Molecular analyses of resistance and sensitivity mechanisms to anti-EGFR directed tumor therapy

This study characterized a panel of NSCLC cell lines as well as a
stably transfected cell model expressing wild-type and mutated EGFR
variants in terms of response to the EGFR-targeting drugs,
gefitinib and cetuximab. The examinations support the notion that
response to gefitinib is neither exclusive nor strictly determined
by the presence of EGFR kinase mutations. Yet, cells expressing
EGFR kinase domain mutations tended to generally respond better to
treatments with gefitinib compared to those with wild-type EGFR. On
the other hand, preliminary studies suggesting that cellular
sensitivity to cetuximab is determined by factors other than EGFR
kinase domain mutations could be substantiated through a robust set
of data. Moreover, several promising candidate genes differentially
expressed in gefitinib sensitive and resistant NSCLC cell lines
were revealed by a global mRNA expression analyses. In addition,
though statistically questionable, several biologically interesting
genes that are possibly involved in determining in vitro response
of NSCLC cells to cetuximab have been postulated. In this work,
four cancer cell models, which are long-term exposed to gefitinib
or cetuximab were established and characterized in terms of
gain-of-resistance towards EGFR-targeting compounds, as well as in
regard to biological and molecular alterations caused by long-term
treatments. It was found that gefitinib long-term treatment of
primary sensitive A431 cells confered growth-resistance to this
TKI, but not to cetuximab. This observation may have clinical
implications for patients that relapsed on gefitinib as it suggests
that they might still profit from cetuximab therapy. On the other
hand long-term exposure of A431 cells to cetuximab did not render
cells resistant to neither the antibody nor gefitinib in regard to
in vitro growth-inhibition. Furthermore, it appeared that long-term
gefitinib-treated A431 cells downregulate overall EGFR levels,
while long-term cetuximab exposed cells displayed decreased EGFR
surface levels but constant overall expression. In addition, a
candidate-based approach identified genes that are differentially
expressed in cancer cells with primary or secondary resistance to
gefitinib or cetuximab. Finally, this study for the first time
provided evidence that cetuximab may block metastasis-facilitating
epithelial-mesenchymal-transition (EMT) in an epithelial cell line.
Moreover, it was suggested that activation of the HGFR may
compensate for cetuximab-mediated blockage of EGFR. This was also
reflected by an increased response of this population towards
treatment with HGFR inhibitors.