The Role of Hedgehog Signalling Pathway Inhibitors in Squamous Cell Carcinoma of Head and Neck

Introduction

Squamous cell carcinoma of the head and neck (HNSCC), encompassing cancers of the oral cavity, oropharynx, larynx, and salivary glands, represents a significant global health burden.¹ Despite advancements in surgery, radiation therapy, and chemotherapy, the prognosis for patients with advanced HNSCC remains poor. This underscores the need for novel, targeted therapies to improve treatment outcomes.²

The Hedgehog (Hh) signalling pathway is a highly conserved developmental pathway crucial for cell growth, differentiation, and pattern formation. In adults, the pathway is generally quiescent but can be reactivated in certain circumstances, including tissue repair and tumorigenesis.^2,3 Aberrant activation of the Hh pathway has been associated with the progression of various cancers, including basal cell carcinoma (BCC) of skin, medulloblastoma, and, increasingly, HNSCC.⁴ This review focused on the role of the Hh signalling pathway in HNSCC and the potential of Hh pathway inhibitors as a therapeutic strategy.

The Hedgehog Signalling Pathway: A Brief Overview

The Hh signalling pathway is initiated by the binding of a Hh ligand (Sonic Hedgehog - SHH, Indian Hedgehog - IHH, or Desert Hedgehog - DHH) to the Patched (PTCH1) receptor. With the absence of Hh ligand, PTCH1 inhibits the activity of Smoothened (SMO), a 7-transmembrane protein.^2,5,6 Binding of Hh ligand to PTCH1 relieves this inhibition, allowing SMO activation.⁷ Activated SMO triggers a signalling cascade that ultimately activates GLI transcription factors (GLI1, GLI2, and GLI3). These GLI factors then translocate to the nucleus and regulate the expression of target genes associated with cell proliferation, survival, and differentiation.⁸

In HNSCC, the Hh signalling pathway may be activated through different mechanisms, including:

• Ligand-dependent activation: Autocrineor para-crine signalling where tumour cells themselves produce Hh ligands, leading to pathway activation.^7,8

• Ligand-independent activation: Mutations in PTCH1 or SMO, resulting in combined activation of the pathway, even in the absence of Hh ligand.⁸

• Other mechanisms: Overexpression of GLI tran-scription factors or alterations in downstream components of the pathway (Figure 1).⁹

Role of the Hedgehog signalling Pathway in HNSCC

Emerging evidence reveals that the Hh  signalling pathway has an extremely important role in the initiation, progression, and metastasis of HNSCC. Hh  signalling promotes the proliferation of tumour cells and protects them from apoptosis. Hh pathway activation can contribute to the maintenance and regulation of stem cells of cancer (CSCs), which are believed to be the culprits for tumour initiation, recurrence, and resistance to the therapeutic modalities.^2,6,10-13 Hh signalling can promote angiogenesis, the formation of new blood vessels, which is essential for tumour growth and metastasis.¹³ The Hh pathway is believed to induce epithelial-mesenchymal transition (EMT), a process in which epithelial cells lose their cell-cell adhesion and acquire a more migratory and invasive phenotype.¹⁴ Hh pathway activation has been linked with resistance to chemotherapy and radiation therapy in HNSCC (Figure 2).

Hedgehog Pathway Inhibitors: Mechanisms of Action and Clinical Development

Several drugs that target the Hh signalling pathway have been developed, primarily targeting the SMO protein. These are broadly classified as SMO inhibitors. Two SMO inhibitors, Vismodegib and Sonidegib, are approved by the Food and Drug Administration (FDA) for the management of basal cell carcinoma. However, their efficacy in other cancers, including HNSCC, has been less promising.¹⁵ Vismodegib (Erivedge) binds to and inhibits SMO, thereby preventing activation of downstream signalling. Sonidegib (Odomzo) also acts through the same mechanism of SMO inhibition.¹⁶

Several clinical trials have estimated the efficacy and potency of SMO inhibitors in HNSCC, both as monotherapy and in combination with other therapies. The results of these trials have been mixed, with some showing modest activity in a subset of patients, while others have shown no significant benefit.^16,17

Clinical Trial Data on Hedgehog Inhibitors in HNSCC

Several clinical trials have investigated the role of Hh signalling inhibitors in HNSCC, often with the combination of standard therapies.

• Single-Agent-Studies: Early phase trials demon­strated modest clinical activity of Vismodegib and Sonidegib as monotherapy in advanced HNSCC. Responses were generally transient, and several pa­tients developed resistance.¹⁸

• Combination Therapy Studies: Clinical trials combining SMO inhibitors with chemotherapy (e.g., platinum-based regimens), radiation therapy, or Epidermal Growth Factor Receptor (EGFR) inhibitors (e.g., cetuximab) have shown some promise, but improvements in overall survival remain limited. Key challenges include increased toxicity and development of resistance.^19,20

• Biomarker-Driven Trials: Attempts to identify specific biomarkers that predict response to Hh signalling inhibitors in HNSCC have been ongoing. Although some studies have proved that increased expression of GLI1 or other Hh target genes may correlate with treatment sensitivity, definitive predictive biomarkers have not yet been identified.²⁰

Challenges and Limitations of Hedgehog Inhibitor Therapy in HNSCC

Several factors contribute to the limited success of Hh signalling inhibitors in HNSCC.

• Mechanisms of Resistance: Resistance to SMO inhibitors is a major hurdle. Various pathways of resistance have been identified, including.²⁰

a) Mutations in SMO: Mutations in the SMO binding pocket can prevent the drug from binding effectively.²¹

b) Amplification of GLI genes: Increased expression of GLI transcription factors can bypass the need for SMO activation.²²

c) Activation of alternative signalling pathways: Cancer cells can activate other signalling pathways, such as the PI3K/AKT or MAPK pathways, to compensate for the inhibition of the Hh signalling pathway.^20,22

• Tumour Heterogeneity: HNSCC is a highly hetero-geneous disease, with different subtypes exhibiting varying degrees of activation of Hh signalling pathway. Only a small group of tumours may be truly dependent on Hh signalling for their growth and survival.^17,21,23,24

• Off-Target Effects and Toxicity: SMO inhibitors can cause various side effects, including muscle cramps, alopecia, dysgeusia (altered taste), and fatigue, which can limit their tolerability.²⁵

• Limited Penetration of Hh Inhibitors: The tumour microenvironment in HNSCC can be dense and poorly vascularized, which may limit the penetration of Hh inhibitors into tumour cells.²⁶

Future Directions and Potential Strategies

To improve the efficacy of Hh signalling inhibitors in HNSCC, several strategies are being explored.

• Development of Novel Hh Inhibitors: New inhibitors targeting other components of the Hh signalling pathway, including GLI inhibitors, are being developed to overcome SMO inhibitor resistance.^26,27

• Combination Therapies: Combining Hh inhibitors with other targeted therapies, including EGFR inhibitors or immunotherapies, may enhance their efficacy.²⁸

• Personalized Approach: Identifying biomarkers that predict response to Hh inhibitors will allow for a more personalized approach to treatment.²⁹

• Targeting the Tumour Microenvironment: Combin­ing Hh inhibitors with drugs that disrupt the tumour microenvironment may improve drug delivery and efficacy.³⁰

• Overcoming Resistance Mechanisms: Developing strategies to prevent or overcome acquired resistance to the inhibitors of SMO is crucial. This includes agents that target the downstream GLI transcription factors, or agents that target other related signalling pathways.^31,32

• Investigating the role of non-canonical Hh signalling: Emerging research suggests that Hh ligands can activate signalling pathways independent of SMO and GLI. Future studies need to clarify the role of non-canonical Hh signalling in HNSCC and its potential contribution to resistance.³³

Conclusion

The Hh signalling pathway plays a complex role in the initiation and progression of HNSCC. While SMO inhibitors have shown some promise in preclinical and early clinical trials, their clinical efficacy as monotherapy has been limited by resistance mechanisms, tumour heterogeneity, and toxicity. Future research should focus on developing novel Hh inhibitors, identifying predictive biomarkers, and exploring combination therapies to overcome these limitations and improve outcomes for patients with HNSCC. A deeper understanding of the intricacies of Hh signalling in HNSCC, particularly non-canonical signalling, is essential for designing more effective targeted therapies. The future of Hh inhibitor therapy in HNSCC lies in personalized medicine, where patients are selected based on the biological characteristics of the tumour and treated with the most appropriate combination of therapies.