Aims: Invasive lobular carcinoma (ILC) may show targetable vulnerabilities secondary to the characteristic loss of the cell adhesion protein E-cadherin. Specifically, a synthetic lethal interaction was identified between E-cadherin loss and ROS1 inhibition. Several clinical trials are currently underway to assess the efficacy of ROS1 inhibitors in ILC, however, ROS1 expression has not been confirmed in ILC tumours and ROS1 has not been validated as a biomarker in the breast cancer setting. This study aimed to (i) examine ROS1 expression in a large cohort of breast cancer cases and (ii) investigate the biology and clinical significance of ROS1 positivity in breast cancer. Methods: ROS1 immunohistochemistry was performed on a large cohort of ILC (n=274) and invasive carcinoma of no special type (NST; n=431) cases with extensive clinicopathological data. The staining performance of four ROS1 antibody clones was compared. Results: There was marked variation in ROS1 status according to antibody clone. D4D6 and SP384 were negative in almost all breast cancer cases, whereas EP282 and EPMGHR2 were positive in 37% and 47% of ILC cases, and 49% and 74% of NST cases, respectively. Only data from clones D4D6 and SP384 were highly concordant, while EP282 and EPMGHR2 were positive in distinct breast cancer subtypes. Conclusions: Assessment of ROS1 status in breast cancer appears to be highly antibody clone dependent. ROS1 antibody clone selection will be an important consideration in the design of clinical trials investigating the clinical validity of ROS1 as a predictive biomarker in breast cancer.

The authors have declared no competing interest.

Anna Sokolova received fellowship support from Breast Cancer Trials.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Ethics committees of The University of Queensland (2005000785) gave ethical approval for this work. Royal Brisbane and Womens Hospital (RBWH 2005/022) gave ethical approval for this work. UnitingCare Health (2006-39) gave ethical approval for this work. Public Health Act from the Queensland Government (CT_4352) gave approval for this study.

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