In vivo models for cancer stem cell research: a practical guide for frequently used animal models and available biomarkers.

The identification of a rare population of cancer stem cells whose presence in tumors is believed to determine their growth and metastatic activity, has provided a novel approach for targeted anti-cancer therapy. At the in vivo stage of the development of new therapeutic approaches for killing cancer stem cells, the most significant issues are the appropriate choice of rational animal models that offer the option to select animal species, strains and substrains, essential techniques for the inoculation of tumors, and methods of tumor detection in animals. The identification and validation of various types of cancer stem cell markers, which could serve as potential marker(s) of therapeutic efficacy of applied drugs, is a considerable challenge. The aim of this review is to provide a guide for the in vivo study of novel therapeutics that target cancer stem cells. This review describes frequently used mouse solid tumor models and evaluates their usefulness for cancer stem cell research. The classification of existing compounds that are used in today's experimental anti-cancer stem cell therapy and examples of exploratory first-in-human studies using these compounds for selective elimination of cancer stem cells will also be discussed. Finally, this review will examine the current status of available cancer stem cell markers, and highlight several important cancer stem cell properties that are still not well understood, but could influence the anti-cancer drug development process.