PATHWAY anti-HER2/neu (4B5) Rabbit Monoclonal Primary Antibody Identifying HER2-low expression

HER2-low is a recently defined subtype of breast cancer categorized as “...faint, partial staining of the membrane in greater than 10% of the cancer cells” or “...weak to moderate complete staining of the membrane in greater than 10% of the cancer cells…” and has a HER2 /chromosome 17 ratio of less than 2.0 by a in situ hybridization assay.^2-4  In current guidelines, these categories are called 1+ or 2+, non-amplified.⁶ Although both 1+ and 2+ have historically been considered ineligible for HER2 targeted therapies, such as trastuzumab, data from the pivotal Phase III DESTINY-Breast04 clinical trial data demonstrates that patients with metastatic breast cancer classified as HER2-low derive substantial clinical benefit from fam-trastuzumab deruxtecan (ENHERTU^®) in the second line setting⁸ and enables a new treatment approach for patients that lacked treatment options.

Roche recommends that a freshly cut section of archived or recently collected sample of FFPE breast cancer be stained using the PATHWAY HER2 4B5 assay, using the locked staining procedure, U PATHWAY HER2 4B5 for the assessment of HER2-low.¹¹

• All patients will have likely received previous diagnostic testing to determine their HER2 and HR status.  These previous findings will be documented in the patient record and depending on institutional practices, stained glass slides may be archived.  While the patient's historic record will provide a HER2 score and archived slides could be requested for review, but there are potential issues that may prevent accurate assessment of HER2-low with archived results and slides. 

Roche recommends that a freshly cut section of archived or recently collected sample of FFPE breast cancer be stained using the PATHWAY HER2 4B5 assay, using the locked staining procedure, U PATHWAY HER2 4B5 for the assessment of HER2-low.¹¹

• All patients will have likely received previous diagnostic testing to determine their HER2 and HR status.  These previous findings will be documented in the patient record and depending on institutional practices, stained glass slides may be archived.  While the patient's historic record will provide a HER2 score and archived slides could be requested for review, but there are potential issues that may prevent accurate assessment of HER2-low with archived results and slides.
• Since the first publication of the practice guidelines for the diagnostic assessment of HER2 in breast cancer in 2007⁷, the guidelines have been updated twice in 2013 and 2018^1,6. 
• Several HER2 IHC assays have been used routinely since the first evidence of IHC testing for HER2 positive targeted therapy⁸.  As such, historic HER2 scores may have been determined using a HER2 IHC assay other than the PATHWAY HER2 4B5 assay.
• Staining protocols outside of the currently validated staining protocol for the PATHWAY HER2 4B5 assay may have been implemented at the time of diagnosis. 
• Although DAB is a stable relative to other chromogens used for IHC, fading does occur over time^9-10. Based on this potential limitation, archived glass slides stained with the PATHWAY HER2 4B5 assay may not have the same staining intensity as when they were initially reviewed. 
• The inability to accurately evaluate the role that these factors play in determining the initial diagnosis should be considered before using archived slides or the historic HER2 IHC score to determine HER2-low status.

The PATHWAY anti-HER-2/neu (4B5) Rabbit Monoclonal Primary Antibody was the only HER2 IHC assay used to select patients for the pivotal DESTINY-Breast04 clinical trial⁴ and is the only HER2 IHC assay approved by the FDA for determining patient eligibility for fam-trastuzumab deruxtecan-nxki treatment.  Other HER2 IHC assays have not been validated in prospective clinical trials for fam-trastuzumab deruxtecan-nxki treatment.

While Roche has not evaluated the Pathway HER2 4B5 assay against other HER2 antibodies, data has shown that the Pathway HER2 4B5 antibody displays more sensitivity at the 1+ bin than another HER2 assay⁵.