br Given that CD plays a pivotal role in regulating
Given that CD133 plays a pivotal role in regulating cancer metas-tasis and therapeutic resistance and that both of cancer metastasis and drug resistance are the major contributors to cancer death, targeting CD133 in cancer patients who have metastatic disease would be the best strategy to bring down the death toll of cancer. To directly tar-geting CD133 in cancer cells, specifically blockade or knockdown of CD133 can be achieved by its neutralizing antibodies, small molecule inhibitors, and short hairpins of CD133. In addition to contributing to cancer death as mentioned previously, CD133 is also present in the normal tissue stem cells and circulating endothelial progenitors, both of which repair the damaged tissues (Brossa et al., 2018; Handgretinger and Kuci, 2013; Li, 2013; Miraglia et al., 1997; Yin et al., 1997). Therefore, it is crucial for a therapeutic strategy to specifically target the CD133+ CSCs than other CD133+ stem cells. There are several reported approaches to specifically target CD133-expressing cancer cells in vitro, in animal models, and in clinical trials. These methods are to use CD133-drug conjugates, asymmetric bispecific Angeli\'s Salt or the T cells that express chimeric antigen receptors of CD133 (CART-CD133).
5.1. CD133-drug conjugates to selectively target CD133-expressing tumors
Pancreatic, gastric and intrahepatic cholangiocarcinoma cells have higher levels of CD133 as compared to normal epithelial cells. Treating Hep3B liver cancer cells and KATO III gastric cancer cells with a cy-totoxic drug monomethyl auristatin (MMAF) that is conjugated with a murine anti-human CD133 antibody resulted in caspase activation and International Journal of Biochemistry and Cell Biology 106 (2019) 1–7
subsequent apoptosis (Smith et al., 2008). One of the mechanisms mediated by the CD133-MMAF conjugates to kill these gastric and liver CSCs is through lysosomes. The in vitro eﬀect of CD133-MMAF con-jugates was verified in Hep3B xenografted SCID mice that anti-CD133-MMAF treatment delayed tumor growth in vivo. Another invention includes to conjugate anti-CD133 monoclonal antibody to nanoparticles that were loaded with anti-cancer drug paclitaxel. It has been shown that treating Caco-2 cells that highly expressed CD133 with CD133-targeted paclitaxel compounds reduced tumor initiating cells as judged by the mammosphere formation and soft-agar colony formation assays (Swaminathan et al., 2013). Furthermore, CD133-targeted paclitaxel compounds had a better eﬀect on inhibiting cancer growth in the breast cancer MDA-MB-231 xenograft mice as compared to paclitaxel treat-ment.
5.2. Asymmetric bispecific antibodies to selectively target CD133-expressing tumors
Use of asymmetric bispecific antibodies is another newly developed method to specifically target CSCs in therapeutics. It has been reported that asymmetric bispecific antibodies that consist monomers of CD133 monoclonal antibody and a single chain of humanized OKT3 antibody selectively induced cell death in CD133high colorectal cancer cells via engaging T cell activation to the CSCs of colorectal cancer (Zhao et al., 2015).
5.3. CAR T cells that selectively target CD133-expressing tumors
Genetic engineering of the T cells of the cancer patients to express chimeric antigen receptors (CAR) that bind to cancer cells is another immunotherapy strategy that can target CD133 expressing CSCs. CART-CD133 treatment has been tested in CD133high glioblastoma stem cells in vitro and in an orthotopic tumor model in vivo (Zhu et al., 2015). Furthermore, when CD133-CAR T cells contacted by the patient derived glioblastoma stem cells, an induced senescence of the activated T cells was detected as judged by an upregulation of CD57, a T cell aging maker. This eﬀect is specifically mediated by the patient-derived CD57+ CSCs. So far, the mechanisms that these CD57+ glioblastoma CSCs utilize to render the activated T cells senescent remains unclear. In a case report from a clinical trial, a metastatic cholangiocarcinoma patient who received therapy of CART-EGFR followed by CART-CD133 responded to the CART cocktail treatment (Feng et al., 2017). In the most recently reported phase I clinical trial for CART-CD133 therapy, 23 patients with metastatic and CD133+ tumors of hepatocellular carcinoma, pancreatic cancer or colorectal cancer were repeatedly given CART-CD133 infusions (Wang et al., 2018). No severe cytotoxi-city was observed among these patients, and 21 of them had no de-tectable de novo tumor cells after CART-CD133 treatments. In addition, the immunohistochemistry results of the post-treated biopsied tissues from these patients indicated a complete elimination of CD133+ CSCs, which resulted in an average 5-month progression free survival of the cancer patients.