Supporting Information Checklist S1 CONSORT Checklist. (DOC) Click here for additional data file.(222K, doc) Axitinib melanoma Protocol S1 Trial Protocol. (DOC) Click here for additional data file.(147K, doc) Footnotes The authors have declared that no competing interests exist. The authors are grateful to the Velux Foundation and the Swiss National Science Foundation (project no. PPOOA-114941) for financial support of this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Increasing evidence suggests that, similar to normal tissues, cancers might also be hierarchically organised. Only a minority of tumour cells, endowed with stem cell-like features, and thus termed cancer stem cells (CSCs), might be responsible for tumour initiation and maintenance (Reya et al, 2001; Pardal et al, 2003; Dalerba et al, 2007a; Visvader and Lindeman, 2008).

Notably, owing to their high expression of DNA repair mechanisms, detoxifying enzymes, such as aldehyde dehydrogenase-1 (ALDH1), and molecular pumps, CSCs might survive radiochemotherapies; thus, possibly causing local recurrences and metastasis formation despite treatment (Dean et al, 2005; Dalerba et al, 2007a; Zhou et al, 2009). Putative CSC populations have been identified in several types of solid tumours, on the basis of the expression of specific markers and on functional stem cell-like properties, including high clonogenicity, differentiation capacity, spheroid formation, and, critically, the ability to reproduce the original tumour on transplantation in immunodeficient mice (Dalerba et al, 2007a; Visvader and Lindeman, 2008).

Phenotypic characterisation of CSCs derived from colorectal cancers is still debated. While initial works identified CD133 molecule as a reliable CSC marker in primary human colorectal cancers (O’Brien et al, 2007; Ricci-Vitiani et al, 2007), a subsequent study has shown that in both mouse and human colorectal cancers, CD133 expression is not restricted to rare cell subsets, but it is detectable in a large majority of tumour cells, irrespective of their tumourigenicity (Shmelkov et al, 2008). Alternatively, the co-expression on tumour cells of CD44, CD166, and EpCAM molecules, has been reported to identify the CSC pool more precisely than CD133 expression alone (Dalerba et al, 2007b). Despite the potentially high clinical relevance of CSCs, little is known about the prognostic value of the expression of putative CSC markers GSK-3 in colorectal cancers. Contradictory findings have been reported about the association between the expression of CD44, in particular of its v6 splicing variant, and tumour progression (Mulder et al, 1994; Herrlich et al, 1995; Weg-Remers et al, 1998).