Cancer stem cells of head and neck squamous cell carcinoma; distance towards clinical application; a systematic review of literature

Abstract

Head and neck squamous cell carcinoma (HNSCC) is the major pathological type of head and neck cancer (HNC). The disease ranks sixth among the most common malignancies worldwide, with an increasing incidence rate yearly. Despite the development of therapy, the prognosis of HNSCC remains unsatisfactory, which may be attributed to the resistance to traditional radio-chemotherapy, relapse, and metastasis. To improve the diagnosis and treatment, the targeted therapy for HNSCC may be successful as that for some other tumors. Nanocarriers are the most effective system to deliver the anti-cancerous agent at the site of interest using passive or active targeting approaches. The system enhances the drug concentration in HCN target cells, increases retention, and reduces toxicity to normal cells. Among the different techniques in nanotechnology, quantum dots (QDs) possess multiple fluorescent colors emissions under single-source excitation and size-tunable light emission. Dendrimers are the most attractive nanocarriers, which possess the desired properties of drug retention, release, unaffecting by the immune system, blood circulation time enhancing, and cells or organs specific targeting properties. In this review, we have discussed the up-to-date knowledge of the Cancer Stem Cells of Head and Neck Squamous Cell Carcinoma. Although a lot of data is available, still much more efforts remain to be made to improve the treatment of HNSCC.

Introduction

Head and neck cancer (HNC) is the malignancy arising from the epithelium of the upper digestive tract and upper respiratory tract, including the nasal cavity, paranasal sinus, pharynx, oral cavity, larynx, and cervical esophagus [1], as shown in Figure 1. Head and neck squamous cell carcinoma (HNSCC) are the major pathological type of HNC [2,3]. Only 40-50% of people with HNSCC live for five years after being diagnosed [4]. The main causes of HNSCC are exposure to alcohol, betel quid products, and tobacco; where the risk factors include carcinogens, tobacco smoking, alcohol, human papillomavirus (HPV) infection, and genetic predisposition [5-7]. The major issue in HNSCC pathogenesis is carcinomas development in mucosal epithelium preneoplastic fields which are made up of genetically altered cells and clonally similar to carcinoma [8]. On the tumours excised state, it may extend into the surgical margins, causing second primary tumors. The dissemination of HNSCC occurs in lymph nodes in the neck region. Here, the unravelling of molecular and biological process of HNSCC may be useful for better management and development of personalized therapies [9,10]. The disease ranks the sixth most common malignancy worldwide, with an increasing incidence rate yearly [11]. Squamous cell carcinoma (SCC) accounts for over 90% of all head and neck malignancies [12,13]. Despite various interdisciplinary therapy, HNSCC treatment is ineffective [14-16]. The theory of cancer stem cells (CSCs) is one of the milestones of cancer therapy. According to this theory, CSCs are a small subpopulation of tumor bulk with the abilities of self-renewal and differentiation into secondary heterotypic groups [17]. Eliminating CSCs may help to cure cancer. The Stemness Phenotype Model proposed a model and stated that CSCs have no specific subpopulation of tumors and cancer cells possess plasticity in CSCs and non-CSCs stemness that can interconvert into each other in different microenvironments. This model predicts a pure CSC phenotype cancer cell to pure non-CSC [18-20]. The dissemination from primary tumors and seeding of new tumors from other distant body places involves an invasion-metastasis cascade. Carcinoma dissemination may occur by two mechanisms - single cell dissemination and collective dissemination of tumor clusters. CSCs are believed to be the origin of a tumor and the source of metastasis. CSCs have more substantial potential for stemness, epithelial-to-mesenchymal transition (EMT), therapeutic resistance, and immune escape [21,22]. Cancer-associated fibroblasts (CAFs) are major components of their microenvironment, performing numerous functions including remodeling, matrix deposition, signaling, and crosstalk with host immunity [23]. New markers and their molecular interactions may be identified for more tailored cancer therapies. Post-therapy relapse may be attributed to inadequate elimination of CSCs [24]. The stem-like cells have been isolated and identified from most solid tumors, including HNSCC. The cluster of differentiation (CD) molecules including CD44, CD133, and ALDH (Aldehyde dehydrogenase) have been identified as specific markers of CSCs in HNSCC. Side population (SP) cells and cells with the ability of sphere formation in a medium contained in EGF are also identified as stem-like cells of HNSCC [25]. The PI3K pathway in HNSCC shows significant activating genetic events which may be useful for further studies [26]. This will also unravel the molecular events behind it. In molecular biology, HNC is a manyfold complex process proceeding from carcinogen causing a single mutation to the dysregulation of many metabolic processes in signaling pathways where these events occur in different conditions and states. The molecular biology of HNS still needs much work to be recognized at every step and stage for a better understanding [27]. Many signaling pathways have been identified involved in HNC which may be a potential target and also inhibition of epidermal growth factor receptor (EGFR) for therapeutic strategy is one of the key targets.

Figure 1.

Major anatomical sites of HNSCC. Organs involved in HNC are the nasal cavity, paranasal sinus, pharynx, oral cavity, larynx, and cervical esophagus. With permission from Chow, 2020.

The HNSCC microenvironment is permissive and seems more aggressive in nature, and the response to tumour stress and hypoxia by the immune system are still needed to be explored. Solving these phenomena may increase our knowledge of molecular biology for better management of HNSCC in future.

Potential regulators of HNSCC CSCs: potential targets and current bottleneck

We searched PUBMED’s literature on HNSCC and CSCs and collected data from CSCs regulators (Tables S1, S2 and S3). These regulators include: Phosphoinositide 3 kinase (PI3K) [28], mTOR signaling pathway [29], Hyaluronan (HA) [30], Snail [31], Human papillomavirus type 16 (HPV16) [32], Maternal embryonic leucine zipper kinase (MELK) [33], Renin-angiotensin system (RAS) [34], Short palate, lung and nasal epithelium clone 1 (SPLUNC1) and Mixed lineage leukemia-3 (MLL3) [35], X-linked inhibitor of apoptosis protein (XIAP) [36], Moloney murine leukemia virus insertion site 1 (BMI1) [37], Mitogen-activated protein kinases (p38 MAPK) [38], Wingless/Integrated (Wnt)/β-catenin [39], Sry-like high-mobility group box (SOX8) [40], Sry-like high-mobility group box (SOX2) [41], mitotic arrest deficient 1 (RARS-MAD1L1) [42], LIN28 proteins [43], heat shock protein 90 (HSP90) [44], 5T4 (an N-glycosylated transmembrane protein whose gene is found on chromosome 6q14-15) [45], c-Met (a proto-oncogene) [46,47], metastasis-associated colon cancer-1 (MACC1) [48], The Hippo-TAZ [49], Oct-4 [50], RXRα [51], epidermal growth factor receptor (EGFR) [52,53], Notch1 [54,55], Disruptor of telomeric silencing 1 (DOT1L) [56], Nucleotide-binding domain (NOD)-like receptor protein 3 inflammasome (NLRP3) [57], Tumor necrosis factor receptor-associated factor 6 (TRAF6) [58], glucose-regulated protein 78 (GRP78) [59], S100 Calcium Binding Protein A4 (S100A4) [60], RhoC (a member Rho family of GTPases) [61], Glycogen synthase kinase-3 beta (GSK3β) [62], c-Fos (a proto-oncogene) [63], G9a (also called EHMT2 or KMT1C, is a major euchromatic methyltransferase) [64], histone deacetylase (HDAC) [65], Senescence-associated secretory phenotype (SASP) [66], PinX1 (a potent telomerase regulator) [67], Sialyl Lewis X (sLeX) [68], fucosylation [69], CD200 [70], casein kinase 2 (CK2, a constitutively active Ser/Thr protein kinase) [71], CD10 [72], SDF-1a/CXCR4 (Stromal cell-derived factor-1) [73], Smad ubiquitination regulatory factor (SMURF1) [74], Procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2 (PLOD2) [75], GLI family zinc finger 3 (GLI3); a mediator of genetic diseases [76], Zinc finger E-box binding homeobox 1 (ZEB1/ZEB2) [77], Inhibitor of binding/differentiation 2 (Id2) [78], Bone morphogenetic protein 4 (BMP4) [79], Interferon-stimulated gene 15 (ISG15) [80], ecotropic viral integration site 1 (EVI1) [81], Signal transducer and activator of transcription 3 (STAT3) [82], S-phase kinase associated protein 2 (Skp2) [83], Latent membrane protein 2A (LMP2A) [84], Olfactomedin-4 (OLFM4) [85], topoisomerases [86], JARID1B (also known as PLU-1, is a Retinoblastoma-Binding Protein 2 (RBP2) Homolog, and a member of the jumonji, AT rich interactive domain (JARID) family with H3K4 demethylase activity) [87], Slug (an epithelial mesenchymal transition master gene) [88], CC-chemokine receptors (CCL21/CCR7) [89], Metastasis-associated Protein 3 (MTA3) [90], CMTM6 (belongs to the CKLF-like MARVEL transmembrane domain-containing family; CMTM1-8) [91], phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) [92], Secreted Frizzled-related Protein 1 (SFRP1) [93], Nuclear factor-erythroid factor 2-related factor 2 (Nrf2) [94], Glioma-associated oncogene homologue 1 (Glia1) [95], Cd47-Signal Regulatory Protein α (CD47-SIRPα) [96], Wolf-Hirschhorn syndrome candidate 1 (WHSC1) [97], Zinc finger and SCAN domain containing 4 (ZSCAN4) [98], cystine transporter (xCT) [99], Mediator Complex Subunit 28 (RCOR1/MED28) [100], Wnt Family Member 5A (WNT5A) [101], the estrogen-regulated anterior gradient 2 (AGR2) [102], Cathepsins [103], cytochrome (CYP1B1) [104], signaling promotes regenerative proliferation (VAV2) [105], Tetraspanin 1 (TSPAN1) [106], Tropomyosin-related tyrosine kinase B (TrkB) [107], phosphoglycerate kinase (PGK1) [108], insulin-like growth factor 1 (IGF-1) [109], super-enhancers (SEs) [110], HOXA10-AS (a regulator of homeobox A10) [111], hydroxysteroid 17-β dehydrogenase 7, hydroxysteroid 17-β dehydrogenase 7 (HSD17B7) [112], Prostaglandin E₂ [113]. Aberrant miRNAs (lower level) miR-204 [114], miR-34a [115], microRNA-200c [116], MiR-520b [117], microRNA let-7a [118], Let-7c [119], miR-34a [120] and Longnoncoding RNA-Pvt1 (LncRNA-Pvt1) [121], Long intergenic non-protein coding RNA, p53 induced transcript (LINC-PINT) [122], Hematopoietic cell-specific Lyn substrate-associated protein X-1 (miR-125a/HAX-1) [123], LINC00963 [124], miR-495 (low) [125], CCNG2 (miR-1246/CCNG2) [126] could regulate HNSCC CSCs. The niche associated factors; hypoxia [127], interleukin-6 (IL-6) [128], IL-4 [129], IL-1β [130], EGF [131], TGF-β [132], tumor associated markers (TAM) markers [133], cancer-associated fibroblast [134] also promote the stemness of HNSCC CSCs. In addition, chewing tobacco [135], arecoline-exposure [136], nicotine [137], and cigarette smoke [138] could induce and activate malignant phenotypes and stemness. It may suggest that HNSCC patients need a lifelong ban on tobacco and arecoline.

From data (Tables S1, S2 and S3), we have summarized the following two basic pieces of information 1). Most studies used HNSCC cell lines or available squamous cell carcinoma cell lines of a particular organ as in vitro research subjects; 2). Most studies have applied CSCs markers (CD44, ALDH, CD133), common CSCs related factors, SP traits, and sphere formation to identify HNSCC CSCs. However, in solid tumors, CSCs may not express a single marker or even none. In addition, CSCs isolated from cancer cell lines are not representative of solid tumors surrounded by niches. Although we have thoroughly reviewed the potential regulators of HNSCC CSCs, it remains unclear which pathway is dominant for HNSCC CSCs. In addition, CSCs may evolve through genetic instability leading to dynamic expression of markers and regulators. In a recent study, Salazar-García et al. [139] performed whole-exome sequencing to analyze the germinal line, tumor cells, and CSC ALDH⁺ samples from different HNSCC patients. They found that the difference in genes of oncogenic pathways. Therefore, exploring new ideas and novel strategies is essential to target HNSCC CSCs effectively.

Key molecules involved in the transcription of cancer stem cell genes and premetastatic genes in cancer stem cells

The distinct CSCs population in cancers is different. However, in HNSCC the specific CSCs population are CD133, CD44, CD98, ALDH, CD200, ALDH, CD44, GRP78 and BMI1 [140]. Similarly, CSC various abnormal extrinsic and intrinsic signals including niche, and mutations are involved in pathways deregulation that leading to their maintenance.

The Wnt/β-catenin canonical pathway is initiated when Wnt ligand binds to Frizzled receptor and 5/6 coreceptors protein. This causes the recruitment of Axin and disheveled which prevents the degradation to protection of β-catenin. Free β-catenin complexes with TCF/LEF (T-cell/lymphoid enhancer factors) and regulate the Wnt target stem cell genes. Phosphorylation of CD133 also results to AKT activation and the NF-kB pathway, leading to stemness genes activation. Hypoxia inducible factor-1-α (HIF1α) and c-Myc (myelocytomatosis) are involved in increase of glycolysis and prevent the oxidative phosphorylation in CSCs. Activation of pyruvate dehydrogenase kinases (PDK1-3) is brought by HIF1α, to stop pyruvate dehydrogenase (PDH), leading to oxidative phosphorylation inhibition. C-Myc has a role in activation of the hexokinase, Glucose transporter -1 receptor, and phosphofructokinase which are in favor of glycolysis [140].

Moreover, the activities of CSCs are governed by numerous pluripotent transcription factors. Some of these factors are Oct4, Sox2, Nanog, Kruppel-like factor 4 (KLF4) and Myc. Besides these pluripotent transcription factors, several intracellular signaling pathways, such as Wnt, NF-κB, Notch, Hedgehog, JAK-STAT, TGF/SMAD, and PPAR along with external factors like vascular niches, hypoxia, tumor-associated macrophages, cancer-associated fibroblasts or mesenchymal stem cells, extracellular matrix, and exosomes play vital roles in the regulation of CSCs [141]. A favorable microenvironment is required for cancer development. Some studies showed intercellular communications are mediated by microvesicles (MVs) released by cells. Large MVs are produced by the tumor cells and released in the circulation and other biological fluids [142,143]. These MVs have the pleiotropic effect, enabling their involvement in cancer development, progression, and premetastatic niche formation [144]. Normal stem cells are an important source of MVs that may act as paracrine mediators of genetic information through horizontal transfer [145-148]. The transmembrane glycoprotein CD44, a common stem cell niche component, integrates signaling in normal stem cells, cancer stem cells, and (pre)metastatic niches [149]. The biological markers of stem cells in pancreatic CSCs (Pan-CSC) include CD44v6, c-Met, Tspan8, alpha6beta4, C-X-C-chemokine receptor 4 (CXCR4), CD133, epithelial cellular-adhesion factor (EpCAM) and claudin7 [150], breast stem cancer cells such as ALDH1A1 (aldehyde-dehydrogenase 1 family member-A1) [151,152], and brain CSCs such as CD133 and in HNSCC such as ALDH⁺, CD44⁺, CD166⁺ [153-155] have been investigated. Cancer stem celllike phenotype and the difference in the gene expression pattern responsible for diverse biological roles in HNSCC has been studied recently. Hypoxia-regulated genes have been shown to be helpful for the prediction of radiotherapy responses in HNSCC patients [156]. The biological role of polycomb (PcG) genes Bmi1 and TERT in tumorigenesis in human stromal cells which were immortalized as the HNSCC model studied recently. It was found that Bmi1 was predominantly expressed in early head and neck squamous cell carcinoma, indicating that the PcG is essential in early cancer development [157]. The role of the p53-p16/RB pathway in HNSCC can be focused on interpreting the early carcinogenesis of HNSCC for better management. Besides, m6A modulators have been emerging cancer progression molecular mechanisms [158].

Stem cells regulators

CSCs have the ability of self-renewal, that may lead them to tumorigenesis [159]. The symmetrical division of CSCs divide them into two CSCs or, in other hand into one CSC and one daughter cell [160]. The symmetrical splitting manner of CSCs expand them to excessively increase cell growth, resulting in formation of tumor [161]. Normal stem cells and CSCs follow same regulatory signaling pathways, for example the Wnt/β-catenin [162], Sonic Hedgehog (Hh) [163], and Notch pathways. These pathways are involved in the self-renewal process [164]. Besides, other signaling molecules, for example PTEN and the polycomb family, are involved in the regulation of CSCs cycle [165].

Several factors are involved in the regulation of stem cells. These might be transcription factors which play a basic role in the proliferation of stem cells. Reprogramming of somatic stem cells can be can be carried out to generate iPSC by increasing the expression of the transcription factors such as Octamer transcription factor4 (Oct4), Sox2, Nanog, KLF4, and Myc [166,167]. Besides, SRY and Oct4 are considered as potential differentiation therapy targets in stem cells. The over-expression of Sox2 transcription factor in basal-like breast cancer may be supportive to characterize the cell phenotypes of poorly differentiated/stem cells [168]. The deletion p53 and Myc synergizes to induce proliferation and tumorigenesis in hepatocytes [169]. Besides the loss of p53, Bcl-2 and BMI-1 overexpression and deletion of p19ARF also shown the regulation of Myc in CSCs survival and proliferation [170]. In addition, the perturbance of Myc results in Hepatocellular carcinoma stem cells differentiating into hepatocytes and biliary duct cells resulting the formation of bile duct structures [171].

CSCs have multipotential characteristics. They can also differentiate into other cell types in addition to their self-renewal capabilities. Bonnet and Dick [172] showed that CD34⁺⁺/CD38^- LSCs (Leukemia stem cells) were able to differentiate and proliferate in severe combined immunodeficient mice. CSCs isolated from the brain of patients are positive for the markers CD133 and nestin [173]. CSCs from the breast indicated varied expression patterns of surface biomarkers. Some of these surface biomarkers are CD44⁺, CD24^-, SP, and ALDH⁺ [174-176]. CD271^- or CD271⁺ melanoma stem cells were able to generate tumors in SCID mice [177].

Initial steps on targeting CSCs

Post-therapy recurrence and metastasis are related to residual CSCs. Therefore, effectively eliminating the subpopulation CSCs is essential in anti-cancer activities [178]. Therefore, much effort is needed to target HNSCC CSCs in preclinical studies.

Small molecular target drugs

Some authors reported that some molecular target drugs could eliminate HNSCC CSCs in vitro (Table S2). EGFR is one of the four members of the HER tyrosine kinase (RTK) receptor family, composed of EGFR/HER1/erbB1, HER2/erbB2, HER3/erbB3, and HER4/erbB4. The receptors of RTK, such as epidermal growth factor (EGF) and transforming growth factor alpha (TGF-α), can activate intracellular signaling pathways that control growth, differentiation, survival, and invasion [179]. The overexpression of EGFR is a frequent molecular alteration associated with aggressiveness, resistance to treatment, and poor clinical outcomes in HNSCC [180]. Therefore, the EGFR-targeted drug cetuximab has been recommended as a clinical combination with dihydropyrimidine dehydeogenase (DDP)-based chemotherapy as an anti-HNSCC treatment in NCCN guidelines. However, resistance to chemotherapy still exists. Recently, it’s been reported that combining Cetuximab and Erlotinib could induce CSCs differentiation and transit EMT-CSCs back to the epithelial phenotype, which sensitizes treatment and restricts local invasion and metastasis [181]. More recently, Roy S et al. [182] found that Afatinib, the second generation of FDA-approved pan-EGFR inhibitor, could inhibit the growth of HNSCC CSCs in vitro and in vivo by inducing severe apoptosis and an uncommon weak protective autophagic response preferentially in stem-like HNSCC cells [183]. However, CSCs usually have low EGFR expression and overexpress the anti-apoptotic Bcl-2 protein [184]. Anti-apoptosis members of the Bcl-2 family are associated with a poor prognosis of HNSCC. ABT-737 is a well-characterized BH3 mimetic that prevents binding ligands to anti-apoptotic Bcl-2 family protein and indirectly activates the pro-apoptotic Bcl-2 family members. Marion Gilormini et al. [185] suggested that ABT-737, alone or in synergism with radiation, can efficiently eliminate stem-like quiescent HNSCC cells in vitro and synergistically inhibit the growth of xenograft tumours. Another group observed the combined effect of ABT-199, Bcl-2 inhibitor, cetuximab, and radiation as anti-CSCs [186]. The combination significantly inhibited proliferation, invasion/migration, and resistance to apoptosis of HNSCC CSCs in vitro and strongly reduced the tumor growth and increased in vivo survival without side effects. Like EGFR, Lin28, an essential RNA-binding protein, also plays a critical role in regulating the balance between stemness and differentiation in embryonic stem cells (ESC) [187]. Chen and coauthors [188] showed that the combination of C1632 (Lin28 inhibitor) and metformin (anti-CSCs hypoglycemic medication) exerts synergistic anti-tumor effects in OSCC cell lines and xenograft tumor growth. Bruton’s tyrosine kinase (BTK), a cytoplasmic non-receptor tyrosine kinase, is upstream of the phosphoinositide 3-kinase (PI3K-AKT) pathway, phospholipase-C, protein kinase-C, and NF-κB, performing many functions, including cellular differentiation, proliferation, and adhesion to innate and adaptive immune responses [189]. Although BTK is mostly involved in the hematologic tumor, it is expressed aberrantly in concurrent chemoradiotherapy (CCRT) resistant OSCC tissues, correlated with stemness and EMT factors, and influences survival rate. The Ibrutinib, a first-class BTK inhibitor, reduced CSCs number and increased the DDP sensitivity of OSCC SP-derived cells [190]. Glycogen synthase kinase 3β (GSK3β) controls the shift from EMT-CSCs to CSCs-epi. Hideo et al. demonstrated that GSK3β inhibition induced mesenchymal-to-epithelial transition (MET) from CD44 (high)/ESA (low) cells to CD44 (high)/ESA (high) cells and pre-existing CD44 (high)/ESA (high) cells to differentiate. The CD44 (high)/ESA (low) cells overexpressed dihydropyrimidine dehydrogenase (DPD), a factor affecting the therapeutic sensitivity to 5-FU. Combination of both DPD inhibitor, 5-chloro-2,4-dihydroxypyridine (CDHP) and GSK3β inhibitors markedly enhanced 5-FU-induced apoptosis of CD44 (high)/ESA (low) cells [191]. In addition, the Wnt/β-catenin signal is another CSCs regulating pathway. Tankyrases are members of the poly (ADP-ribose) polymerase (PARP) family proteins, which serve as regulators of the canonical Wnt/β-catenin signaling [192]. XAV-939, a small molecule of tankyrase inhibitor, reduced CSCs-mediated chemoresistance in DDP-resistant HNSCC cell lines combined with DDP via DNA damage [193]. Similarly, a recent study identified LF3, a 4-thioureido-benzenesulfonamide derivative, as a potent and specific inhibitor of activated Wnt/β-catenin signals [194]. In this study, the self-renewal capacity of head neck CSCs was blocked by LF3, as examined by sphere formation. Beside, secreted frizzled-related protein 4 (sFRP4) is one of five members of the sFRP family and a naturally extracellular inhibitor of Wnt signaling [195]. Warrier’s group showed that sFRP4 decreased the expression of CSCs markers (CD44 and ALDH) and inhibited proliferation, EMT, and enhanced chemosensitivity of HNSCC CSCs [196]. Moreover, histone deacetylases (HDACs) regulate several genes involved in cancer initiation and aggressiveness [197]. Similarly, Royal jelly acid showed suppression in HCC tumorigenicity that inhibited H3 histone lactylation targeted H3K9la and H3K14la sites [198]. In addition, transcriptome analysis of transgenic mice models predicted several oncogenes in brain [199], lungs [200].

Studies demonstrated that HDACs inhibitors (HDACi), suberoylanilide hydroxamic acid (SAHA), and trichostatin A (TSA), inhibited the stemness of HNSCC CSCs, and enhanced the DDP sensitivity, which may be attributed to reduced NANOG and Survivin expression [201,202]. Similarly, valproic acid (VPA), another HDACi, inhibited the self-renewal abilities of HNSCC CSCs with decreased expression of CSCs markers, such as Oct4, Sox2, and CD44, and enhanced sensitivity to DDP via reducing ABCC2, six and inducing apoptosis. The VPA combined with DDP attenuated xenograft tumor growth [203]. Entinostat, another HDACi, could induce cycle arrest (G0/G1 phase), tumor apoptosis and increase in ROS production, and significant reductions in HNSCC CSCs [204]. In addition, cancer cells have a super capacity to ROS scavenger via redox enzyme. Therefore, combining dimethyl fumarate (DMF), a GSH (glutathione) inhibitor, and Buthionine sulfoximine (BSO), a GSH synthesis inhibitor, could sensitize HNSCC CSCs to radiotherapy. It suggests that reduced antioxidant capacity may be a striking strategy to target CSCs [205].

Another reason for the radio-resistance of HNSCC CSCs is an extended G2/M arrest phase. Therefore, UCN-01, a checkpoint kinase (Chk1) inhibitor, and all-trans retinoic acid (ATRA), an inducer of differentiation, combined with irradiation drastically decreased the surviving fraction of HNSCC CSCs [206]. MEDI5117 is an IL-6 inhibitor. Finkel and coauthors found that low-dose MEDI5117 antibodies decreased the CSCs fraction in three low-passage patient-derived xenografts (PDX) models of HNSCC [207]. They conducted a clinical trial in which MEDI5117 prevented tumor recurrence when used in the adjuvant setting. BMI-1, downstream of IL-6, is a CSCs-related factor. Jia et al. [208] demonstrated BMI-1⁺ CSCs contributed to the failure of PD-1 and DDP treatment in an HNSCC mouse model. PTC209 plus PD-1 inhibitor could eliminate CSCs via recruiting CD8⁺ T cells and prevent the progression and relapse of HNSCC. Meanwhile, an inhibitor of the IL-6R/BMI-1 axis, Tocilizumab, could target HNSCC CSCs via reversing DDP-induced self-renewal and chemoresistance in DDP-resistant HNSCC cells [209]. COX-2 is an inducible enzyme that triggers the biosynthesis of prostaglandins. Celecoxib, a COX-2 inhibitor, inhibited RNA expression of stemness-related genes and sphere formation in HNSCC cell lines [210].

Compounds extracted from natural herbs

Although targeted molecular drugs have shown the ability of anti-HNSCC CSCs experimentally, resistance is a challenging problem clinically. These causes include activating mutations in the target itself and activating various compensatory pathways and EMT as a major mechanism of resistance [211]. Emerging evidence demonstrates that pure compounds extracted from natural herbs or plants exhibit features of multi-targets, anti-CSCs, and less toxicity. Ovatodiolide (OV), a bioactive chemical substance purified from Anisomeles indica (L.) Kuntze (Labiatae) could inhibit NPC tumor sphere formation, attenuate NPC stem cell tumorigenicity, and enhance the sensitivity via reducing the expression of p-FAK, p-PXN, F-actin, slug proteins, SOX2, OCT4, and JAK-STAT signaling pathway [212]. In addition, Epigallocatechin-3-gallate (EGCG) is active polyphenolic catechin purified from green tea. Lee et al. examined the anti-tumor effect of EGCG on HNSCC CSCs. They demonstrated that EGCG inhibits the self-renewal capacity of HNSC CSCs via inhibition of stem cell markers, such as Oct4, Sox2, Nanog, CD44, ATP-binding cassette subfamily-G member-2 (ABCG2), and Notch signaling [213].

Quercetin is a polyphenolic flavonoid compound in nuts, teas, vegetables, herbs, and people’s daily diets [214]. Chang et al. found that Quercetin could reduce the stemness of HNSCC CSCs through the decreased expression of Twist, N-cadherin, and Vimentin [215]. Besides, Deng’s laboratory identified a new gamboge derivative compound 2 (C2). In their study, C2 treatment reduced colony formation of HNSCC CSCs and inhibited expression of CSCs markers (CD49f, CD133, and CD44) more significantly than DDP with less toxicity. The inhibition effect of C2 on CSCs was attributed to targeting Ki-67, phosphor-EGFR, CD49f, and CD133 [216]. Cucurbitacin I is a natural triterpenoid isolated from the Cucurbitaceae family plants and other plant types. Chen et al. found that Cucurbitacin I could inhibit the proliferation, tumor aggressiveness, and stemness signatures and induce apoptosis, differentiation, and radiosensitivity of HNSCC CSCs via suppression of STAT3, Janus-activated kinase 2 (JAK2), Bcl-2, Bcl-xL, and survivin [217].

Chang et al. screened for active components and discovered YMGKI-1 and YMGKI-2 from Antrodia cinnamomea Mycelia (ACM) natural products. They demonstrated that both components inhibited stemness, decreased expression of CSC markers, and promoted radiosensitivity of HNSCC CSCs by downregulating the activated autophagic signaling pathways, STAT3 and Src [218,219].

In addition, Plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone, PLB) is a small molecular compound derived from the root of Plumbago zeylanica L, Juglans regia, Juglans cinerea, and Juglans nigra, with a variety of pharmacological activities. Recently Pan et al. [220] reported that PLB-induced apoptosis inhibited EMT and stemness and promoted MET via mediating multiple targets on tongue squamous cell carcinoma (TSCC) cell line SCC25 cells. In addition, translation inhibition could disrupt stemness properties [220]. SVC112 is a synthetic derivative of the cyclic hexapeptide bouvardin, a plant-derived translation elongation inhibitor with less toxicity. Keysar et al. demonstrated that SVC112 inhibits tumorsphere growth and enhances radiosensitivity in vitro by suppressing Myc, Cyclin D1, Myc, and Sox2. SVC112 alone and with radiation inhibits the growth of tumours and CSC in vivo [221]. Besides this, Lovastatin (LV) is a natural lipophilic statin derived from Monascus or Aspergillus-fermented rice and Dioscorea. In Peng et al.’s study, LV inhibited proliferation and self-renewal and induced apoptosis and cell cycle arrest of NPC CSCs. LV could also synergistically enhance the sensitivity of NPC CSCs to chemotherapy and photodynamic therapy [222]. Tetrandrine is a bis-benzylisoquinoline alkaloid isolated from Stephania tetrandra and other related species of Menispermaceae. Cui et al. demonstrated that tetrandrine inhibited the cell viability and proliferation of CD133 in Hep-2 cells by impacting the cell cycle and enhancing cell apoptosis via upregulating Bax and caspase-3 and downregulating Bcl-2 [223]. Isoliquiritigenin (ISL) is a natural flavonoid compound derived from the natural herb licorice root (licorice) with significant anti-tumor ability. Hu et al. showed that ISL was more cytotoxic to OSCC CSCs and hindered self-renewal by reducing ALDH1 enzymatic activity and CD44 positivity in OSCC-CSCs. ISL also enhanced sensitivity to DDP via inhibiting ABCG2. Finally, they demonstrated that the anti-CSCs ability of ISL was ascribed to regulating the protein expression of mRNA and membrane GRP78 [224]. Sulforaphane (SF) is an isothiocyanate isolated from broccoli.

Elkashty et al. found that SF-combined treatments inhibited the colony formation of HNSCC CSC and in vivo tumor progression with potential mechanisms including the stimulation of caspase-dependent apoptotic pathway, inhibition of SHH pathway, and decreased expression of SOX2 and OCT4 [225]. Curcumin is a bioactive polyphenolic compound identified in turmeric with significant anti-tumor ability. However, the low solubility in aqueous media, poor bioavailability, and pharmacokinetic profiles limit its therapeutic potential. Therefore, several different formulations have been produced. Recently Basak et al. found that Curcumin-difluorinated (CDF), a synthetic analog of curcumin, was packaged in liposomes and used to evaluate the growth inhibition of DDP-resistant HNSCC cell lines. Treatment with liposomal CDF resulted in a statistically significant tumor growth inhibition in nude mice xenograft and a reduction in the expression of CD44, indicating an inhibitory effect of liposomal CDF on CSCs [226]. In addition, curcumin and metformin combination could prevent 4-nitro quinoline-1-oxide (4NQO) induced oral carcinogenesis in a mice model through an overall downregulation of CSC markers [227]. Isoorientin (3’,4’,5,7-tetrahydroxy-6-C-glucopyranosyl flavone) is a C-glycosyl flavone extracted from Aspalathus linearis and several other plant species. In Liu et al.’s study, Isoorientin could significantly reduce the expression of p-STAT3, Wnt/β-catenin, p-GSK3, and downstream effectors transcription factor-T cell factor 1 (TCF1) and LEF1, enhance DDP toxicity, and inhibit the tumorigenicity and growth of OSCC all in all attributing owing to targeting OSCC-SC-mediated stemness [228]. Apigenin (4’,5,7-trihydroxyflavone) is one of the most studied phenolics abundant in fruits and vegetables. The compound could significantly down-regulated expressions of CSCs markers, CD44, NANOG, and CD105 of HNSCC cells and reduce the number of cells expressing CSCs markers under hypoxia [229]. Resveratrol (trans-3,5,4’-trihydroxystilbene) is a phytoalexin initially found in Polygonum cuspidatum. In Hu et al.’s study, resveratrol reduced the activity of CSCs markers (ALDH1 and CD44) and CSCs-related gene expressions (Oct4, Nanog, and Nestin) in HNC-CSC and regulated EMT-related markers in vitro and in vivo, which may lead to a valuable clinical therapeutics combining with conventional chemotherapy modalities for HNC [230].

Silibinin is a flavonolignan extracted from the fruit and seeds of Milk thistle. It is well-known for its hepatoprotective and anti-carcinogenic effect on various experimental cancer models. Chang et al. showed that Silibinin exerted an inhibitory influence on invasion, stemness, EMT, and anti-apoptosis ability of HNC-CSCs via activation of miR-494-inhibiting Bmi1/ADAM10 expression [231]. Recently in an in vitro study, Propolis could reduce CSCs numbers and decrease CSCs markers specifically [232]. The effects of the compounds mentioned above are summarized in Table S3.

Immunotherapy

In addition to specific formulation, Immunotherapy targeting CSCs provides another promising perspective. Liao T et al. [233] tested responses against putative HNSCC CSCs by an alloantigen-specific model system in vitro. Although CSC populations were less sensitive to major histocompatibility complex (MHC) class I-restricted alloantigen-specific CD8⁺ CTL lysis, IFN-γ pretreatment upregulates molecules essential for antigen processing and presentation, leading to over-proportionally enhanced lysis of CSC-enriched spheroid culture-derived cells (SDC). Moreover, the subset of ALDH^high CSCs presented more sensitivity toward CD8⁺ CTL killing than the ALDH^low SDC. The in vitro experiment by Liao T et al. suggested that Immunotherapy targeting ALDH⁺ CSCs may be a promising approach. In a preclinical study, researchers induced and expanded human leukocyte antigens (HLAA2) restricted, ALDH1A1 peptide-specific CD8⁺ T cells by in vitro stimulation of CD8⁺ T cells isolated from peripheral blood from regular HLA-A2⁺ donors. These HLA-A2-restricted, ALDH1A1 peptide-specific CD8⁺ T cells recognized and eliminated ALDH^bright cells, specifically in vitro and in vivo. They showed that the adoptive transfer of ALDH1A1-specific CD8⁺ T cells inhibited the growth of primary and metastatic tumors in xenografts [234].

Novel radiotherapy

Surgery and radiation are the mainstream in HNSCC therapy. HNSCC CSCs resistant traditional photon radiation and EGFR inhibitors; moreover, IR can activate EMT and the CSC phenotype [235]. However, in recent in vitro research, carbon ion irradiation effectively reduced migration/invasion of HNSCC CSCs and non-CSCs alone or combined with cetuximab [236]. More recently, a study showed that daily photobiomodulation with 3 J/cm² suppressed cellular viability and that 6 J/cm² decreased the number of spheres of OSCC cell lines and the expression of the CSC-related gene BMI1 [237]. Yu et al. demonstrated that topical 5-aminolevulinic acid-mediated photodynamic therapy (ALA-PDT) inhibited the ALDH1 activity of HNSCC cells, eliminated self-renewal capacity, CD44 positivity, stemness signatures, and enhanced chemosensitivity in HNSCC CSCs [238,239].

Potential strategies

Given the therapeutic options above-mentioned, personalized treatment may be a good one to solve the problems. We can identify primary CSCs markers and regulators and then take adequate measures to target CSCs in individual HNSCCs. Based on individualized treatment: 1) Local CSC-targeted DDSs. It may be a promising approach to apply peritumoral injections using pure natural compounds with multi-targets and less toxicity as anti-CSCs formulations, such as nanoparticles, and in combination with radiation. Local CSC-targeted DDSs are better than conventional IVs. It could withstand the barrier of the niche, ensure anti-CSC agents arrive at the targeted CSCs, and can be taken up by CSCs with enhanced permeability and retention effect (EPR). 2) Immunotherapy targeting CSCs in individual HNSCC treatment. 3) The biology of CSCs depends on the niche; double targeting cancer stemness and the niche, such as hypoxia, microcirculation, or immune status, is a possible approach. Chinese traditional medicine (TCM) has its principle of the human environment, for instance, the Yin-Yang theory. According to the theory, illness is due to the imbalance of two opposing forces of energy, Yin and Yang. In addition, the compounds mentioned above extracted from Chinese traditional herbs (TCH) exhibit anti-CSC effects. Pure combined compounds with fewer toxicities like Cocktail therapy, may have a promising prospect, and TCH carried by novel DDSs may be another promising strategy. In general, the anti-CSCs strategy seems a promising therapeutic option, although there is still a long way to go for successful clinical application.

Targeted drug delivery in HNSCC

Although the drug monomer presents an anti-CSCs effect, targeting CSCs requires unique drug delivery systems (DDSs) formulation, as shown in Figure 2. Su et al. [240] synthesized and characterized anti-CD44 antibody-coated superparamagnetic iron oxide nanoparticles (SPIONPs). The exploration of the formulation was dependent on the mechanism of hyperthermia therapy. The CD44-SPIONPs (superparamagnetic iron oxide nanoparticles) target CD44⁺ HNSCC CSCs by endocytosis, which could generate heat through magnetic vector and physical rotation under an alternating magnetic field to kill CSCs. After the AMF treatment, CD44-SPIONPs induced CSCs to undergo programmed death with an inhibitory ratio of 33.43%, significantly inhibiting the growth of grafted Cal-27 tumors in mice. In addition, Miyano et al. [241] used cyclic Arg-Gly-Asp (cRGD) peptide, an HNSCC CSCs marker specific binding to integrin α_vβ₃, on micellar nanomedicines incorporating cisplatin (cRGD-installed DDP/m). The cRGD-installed DDP/m showed significant antitumor activity against primary HNSCC xenograft tumors, with the rapid accumulation of the metastatic lymph nodes leading to prolonged mice survival.

Figure 2.

Overview of targeted drug delivery. The drug molecules (green) are surrounded by amphiphilic ligands with a hydrophilic tail. Attached to the tail of this amphiphilic ligand are markers and antibodies. These antibodies bind with surface-specific receptors on tumor cells in the drug and are released there to initiate apoptosis in HNC.

RNAi therapies remain unsatisfactory due to delivery limitations by many factors, such as easy degradation by enzymes. Lo et al. [242] provided a feasible non-viral gene delivery method, cationic polyurethane-short branch polyethyleneimine (PU-PEI)-based delivery of nuclear localization signal (NLS) pre-conjugated dsDNA encoding siRNAs. In their study, co-administrated PU-PEI vehicles containing NLS-pre-conjugated dsDNA encoding either siEZH2 or siOct4 remarkably achieved gene silencing, which led to diminished CSC-like properties, suppression of EMT, enhanced radiosensitivity, and prevention of metastasis in HNSCC. Meanwhile, nano micelles could load multiple agents to target different subpopulations. Recently, Zhu et al. developed salinomycin (SAL)-loaded poly (ethylene glycol), 2000-di-stearoyl phosphatidyl-ethanolamine (DSPE-PEG)-methotrexate (MTX) nano micelles (M-SAL-MTX), for SAL targeted CD133⁺ CSCs and MTX could kill non-CSCs. In their study, M-SAL-MTX effectively accumulated in tumor tissues compared with a single treatment of SAL or MTX; therefore, M-SAL-MTX exhibits significant anti-CSCs and anti-non-CSCs in vivo [243].

In addition, peritumoral injections are available in HNSCC. Hyaluronic acid (HA) is a particular ligand for the CD44 surface receptors. Peritumoral injections of cisplatin conjugated to nanoscopic (25-100 nM) particles of HA (HA-cisplatin) provide superior antitumor efficacy and CSCs targeting compared to conventional IV cisplatin therapy in a laryngeal cancer xenograft model with less toxicity [244]. It may be useful in nanoparticles targeting CSCs markers with a specific route of administration may be a promising strategy to target CSCs.

Recent advances in cancer genetics, sequencing, and their role in therapeutic efforts have led to precision medicine. Precision medicine is mainly based on the genetic, environmental, and lifestyle characteristics that can lead to identifying the therapy for individual patients. Although this approach is very effective in oncology, some issues are still there, including drug resistance and toxicities. Drug delivery systems have enabled the modulation of pharmacological parameters, including stability, pharmacokinetics, absorption, and exposure to tumors and healthy tissues.

Nanomedicine is very helpful in targeted drug delivery, decreasing the drug toxicity to non-target cells compared to non-carrier drugs [245]. Currently, different types of nanoparticles (NPs) have been reported and approved by US Food and Drug Administration [246] for cancer diagnosis and treatment. These are organic NPs (polymer, dendrimer, ferritin, and micelles) and inorganic (Q dots, silver iron oxide, gold). The NPs technologies have greatly improved controlled drug releases and enhanced the targeting of drug delivery to specific tissues [247-249]. These advantages of NPs drug delivery systems are improving the current treatments and paving the way for new therapy options. To cover the significant issues of medicines, including solubility and bioavailability, long time circulation, and unwanted toxicity to neighboring healthy tissues, phytomedicine integration into nano vehicle is a valuable and productive choice to enhance its biological effects and overcome the physiological barriers [250,251].

Nanotechnology-based targeted drug delivery system for HNC therapy is alternatives treatments that maximize the efficacy and offer good efficacy to the problems compared to conventional therapies. A targeted drug delivery system reduces the rate of delivery failures and cell death and minimizes multidrug resistance. These properties of NPs are promising in HNC treatment because to reach the target, the therapeutic targets need to cross biological barriers, including the blood-brain barrier, which is a significant obstacle and reduces drug delivery to the brain [252]. To overcome the shortcomings of conventional methods in HNC management, using nanocarriers as diagnostic and therapeutic agents has improved efficacy and safety. The NPs guide anti-cancerous drugs to the target cells, increasing the concentrations of drugs in the intracellular environment of the target cells and reducing the toxicity to normal cells. They attach specific receptors to the target cells on the surfaces and are internalized by endocytosis.

Targeting through nanocarriers

Nanocarriers are the most effective system to deliver the anti-cancerous agent at the site of interest using passive or active targeting approaches. The system enhances the drug concentration in HCN target cells, increases retention, and reduces toxicity to normal cells [253,254]. They are targeting through nanocarriers involving active and passive approaches. Active targeting consists of using ligands/drugs to the site of interest in HNC, while passive targeting is without any stimulus or ligand to the target site.

Passive targeting

Passive targeting involves the systemic administration of nanocarriers, which tend to accumulate selectively at the desired location as a result of the enhanced permeability and retention (EPR) phenomenon [255]. The EPR may be influenced by different tumor microenvironment factors (TME), including vasculature, stage, macrophages, interstitial, and lymphatics fluid pressure [256,257]. Thus, the anatomy and physiological conditions of the target are essential for passive targeting. The blood vessels are produced in high quantities in tumoral tissues promoting rapid growth that allows nanocarriers to be easily retained in tumor tissues of HNC [257].

Active targeting

Active targeting involves the specificity and designing of the nanocarrier to attach to the target site [255]. All NPs exhibit a good conjugation capability with target ligands, including antibodies, sugars, nucleic acids, peptides, vitamins, and other small molecules. The nanocarrier is conjugated with a molecule with a good binding affinity to attach firmly to the target tissue [258]. Nanoparticles are attached to targeting ligands and given a reasonable degree of tumor specificity. During the tumor diagnosis/treatment, these drugs/ligands specifically bind to the NP’s target and interact with the target cells receptors (which are tumor markers), and endocytosis carries the ligand inside the target cells [259]. In active targeting of HNC, the target cells must have high expressed markers compared to healthy tissues. A flowchart of active and passive targeting is provided in Figure 3.

Figure 3.

Flowchart mechanism of active and passive targeting.

Challenges in active targeting

The critical challenge is selecting a targeting agent to minimize the toxicity to surrounding healthy tissues. Physical/triggered targeting is also a type of active targeting that depends on the usage of internal (pH, enzymes, redox potential) or external (temperature, UV light, ultrasound) stimuli to potentiate the nanocarrier to the site of interest for releasing the drugs molecules [260,261]. In one of its kind, the magnetic nanocarriers are driven to the target site using external stimuli of the magnetic field.

Nanocarriers for drug delivery

In this segment, our attention is directed towards the most auspicious nanocarriers intended for drug delivery in the treatment of head and neck cancer (HNC). These include lipid-based, polymer-based, and metallic-based nanocarriers.

Lipid-based nanocarriers

In cancer-targeted drug delivery systems, lipid-based nanocarriers are commonly phospholipids possessing unique properties and self-organization in an aqueous environment to form organized shapes and structures. Lipids may form liposomes, micelles, or bilayers. Micelles and liposomes are the two most commonly used for drug delivery.

Micelles have a hydrophobic core and hydro-carbonated tails surrounded by polar heads. Micelles are formed by amphipathic molecules containing a polar group and a tail with only one hydrocarbon [262,263]. Concerning HNSCC tumor studies, it has been reported that microRNA-107 is downregulated compared to healthy cells. To deliver the pre-miR-107 successfully, a cationic lipid nanoparticle was developed, consisting of DDAB (dimethyl octadecyl ammonium bromide), cholesterol, and α-Tocopheryl polyethylene glycol 1000 succinate [264].

Liposomes have been used extensively as nanocarriers in cancer therapy. Each liposome consists of a phospholipid bilayer and an aqueous inner cavity, which can encapsulate different types of polar molecules [265,266]. The nano-formulations are in clinical trials; some have been marketed to treat HNC. Liposomes are an ideal system of nanoencapsulation to carry drugs, overcome pharmacokinetics problems, stability in vivo, and toxicity to healthy tissues. One important example is the encapsulation of curcumin in liposomes.

Curcumin is a component of Curcuma longa, possessing the potential property of antibiotic, anti-inflammatory, and antioxidant agent, and it also demonstrated good anticancer activity [267].

Polymer-based nanocarriers

These nanocarriers can be produced from synthetic or natural polymers [268,269], which are biocompatible, stable, possess toxicity, have no side effects, and are entirely metabolized in the human body. In a previous study, multifunctional polymer-based nanoparticles (Linear dendritic mPEG-BMA4) for targeted delivery of saracatinib (kinase inhibitor) into HNC cells in vivo. Compared with free drugs, the polymeric nanoparticles loaded with saracatinib demonstrated good anticancer activity [270]. Chen et al. reported an injectable, biodegradable polymer, cisplatin, for the human HNSCC treatment. This polymer exhibited a well released (80%) of cisplatin and was significantly involved in tumor suppression compared to free cisplatin [271].

Folate-targeted treatment with methotrexate (MTX) is commonly considered in HNSCC; however, its severe side effects are very severe [272]. Dendrimer-targeted delivery can minimize toxicity and enhance drug efficacy. Ward et al. used cell lines with null, intermediate, and high expression folate receptors and evaluated in vivo efficacy of G5 poly-amidoamine dendrimer-based targeted treatment. The targeted system was more effective against cell lines with high folate receptor expression with increased effectiveness compared to free MTX and control [273].

Metallic nano-carrier

Metallic-based nanocarriers were also found effective in HNC therapy. Zhang et al. used superparamagnetic nanoparticles as a novel targeted drug delivery system for HNCs therapy. A biocompatible mesoporous Fe₃O₄ NPs attached with superparamagnetic polyacrylic acid was developed. These mesoporous Fe₃O₄ NPs delivered bleomycin to the tumor tissue, starting its slow release and the tumor cells apoptosis. The drug also showed significantly reduced side effects of bleomycin to healthy cells. This new approach provided potential applications of mesoporous Fe₃O₄ NPs in HNC treatment using simple technologies, fewer side effects, and more efficacy [274].

Applications of quantum dots (QDs) in HNC

The current applications of magnetic resonance imaging (MRI), X-ray, ultrasound, and radionuclide imaging, to detect and diagnose tumors have limitations. These techniques are less sensitive in detecting malignant cells when small in number, unable to detect biomarkers specific to cancer cells’ surface and exhibit hazardous effects to different levels. Thus, in investigating advanced and novel approaches with fewer dangerous effects and high sensitivity, specificity is of prime importance and urgently required.

QDs, also known as “artificial atoms”, is today’s most attractive topic in nanobiology. Several researchers are interested in using QDs in cancer diagnostics [275,276] because; they have excellent resistance to photo-bleaching; secondly, they have good optical properties with superior fluorescence intensity; thirdly, QDs possess multiple fluorescent colors emission under single-source excitation and size-tunable light emission. Furthermore, during the synthesis process, the wavelengths emitted could be tuned and controlled precisely by size and shape. This property is beneficial in performing nanometer resolution and co-localization of multicolor QDs using confocal microscopy. This is also important in reducing the slices of tissue that must be cut for biomarker observation [275-277].

Application of dendrimer nanoparticles in HNC

Dendrimers are synthetic polymers playing an important role in drug discovery and carrier systems [278]. They are “smart” nanocarriers in medicine with multifunctional that can be used in targeted drug delivery of one or more agents selectively to tumor cells with more safety and also to intracellular gene-specific targeting [279,280]. Dendrimers with nano polymeric designs have been considered a highly specific class delivery system for drugs and genes [281,282]. Over the past decade, gene therapy has been used in clinical trials. Although there are some drug and gene delivery concepts, including liposomes, viral vectors, cationic polymers, gold, and magnetic nanoparticles [283], however, dendrimers are the most attractive nowadays for their good safety and specificity to the target site [284]. Dendrimers which are 1-100 nm in size, are globular macromolecules consisting of a central core domain, a hyperbranched mantle domain, and a domain of corona with exterior reactive functional groups [285]. These are perfect (spherical) molecules as nanocarriers with specific properties for cell-specific targeting. Dendrimers may be of different kinds, including melamine, poly(propylene imine) (PPI), poly-amidoamine (PAMAM), poly(ethylene glycol), poly(glycerol-co-succinic acid), poly-l-lysine (PLL), triazine, poly(glycerol), poly[2,2-bis(hydroxymethyl) propionic acid], (PEG), and citric acid-based ones [286,287]. PAMAM and PPI vectors have been extensively examined for medical use [288,289]. Both have amine-terminated end and pH-dependent drug release properties, making them most suitable for HNC treatment. The dendrimer’s ‘back folding’ or collapse on itself is the most attractive property of dendrimers due to the tertiary amine groups deprotonated at elevated pH [288]. The dendrimer scan traverse several barriers using active and passive tumor targeting.

Recently a poly-amidoamine generation 4 (G4) dendrimer and fluorescently labeled for gene delivery and folic acid-decorated conjugates in HNSCC-targeted have been reported [290]. The G4 dendrimer delivery system is conjugated with folic acid (FA) and has the properties of targeting the moiety of HNSCC. In HNSCC cells, complexing this G4 dendrimer with siRNA or plasmid significantly increases the knockdown system’s gene transfection or efficiency. In HNSCC, the G4-FA vector exhibited excellent tumor targeting capability, biocompatibility, sustained retention, and high uptake in a gene therapy approach.

Dendrimers have been synthetically engineered with nanodevices in nanocarrier drug delivery systems. The terminal moieties are responsible for dendrimers’ biological effect and global efficiency. Dendrimers in classical drugs overcome the physicochemical limitations, including solubility, stability, specificity, biodistribution, and therapeutic efficiency, Figure 4. They have the property to reach the right targets by immune clearance, penetration into cells, and interactions in off-target [291]. All the dendrimers have the desired properties of drug retention, release of the therapeutic agent, unaffecting by the immune system, blood circulation time enhancing, and cells or organs specific targeting [292]. An overview is provided in Figure 5.

Figure 4.

Synthesis of dendrimers and drug conjugation.

Figure 5.

A. Dendrimer’s structure, synthesis, and mechanism of drug release. B. Dendrimer.

Different intervention plans in HNC patients

Significant physical and psychological morbidity have been experienced by the HNC patients during radiotherapy (XRT) that not only resulted in the interruption of the treatment, but also quality of life. Intensive radiotherapy (XRT) is carried out in these patients either alone or in combination of other treatments [293]. Different interventions have been applied for cancers prevention and control. However, for HNC survivors the interventions so for, may not address the general health and cancer related needs. A study developed a couple-based intervention called “Spouses coping with the Head and neck Radiation Experience” abbreviated as SHARE, was delivered through phone. This intervention supports psychoeducation that encourages self-management and teaches strategies to improve teamwork and coping. The study evaluated couples on self-management and coordination of care and support at the start of XRT to control/alleviate symptom burden (physical and psychological) and improve both partner’s adjustment. The results of the study supported the feasibility, acceptability, and preliminary efficacy of SHARE [294]. The successful treatment of HNCs can be based on the TNM (tumor, node, metastasis) staging system. A study based on Polish patients found concordance between clinical and pathological T and N stages in patients with HNCs. It was found that there is a moderate agreement between the clinical and pathological stages for stage T, while substantial agreement was found for stage N [295].

Focused on improving quality of life (QOL) and/or mood in HNC patients, a team of researchers reviewed the available literature and targeted the types of interventions such as educational, psychosocial, physical, and psychological symptom management, mindfulness, pharmacologic, exercise, and telemedicine. Preliminary feasibility and acceptability with some positive impacts on QOL and/or mood were found in HNC patients [296]. PRO-ACTIVE trial intervention in HNC patients suggested useful modifications in telehealth [297].

A more recent study investigation the impact of cognitive behavioral intervention on HNC and treatment on eating and talking, and health-related quality of life of survivorship [298]. This study reports that impact treatment can be particularly distressing and markedly changed activities among survivors. Engaging in therapeutic approaches to manage distress in treatment time may influence quality of life and mood of survivorship phase.

Another study applied Navigation for Disparities and Untimely Radiation Therapy, a multilevel intervention to evaluate feasibility, preliminary efficacy, and acceptability in HNC. They found that the potential postoperative radiation therapy has been improved [299].

A previous study also applied the telehealth intervention in HNC treatment. It was observed that the telehealth intervention is also feasible and acceptable for better management of treatment for HNC [300].

Exercise has also been regarded as a potential intervention in prevention of different types of cancer. During the investigation regular exercise was also found to be a promising intervention in HNC patients. The patients were active participants in a six-week supervised exercise intervention in HNC treatment [301].

Conclusion

HNC has been ranked sixth among the most common cancer worldwide, and its occurrence is still increasing in the future. Due to the failure of HNC treatment, there is an urgent need to design innovative techniques for better management of HNC. Using the advanced approaches of NPs, drug concentration may be increased at the target site. There are several studies available who have evaluated HNSCC patients’ tumors or tissues, but the outcomes of the clinical data are not satisfactory. To establish clinically significant CSC markers in the head and neck regions, the primary barrier is that this region is secondary to the convention of amassing malignancies from different upper aerodigestive regions with diverse embryological and biological features. As a result, there is little definitive data about clinical implications of CSCs within HNSCC, the primary exception being prognostic value. Another reason might be due to no single biomarker for CSCs in HNSCC is available. In this context, nanomedicine emerged as an alternative and potential approach to using nanocarriers, which the body’s immune system could not sense. NPs have the potential to improve treatment efficiency without harming normal cells. Moreover, nanocarriers are essential to combat tumor resistance in various targeting strategies. Nanotechnology may shift the management of HNC through theragnostic approaches, simultaneously allowing diagnosis and therapy. QDs in HNC diagnostics may be useful as they have good optical properties and superior fluorescence intensity. Very limited information is available about the applications of dendrimers in HNC therapy. The contribution of dendrimers for targeted drug delivery in HNC may be more useful. However, for better management of HNC, the NPs’ biocompatibility, toxicity, and long-term implications need further trials and understanding before it is applied on a large scale.

Disclosure of conflict of interest

None.