Cetuximab effect on human colon cancer stem cells

Cancer Stem cells (CSCs), recently identified in the majority of solid tumors, are thought to drive tumor growth, giving rise to a cascade of differentiated cells composing the bulk of the tumor. Tumor relapse that most often follows treatment with anti-proliferative and cytotoxic drugs would be explained by selective resistance of CSC to these agents. Colon cancer stem cells (cCSCs), first isolated in the host laboratory from surgical specimens, can be grown in vitro as clusters called tumor spheres that maintain an undifferentiated state and are able, upon injection in immunodeficient mice, to generate a xenograft identical to the parental tumor, in terms of both antigen expression and histological tissue organization. Because of all these features cCSCs may represent predictive tools for patient s therapeutic response. Cetuximab (Erbitux), currently in use for metastatic colorectal cancer, is a recombinant chimeric human:murine immunoglobulin IgG1 that binds to EGFR displacing its natural ligands. Cetuximab also induces receptor internalization and degradation. Mutations in signaling pathway mediators acting downstream of EGFR, including KRAS, BRAF, NRAS, or PIK3CA are believed to determine resistance to the drug. In particular, KRAS-mutated patients are currently excluded from treatment. In order to verify whether Cetuximab treatment affects the stem cell compartment within tumors, in this study I analyzed its effect in a panel of cCSCs generated by individual patients, both in vitro and in xenografts. The data show that the effect of Cetuximab on individual cCSCs reflects the known clinical data on individual tumor mutations in the EGFR signaling pathway molecules. The study therefore confirms that panels of cCSCs generated by individual patients represent good predictive tools for the preclinical screening of pathway-oriented, cancer stem cell-directed therapeutics. Most importantly, the analysis of stem cell content in Cetuximab-treated xenografts by cytofluorimetry, agarose assay, and serial re-transplantation into secondary hosts clearly demonstrate that Cetuximab, differently than the classical chemotherapeutics currently in use for colon cancer, is able to effectively hit cCSC populations included in the tumors.