Potential Alternative Therapeutic Modalities for Management Head and Neck Squamous Cell Carcinoma: A Review

Abstract

Head and neck squamous cell carcinoma (HNSCC) includes malignancies of the lip and oral cavity, oropharynx, nasopharynx, larynx, and hypopharynx. It is among the most common malignancy worldwide, affecting nearly 1 million people annually. The traditional treatment options for HNSCC include surgery, radiotherapy, and conventional chemotherapy. However, these treatment options have their specific sequelae, which produce high rates of recurrence and severe treatment-related disabilities. Recent technological advancements have led to tremendous progress in understanding tumor biology, and hence the emergence of several alternative therapeutic modalities for managing cancers (including HNSCC). These treatment options are stem cell targeted therapy, gene therapy, and immunotherapy. Therefore, this review article aims to provide an overview of these alternative treatments of HNSCC.

Introduction

According to the WHO, approximately 900,000 new cases of cancers of the head and neck region, and more than 400,000 deaths are reported worldwide yearly.^1,2 Most of these cancers of the head and neck region are squamous cell carcinoma. Head and neck squamous cell carcinoma (HNSCC) arises from the mucosal epithelium of the oral cavity, pharynx, larynx, and sinuses. ³ It may as well arise from the keratinocytes in the spinous layer of the epidermis of the skin covering the head and neck region.^4,5 Despite the common origin of the HNSCC, there are several sources for its heterogeneity due to varying etiological, risk, and biological factors. As a result, treatment approaches and outcomes differ significantly among the different subsites. ⁶

The primary aim of the treatment of HNSCC is eradicating cancer, preventing recurrence, and restoring the form and function of the affected parts. ⁷ Traditionally, the treatment options for HNSCC include surgery, radiotherapy, and conventional chemotherapy.^8,9 However, these treatment options continue to have their specific sequelae, which produce high rates of recurrence as well as severe treatment-related disabilities (including physical, emotional, functional, and occupational dysfunction) for long-term survivors.^8,10 For example, the excision of tumor tissues in the head and neck region causes damage to adjacent or underlying anatomical structures, which interferes with functions like chewing, swallowing, and breathing. On the other hand, conventional chemotherapy may show poor systemic stability and unwanted drug-related side effects (bone marrow depression and nephrotoxicity). Whereas radiation therapy has long-term side effects on healthy cells like xerostomia, fibrosis, and osteoradionecrosis. ¹⁰

Taking into consideration the challenges of traditional treatment, coupled with the emergence of new models of cancer pathogenesis, some alternative treatment methods for managing cancers, including the HNSCC, have been introduced.^6,8 Some of these modalities include stem cell targeted therapy, gene therapy, and immunotherapy. These new alternative treatment options may be beneficial in improving survival and decreasing treatment-related morbidity caused by HNSCC. ⁸

Tumorigenesis and Cancer Stem Cells of Head and Neck Squamous Cell Carcinoma

It is important to appreciate the tumorigenesis and molecular pathogenesis of head and neck squamous cell carcinoma before understanding how various treatment modalities work against it. Under normal circumstances, TP63 and NOTCH1 control the morphogenesis of squamous cells in an adult’s mucosa. TP63 (expressed in keratinocytes) maintains the proliferative potential of the basal layer by controlling the expression of basal marker keratin 5/14 (K5 and K14). ⁶ The NOTCH1 expression results in the terminal differentiation of cells in the basal layer into cells of the spinous and granular layers. NOTCH signaling mediates cell-to-cell communication by direct transmission from the cell membrane to the nucleus.^6,11 For prevention of abnormal proliferation, the abnormal cell will either be removed from the differentiation-associated cell cycle or will undergo programmed cell death. ⁶

Tumorigenesis of HNSCC involves a progressive accumulation of genetic alterations in genes controlling the cell cycle progression, signaling pathways (for mitosis and differentiation), angiogenesis, and cell death. ¹² The accumulation of these abnormalities in oncogenes, proto-oncogenes, and tumor suppressors can thus lead to the formation of malignancy. ¹³ Some critically altered pathways in HNSCC include those involving tumor suppressors (like SMAD4, TGFBR2, CDKN2A, TP53, TP63, CASP8, IRF6, NOTCH1, and FAT1), oncogenes (CCND1, RAS, EGFR, HRSA, and PIK3CA), and protein involved in tumorigenesis (including Rb, CDK2/4/6, K1, K14, TGF-β, P14, P16, and P21).^6,12-14 Loss of tumor suppressors (TP53, TGFBR2, SMAD4, and CDKN2A) and amplification of oncogene (CCND1) lead to abnormal cell proliferation and limited apoptosis. Furthermore, loss and/or abnormal expression of tumor suppressors (NOTCH1 and TP63) and alterations in “survival” genes (CASP8, PIK3CA, EGFR) lead to the removal of additional barriers to tumor cell proliferation and survival.^6,12,13,15 The most commonly observed abnormalities in HNSCC tissues include dysregulation of the P53 gene and mutation of CDKN2A. ¹⁵ The molecular biology of HNSCC tumorigenesis is summarized in Figure 1.

Figure 1.

Schematic presentation of tumorigenesis of head and neck squamous cell carcinoma.

Cancer Stem Cells

Once the cell becomes neoplastic, a small subpopulation of these cells known as cancer stem cells (CSC) develops a unique ability to asymmetrically divide, capable of self-renewal and differentiation into progenitor cells that produce a heterogeneous population of cancer cells.^16-18 The CSCs are principally in the inactive G₀ phase and thus avoid destruction by chemotherapy and radiotherapy (which often target active cells). ¹⁷ Consequently, CSC and “Stemness” markers are responsible for regulating the tumorigenesis, proliferation, aggressiveness, chemo-radioresistance, recurrence, and metastasis of HNSCC. ¹⁶

Potential Alternative Treatment Options for Head and Neck Squamous Cell Carcinoma

Cancer Stem Cell Target Therapy in HNSCC

Growing evidence supports the idea of cancer stem cells (CSCs) being responsible for tumor survival and recurrence following radiation or cytotoxic agents-induced depopulation.^19,20 Since CSCs show a tendency to resist conventional therapy and remain inactive for long periods, it is hypothesized that the identification of CSCs can be useful in the management of cancer (including the HNSCC) through biological interference targeting this very cancer cell population. ²⁰

The CSCs tend to express several protein markers; therefore, the majority of CSC characterization relies on using combinations of these markers. ¹⁷ Some of the markers used to identify HNSCC CSCs include CD44, CD24, CD133, CD 326, ALDH1, OCT4, SOX2, NANOG, STAT3, Musashi-1, and components of Renin-Angiotensin System (PRR, ATIIR1, and ATIIR2).^16-18 In their study, Koukourakis et al ²⁰ found that CD44 was linked with the better differentiated HNSCC. Moreover, CD44, Oct4, and integrin-b1 were all associated with poor prognosis, while ALDH1 was associated with a favorable prognosis.

By identifying the markers, the HNSCC CSC expressing these molecules can be targeted. Targeted therapies occur when antibodies specifically recognize and bind to certain CSC surface markers or signaling pathways. ²¹ Thus by targeting these proteins, cancer cells' growth, division, and spread are controlled. Amongst the most common and most studied CSC markers in HNSCC are CD44, BMI-1, and ALDH1.^9,20,22,23

CD44⁺

The cluster of differentiation 44 (CD44) is a complex type I transmembrane adhesion glycoprotein involved in intercellular interactions, cell adhesion, and cell migration.^9,24 CD44 is expressed in several cell types in humans, including embryonic stem cells, differentiated cells, and cancer cells. ²⁴ Targeting CD44⁺ CSC is one of the avenues for managing HNSCC considering that it binds growth factors (eg. EGFR) and metalloproteinases (eg. MMP9) thereby inducing angiogenesis (a hallmark of HNSCC progression), reducing apoptosis, and enhancing cancer cell invasion.^22,25

Nanbu et  al ²⁵ combined SN-38, an active metabolite produced from irinotecan hydrochloride—a type I DNA topoisomerase inhibitor and gefitinib (an EGFR tyrosine kinase inhibitor) to inhibit HNSCC cell growth. In in vivo xenograft mouse model, they found no significant difference in tumor growth inhibition between treatment (combined gefitinib and SN-38) and control (gefitinib) groups. However, continuing tumor measurements post-chemotherapy revealed unlike in the treatment group, tumor growth resumed in the control group. They found that a combination of SN-38 and gefitinib inhibit tumor metastasis and recurrence through enhancement of the lysosomal degradation of CD44.

There are limited studies that target CD44⁺ HNSCC CSC, but some authors have reported on the effect of targeting CD44⁺ CSC in other malignancies including ovarian epithelial cancer, ²⁶ leukemia, ²⁷ lung cancer, ²¹ and breast cancer. ²⁸ It might be worth it to assess the effect of medications used in targeting CSC of other malignancies against CSC of HNSCC. Girda et al ²⁶ studied the efficacy of SPL-108 (an 8 amino acid peptide that binds to CD44 and modulates its actions) on ovarian epithelial cancer. In their study, SPL-108 which has antiangiogenic, anti-migratory, and anti-invasive properties against cancer cells was shown to affect the ABCB1 expression, a known downstream target of CD44. The results of the study depicted that of the 14 patients enrolled, four (29%) had a progression of the disease.

Vey et al ²⁷ investigated the effect of drug RG7356, a recombinant immunoglobulin G1 humanized anti-CD44 monoclonal antibodies that specifically bind to the standard region of CD44 near the HA binding domain. In vitro, the RG7356 mediates disruption of the tumor microenvironment and triggers the release of specific chemo-attractants that recruit and activate macrophages, causing phagocytosis of RG7356-opsonized tumor cells. ²⁷ However, the authors concluded that the limited clinical activity noted in their study does not support the use of RG7356 as monotherapy in patients with acute myeloid leukemia.

In another study, Xu et al ²⁸ investigated the effect of pyrvinium pamoate on CD44⁺ CSC in breast cancer. The results of the flow cytometric assay depicted that after 3-day treatment, pyrvinium pamoate markedly reduced the CD44⁺ population in different breast cancer cell lines.

Shitara et al ²⁹ conducted a study targeting CD44⁺ cancer stem cells with sulfasalazine in 11 patients with advanced gastric cancer. They concluded that an 8 g/day dose of sulfasalazine induced a decrease in the proportion of CD44⁺ positive cells.

ALDH1 (aldehyde dehydrogenase 1)

Aldehyde dehydrogenase 1 (ALDH1) is an intracellular enzyme responsible for the maintenance and differentiation of stem cells. ²³ The enzyme also plays part in epithelial–mesenchymal transition and enhances resistance to radiotherapy and chemotherapy.^9,30 High ALDH1 expression has been found to cause advanced tumor stage, increased size, and metastasis. ²³ Therefore, targeting ALDH 1 is among the therapeutic options for HNSCC.

To target ALDH 1 positive HNSCC, Kim et al ³¹ used a novel small-molecule ALDH inhibitor (Aldi-6), a 3-(Dimethylamino)-4ʹ bromopropiophenone. They injected HNSCC cells subcutaneously injected into the flanks of immunodeficient NOD-scid IL2Rgamma mice. Subsequently, these mice were treated with Aldi-6 (24 mg/kg/day). They found that Aldi-6 alone reduced the final tumor volume by 60% compared to the control group. A combination of Aldi-6 and cisplatin reduced the final tumor volume by 75% compared to the tumors treated with cisplatin alone.

Yao et al ³² investigated the role of ALDH inhibitor disulfiram (DSF) in enhancing the sensitivity of HNSCC to chemotherapy. Their findings indicated that DSF enhanced the radio-chemo sensitivity of HNSCC by inducing apoptosis. As a potential radio-chemosensitizer, the authors suggested assessing disulfiram’s therapeutic potential in a clinical setting.

In recent years, the inhibition of ALDH 1 in ovarian cancer was studied by Nwani et al. ³³ They used a small-molecule, CM37, with inhibitory activity against ALDH1A1. The results of the study portrayed that by inhibiting ALDH1A1, CM37 amplified intracellular reactive oxygen species buildup, which consequently caused DNA damage and reduced ovarian cancer cell viability.

BMI-1

The B lymphoma Mo-MLV insertion region 1 homolog (BMI-1) is often overexpressed in human malignancies as it is critical for stem cell homeostasis and tumorigenesis.^34,35 It is responsible for mediating epigenetic transcriptional silencing through the modification of chromatin structure. ³⁵ It is essential for epithelial–mesenchymal transition during tumor development in head and neck cancer patients as well as protection of tumor cells from apoptosis. ³⁴ Inhibition of BMI-1 may lead to suppression of the CSC.

Herzog and colleagues ³⁶ studied the effect of Tocilizumab, a humanized monoclonal anti-IL-6R antibody that inhibits BMI-1 function. They used UM-SCC-22B-naive and UM-SCC-22BCis6-resistant cells in biodegradable scaffolds embedded in mice. Weekly treatment with tocilizumab (± cisplatin) slowed down tumor growth in both UM-SCC-22B and UM-SCC-22BCis6. Expression of both STAT3 and BMI-1 was inhibited in UM-SCC-22B tumors after treatment with tocilizumab.

Some authors have investigated the role of QW24, a novel synthetic small-molecule compound, in inhibiting BMI-1 in colorectal carcinoma. ³⁷ They demonstrated that in vivo, QW24 exerts potent anti-tumor activity by down-regulating BMI-1 without obvious toxicity.

To study various inhibitors of BMI-1, Wang et al ³⁸ assessed the effect of PTC-209 on nude mice transplanted with HNSCC cells. In vivo, they found PTC-209 to inhibit BMI-1. Similarly, in PTC-209-treated samples, the cancer cell proliferation was compromised (demonstrated by weaker Ki-67 staining). PTC-209 treatment also downregulated other CSC markers and modulators including CD44, CD133, and Sox2.

OCT4

Octamer-binding transcription factor 4 (OCT4) is among the key transcription factors involved in the regulation of self-renewal and maintenance of pluripotent embryonic stem cells.^39,40 Some studies indicate OCT4 is expressed in CSCs, especially in the early stages of tumorigenesis. ⁴¹ Thus, it is one of the areas scientists have been targeting in seeking a cure for cancers.

Chen and colleagues ⁴⁰ assessed the effectiveness of the OCT4 vaccine as an anti-tumor using BALB/c mice vaccinated with the combination vaccine (OCT4-3 + TLR9). Their findings revealed that in the OCT4-3 + TLR9 vaccine group, the tumors grew slowly and shrunk significantly. Moreover, they immunized the BABL/c mice and challenged them with cancer cells to investigate the prophylactic efficacy of OCT4-3 + TLR9. They found that 90 days post-tumor implantation, 20% of mice exhibited no tumors in the OCT4 + TLR9 group. It may be worth conducting clinical trials of such a vaccine in head and neck squamous cell carcinoma as well.

STAT3

Signal transducer and activator of transcription 3 (STAT3) is an oncogene that promotes cell proliferation and resistance to apoptosis, angiogenesis, invasion, and migration. ¹⁷ Since it is well established that cellular transformation and tumor progression are due to chronic activation/phosphorylation of STAT3, ⁴² Sulaiman et al ⁴³ used this context to examine the anti-cancer effect of PCT-209 in the modulation of the STAT3 pathway. They demonstrated that PTC-209 inhibited the STAT3 signaling pathway, thereby inhibiting cellular proliferation and induction of cellular death.

In another study, Jia et al ⁴² explored the role of dihydroartemisinin (DHA) in inhibiting STAT3 in HNSCC. The team studied tumor-bearing BALB/c mice that were injected intraperitoneally with DHA at a dosage of 50 mg/kg once daily, 5 times per week, for 4 weeks. They found DHA inhibited phosphorylation, activation, and proliferation of STAT3 in HNSCC cells.

Sen and her team ⁴⁴ evaluated the efficacy of AZD1480, an ATP-competitive small-molecule inhibitor of JAK2 kinase, in HNSCC both in vitro and in vivo. They reported that in vitro, AZD1480 decreased STAT3 phosphorylation, thus HNSCC cell proliferation. Whereas in vivo, it significantly slowed tumor growth in heterotopic xenograft tumor models.

SOX2

Sry-related high-mobility box 2 (SOX2), a critical transcription factor, has a significant role in various phases of embryonic development together with the maintenance of undifferentiated embryonic stem cells. ⁴⁵ Dysregulation of SOX2 expression is thus a contributing factor in cancer pathogenesis. ⁴⁶ Although SOX2 is crucial in maintaining the CSCs' pluripotency and differentiation and plays a critical role in cancer initiation, progression, and development of drug resistance, ⁴⁷ directly targeting it has been proven to be difficult.^45,47 Therefore, targeting signal molecules upstream or downstream of SOX2 is an alternative approach. Since SOX2 expression is promoted by the Epidermal Growth Factor (EGFR)-mediated signaling pathway, inhibition of EGFR can downregulate SOX2. Singh et al ⁴⁸ found that EGFR inhibitors, gefitinib, and erlotinib reduced the levels of SOX2 by blocking the EGFR/SRC/AKT signaling, eventually suppressing the self-renewal properties of cancer stem cells.

Yin and his team ⁴⁹ studied the effect of MLN4924/pevonedistat, a small molecule that inhibits neddylation (a process by which the ubiquitin-like protein NEDD8 is conjugated to its target proteins). They found that MLN4924 downregulates SOX2 expression in a dose-dependent manner by inducing the accumulation of MSX2, a known transcription repressor of SOX2.

Gene Therapy in HNSCC

Gene therapy entails the replacement of the defective gene with a therapeutic gene in the cells of a patient to alter or repair an acquired genetic abnormality. ⁵⁰ This therapeutical modality has found its way into the management of cancers including HNSCC. Gene therapy for managing cancers involves several strategies including the addition of a tumor-suppressor gene, deletion of a defective tumor gene, and downregulation of the expression of genes that stimulate tumor growth.^51,52 Other modalities are the enhancement of immune surveillance, activation of prodrugs that have a chemotherapeutic effect (“suicide" gene therapy), the introduction of viruses that destroy tumor cells, and the introduction of genes to inhibit tumor angiogenesis.^50,51

For gene therapy to be carried out, it is crucial to identify the genes responsible for transforming a normal cell into a malignant cell. Agarwal et al ⁵³ analyzed the sequence genes in HNSCC. They found somatic mutations in TP53, NOTCH1, CDKN2A, PIK3CA, FBXW7, and HRAS genes. The inactivating mutations arose due to nonsense, insertions or deletions (“indels”), splice-site mutations, and missense. On performing whole-exome sequencing of head and neck squamous cell carcinoma in patients, Patel et al ⁵⁴ and Lawrence et al ⁵⁵ identified frequently mutated genes TP53, NOTCH1, CASP8, RYR2, LRP2, PIK3CA, CDKN2A, and ATM. In another study designed to discover HNSCC genetic mutations by deep exon sequencing, it was observed that the common gene mutations included TP53, NOTCH1, and PIK3CA. Others were CDH1, CEBPA, and FES. ⁵⁶ Ju et al ⁵⁷ studied the genomic abnormalities underlying HNSCC in the Chinese population. Results of the study depicted that the highly mutated genes were TP53, FAT1, KMT2D, CDKN2A, LRP1B, NOTCH1, and CHD4. In a study involving HNSCC patients of South Asian origin, mutations were noted in various genes including TP53, PIK3CA, FGFR2, ARID2, MLL3, MYC, and ALK. Novel recurrent mutations in ASNS (asparagine synthetase) were also found in this population. ⁵⁸

In vivo, the therapeutic gene is delivered to the target cells by direct insertion of genetic material into the affected site. ⁵⁰ The vector for transferring therapeutic genes can either be viral or non-viral.^50,59 Viral vectors include retrovirus, adenovirus, herpes simplex virus, and adeno-associated virus^50,59 (Figure 2). Non-viral vectors can be either chemicals (oligonucleotides, lipoplexes, polyplexes, dendrimers, and nanoparticles like gold) ⁵⁰ or physical (electroporation, bio-ballistics, sonoporation, magnetoporation, and high-pressure hydrodynamics).^50,59

Figure 2.

Schematic presentation of gene therapy using a viral vector. 1: The viral vector incorporated with therapeutic gene. 2: Viral vector entering the cancerous cell. 3: Viral vector encased in the vesicle. 4: Disruption of the vesicle and release of the virion. 5: Release of modified viral DNA into the nucleus of the tumor cell. 6: Modified DNA (Therapeutic gene) incorporated in mutated gene to facilitate either gene downregulation or apoptosis.

TP53 gene therapy

TP53 is the gene that codes for the p53 tumor protein. It is a nuclear transcription factor that transactivates numerous target genes, which have an unprecedented role in cell cycle regulatory pathways by inducing cell cycle arrest and/or apoptosis.^60,61 Under normal circumstances, p53 is expressed at an extremely low level unless external events are causing impending stress that destabilizes the genome to become the source of tumourigenesis.^60,61 Once DNA is damaged, TP53 allows cells to repair damaged DNA by mediating cell cycle arrest, thus referred to as the “Guardian of the genome.” ⁶⁰

Dysfunction in the TP53 tumor-suppressor gene (located at 17p13) is implicated in many cancers, including HNSCC, ⁶² and is the most frequently mutated gene in HNSCC, existing in 60–70% of patients.^63,64 Therapeutic strategies targeting p53 in HNSCC can be grouped into those encompassing wild-type p53, mutated p53, and HPV-positive HNSCC.^65,66 Compounds such as PM2, RITA, Nutlin-3, and CH1iB either degrade or directly inhibit WT p53, and achieve p53 reactivation by affecting p53 inhibitors such as MDM2 and MDMX/4.^65,66 Some compounds (PRIMA-1, MIRA-1, RETRA, APR-246, COTI-2, CP-31398) target mutated p53 by binding it and restoring the wild-type p53 conformation and transcriptional activity.^65,66 Bortezomib and Ad-E6/E7-As target the viral enzymes E6/E7 which are responsible for the breakdown of p53 and in the cases of HPV + HNSCC cells. ⁶⁶

Arya et al ⁶⁷ investigated the role of Nutlin-3 in radiosensitizing laryngeal squamous cell carcinoma (LSCC). Their findings pointed out that Nutlin-3 caused significant radiosensitization in LSCC cells harboring wild-type p53, increased cell cycle arrest, and significantly increased senescence in the absence of increased apoptosis.

In one study, Roh and colleagues ⁶⁸ tested the effect of PRIMA-1, CP-31398, RITA, and Nutlin-3 on four human HNSCC cell lines differing in TP53 status. They observed a noticeable p53 reactivation in mutant TP53-bearing tumor cell lines treated with PRIMA-1 or CP-31398 and in wild-type TP53-bearing cell lines treated with Nutlin-3. Nutlin-3 further expressed significant growth suppression in tumor cells (indicating MDM2-dependent p53 degradation). Chuang et al ⁶⁹ found that RITA exerts significant effects on HNSCC cells by inducing a conformational change in p53, causing activation of its downstream targets. In a phase II trial of intratumoral administration of ONYX-015 (oncolytic adenoviruses targeting p53), in patients with refractory HNSCC, it was found that 10% complete response, 62% stable disease, and 29% progressive disease rates. ⁷⁰

NOTCH1 gene therapy

NOTCH signaling has a significant complex, context-dependent manner role in the accomplishment of differentiation, proliferation, and self-renewal in disease development.^71,72 NOTCH1 is reported to have oncogenic functions and promote cancer growth, and in HNSCC, there is significantly higher NOTCH1 expression than in normal tissue. ⁷³ NOTCH1 gene mutations are present in about 10% to 50% of HNSCCs. ⁷¹

To inhibit NOTCH1 in ovarian cancer, Zhao and colleagues ⁷⁴ utilized a novel cationic cholesterol derivative-based liposome for efficient delivery of siRNA into human SKOV3 ovarian cancer cells to inhibit NOTCH1 gene expression. They found cationic cholesterol derivatives to be efficient non-viral siRNA carriers for the suppression of NOTCH1 activation in ovarian cancer cells. In another study, Ye et al ⁷⁵ used small interfering RNA (siRNA) to successfully inhibit NOTCH1 signaling in prostate cancer cells. Though no study looked at the role of inhibiting NOTCH1 in HNSCC, the possibility of targeting it should not be underlooked on considering genetic therapy for HNSCC.

Targeting PI3K/AKT signaling pathway

Phosphoinositide kinases (PIKs) are a group of lipid kinases that are significant upstream activators of numerous signaling pathways that drive oncogenesis, and mutations of the PIK3CA gene are associated with poor prognosis of solid malignancies. ⁷⁶ The PIK3CA mutation is one of the most common mutations in HNSCC affecting both HPV-positive and negative diseases.^53,56,58,77 Normally, the PI3K signaling pathway regulates normal cell proliferation, survival, and apoptosis. ⁷⁸ PI3K signaling pathways AKT and PTEN are associated with the development and progression of HNSCC. ⁷⁹ Dunn et al ⁸⁰ demonstrated 250 mg/day BYL719/Alpelisib (an α-specific PI3K inhibitor) in combination with cetuximab and intensity-modulated radiation therapy in patients with locally advanced HNSCC to be effective. Smolensky and colleagues ⁸¹ assessed the efficacy of PI3K inhibitor LY294002 (LY) on oral squamous cell carcinoma (OSCC). They noted that inhibition of PI3K/AKT with LY further decreased cell proliferation and increased apoptosis in OSCC cells that were treated with doxorubicin or AD198. In their study, Liu et al ⁸² demonstrated that a novel tumor-suppressor protein histidine phosphatase (LHPP) inhibited the progression of OSCC through the PI3K/AKT signaling pathway, and promotes the apoptosis of OSCC by decreasing the transcriptional activity of p-PI3K and p-Akt.

Immunotherapy in HNSCC

It is theorized that the (pre)cancerous cells can be identified and destroyed by the body’s immune system since they tend to express antigens that can be targeted by immune cells.^83,84 Thus, taking this concept into consideration, cancer immunotherapy is an area of interest in the management of HNSCC. ⁸⁴

Under normal circumstances, cancer cells adapt the immune surveillance by either generation of inhibitory immune cells or the release of suppressive cytokines and mediators that allow immune escape.^83,85 Acquisition of traits that allow cancer cells to evade immune surveillance and an effective immune response consequently leads to tumor progression. ⁸⁶ Compared to normal tissues, HNSCC tissues are in an immunosuppressive state (cold tumor) due to fewer lymphocytes, decreased natural killer (NK) cells function, and poor specific antigens presentation.^14,87 Moreover, HNSCC has a relatively high tumor mutation burden (TMB); hence, they deploy the body’s immune system thereby promoting the production of immunosuppressive mediators. ¹⁴

The immunosuppressive nature of HNSCC gives a justification for using immunotherapy in its management. ¹⁴ HNSCC immunomodulatory drugs target immune-suppressive pathways that are involved in the interplay between tumor cells and T lymphocytes. ⁸⁴ Among the immune checkpoints involved in cancers (including HNSCC) are cytotoxic T lymphocyte antigen 4 (CTLA-4), programmed cell death protein-1/programmed death-ligand 1 (PD-1/PD-L1), and T-cell immunoglobulin mucin-3 (TIM-3)^87-89. Others include lymphocyte activated gene-3 (LAG-3), T-cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif (TIGIT), glucocorticoid-induced TNFR family-related gene (GITR), and V-domain Ig suppressor of T-cell activation (VISTA).^87,88

Immune checkpoint inhibitors (ICIs) can revive the T cells mediated immunity and activate the body’s immune system to eliminate cancer cells. ⁹⁰ Of recent times, several clinical trials are being undertaken for the use of immunotherapy against HNSCC. The significant immune checkpoints currently being targeted for cancer immunotherapy are CTLA-4, PD-L1, and PD-1.^84,90,91

PD-1, a member of the CD28 receptor family, is expressed on activated T- and B-cells, natural killer (NK) cells, and macrophages.^84,88 PD-L1 and PD-L2 are PD-1 ligands expressed on endothelial and epithelial antigen-presenting cells and activated lymphocytes. ⁸⁷ In the HNSCC tissues, PD-L1 is produced via an abnormal PD-1 signaling pathway and is responsible for tumor immunosuppression. ⁸⁴ On the other hand, CTLA-4 is a CD28 homolog expressed by both CD4⁺ and CD8⁺ T cells. It binds to CD80/CD86 (B7 ligands) which mediate opposing functions in T-cell activation ⁹² (Figure 3).

Figure 3.

A: Cancer cell’s immune checkpoint inhibitors (eg. PD-)1 bind to T-lymphocyte’s receptors (eg. PD-L1, PD-L2) thus preventing the T-cell from killing the cancer cell. B: The antibodies bind to the T-lymphocyte receptors/ligands and the immune checkpoint of cancer cells allowing the T-cell to kill the cancerous cell.

Most current immunotherapy against HNSCC clinical trials (from stage I to III) rely on antibodies that act as immune checkpoint inhibitors (ICIs) against PD-1, PD-L1, and CTLA-4. Some examples of such are atezolizumab, durvalumab, avelumab, and cemiplimab (PD-L1 antibodies), ipilimumab and tremelimumab (CTLA-4 antibodies), and PD-1 antibodies (nivolumab and pembrolizumab).^83,90,91

In the majority of clinical trials, investigators explore the efficacy of combining ICIs with other treatment strategies. ⁸⁵ Areas of exploration include ICIs combination with other ICIs, TLR9 agonists, cetuximab (the anti-EGFR antibody), lenvatinib (anti-VEGF multiple kinase inhibitor), Epacadostat (IDO1 inhibitor), oncolytic viruses (eg. T-vec), vorinostat (HDAC inhibitor), and radiation therapy.^83-85,87,90

In a randomized control trial study involving 361 patients with recurrent HNSCC whose disease had progressed within 6 months after platinum-based chemotherapy, Ferris et al ⁹³ used nivolumab, a PD-1 monoclonal antibody, to assess its efficacy. They found that compared to standard therapy, nivolumab prolonged survival, and was associated with fewer toxic effects of grade 3 or 4.

Schoenfeld and colleagues ⁹⁴ designed a randomized phase 2 clinical trial involving 29 patients with untreated squamous cell carcinoma of the oral cavity (≥T2, or clinically node-positive). The intervention was nivolumab, 3 mg/kg, weeks 1 and 3, or nivolumab and ipilimumab (ipilimumab, 1 mg/kg, given week 1 only), and patients underwent surgery 3 to 7 days following cycle 2. The results pointed out that there was evidence of response in both the N and N + I arms (volumetric response 50%, 53%; pathologic downstaging 53%, 69%; RECIST response 13%, 38%; and pathologic response 54%, 73%, respectively). Four patients had major/complete pathologic response greater than 90% (N, n = 1; N + I, n = 3). With 14.2 months median follow-up, 1-year progression-free survival was 85% and overall survival was 89%.

Ferrarotto and colleagues ⁹⁵ compared the impact of neoadjuvant durvalumab (PD-L1 inhibitor) with durvalumab plus tremelimumab (CTLA-4 inhibitor) on CD8 + TIL density, safety, and efficacy in 28 patients with oropharyngeal carcinoma. They concluded that neoadjuvant durvalumab with or without tremelimumab was safe to administer to oropharyngeal carcinoma patients. Compared to single-agent durvalumab, combination treatment did not lead to a greater increase in CD8 + TIL density or better responses.

In recent years, Uppaluri et al ⁹⁶ designed a study to determine whether pembrolizumab would be safe, result in pathologic tumor response (pTR), and lower the relapse rate in patients with resectable human papillomavirus (HPV) -unrelated HNSCC. They enrolled 36 patients and after the results, they found that neoadjuvant pembrolizumab administered to patients with resectable locally advanced, HPV-unrelated HNSCC was safe, and resulted in pTR in 44% of patients and a one-year relapse rate lower than historical in patients with high-risk pathology. High-depth, multisector genome sequencing of surgical specimens documented tumor clonal loss after neoadjuvant pembrolizumab.

Sun et al ⁹⁷ studied safety and efficacy of pembrolizumab with radiotherapy and/or chemoradiotherapy. The phase II PembroRad trial compared pembrolizumab-RT vs SoC cetuximab-RT in 133 patients with unresectable stage III-IVa-b LA HNSCC unfit for high-dose cisplatin. In both arms, patients received intensity-modulated radiation therapy (IMRT) at 69.9 Gy in 33 fractions and the compliance to RT was not different (86% and 88%, respectively). Preliminary safety results indicated that pembrolizumab-RT was more tolerable, with mucositis and radiation field dermatitis ≥ G3 in 57% of patients with cetuximab vs 24% with pembrolizumab (P = .004) and 49% vs 17%, respectively (P = .003).

Challenges of Alternative Treatment

Despite a better scientific understanding of the molecular biology of tumorigenesis and a vast array of treatment trials for HNSCC, many challenges are roadblocks in the successful implementation of alternative therapeutic modalities for HNSCC. Generally, these challenges include the selection of appropriate patients for clinical trials, lack of reliable predictive biomarkers for anticipating response rates, technological challenges in producing antibodies, genetic instability of stem cells, the pharmacokinetic behavior of therapeutic molecules, and delivery of a cancer-therapeutic gene to diseased tissue at efficacious doses.^83,98-100 Other challenges include the longer duration some of the modalities (e.g immunotherapy) take to achieve a clinical response, ethical, policy, and financial issues, lack of skilled human resources, and the mindset of the society (especially in developing nations) that predominantly believe in traditional therapeutic methodology which are currently practiced.^87,98,99

Conclusion

Stem cell targeted therapy, gene therapy, and immunotherapy are promising approaches for treating HNSCC. Though we might still be in the early stages of understanding the potential of these alternative therapies, rapid developments in understanding the various aspects of HNSCC cells continues to deepen. The amalgamation of knowledge about tumor biology and findings of several clinical trials will enable the scientific committee to develop guidelines that will dictate the comprehensive treatment of HNSCC.