Advancing Education, Research, and Quality of Care for the Head and Neck oncology patient.
Introduction: The standard of care for head and neck squamous cell carcinoma (HNSCC) has remained largely unchanged over the past few decades. Advances in understanding HNSCC tumor biology have led to the development of investigational therapies, including antagonists of inhibitors of apoptosis proteins (IAPs) like the oral drug xevinapant (Debio-1143). While xevinapant initially showed promise, its clinical utility remains uncertain following the discontinuation of the Phase III randomized TrilynX study, which evaluated its use with chemoradiotherapy for locally advanced HNSCC. Investigations into additional mechanisms of this drug may contribute to the understanding of its clinical utility. Interestingly, in a phase II trial of xevinapant, one patient withdrew due to the drug's bitterness. Bitter taste receptors (T2Rs), while primarily known for their role in taste perception on the tongue, are also expressed in various tissues throughout the body and in HNSCC tumors. In HNSCC cells, activation of T2Rs, including the isoform T2R14, by bitter agonists can trigger intracellular calcium release, mitochondrial depolarization, proteasome inhibition, and apoptosis. High expression of the T2R14 gene in HNSCC tumors was shown to be associated with HPV-positive tumors, further suggesting a role for this receptor as a therapeutic target for certain tumors. We hypothesize that xevinapant may act as a bitter agonist of T2R14, causing an intracellular calcium response and inducing apoptosis in HNSCC cells through T2R-mediated mechanisms independent of its known effects on IAPs.
Methods: BitterPredict, a structure-based tool for predicting bitterness, was used to assess the likelihood of xevinapant being a T2R agonist. In vitro responses to xevinapant were assessed in UM-SCC47 cells using live cell imaging for calcium (Fluo-4) and cell viability (crystal violet assay). T2R14 specificity was tested using the antagonists LF1 and LF22.
Results: BitterPredict indicated that xevinapant was likely bitter with high confidence. UM-SCC47 cells showed significant intracellular calcium responses to 1 mM xevinapant. Of note, these responses were reduced with LF22 and completely inhibited by LF1, suggesting T2R14 involvement. Xevinapant treatment over 24 hours decreased cell viability in a dose-dependent manner, as measured by crystal violet assay.
Conclusions: These results demonstrate that Xevinapant, an IAP inhibitor, activates T2R14-mediated pathways, leading to intracellular calcium release in HNSCC cells in vitro. Prior work in HNSCC cells has shown that activation of T2R14 by bitter agonists leads to apoptosis. Future directions will determine if the observed decreased cell viability in UM-SCC47 cells is mediated through this T2R14-mediated response caused by xevinapant by performing cell viability or apoptosis assays with T2R14 antagonists or knockdown models. This activation of T2R14 may be independent of, or synergistic with, xevinapant's antagonistic effects on IAPs. Given that T2R14 is expressed both intracellularly and extracellularly, new therapeutic strategies for xevinapant could include topical or injectable formulations targeting T2R pathways. Additionally, T2R-mediated off-target effects of xevinapant should be considered in evaluating drug toxicity, as T2Rs are expressed in various normal tissues outside of the oral cavity.