The antibody-drug conjugate sacituzumab govitecan has been approved by the Food and Drug Administration (FDA) for the treatment of triple-negative breast cancer (TNBC), hormone receptor-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer, and urothelial carcinoma [1]. Sacituzumab govitecan specifically targets trophoblast cell-surface antigen 2 (TROP2), the protein product of the TACSTD2 gene [2,3]. TROP2 has been shown to play a role in tumorigenesis and cancer progression [4] and is highly expressed in most human solid epithelial tumors [2,5,6], including breast cancer [4,[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]], with a resulting adverse impact on prognosis in patients with various solid tumors [6,24], including carcinoma of the breast [9,10,22].

However, there is relatively little detailed information regarding the expression of TROP2 in the different molecular subtypes of breast carcinoma, either at the mRNA [7,8,14,23] or protein expression level [[13], [14], [15], [16],[18], [19], [20], [21]]. Such analyses have often been limited to comparing TROP2 expression levels between triple-negative and non-triple-negative tumors or correlating TROP2 expression levels with individual biomarkers (estrogen receptor [ER], progesterone receptor [PR], and HER2) [8,13,14,16,20,21], rather than comparing the expression of TROP2 between individual molecular subtypes of breast cancer [15,18,19,23]. Moreover, published immunohistochemical (IHC) studies may have included both resection and core biopsy specimens, previously treated and untreated tumors, and both primary and metastatic tumors, with IHC assessment performed using either tissue microarrays or whole tissue sections. Such variations in study design may obscure or distort true intrinsic differences in TROP2 expression between the molecular subtypes of breast carcinoma and may also limit the ability to compare results between studies. More specifically, the use of core biopsies or tissue microarrays may not adequately assess intratumoral heterogeneity of TROP2 expression [10,25], prior treatment may alter TROP2 expression [26], and apparent “shifts” in the molecular subtype of a carcinoma may occur during a patient's clinical course, reflecting either intratumoral heterogeneity, spontaneous evolution of tumor biology, or effects of prior therapy [[27], [28], [29]]. Additional methodological differences which may contribute to disparate results between prior studies have included the use of different TROP2 antibodies, other variations in IHC techniques, and various scoring methods for assessment of IHC expression of TROP2, as well as inconsistent “cut-off” levels for determining positivity or for classifying positive staining into high vs. intermediate vs. low (or high vs. low) expression level categories.

In view of the targeted nature of therapy with sacituzumab govitecan, one might anticipate that the level of TROP2 expression by tumor would be a useful biomarker for selecting patients who might mostly likely respond to this agent. However, measurement of the IHC expression of TROP2 on patients' tumors was not a requirement for enrollment in any of the clinical trials leading to FDA approval of sacituzumab govitecan for therapy of breast carcinoma or urothelial carcinoma [12,13,[30], [31], [32], [33], [34], [35]], nor is testing for IHC expression of TROP2 recommended by the manufacturer of sacituzumab govitecan (Trodelvy®) [36]. In the case of breast carcinoma, it has been argued that the majority of cancers exhibit moderate to high expression of TROP2 and that clinical responses are seen across the full spectrum of TROP2 expression, including low TROP2 expression, thus obviating the need to measure TROP2 expression prior to therapy [12,17,37]. However, only small numbers of patients with low or absent TROP2 expression have been included in clinical trials of sacituzumab govitecan in breast carcinoma [12,17,37], thus limiting the ability to draw definitive conclusions regarding the relative efficacy of sacituzumab govitecan in patients whose tumors lie at the lower end of the spectrum of TROP2 expression.

If the degree of TROP2 expression does prove to be predictive of responsiveness to sacituzumab govitecan in patients with breast cancer, then more detailed knowledge of the expression of TROP2 among the different molecular subtypes of breast cancer may help guide future clinical trials of sacituzumab govitecan in these patients, and IHC staining for TROP2 may be useful not only for identifying those patients who are most likely to respond to this targeted therapy, but also those for whom other potentially more effective therapies should be considered.

Accordingly, we sought to compare the IHC expression of TROP2 in the different molecular subtypes of invasive carcinoma, using a well characterized study cohort limited to resections of therapy-naive primary invasive tumors. We also investigated possible correlations between TROP2 expression and various clinicopathologic parameters in the study cohort.