Stem cells of normal tissues have resistance mechanisms that allow them to survive genotoxic insults. The stem cell–like cells of tumors are defined by their tumor-initiating capacity and may have retained these resistance mechanisms, making them resistant to chemotherapy. We studied the relationship between resistance to the topoisomerase I inhibitor irinotecan and tumor-initiating potential in human colonosphere cultures and in mice with colorectal xenograft tumors.

Colonosphere cultures were established from human colorectal tumor specimens obtained from patients who underwent colon or liver resection for primary or metastatic adenocarcinoma. Stem cell and differentiation markers were analyzed by immunoblotting and fluorescence-activated cell sorting. Clone- and tumor-initiating capacities were assessed by single-cell cloning and in immune-deficient mice. Sensitivity to irinotecan was assessed in vitro and in tumor-bearing mice. The relationship between drug resistance and tumor-initiating capacity was tested by fluorescence-activated cell sorting of colonosphere cells, based on expression of ABCB1 and aldehyde dehydrogenase (ALDH) activity.

Colonosphere cultures had a high capacity to initiate tumors in mice and were resistant to irinotecan. Inhibition of the drug-efflux pump ABCB1 by PSC-833 allowed irinotecan to eradicate tumor-initiating cells. However, ABCB1 was expressed only by a subpopulation of differentiated tumor cells that did not form clones or tumors. Conversely, tumor-initiating cells were ABCB1-negative and were identified by high ALDH activity. Tumorigenic ALDH^high/ABCB1^negative cells generated nontumorigenic ALDH^low/ABCB1^positive daughter cells in vitro and in tumor xenografts. PSC-833 increased the antitumor efficacy of irinotecan in mice.

The resistance of colorectal tumors to irinotecan requires the cooperative action of tumor-initiating ALDH^high/ABCB1^negative cells and their differentiated, drug-expelling, ALDH^low/ABCB1^positive daughter cells.