# Recent Advances in the Targeting of Head and Neck Cancer Stem Cells
## 6 Therapeutic Targeting of CSCs
### Targeting Other Stem Cell Markers  
Bmi-1 is an important factor in HNSCC CSCs self-renewal and maintenance, and HNSCC progression is associated with this protein [85; 132]. Therefore, it has been recognized as a potential therapeutic target for CSC eradication [133; 134]. For example, the combination treatment of anti-PD1 and cisplatin, enriched Bmi-1+ CSCs but also inhibited HNSCC growth. Additionally, the inhibition of Bmi-1 in combination with anti-PD1 eliminates CSCs and inhibits HNSCC progression [135]. Also, targeting Bmi-1+ CSCs decreases chemoresistance to cisplatin and eliminates metastases in HNSCC [52]. As ALDH is an important CSC marker, its inhibition has also been investigated in terms of CSC therapy. Disulfiram, an ALDH inhibitor, has been shown to increase radio- and chemo-sensitization in HNSCC CSCs while also inhibiting spheroid formation [136]. Oncofetal antigen 5T4 has been demonstrated as a cancer stem cell marker and two studies have been performed to explore the effect of its inhibition on HNSCC CSCs. The first study explored the strategy of 5T4 inhibition through an innovative antibody-drug conjugate targeted to 5T4 and carrying a DNA-damaging \"payload\" (pyrrolobenzodiazepapine). The authors showed that this inhibition decreases the fraction of HNSCC CSCs in vitro and in vivo and prevents local recurrence in a patient-derived xenograft (PDX)  
Figure 3: Strategies for CSC eradication. The importance of CSC-targeted therapy is shown in (**A**). Classical chemotherapy only reduces tumor bulk but does not destroy CSCs, resulting in cancer relapse. CSC-targeted therapy specifically targets these cells resulting in cancer regression. (**B**) Aiming specifically at CSCs is feasible by targeting their specific markers, signaling pathways, and particular receptors important for their maintenance, by using PD-1 blockade or by using natural compounds in therapy. model [137]. Another study aimed at nasopharyngeal carcinoma (NPC) explored chimeric antigen receptor (CAR)-engineered cytokine-induced killer (CIK) cell therapy against NPC. CIK cells are created ex vivo by stimulating peripheral blood mononuclear cells (PBMCs) with IFN\(\gamma\), anti-human CD3 antibody, and IL-2. This chimeric CAR construct was specific to 5T4, and the co-culture of CIK cells generated using this construct with NPC spheroids successfully eliminated the spheroids [138]. Targeting histone deacetylases has been proposed as a reasonable strategy for CSC suppression. Consequently, pharmacological inhibition of histone deacetylase 6 (HDAC6) decreases cisplatin resistance and inhibits oral CSCs [139]. Another HDAC inhibitor, entinostat, also reduced the number of oral CSCs [140]. Other well-known proteins that have been connected with CSC features and maintenance have also been studied as potential targets for CSC suppression. The importance of integrin \(\beta\)1 in the regulation of stemness and radioresistance has been demonstrated in a recent study [118], suggesting this protein could be the next candidate for CSC therapy. Another recent study by [PERSON] et al. identified that higher expressions of MET, STAT3, and AKT were connected with poor overall survival in HNSCC patients, so they synthesized a small molecule HNC018 that targets these molecules. HNC018 decreased sphere formation and also increased the response to cisplatin and suppressed tumor growth in vivo [141]. Another study investigating MET as a CSC therapeutic target showed that MET inhibition induces the radiosensitization of HNSCC [142]. [PERSON] et al. demonstrated recently that \(\beta\)-catenin and histone methyltransferase Enhancer of Zest Homolog 2 (EZH2) were accumulated in cisplatin-resistant and CSC populations and proposed that their inhibition could be a strategy for CSC suppression [143]. The high mobility group AT-hook 2 (HMGA2)-Snai2 axis was shown to be important in tumorigenicity and stemness of HNSCC CSCs, so targeting these two proteins might also be a suitable approach [144]. [PERSON] et al.