ALDH+ tumor-initiating cells exhibiting gain in NOTCH1 gene copy number have enhanced regrowth sensitivity to a γ-secretase inhibitor and irinotecan in colorectal cancer

The Notch signaling pathway has been shown to be upregulated in colorectal cancer (CRC) and important for the self-renewal of cancer stem cells. In this study, we evaluated the efficacy of PF-03084014, a gamma-secretase inhibitor, in combination with irinotecan to identify the effects of treatment on tumor recurrence and the tumor-initiating population in our CRC preclinical explant model. The combination of PF-03084014 and irinotecan had the greatest effect at reducing tumor growth on four CRC tumors when compared with treatment with PF-03084014 or irinotecan alone. The combination significantly reduced tumor recurrence in two CRC explants (CRC001 and CRCO36) after treatment was discontinued. Both of these tumors exhibited elevated baseline levels of Notch pathway activation as well as an increase in NOTCH1 gene copy number when compared with the two CRC explants (CRCO26 and CRCO27) where tumors reappeared quickly after termination of treatment. Isolation and injection of aldehyde dehydrogenase (ALDH(+) and ALDH(-)) cells in an in vivo explant model demonstrated that the ALDH(+) cell population were tumorigenic. Evaluation of the ALDH(+) cells after 28 days of treatment showed that the combination reduced the ALDH(+) population in the tumors that did not regrow. Furthermore, ALDH+ cells from CRC001 and CRCO27 were injected in vivo and treated immediately for 28 days. Two months after treatment, tumors were evident in the combination treatment group for CRCO27 but not for CRCO36. These results indicate the combination of PF-03084014 and irinotecan may be effective in reducing tumor recurrence in CRC patients whose tumors exhibit elevated levels of the Notch pathway. (C) 2012 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.