New standards in HER2-low testing: the CASI-01 comparative methods study.

Area of Science:

• Oncology
• Pathology
• Biomarker Detection

Background:

• Current immunohistochemistry (IHC) assays for HER2 overexpression are inadequate for detecting HER2-low expression required for Trastuzumab deruxtecan (T-Dxd) eligibility.
• No HER2-low assay has been validated against HER2 protein expression, creating a critical gap in T-Dxd patient selection.

Purpose of the Study:

• To identify optimal assay conditions for accurate HER2 testing, differentiating HER2 overexpression (3+) and HER2-low (1+/ultra-low) expression.
• To compare conventional FDA-cleared HER2 assays with higher-sensitivity assays using pathologist and image analysis readouts.

Main Methods:

• The CASI-01 study involved 54 IHC laboratories across Europe and the U.S.
• HER2 overexpression was validated against HER2 gene amplification using in situ hybridisation (ISH).
• HER2-low accuracy was assessed using novel HER2 reference standards and a dynamic range parameter.

Main Results:

• Predicate HER2 assays showed variability in detection thresholds (30,000-60,000 molecules/cell) but high accuracy for HER2 overexpression (3+).
• These assays demonstrated poor dynamic range for detecting HER2-low scores.
• Enhanced IHC sensitivity with image analysis improved HER2-low score detection six-fold (p=0.0017).

Conclusions:

• Standard IHC assays accurately identify HER2 3+ but lack the dynamic range for precise HER2-low scoring.
• Enhanced IHC analytic sensitivity combined with image analysis is essential for accurate HER2-low testing.
• This study highlights the importance of reporting IHC analytic sensitivity and assay dynamic range, and the potential of image analysis in precision medicine.