As described above, targeting the EGFR has been intensely pursued in the last decade as a cancer treatment strategy. Clinical trials to investigate the activity of EGFR inhibitors commonly identify 10-20% major response rates. Although highly valuable for those patients that respond, approximately 80% of patients tumors show no response (intrinsic resistance) to EGFR inhibition strategies. Further, as increasing numbers of patients are treated with EGFR inhibitors, the emergence of acquired resistance following initial favorable response has been identified. Preclinical models of resistance to EGFR inhibitors have been recently established and are providing new insights regarding mechanisms of response to EGFR agents.
One approach to inhibit the activity of the EGFR involves the use of small molecule tyrosine kinase inhibitors (TKIs) that bind to the ATP-binding site in the tyrosine kinase domain (TKD) of the EGFR. These agents inhibit EGFR autophosphorylation and ultimately lead to blockade of downstream signaling and cellular proliferation. To date, three anti-EGFR TKIs have been approved by the FDA for use in oncology; erlotinib (OSI-774, Tarceva), gefitinib (ZD1839, Iressa) and lapatinib (GW572016, Tykerb). The identification of mutations in the TKD of the EGFR that predict response to EGFR-TKIs in selected lung cancer patients represents a landmark development in the EGFR field (42
). Mutation in exon 21 of the EGFR TKD, L858R, may predict increased sensitivity to TKIs, whereas the T790M mutation in exon 20 is associated with acquired resistance to TKI therapy (43
). These findings suggest that patient selection may be critical for successful therapies using EGFR TKIs (44
). Although TKD mutations have not been identified with high frequency in HNSCC, there may well emerge mutations in other key signaling pathways that prove more pertinent to HNSCC growth behavior.
Other receptor tyrosine kinases (RTKs) with overlapping signal transduction pathways with the EGFR can promote resistance in EGFR driven cancers (45
). cMET (HGFR) is a RTK that regulates cell cycle progression, migration, angiogenesis and cell survival. Increasing evidence identifies cMET as an attractive target for molecular-targeted cancer therapy (46
). Previous studies identified cross-talk between the EGFR and cMET in transformed cells (47
). However, until recently there has been no clear evidence demonstrating that cMET is involved in regulating acquired resistance to EGFR targeting agents. Engelman et al
recently reported that NSCLC HCC827 cells chronically exposed to gefitinib in vitro
led to the amplification of cMET. This increased activity of cMET resulted in the constitutive activation of the HER3-Akt signaling pathway in gefitinib-resistant cells and was abrogated by the selective cMET inhibitor PHA665752 thus restoring the sensitivity of resistant cells to gefitinib (45
). Taken together, these data indicate that mutations in the EGFR or altered signaling can lead to acquired resistance to EGFR TKIs.
A second approach to inhibit the activity of the EGFR uses monoclonal antibodies (mAbs) that target the extracellular domain of the EGFR. Cetuximab (IMC-C225, Erbitux) blocks natural ligand binding (48
), prevents receptor activation and dimerization and ultimately induces receptor internalization and downregulation (49
). Cetuximab exhibits promising anti-tumor activity as monotherapy or in combination with chemotherapy and/or radiation (50
). A series of clinical trials demonstrating clinical benefit led to the FDA approval of cetuximab for use in patients with HNSCC and in metastatic colorectal cancer (52
). Another anti-EGFR monoclonal antibody, panitumumab, has also gained recent FDA approval for use in the metastatic colorectal cancer setting (54
). Although EGFR TKD mutations appear to correlate with response to the TKIs erlotinib and gefitinib, no such correlation exists for cetuximab response (55
). This indicates that other molecular based mechanisms exist for resistance to cetuximab therapy.
One of the first reports on acquired-resistance to cetuximab suggested that altered control of angiogenesis could be one mechanism responsible for resistance to EGFR targeting agents. Ciardiello et al
found a 5- to 10-fold increase in VEGF production and secretion in cetuximab-resistant cell lines established from GEO colon cancer xenograft. Growth of EGFR inhibitor-resistant tumors could be inhibited by ZD6474, a dual EGFR/VEGFR TKI. (56
). In addition, using A431 xenograft, Viloria-Petit et al
generated six variant cell lines resistant to anti-EGFR antibody and found that these cells produced significant amounts of VEGF when compared to parental cells (57
). Collectively these findings indicate that receptor ligands play a critical role in EGFR inhibitor resistance.
Another pre-clinical study addressing acquired-resistance to cetuximab in vitro
utilized high-throughput screening to examine the activity of RTKs in cetuximab-resistant tumor cells following chronic exposure to cetuximab (58
). The findings suggested that cells developing acquired-resistance to cetuximab exhibited increased steady-state EGFR expression secondary to alterations in trafficking and degradation. EGFR upregulation promoted increased dimerization with HER2 and HER3 leading to their transactivation and subsequent activation of the PI(3)K/Akt pathway. Blockade of EGFR and HER2 led to loss of HER3 and PI(3)K/Akt activity. These data suggest that acquired-resistance to cetuximab is accompanied by dysregulation of EGFR internalization/degradation and subsequent EGFR-dependent activation of HER3. These findings suggest a rationale for the clinical evaluation of combinatorial anti-HER targeting approaches in tumors manifesting acquired resistance to cetuximab (58
). Further investigations of this model of acquired resistance to cetuximab indicated that Src family kinases (SFKs) are highly activated in cetuximab-resistant cells and enhance EGFR activation of HER3 and PI(3)K/Akt. Studies using the FDA approved Src kinase inhibitor dasatinib decreased HER3 activity followed by loss of PI(3)K/Akt (unpublished data). In addition, dasatinib therapy re-sensitized cetuximab resistant cells to cetuximab therapy (
). These results indicate that SFKs and the EGFR cooperate in acquired-resistance to cetuximab and suggest that dasatinib therapy in combination with cetuximab may have strong clinical benefit (
Dasatinib re-sensitizes cetuximab-resistant cells to cetuximab therapy
Potential mechanisms of acquired-resistance to cetuximab