Head & Neck Cancer Overview - Signaling Pathway. Diagnostics Marker. Targeted Therapy and Clinical Trials.

An introduction to Head & Neck Cancer

Head and neck cancer is a group of cancer that originates from the oral cavity, throat, nose, sinuses, or salivary glands. Symptoms of cancer may include a sore throat or a lump that lead to swallowing or breathing trouble, unusual bleeding, and facial swelling. The major risk factors of head and neck cancer include the use of alcohol or tobacco, as well as smokeless tobacco. Additionally, infection of human papillomavirus (HPV), especially HPV-16, is associated with the increased risk of oropharyngeal cancer. Head and neck cancer is the seventh most common cancer worldwide, accounting for nearly 4% of all cancer in the United States, most of which are classified as squamous cell cancers and called head and neck squamous cell carcinoma (HNSCC). In 2018, there were 890,000 new cases and 450,000 deaths of the disease worldwide. Notably, patients with head and neck cancer in association with heavy use of tobacco and alcohol are on the decline due to decreased use of tobacco. However, HPV-associated oropharyngeal cancer caused by HPV-16 is rising, especially among younger people in westernized countries. In the United States, the proportion of head and neck cancers diagnosed as HPV-associated oropharyngeal cancers increased from 16.3% in the 1980s to about 72.7% in the 2000s. With the development of radiotherapy, chemoradiotherapy, and target therapy, patients with head and neck cancer especially HPV-associated oropharyngeal cancer is receiving a better prognosis.

Figure 1. Head & Neck Cancer Signaling Pathway

1 Main Signaling Pathways in Head & Neck Cancer Therapy

1.1 EGFR signaling cascade

EGFR is a 170 kDa transmembrane glycoprotein cell surface receptor of the ErbB/HER family, which also includes ErbB2 (HER2/neu), ErbB3 (HER3), and ErbB4 (HER4). Six main ligands include EGF, heparin binding-EGF, TGF-α, amphiregulin, betacellulin, and epiregulin2. The binding of the ligands to EGFR homodimerizes or heterodimerizes with other HER receptors or receptor tyrosine kinases (RTKs) such as MET or IGK-1 receptor. The activated EGFR mainly participates in the following signaling pathways: MAPK, PI3K/Akt/mTOR, JAK/STAT. Overexpression of EGFR together with its ligands is commonly found in head and neck cancer patients, which is associated with decreased survival, radiation resistance, and distant metastasis. Smoking is a common carcinogenic factor of head and neck cancer, it can induce the production of amphiregulin and TGF-α that result in EGFR activation. After that, the expression of COX2 and downstream product PEG2 is up-regulated, which in turn transactivates EGFR and forms a positive feedback loop.

1.2 Wnt/β-catenin signaling cascade

The Wnt/β-catenin signaling pathway is regulated by a complex of proteins that include adenomatous polyposis coli (APC), the scaffold protein Axin1, glycogen synthase 3β (GSK3β), casein kinase 1α (CK1α), and protein phosphatase 2A (PP2A). Activation of the pathway is initiated by the binding of Wnt ligand to the transmembrane Frizzled (Fzd) receptor and its coreceptor, the low-density lipoprotein receptor-related protein (LRP5/6). Then the LRP5/6 is phosphorylated, leading to inactivation of the destruction complex. The β-catenin accumulates in the cytoplasm in the absence of the complex and translocates to the nucleus It displaces repressors from the promoters and interacts with the T-cell factor (TCF) and lymphoid enhancer-binding factor (LEF1) for the expression of downstream genes. Activation of Wnt in head and neck cancer is more frequent than genetic findings, which is caused by the molecular cross-talk between pathways. It has been reported that β-catenin can be activated by the up-regulation of EGFR or PI3K signaling, both of which are the most frequently dysregulated signaling pathways in head and neck cancer.

1.3 PI3K signaling cascade

The PI3K signaling pathway is regulated by receptor tyrosine kinases (RTKs) such as ErbB family receptors, insulin-like growth factor 1 receptors, as well as G protein-coupled receptors. On the activation of RTKs, class I PI3K translocate to the membrane and phosphorylate phosphatidylinositol 4,5-bisphosphate (PIP₂) to phosphatidylinositol 3,4,5-trisphosphate (PIP₃), which in turn act as a second messenger and triggers the downstream signaling pathways, for example, the Akt signaling pathway. The activated Akt could induce the activation of the mammalian target of rapamycin (mTOR) and regulate cellular processes such as proliferation, metabolism, and protein translation. Studies have shown that activation of the PI3K signaling pathway plays an important role not only in the initiation of head and neck cancer but also in the invasion of cancer.

1.4 Hippo/YAP signaling cascade

Hippo/YAP is one of the canonical oncogenic signaling pathways and participates in the regulation of gene expression associated with cell proliferation, survival, and migration. The pathway consists of two main components, one integrates different stimuli while the other regulates the activity of downstream elements. The cytosolic multimeric signaling complex responsible for integration is composed of serine and threonine kinases MST1 and MST2 (mammalian STE20-like protein kinase 1 and 2), and LATS1 and LATS2 (large tumor suppressor kinase 1 and 2), which interact with the adaptor proteins SAV1 (Salvador family WW domain containing protein 1) and MOB1 (MOB kinase activator 1), respectively. When the Hippo pathway is activated, MST1/2 is phosphorylated and activates LATS1/2, which in turn leads to phosphorylation of the transcriptional coactivators YAP. Thus, the YAP is prevented from nuclear translocation and TEAD (TEA domain transcription factor) binding, which finally switch off the expression of target genes.

2 Head and neck cancer diagnosis

Head and neck cancer includes all malignancies that arise in the nasal, oral cavity, pharynx, larynx, and paranasal sinuses, most of which are squamous cell carcinomas. The head and neck squamous cell carcinoma (HNSCC) is reported to be the sixth leading cause of cancer death worldwide. Most of the cancers are caused by tobacco, alcohol, and HPV infection. Though continuous studies were conducted in the field of HNSCC diagnosis and therapy, the mortality remains unchanged, which may be due to the failure of early diagnosis and effectiveness of treatment. Therefore, it is of great significance to diagnose it at an early stage.

2.1 Molecular Markers for Head and Neck Cancer

Diagnosis and intervention of head and neck cancer at an early stage are crucial to the control of cancer progression. Biomarkers could be essential tools for diagnosis of the disease timely. Research has reported a series of biomarkers that can be analyzed in the tissue, plasma, or other body fluids for the diagnosis of head and neck cancer.

HPV, especially the high risk HPV16 and HPV18, is considered as one of the common carcinogenic factor for HNSCC. HPV DNA is detected in 15-25% of HNSCC, 45-67% of tonsil cancer, 13-25% of hypopharyngeal cancer, 12-18% of oral cavity cancer, and 3-7% of carcinoma larynx. HPV16 and HPV18 dysregulate the transformation of cells and alter the control of the cell cycle by producing oncoprotein E6 and E7, which could be diagnostic and prognostic markers in head and neck cancer. Increased gene activation caused by hypermethylation is frequently found in the early stages of HNSCC. The specificity of methylation-specific polymerase chain reaction for the detection of HNSCC was reported to be 90% in salivary samples and 72% in serum samples. Additionally, interleukin (IL)-6 and IL-8 are associated with tumor progression and metastasis, which also participate in the tumorigenesis of head and neck cancer. IL-6 is a potential biomarker in serum samples for the detection of oral cavity or oropharynx squamous cell carcinoma while IL-8 could also be an early biomarker of cancer.

2.2 Protein Markers for Head and Neck Cancer

Recent studies have shown the importance of chemokines and their receptors in head and neck cancers. For example, the expression of CXC chemokine receptor 2 (CXCR2) in tumor tissue is higher than that of the paraneoplastic tissue in squamous cell carcinoma of the larynx. The upregulation of CXC2 is significantly associated with node metastasis and histological grade, which could be a potential prognostic marker for laryngeal squamous cell carcinoma. Similarly, another potential marker CXCR4 is an important factor in tumor progression and metastasis of HNSCC and is found upregulated in nasopharyngeal carcinoma tissues. Cytokeratins (CKs) are major components of the intracellular filament network. Overexpression of CKs is reported to be found in oropharynx squamous cell carcinoma (OSCC), indicating the relationship between cytokeratins overexpression and tumor progression and prognosis. The subtypes of CKs, CK-6, and CK-16 are constitutively expressed in mucosal stratified squamous epithelia, which may be considered as markers of cellular hyperproliferation. However, the results of the markers for clinical cases detection are not uniform. Intensive studies with larger samples regarding the specificity of CK-6 and CK-16 are needed before being used as biomarkers for head and neck cancer.

3 Targeted therapy for head and neck cancer

3.1 Head and neck cancer therapy for EGFR therapy

There are two main types of inhibitors that target EGFR for the treatment of head and neck cancer. The EGFR targeted monoclonal antibodies (mAbs) could block ligand-receptor binding of EGFR by occupying the extracellular domain of EGFR, which in turn abrogates the dimerization of EGFR. The mAb-receptor complex is then internalized and degraded, leading to the downregulation of EGFR overexpression. Common EGFR targeted mAbs cetuximab has been approved for the treatment of head and neck cancer. While other mAbs, such as pertuzumab, panitumumab, and trastuzumab, is still under evaluation. Another EGFR targeted agent, EGFR tyrosine kinase inhibitors (TKIs) could compete with ATP to eliminate the downstream signaling of EGFR. Although TKIs are normally short-acting drugs with much shorter half-life than the mAbs, there are advantages that TKIs have over mAbs, such as oral administration and fewer hypersensitivity reactions. Several TKIs, including lapatinib, afatinib, and dacomitinib have shown promise in clinical trials for the treatment of head and neck cancer.

Table 1 Clinical trials of EGFR mAb Cetuximab

Table 2 Clinical trials of EGFR mAb Panitumumab

Table 3 Clinical trials of EGFR inhibitor Panitumumab

Table 4 Clinical trials of EGFR inhibitor Afatinib

3.2 Head and neck cancer therapy for Wnt therapy

Wnt/β-catenin signaling is attracting more and more attention as a target for HNSCC therapy. Various Wnt inhibitors have been developed and tested in preclinical models or carried out in clinical trials. Inhibitors targeting acyl-transferase Porcupine, a key member of Wnt cascade, such as IWP, LGK974, and ETC-159 have been developed. A recent study has shown the HNSCC growth and metastasis inhibition of LGK974 in the chick chorioallantoic membrane assay. Another small molecule that inhibits the interaction between β-catenin and CBP, ICG001, is proved to suppress HNSCC progression in preclinical models. Additionally, the third generation derivative of ICG001, E7386 is now in phase I trials for the treatment of HNSCC.

Table 5 Clinical trials of Wnt inhibitors

3.3 Head and neck cancer therapy for PI3K therapy

Given that PIK3CA is the most frequently mutated oncogene in human cancers, multiple PI3K pathway targeted inhibitors have been developed. Inhibitors such as buparlisib, BYL-719, or PX-886 are under investigation in combination with chemotherapy or cetuximab for the treatment of HNSCC. Due to the similarities between PI3K and mTORC1, some compounds are developed to inhibit Class I PI3K isoforms, mTORC1, and mTORC2. Everolimus and temsirolimus are mTOR inhibitors being evaluated in patients with HNSCC, which have already been approved for the treatment of multiple cancers. A phase II trial using temsirolimus in combination with low-dose weekly carboplatin and paclitaxel was well tolerated in patients with recurrent and/or metastatic HNSCC.

Table 6 Clinical trials of PI3K inhibitor Buparlisib

3.4 Head and neck cancer therapy for Hippo therapy

Different inhibitors have been developed to directly target YAP/TAZ of the Hippo pathway, as well as their upstream regulators or downstream effectors. The direct inhibitors include verteporfin and peptide called “super-TDU” that can block the interaction between YAP and TEAD. Statins was another category of candidate drugs that can inhibit YAP/TAZ activity in cancer cells. Although few studies evaluating their YAP/TAZ inhibition ability in HNSCC are reported, it is worth mentioning that a study using the oral squamous cell carcinoma (OSCC) model has demonstrated the inhibitory role of simvastatin.

Features

• An Introduction to Head & Neck Cancer
• Main Signaling Pathways in Head & Neck Cancer Therapy
• EGFR signaling cascade
• Wnt/β-catenin signaling cascade
• PI3K signaling cascade
• Hippo/YAP signaling cascade
• Head & Neck Cancer Diagnosis
• Molecular Markers for Head & Neck Cancer
• Protein Markers for Head & Neck Cancer
• Targeted Therapy for Head & Neck Cancer
• Head & Neck Cancer for EGFR pathway
• Head & Neck Cancer for Wnt pathway
• Head & Neck Cancer for PI3K pathway
• Head & Neck Cancer for Hippo pathway