADC use into earlier treatment stages and explore new targets and payloads.
36