We’ve been talking about the potentially relevant molecular markers for EGFR, and the importance of EGFR as a cancer target (see prior post), without really describing what these markers are.   There are three main aspects of EGFR biology that have been studied for their potential predictive value in consideration of EGFR inhibitor therapy, whether the oral tyrosine kinase inhibitors like tarceva (erlotinib) or iressa (gefitinib), or the monoclonal antibodies against EGFR such as erbitux (cetuximab).  What are these markers?

(Click to enlarge)

  The first and most readily available is immunohistochemistry, or IHC.  This technique measures whether the tumor cells show the EGFR protein on their surface, and the result is typically reported as between 0 and 3+, with the higher numbers relating to a higher proportion of cells having the protein, and also a higher density of the protein, which leads to a greater intensity of the stain, which appears brown in the picture at the top of the figure above.   One problem is that IHC is quite technique and interpreter dependent, so there are variables such as how good the staining antibody is, how a pathologist interprets the prevalence of positive cells and the intensity of the stain, etc.  This hasn’t been very consistently correlated with the activity of EGFR TKIs, but it has sometimes been felt to important for the EGFR monoclonal antibodies, since they are supposed to target this protein on the surface of the cell.  If the protein isn’t there and the tumor is IHC negative for EGFR, you’d presume that a drug like erbitux wouldn’t be very effective.  Still, results of trials that excluded patients with EGFR IHC negative tumors aren’t very different from the results seen in trials with EGFR IHC positive trials.

The next technique is EGFR fluorescence in situ hybridization, which measures the number of copies of the EGFR gene inside tumor cells.  That should normally be two, since there are a pair of matching chromosomes inside normal cells, but in tumor cells, you sometimes see a bunch of extra copies of the gene in the cell, so called gene amplification.  This technique for testing isn’t practiced as commonly or in as many centers as IHC for EGFR, but there have been some studies that have suggested the potential importance of EGFR FISH, showing response and survival after an EGFR tyrosine kinase inhibitor to be more favorable in patients with EGFR FISH positive tumors, or either EGFR IHC or FISH positive, compared with patients who have neither IHC or FISH positivity for EGFR.  Still, it’s a complex issue with mixed results that merit a full discussion in the future.

Finally, there’s mutation of the EGFR gene itself by a deletion or substitution of an individual amino acid in the sequence that makes the EGFR protein.  This story started in the spring f 2004, when two labs in Boston, one at Massachusetts General Hospital and the other at the Dana Farber Cancer Institute, independently identified these mutations within a week or so of each other.  These results were published in the leading journals:

Even the loss or substitution of a single amino acid in the protein chain can lead to important changes in how the protein folds, basically changing the shape of the receptor and how it interacts with EGFR tyrosine kinase inhibitors (TKIs).   This actually leads to especially tight binding of the EGFR TKIs and very potent inhibition of EGFR.

The exciting part was that in the original paper in the New England Journal of Medicine by Tom Lynch and colleagues from Mass General (abstract here), 8 of the 9 people they had identified as having a great and long response to iressa (the EGFR TKI first available and widely used in patients back in 2003-2004) also had an EGFR “activating mutation”:

You may also notice that 6 of the 9 patients were women, all patients had an adenocarcinoma or BAC, and that 6 of 9 patients were never smokers.   This question of whether mutations really added much to the clinical predictors of female sex, adeno/BAC tumor subtype, and smoking status has been a subject of ongoing debate for years.  However, as I’ve written in a relatively recent post, the emerging data on EGFR mutations has turned me into a convert from a “clinical selector” to someone who sees the importance of molecular variables and especially EGFR mutation in clinical decision making for many patients with advanced NSCLC.

We’ll talk more about the results in patients selected based on EGFR mutations next.

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