Currently, there are multiple potential anti-CSC agents in pre-clinical and clinical trials, including Hh, NOTCH, AKT, and CXCR1 inhibitors. While these are promising agents, the likelihood of clinical success will depend on many aspects, including safety, trial design, and rational endpoints. High-throughput screens represent new avenues for drug discovery and identification of novel pathways regulating CSCs. Breast CSCs screens may be based on methods to enrich for CSCs, such as CD44+
, or sphere formation from cell lines, primary breast cancer xenografts, or primary samples (). These CSC enriched populations can be used to screen siRNA, lentiviral shRNA, or small molecule libraries using various cellular endpoints including growth inhibition (Gupta et al., 2009
), flow cytometry (Krutzik and Nolan, 2006
), sphere formation (Diamandis et al., 2007
), or cell migration (Wurdak et al., 2010
High-throughput approaches to target breast CSC
High-throughput screens provide a larger pool of compounds for testing in pre-clinical and clinical models given the high attrition rate and decreasing productivity within the pharmaceutical industry (Booth and Zemmel, 2004
; Munos, 2009
). While some side-effects from chemotherapy agents are poorly understood, it is increasing clear that others are due to toxic effects on normal stem cells in various tissues, such as hematopoietic, skin and gastrointestinal (see ‘The realities of targeting CSCs’ above). Thus, agents identified in primary high-throughput screens need to tested on normal stem cells to remove potentially cytotoxic agents with adverse side-effects (). These “confirmed” agents can then be tested in primary tumor xenograft mouse models, However, current models, based primarily on tumor growth from rapidly dividing progenitor cells, are poorly adept to identify anti-CSC agents, which may have modest growth inhibition effects as a sole agent (). Rather, testing agents in an early (i.e. adjuvant) setting would identify those agents with anti-CSC activity since early tumor formation is stem cell dependent.
Validation of potential “hits” as anti-CSC agents
New therapies might come from traditional or existing medicines that were not tested for anti-cancer efficacy and can be rapidly “repurposed” (Chen et al., 2008
; Chong and Sullivan, 2007
; Eberhard et al., 2009
; Ginestier et al., 2010
). The CXRC1 inhibitor repertaxin was developed to block organ transplant rejection, the gamma-secretase inhibitors initially developed to treat Alzheimer's disease, and anti-fungal ciclopirox olamine recently shown to target AML LSCs are examples of “repurposing” existing medicines.
Many proteins are considered “undruggable” based on cellular location, binding, and function (i.e. transcription factors) (Overington et al., 2006
). The promise of RNAi is great considering the ability to target these “undruggable” proteins. However, RNAi has limitations, including off-target effects, immune system modulation, and issues related to in vivo
delivery (Jackson and Linsley, 2010
; Whitehead et al., 2009
). Systemic delivery, via intravenous injection, is likely required for treatment of disseminated disease and requires the ability to avoid non-target tissues and efficient delivery to CSCs. The ability to bypass kidney filtration, phagocytosis, aggregation in serum, and enzymatic degradation are key to successful RNAi therapy with numerous groups and companies actively trying to solve these issues. Indeed, RNAi therapeutics are currently in clinical trials for various diseases, such as age-related macular degeneration, but most current trials rely on localized delivery instead of using delivery agents (Whitehead et al., 2009
Much progress has been made in targeting CSC since the “original” discovery in AML (Lapidot et al., 1994
) and the identification of parthenolide as an anti-LSC agent (Guzman et al., 2005
). Recent advances in understanding the pathways utilized by CSC, such as Hh, NOTCH, and Wnt, have led to exciting pre-clinical and phase I clinical trials to test the clinical relevance of CSC. In addition, improved in vitro
culture methods to maintain CSC activity from primary tumor samples opens the arena of high-throughput screening of small molecule and siRNA libraries. Initial results from these screens have provided new clues and leads in the fight against cancer. Taken together, these studies provide novel agents to target this critical cell population. It is hoped that successful targeting of CSC will significantly improve the outcome for patients with cancer.