HER2 Testing and Correlation With Efficacy of Trastuzumab Therapy

As a result of the availability and clinical efficacy
of trastuzumab (Herceptin), clinicians are now faced with a dilemma regarding
the accurate identification of patients with HER2 overexpression. In the October
2002 issue of ONCOLOGY, Monica Fornier and colleagues critically reviewed the
methodologies currently available for HER2 testing in their article, "HER2
Testing and Correlation With Efficacy of Trastuzumab Therapy."[1]

The most widely used technique for detection of HER2 overexpression at the
protein level is immunohistochemistry (IHC). However, the problems with this
technique are, as correctly stated by the authors, that multiple primary
antibodies—each of which binds to different epitopes and has differing
sensitivities and specificities—are currently in clinical use. Moreover,
different cutoff values are used to distinguish overexpressing form non-overexpressing
tumors.[2] Detection of HER2/neu alteration is therefore not well standardized,
and although IHC detection methods are widely commercially available, some of
these assays may be suboptimal for this purpose.

Other IHC Limitations

Another concern is that the degree of IHC staining intensity for HER2 is
subjective and qualitative. As a consequence, it may be difficult to know
whether a sample scored as HER2-positive in one laboratory will be confirmed as
HER2-positive by another laboratory. Furthermore, formalin fixation of tumor
samples and storage in paraffin can result in epitope degradation so that
sensitivity is lost over time in archival clinical material.[3-5]

To overcome this problem, some assays use a technique called antigen
retrieval, which consists of boiling the tumor sample using heat or microwave
radiation, in order to rehabilitate epitopes that have been lost during tissue
processing and storage. Many believe that the loss of antigenicity in
paraffin-embedded specimens can be corrected by application of heat-induced
antigen retrieval to the paraffin-embedded tissue sections. However, this
approach has not been validated, before introduction of these reagents and
methods, by systematic evaluation of clinical breast cancer specimens with known
amplification/overexpression levels.

Detection Rate Variability

As summarized in the review by Fornier et al, reported detection rates for
IHC differ among clinical laboratories. This discrepancy has generated
considerable confusion among clinicians, pathologists, and patients regarding
the appropriate methods for HER2 testing. To address this issue, we recently
used each of the four US Food and Drug Administration (FDA)-approved assays for
detecting HER2 alterations, to determine which is the most accurate. We tested
these assays on breast cancer specimens in which we had previously determined
amplification and overexpression levels using solid matrix blotting techniques
as a "gold standard."