Epitope Expression Persists in Circulating Tumor Cells as Breast Cancers Acquire Resistance to Antibody Drug Conjugates

Antibody-drug conjugates (ADCs) targeting cell surface proteins TROP2 or HER2 are effective in metastatic breast cancer, but the precise clinical contribution of epitope expression is uncertain. We prospectively monitored circulating tumor cells (CTCs) in 33 patients receiving ADC therapies using quantitative imaging. The expression of TROP2 and HER2 are heterogeneous across single CTCs from untreated patients, comparable to matched tumor biopsies, and display poor association with clinical response. Within three weeks of treatment initiation, declining CTC numbers correlate with a durable response (TROP2: median time to progression 391 versus 97 days, HR 4.15, P=0.0046; HER2: 322 versus 66 days, HR 9.12, P=0.0002). Neither TROP2 nor HER2 expression is reduced at progression, compared to matched pretreatment CTCs, and switching ADC epitope while maintaining a similar payload shows poor efficacy. Thus, epitope downregulation is not a common driver of acquired resistance to TROP2 or HER2 ADCs, and second-line ADC therapies may benefit from distinct payloads. ADCs target tumor-associated antigens, followed by internalization and release of drug payloads. However, clinical studies of epithelial-targeting ADCs show efficacy despite low tumor epitope expression. Our finding that epitope downregulation does not commonly accompany acquired resistance suggests alternative drivers of clinical efficacy and the need for testing non-cross-resistant payloads to overcome resistance.