Tigecycline suppresses colon cancer stem cells and impairs tumor engraftment by targeting SNAI1-regulated epithelial-mesenchymal transition.

Colon Cancer: An Update and Future Directions

The inflammatory milieu of colon cancer stem cell (CSC) niche is also an important growth regulator for both CSC and progenitor cell population. Inflammatory signals, e.g., cytokines arising in CSC niche, due to inflammatory cells in microenvironment, also network with other regulatory pathways to influence the expansion of both cell types. With this background, we aimed to assess whether silibinin, a nontoxic, naturally occurring chemopreventive agent with established preclinical efficacy against growth and progression of colorectal cancer (CRC), has the potential to target colon CSC and associated inflammatory niche during CRC inhibition. Our results found that silibinin strongly decreases the percentage of colonosphere formation (a stem cell characteristic) of CRC cells, and that this effect on colon CSC is mediated via blocking IL-4/6 signaling in CRC cell lines. Silibinin also strongly decreased IL-4/6 induced activation of STAT-3 and NF-kB transcriptional activity, which is associated with decreased mRNA levels of various CSC regulatory molecules, and CSC-associated markers. In other studies, silibinin significantly reduced the booster signals of macrophages towards colon CSC, resulting in decreased colonosphere numbers under both normoxic and hypoxic conditions. Furthermore, we used a colitis-related AOM/DSS-induced colon tumorigenesis model to assess the role of inflammatory conditions on colon CSC generation and expansion, and their modulation by silibinin. Our data indicated the protective effect of oral silibinin feeding in this model in terms of an absence of large macroadenomas (>2-3 mm) in the colon. This effect was accompanied by minimal colonic inflammation (decrease in recruitment of inflammatory cells); tissue analysis indicated a decrease in the expression of proinflammatory cytokines, and associated transcription factors: STAT-3 and NF-kB in the colonic tissues from silibinin group. A decrease in transformed stem cell population (for CSC pool expansion) was also identified by dual staining for BMI-1 and CD44 in the colonic tissues. Together, these results highlight silibinin's potential to interfere with CSC pool expansion by targeting both colon CSC and their inflammatory niche, as well as associated signals involved with the survival and multiplication of colon CSC pool. These findings further support the translational potential and clinical usefulness of silibinin in CRC intervention and therapy. (Supported by R01 CA112304.) Citation Format: Rajesh Agarwal, Chapla Agarwal. Translational potential of a small-molecule silibinin in colorectal cancer: Targeting cancer stem cells and their inflammatory niche [abstract]. In: Proceedings of the AACR International Conference held in cooperation with the Latin American Cooperative Oncology Group (LACOG) on Translational Cancer Medicine; May 4-6, 2017; São Paulo, Brazil. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(1_Suppl):Abstract nr B32.

Methylation of Cancer-Stem-Cell-Associated Wnt Target Genes Predicts Poor Prognosis in Colorectal Cancer Patients

Constitutive activation of signal transducers and activators of transcription 3 (STAT3) signaling is frequently detected in human cancer including colon cancer, and has emerged as an attractive molecular target for cancer prevention. Recent experimental evidence suggests that the existence of a small population of tumorigenic stem/progenitor cells may be responsible for tumor initiation, chemotherapy and radiation resistance, invasion, recurrence, and metastasis. An increasing body of evidence suggests that the cancer stem cell concept is also relevant to colorectal cancer. To date, however, whether STAT3 is activated in colon cancer stem cells or cancer-initiating cells and what the role of STAT3 signaling may play in these cancer stem cells is still unknown. If STAT3 is activated in colorectal cancer stem cells, inhibiting STAT3 may offer a promising opportunity to target colorectal cancer stem cells and prevent the recurrence of cancer. We utilized the colon cancer stem cells, which are characterized by an aldehyde dehydrogenase (ALDH)-positive (ALDH+) and CD133-positive (CD133+) subpopulation. We demonstrated that ALDH+/CD133+ cells have capacity than ALDH-/CD133- cells to form more tumorspheres and to exhibit more potent tumor-initiating ability in mice. We then examined the STAT3 activation in these colon cancer cells. Interestingly, we observed that the ALDH+/CD133+ subpopulation of colon cancer cells expressed higher levels of phosphorylated STAT3, an active form of STAT3, compared to the ALDH-/CD133- subpopulation and un-separated colon cancer cells, suggesting that STAT3 is activated in colon cancer stem cells. We demonstrated that dietary agent, curcumin and a new curcumin analog, GO-Y030 can inhibit STAT3 phosphorylation, cell viability, and induced apoptosis in colon cancer stem cells. The inhibition of STAT3 was confirmed using a novel STAT3-selecitve curcumin analog, FLLL32, STAT3 ShRNA, as well as STAT3 inhibitors, LLL12 and Stattic. All these STAT3 inhibitors, inhibited STAT3 phosphorylation, cell viability, and tumorsphere growth in colon cancer stem cells. Furthermore, both GO-Y030 and LLL12 inhibited tumor growth of colon cancer stem cells in NOD/SCID mouse model in vivo. In Summary, this is the first report to demonstrate that persistent STAT3 phosphorylation is expressed in colon cancer stem cells. Our study is also the first attempt to target STAT3 in colon cancer stem cells and we demonstrated for the first time that colon cancer stem cells are indeed sensitive to the inhibition by small molecular STAT3 inhibitors, FLLL32, LLL12, Stattic, STAT3 shRNA, and curcumin analog, GO-Y030. Our results suggest that STAT3 is a novel prevention target in colon cancer stem cells and inhibition of activated STAT3 in cancer stem cells may offer an effective preventive approach for colorectal carcinoma. Citation Information: Cancer Prev Res 2010;3(12 Suppl):A100.

Sex-determining region Y-box2 (SOX2), a master regulator of embryonic and induced pluripotent stem cells, drives cancer stem cells (CSCs) properties, fuels tumor initiation, and contributes to tumor aggressiveness. Our previous study has demonstrated the oncogenic role of SOX2 in colorectal cancer (CRC). In this study, we sought to elucidate the underlying mechanisms. Cell function experiments were performed to detect chemoresistance, proliferation, stemness, migration, and invasion in vitro. Chromatin immunoprecipitation, co-immunoprecipitation, luciferase reporter assay, and immunofluorescence were performed to explore the regulation of ABCC2, β-catenin, and Beclin1 by SOX2. The carcinogenic role of SOX2-β-catenin/Beclin1-ABCC2 axis in vivo was analyzed by CRC tissues and xenograft models. Here, we reported that SOX2 sustained chemoresistance by transcriptional activation of ABCC2 expression. Suppressing either β-catenin or autophagy signaling curbed SOX2-driven chemoresistance, stemness, and epithelial–mesenchymal transition (EMT). Mechanistically, SOX2 combined with β-catenin and increased its nuclear expression and transcriptional activity. Transcriptional activation of Beclin1 expression by SOX2 consequently activating autophagy and inducing malignant phenotype. Furthermore, overexpression of β-catenin or Beclin1 facilitated ABCC2 expression. The clinical analyses showed that high expression of ABCC2 and Beclin1 were positively correlated with SOX2 and were associated with poor prognosis in CRC patients. Finally, xenograft models revealed that inhibition of SOX2 expression and autophagy restrained tumor growth and chemoresistance in vivo. Conclusively, we demonstrated a novel mechanism by which the SOX2-β-catenin/Beclin1/autophagy signaling axis regulates chemoresistance, stemness, and EMT in CRC. Our findings provide novel insights into CRC carcinogenesis and may help develop potential therapeutic candidates for CRC.

Increasing evidence supports the concept that cancer stem cells (CSCs) are responsible for cancer progression and metastasis, therapy resistance and relapse. In addition to conventional therapies for colon cancer, the development of immunotherapies targeting cancer stem cells appears to be a promising strategy to suppress tumor recurrence and metastasis. In the present study, dendritic cells (DCs) were pulsed with whole-tumor cell lysates or total RNA of CD44+ colon cancer stem cells (CCSCs) isolated from mouse colon adenocarcinoma CT-26 cell cultures and investigated for their antitumor immunity against CCSCs in vivo and in vitro. In a model of colon adenocarcinoma using BALB/c mice, a sequential reduction in tumor volume and weight was associated with an extended survival in tumor-bearing mice vaccinated with DCs pulsed with RNA or CCSC lysate. In addition, a lactate dehydrogenase assay indicated that cytotoxic T-cells derived from the treated mice exhibited strong cytotoxic activity. Additionally, an enzyme-linked immunosorbent assay revealed that the cytotoxic T-cells of the treated mice released higher levels of interferon-γ against CCSCs compared with those of the control group. In all experiments, the antitumor efficacy of the lysate-pulsed DC-treated and RNA-pulsed DC-treated groups were significantly higher compared with that of the DC-treated and control groups. The results of the present study indicated the potential use of DCs pulsed with cancer stem cell lysates as a potent therapeutic antigen to target CSCs in colon cancer. Additionally, the results provided a rationale for using lysate-pulsed DCs in vivo to eliminate residual tumor deposits in post-operative patients.

BackgroundSince colon cancer stem cells (CSCs) play an important role in chemoresistance and in tumor recurrence and metastasis, targeting of CSCs has emerged as a sophisticated strategy for cancer therapy. α-mangostin (αM) has been confirmed to have antiproliferative and apoptotic effects on cancer cells. This study aimed to evaluate the selective inhibition of αM on CSCs in colorectal cancer (CRC) and the suppressive effect on 5-fluorouracil (5-FU)-induced CSCs.MethodsThe cell viability assay was performed to determine the optimal concentration of αM. A sphere forming assay and flow cytometry with CSC markers were carried out to evaluate the αM-mediated inhibition of CSCs. Western blot analysis and quantitative real-time PCR were performed to investigate the effects of αM on the Notch signaling pathway and colon CSCs. The in vivo anticancer efficacy of αM in combination with 5-FU was investigated using a xenograft mouse model.ResultsαM inhibited the cell viability and reduced the number of spheres in HT29 and SW620 cells. αM treatment decreased CSCs and suppressed the 5-FU-induced an increase in CSCs on flow cytometry. αM markedly suppressed Notch1, NICD1, and Hes1 in the Notch signaling pathway in a time- and dose-dependent manner. Moreover, αM attenuated CSC markers CD44 and CD133, in a manner similar to that upon DAPT treatment, in HT29 cells. In xenograft mice, the tumor and CSC makers were suppressed in the αM group and in the αM group with 5-FU treatment.ConclusionThis study shows that low-dose αM inhibits CSCs in CRC and suppresses 5-FU–induced augmentation of CSCs via the Notch signaling pathway.

Although chemotherapeutic regimens often suppress tumor growth, cancer patients show high variability in their responses and commonly develop resistance to these drugs and recurrence of the tumor. To explain this phenomenon, the cancer stem cell hypothesis suggests that tumors contain a small number of tumor-forming, self-renewing, cancer stem cells (CSCs) within a population of non-tumor-forming cancer cells. In our study, we have identified that CD133+/CD44+/ALDH1+ colon cancer cells are able to self-renew, form tumors in immunodeficient (rag2-/-gammac-/-) mice and are highly resistant to 5-fluorouracil (5-FU) chemotherapy treatment. Several studies have identified that microRNAs are central regulators of oncogenesis, however their role in CSC formation and maintenance has not been elucidated. In order to identify microRNAs that suppress the growth of CD133+/CD44+/ALDH1+ colon cancer stem HCT-116 cells, we transfected a library of 810 microRNAs in these cells and evaluated their viability 24h post transfection. The microRNA library screen revealed 8 microRNAs that suppressed more than 80% colon CSC growth. Specifically, miR-26b inhibited most effectively (96%) colon CSC growth. In addition, microRNA microarray analysis revealed that miR-26b expression is highly decreased in colon CSCs in comparison to colon non-stem cancer cells (NSCCs), suggesting the importance of miR-26b in colon CSC growth. Bioinformatic and molecular analyses revealed that miR-26b targets directly the 3’UTR of the enhancer of zeste homolog 2 (EZH2). Overexpression of EZH2 in colon NSCCs resulted in formation of colon CSCs, suggesting the importance of EZH2 in colon CSC formation. Furthermore, chromatin-immunoprecipitation-sequencing (ChIP-seq) analysis showed that miR-26b overexpression in colon CSCs reactivates the expression of genes repressed by H3K27 tri-methylation. Interestingly, we have identified that miR-26b promoter area is a direct target of EZH2 suppressed by H3K27 tri-methylation. The transcriptional signature of the miR-26b-EZH2 network was also identified in colon CSCs derived from human colon cancer tumors. In addition, miR-26b targeted selectively colon CSCs and acted together with 5-FU chemotherapy to block tumor growth and prolong remission in colon cancer mouse models. Taken together, these data suggest the identification of novel epigenetic-microRNA network in colon CSCs and provide the rationale and experimental basis for using the combination of miR-26b and chemotherapy drugs to improve treatment of patients with colon cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 972. doi:10.1158/1538-7445.AM2011-972

Curcumin induces apoptotic/autophagic cell death of colon cancer cells, and targets cancer stem cells (CSC). In here, we examined for the first time, possible apoptotic/autophagic effects of curcumin on colon CSCs, positive for DCLK1/CD44/LGR5. Curcumin (20-25µM) caused significant loss in the expression of stem cell markers DCLK1/CD44/ALDHA1/LGR5, along with down-regulation of several pluripotency factors, in tumorspheres and xenografts derived from human colon cancer cells. Surprisingly, curcumin induced proliferation and autophagic survival of a small subset of DCLK1+ve CSCs. Colon cancer tumorspheres were disintegrated by curcumin, but regrew as secondary tumorspheres after 30-40 days, suggesting the novel possibility that autophagy-associated survival may allow CSCs to survive and regrow as tumorspheres. We next examined inhibitory effects of DCLK1-siRNA. Unlike curcumin, DCLK1-siRNA only induced apoptotic cell death, with no autophagic response. Combination of curcumin+DCLK1-siRNA caused massive apoptotic/autophagic cell death of colon cancer cells growing as tumorpsheres or xenografts, and reversed the transformed phenotype of CSCs, as previously reported by us (Int.J.Cancer, 2012). Co-treatment with curcumin+DCLK-siRNA was more effective that either agent alone in: 1)attenuating growth of tumorspheres, 2)shrinking the pre-formed xenografts in vivo, and 3)eliminating relapse (re-growth) of tumorspheres. We recently discovered that colon cancer cells and adenocarcinomas from patients express DCLK1-short (S) isoform , transcribed from intron-V promoter; long (L) isoform was silenced by promoter methylation. Normal intestinal cells and non-transformed epithelial cells, on the other hand, mainly expressed DCLK1-L isoform. Promoter-reporter studies and ChIP assays demonstrated that curcumin significantly reduced expression levels of DCLK1-S isoform in colon cancer cells but paradoxically increased its expression in non-transformed/normal intestinal cells. Thus, the differential effects of curcumin on DCLK1 expression in cancer vs normal cells may explain the underlying mechanisms by which curcumin targets CSCs while sparing normal stem cells. Conclusion. Our studies strongly suggest that, 1) DCLK1 represents a functional protein for colon cancers, 2) combination of curcumin+DCLK1-siRNA may target and eradicate colon cancer stem cells., and 3) identifying small molecules that inhibit expression of S-isoform may allow to specifically target cancer stem cells, while sparing normal stem cells for cancer treatment purposes. This work was supported by NIH Granst to PS (R01CA09795909 and RO1CA0975909-S1). Citation Format: Shubhashish Sarkar, Malaney O'Connell, Carla, Kantara, Pomila Singh. A sub-set of DCLK1+ve colon cancer stem cells (CSCs) survive curcumin induced autophagy, while co-treatment with curcumin +DCLK1-siRNA eliminates CSCs: Role of long and short isofoms of DCLK1. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3903. doi:10.1158/1538-7445.AM2014-3903

miRNAs have important roles in regulating cancer stem cell (CSC) properties and are considered to be potential therapeutic targets. However, few studies have focused on miRNAs which are specifically related to colon CSCs. Here, a PCR-based miRNA profiling analysis of normal colon stem cells (NCSC) and colon CSCs (EpCAM⁺/CD44⁺/CD66a⁻) identified miRNAs which regulate colon CSC properties. Interestingly, miRNA-137 (miR-137) expression was downregulated in the colon CSCs compared with NCSCs, while doublecortin-like kinase 1(DCLK1) mRNA was highly expressed in the colon CSCs but low in the NCSCs. In fact, DCLK1-positive cancer cells were widely distributed in clinically resected colon cancer specimens, while DCLK1-positve epithelial cells were rarely detected in normal colon tissues including the crypt bottoms. Luciferase assay and immunoblot analysis revealed that miR-137 regulated DCLK1 gene expression. Transduction of exogenous miR-137 suppressed the development of colon cancer organoids in vitro and the tumorigenicity of colon cancer cells in vivo without affecting the growth of normal intestinal organoids. Furthermore, the suppression of miR-137 enhanced the organoid development of normal colon cells. These data demonstrate that miR-137 has the capacity to suppress the tumorigenicity of colon CSCs and that maintained expression of miR-137 in NCSCs contributes to suppressing uncontrolled cell proliferation through the inhibition of DCLK1 expression. The miR-137/DCLK1 axis as an important regulator in NCSCs and colon CSCs; further understanding of this axis may foster the development of potential gene therapeutic strategies targeting colon CSCs.

<div>Abstract<p>miRNAs have important roles in regulating cancer stem cell (CSC) properties and are considered to be potential therapeutic targets. However, few studies have focused on miRNAs which are specifically related to colon CSCs. Here, a PCR-based miRNA profiling analysis of normal colon stem cells (NCSC) and colon CSCs (EpCAM<sup>+</sup>/CD44<sup>+</sup>/CD66a<sup>−</sup>) identified miRNAs which regulate colon CSC properties. Interestingly, miRNA-137 (miR-137) expression was downregulated in the colon CSCs compared with NCSCs, while <i>doublecortin-like kinase 1</i> (<i>DCLK1</i>) mRNA was highly expressed in the colon CSCs but low in the NCSCs. In fact, DCLK1-positive cancer cells were widely distributed in clinically resected colon cancer specimens, while DCLK1-positve epithelial cells were rarely detected in normal colon tissues including the crypt bottoms. Luciferase assay and immunoblot analysis revealed that miR-137 regulated <i>DCLK1</i> gene expression. Transduction of exogenous miR-137 suppressed the development of colon cancer organoids <i>in vitro</i> and the tumorigenicity of colon cancer cells <i>in vivo</i> without affecting the growth of normal intestinal organoids. Furthermore, the suppression of miR-137 enhanced the organoid development of normal colon cells. These data demonstrate that miR-137 has the capacity to suppress the tumorigenicity of colon CSCs and that maintained expression of miR-137 in NCSCs contributes to suppressing uncontrolled cell proliferation through the inhibition of DCLK1 expression.</p><p><b>Implications:</b> The miR-137/DCLK1 axis as an important regulator in NCSCs and colon CSCs; further understanding of this axis may foster the development of potential gene therapeutic strategies targeting colon CSCs. <i>Mol Cancer Res; 14(4); 354–62. ©2016 AACR</i>.</p></div>

<div>Abstract<p>miRNAs have important roles in regulating cancer stem cell (CSC) properties and are considered to be potential therapeutic targets. However, few studies have focused on miRNAs which are specifically related to colon CSCs. Here, a PCR-based miRNA profiling analysis of normal colon stem cells (NCSC) and colon CSCs (EpCAM<sup>+</sup>/CD44<sup>+</sup>/CD66a<sup>−</sup>) identified miRNAs which regulate colon CSC properties. Interestingly, miRNA-137 (miR-137) expression was downregulated in the colon CSCs compared with NCSCs, while <i>doublecortin-like kinase 1</i> (<i>DCLK1</i>) mRNA was highly expressed in the colon CSCs but low in the NCSCs. In fact, DCLK1-positive cancer cells were widely distributed in clinically resected colon cancer specimens, while DCLK1-positve epithelial cells were rarely detected in normal colon tissues including the crypt bottoms. Luciferase assay and immunoblot analysis revealed that miR-137 regulated <i>DCLK1</i> gene expression. Transduction of exogenous miR-137 suppressed the development of colon cancer organoids <i>in vitro</i> and the tumorigenicity of colon cancer cells <i>in vivo</i> without affecting the growth of normal intestinal organoids. Furthermore, the suppression of miR-137 enhanced the organoid development of normal colon cells. These data demonstrate that miR-137 has the capacity to suppress the tumorigenicity of colon CSCs and that maintained expression of miR-137 in NCSCs contributes to suppressing uncontrolled cell proliferation through the inhibition of DCLK1 expression.</p><p><b>Implications:</b> The miR-137/DCLK1 axis as an important regulator in NCSCs and colon CSCs; further understanding of this axis may foster the development of potential gene therapeutic strategies targeting colon CSCs. <i>Mol Cancer Res; 14(4); 354–62. ©2016 AACR</i>.</p></div>

In the last 15 years, it has become well established that microRNA play important roles in cancer biology. Due to their ability to regulate the expression of important target genes, aberrant expression of miRNAs has been linked to cancer development and progression. Based on these important functions, there is great interest in developing miRNA based therapeutics. In colorectal cancer, treatment using 5-flurouracil (5-FU) based chemotherapy has improved patient outcomes. However, there remain challenges associated with chemoresistance and recurrence for patients with advanced stage colorectal cancer. miRNA based therapeutics may represent an potential novel therapeutic option for colorectal cancer therapy either alone or in combination with 5-FU based chemotherapy. miR-15a was of the first miRNAs identified to be associated with cancer, and has been shown to have important roles in several tumor types. miR-15a is downregulated in colon cancer and associated with poor patient prognosis. We have identified several important cancer related targets of miR-15a in colon cancer, including YAP1, DCLK1, BMI1 and BCL2. Through the regulation of these targets, miR-15a expression can be used as a potent inhibitor to reduce colon cancer cell proliferation, invasion and improve sensitivity to 5-FU, as well as decreasing tumor growth in vivo mouse colon tumor models using colon cancer stem cells. In the interest of developing miR-15a based colon cancer therapeutics, we have made a modified miR-15a mimic that shows enhanced abilities to disrupt resistant colon cancer cell proliferation and induction of cell cycle arrest when compared to unmodified miR-15a. Following transfection with miR-15a mimic, cell number was reduced by 84% compared to control and 66% compared to miR-15a precursor. Cell cycle analysis showed that G1/S ratio was increased from 1.07 for control to 2.87 for precursor miR-15a and 7.07 for miR-15a mimic. This miR-15a mimic also maintains its ability to regulate these important target genes in colon cancer stem cells. In mouse models using colon cancer stem cells, miR-15a mimic has demonstrated therapeutic potential by reducing tumor growth. Based on these findings, there is potential that modified miR-15a could be adapted for treatment of patients to improve survival of advanced stage colorectal cancer patients. Citation Format: Andrew T. Fesler, Hua Liu, Ning Wu, Jingfang Ju. Developing a novel miR-15a mimic as a potential therapeutic molecule to eliminate resistant colorectal cancer stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5443. doi:10.1158/1538-7445.AM2017-5443

Author response: Comprehensive characterization of tumor microenvironment in colorectal cancer via molecular analysis

Colorectal cancer is commonly treated by tumour resection, as chemotherapy and radiation have proven to be less effective, especially if the tumour has metastasized. Resistance to therapies occurs in almost all patients with colorectal cancer, especially in those with metastatic tumours. Cancer stem cells have the ability to self-renew, and their slow rate of cycling enhances resistance to treatment and increases the likelihood of tumour recurrence. Most metastatic tumours are unable to be surgically removed, thus creating a need for treatment modalities that target cancers directly and destroy cancer stem cells. Photodynamic therapy involves a photosensitizer that when exposed to a light source of a particular wavelength becomes excited and produces a form of oxygen that kills cancer cells. Photodynamic therapy is currently being investigated as a treatment modality for colorectal cancer, and new studies are exploring enhancing photodynamic therapy efficacy with the aid of drug carriers and immune conjugates. These modifications could prove effective in targeting cancer stem cells that are thought to be resistant to photodynamic therapy. In order for photodynamic therapy to be an effective treatment in colorectal cancer, it requires treatment of both primary tumours and the metastatic secondary disease that is caused by colon cancer stem cells. This review focuses on current photodynamic therapy treatments available for colorectal cancer and highlights proposed actively targeted photosynthetic drug uptake mechanisms specifically mediated towards colon cancer stem cells, as well as identify the gaps in research which need to be investigated in order to develop a combinative targeted photodynamic therapy regime that can effectively control colorectal cancer primary and metastatic tumour growth by eliminating colon cancer stem cells.