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

Abstract

Antibody-drug conjugates (ADCs) target surface proteins on cancer cells, leading to internalization and delivery of a drug payload, thereby enhancing selectivity and minimizing toxicity. ADCs against TROP2 (Sacituzumab govitecan) or HER2 (T-DXd) have demonstrated efficacy in metastatic breast cancer, yet paradoxically, outside of HER2-amplified breast cancers, expression levels of these breast cancer-enriched epitopes in tumor biopsies have not been strongly correlated with clinical response. We undertook serial quantitative imaging of circulating tumor cells (CTCs) in a prospective cohort of 35 patients treated with either of these ADCs. At the single-cell level, expression of TROP2 and HER2 within individual patients is highly heterogeneous in both CTCs and paired tumor biopsies. Measurement of these epitopes on CTCs immediately prior to ADC therapy does not predict depth of clinical response. However, absence of CTCs or >80% reduction in CTC numbers after three weeks of treatment (CTCLow) predicts durable response, compared with CTCHigh cases (TROP2: HR 5.15, P = 0.012; HER2: HR 6.01, P<0.001). Targeted epitopes are not commonly downregulated on CTCs at the time of acquired clinical resistance, and switching between TROP2- and HER2-targeting ADCs sharing similar payloads infrequently leads to second-line response. Thus, while CTC burden is correlated with response to these ADCs, the level of TROP2 or HER2 expression is poorly predictive. These findings point to sensitivity to the drug payload as a potential driver of clinical response to currently approved ADCs in breast cancer. SIGNIFICANCE STATEMENTThe clinical efficacy of ADCs may depend both on differential targeting of cancer cells, using antibodies to tumor-enriched epitopes, and on the cleavage and release of drug payloads to which cancer cells are sensitive. ADCs currently used to treat breast cancer target either of two epitopes, TROP2 or HER2, but they share chemically related payloads. While CTC numbers track with response, we find that epitope expression is not strongly predictive. The limited success of sequentially switching between TROP2- or HER2-targeting ADCs as second-line treatment, following progression on a first-line ADC, highlights the need to incorporate non-cross-resistant drug payloads on ADCs to overcome such acquired resistance.