Decoding chemoradiotherapy response in head and neck squamous cell carcinoma

Head and neck squamous cell carcinomas (HNSCC) arise from the mucosal epithelium of the upper aerodigestive tract. While alcohol and tobacco consumption remain the principal risk factors, infection with human papillomavirus (HPV) has emerged as a major etiological factor, predominantly for oropharyngeal squamous cell carcinoma (OPSCC). HPV-positive OPSCC is now recognized as a distinct disease entity with unique molecular features, improved prognosis, and a separate staging system. Despite this, majority of HNSCC are HPV-negative, and approximately 60% of patients present with locoregionally advanced disease, which is associated with poorer outcomes. For patients with advanced-stage HNSCC of the pharynx and larynx, definitive concomitant cisplatin-based chemoradiotherapy (CRT) is the current standard of care, as surgical approaches often result in unacceptable functional morbidity. The administration of cisplatin based chemotherapy at the same time as radiotherapy treatment improves overall and event-free survival, with the standard regimen consisting of high-dose cisplatin (100 mg/m²) every three weeks during radiotherapy. However, HPV-negative disease remains relatively resistant to this approach, and cisplatin-related toxicity frequently prevents patients from completing treatment, adversely affecting outcomes. Consequently, there is a critical need to better understand treatment resistance, toxicity, and alternative therapeutic strategies in this patient population. In Chapter 2, real-world oncological outcomes were analyzed in a large multicenter cohort of patients with advanced-stage HPV-negative HNSCC treated with CRT. This focused approach enabled identification of clinical parameters associated with treatment failure in this more resistant subgroup. The study demonstrated improved survival outcomes compared to historical series, likely reflecting advances in radiotherapy techniques, standardized treatment protocols, and improved patient selection. Nevertheless, outcomes remained suboptimal, and over one-third of patients were unable to complete CRT due to cisplatin toxicity. Multivariable analysis confirmed that achieving a cumulative cisplatin dose of at least 200 mg/m² was strongly associated with treatment success, while substitution with carboplatin in cases of cisplatin intolerance appeared to provide limited benefit. These findings highlight the delicate balance between treatment efficacy and toxicity and underscore the importance of patient support, toxicity prediction, and alternative radiosensitizers. Given the lack of clinically applicable biomarkers to predict response to cisplatin, Chapter 3 explored the theranostic potential of radiolabeled cisplatin ([195mPt]cisplatin; CISSPECT®) to assess drug uptake and retention. Although in vitro differences in cisplatin retention were observed between sensitive and resistant HNSCC cell lines, these differences could not be reliably detected in vivo, likely due to systemic pharmacokinetics and cellular transport mechanisms. As a result, [195mPt]cisplatin was not deemed clinically useful for predicting treatment response, although potential applications in toxicity research or therapeutic radiopharmaceutical development remain. Tumor biology is a key determinant of CRT response, and Chapter 4 evaluated the applicability of RNA-based prognostic gene signatures. Validation of a gene set originally developed in surgically treated oral cavity cancer failed in an HPV-negative OPSCC cohort treated with (chemo)radiotherapy, despite the molecular differences between HPV-negative tumors in the oral cavity and oropharynx being considered minimal. This finding supports the need for treatment-specific biomarkers. In Chapter 5, the clonal relationship between primary tumors and local recurrences after CRT was investigated. Improved analytical methods revealed that a subset of recurrences may be genetically unrelated to the original tumor, raising important questions about tumor heterogeneity, field cancerization, and treatment-driven clonal selection. These findings have implications for prognostic modeling and suggest that some recurrences may need to be approached as biologically distinct tumors. Finally, Chapter 6 identified novel therapeutic opportunities through a genome-wide CRISPR-Cas9 screen, revealing a previously unrecognized role for microtubule-associated genes, particularly CLASP1, in regulating radiosensitivity. Loss of CLASP1 increased radiation-induced cell death by disrupting S-phase processes and mitotic integrity, positioning microtubule dynamics as a promising target for radiosensitization.