Adaptive c-Met-PLXDC2 Signaling Axis Mediates Cancer Stem Cell Plasticity to Confer Radioresistance-associated Aggressiveness in Head and Neck Cancer.

Radiotherapy plays an essential role in the treatment of head and neck squamous cell carcinoma (HNSCC), yet radioresistance remains a major barrier to therapeutic efficacy. A better understanding of the predominant pathways determining radiotherapy response could help develop mechanism-informed therapies to improve cancer management. Here we report that radioresistant HNSCC cells exhibit increased tumor aggressiveness. Using unbiased proteome profiler antibody arrays, we identify that upregulation of c-Met phosphorylation is one of the critical mechanisms for radioresistance in HNSCC cells. We further uncover that radioresistance-associated HNSCC aggressiveness is effectively exacerbated by c-Met but is suppressed by its genetic knockdown and pharmacologic inactivation. Mechanistically, the resulting upregulation of c-Met promotes elevated expression of plexin domain containing 2 (PLXDC2) through activating ERK1/2-ELK1 signaling, which in turn modulates cancer cell plasticity by epithelial-mesenchymal transition (EMT) induction and enrichment of the cancer stem cell (CSC) subpopulation, leading to resistance of HNSCC cells to radiotherapy. Depletion of PLXDC2 overcomes c-Met-mediated radioresistance through reversing the EMT progress and blunting the self-renewal capacity of CSCs. Therapeutically, the addition of SU11274, a selective and potent c-Met inhibitor, to radiation induces tumor shrinkage and limits tumor metastasis to lymph nodes in an orthotopic mouse model. Collectively, these significant findings not only demonstrate a novel mechanism underpinning radioresistance-associated aggressiveness but also provide a possible therapeutic strategy to target radioresistance in patients with HNSCC. Significance: This work provides novel insights into c-Met-PLXDC2 signaling in radioresistance-associated aggressiveness and suggests a new mechanism-informed therapeutic strategy to overcome failure of radiotherapy in patients with HNSCC.

Tipifarnib enhances anti-EGFR activity of cetuximab in non-HRas mutated head and neck squamous cell carcinoma cancer (HNSCC).

OBJECTIVE: To test the potential ability of tipifarnib to impair proliferation and to enhance the activity of the EGFR inhibitor cetuximab in wild-type H-Ras HNSCC, which accounts for the majority of HNSCC. MATERIALS AND METHODS: Cell growth, apoptosis and signaling changes in HNSCC cells following tipifarnib exposure in vitro were assessed by SRB, colony formation assay, annexin V staining and Western blot, respectively. A patient-derived xenograft (PDX) animal model was adopted to evaluate the efficacy of tipifarnib in vivo with and without cetuximab. RESULTS: Treatment of wild-type H-Ras HNSCC cell lines in vitro with tipifarnib reduced cell growth and increased levels of defarnesylated H-Ras in a dose-dependent manner. In a PDX mouse model, treatment with single-agent tipifarnib led to only near-significant growth inhibition. The addition of cetuximab resulted in increased anti-proliferative effect both in culture and in PDX models, which was also mirrored by Western blot and apoptosis assay results. CONCLUSION: Tipifarnib has only a moderate ability to slow tumor growth as a single agent in HNSCC with wild type H-Ras, despite specifically inhibiting the farnesyltransferase upon which the function of H-Ras depends. The combination of cetuximab and tipifarnib appears to enhance the anti-proliferative effect of single-agent tipifarnib and marginally enhance that of single agent cetuximab. These findings deserve further evaluation.

Functional HPV-specific PD-1+ stem-like CD8 T cells in head and neck cancer.

T cells are important in tumour immunity but a better understanding is needed of the differentiation of antigen-specific T cells in human cancer1,2. Here we studied CD8 T cells in patients with human papillomavirus (HPV)-positive head and neck cancer and identified several epitopes derived from HPV E2, E5 and E6 proteins that allowed us to analyse virus-specific CD8 T cells using major histocompatibility complex (MHC) class I tetramers. HPV-specific CD8 T cells expressed PD-1 and were detectable in the tumour at levels that ranged from 0.1% to 10% of tumour-infiltrating CD8 T lymphocytes (TILs) for a given epitope. Single-cell RNA-sequencing analyses of tetramer-sorted HPV-specific PD-1+ CD8 TILs revealed three transcriptionally distinct subsets. One subset expressed TCF7 and other genes associated with PD-1+ stem-like CD8 T cells that are critical for maintaining T cell responses in conditions of antigen persistence. The second subset expressed more effector molecules, representing a transitory cell population, and the third subset was characterized by a terminally differentiated gene signature. T cell receptor clonotypes were shared between the three subsets and pseudotime analysis suggested a hypothetical differentiation trajectory from stem-like to transitory to terminally differentiated cells. More notably, HPV-specific PD-1+TCF-1+ stem-like TILs proliferated and differentiated into more effector-like cells after in vitro stimulation with the cognate HPV peptide, whereas the more terminally differentiated cells did not proliferate. The presence of functional HPV-specific PD-1+TCF-1+CD45RO+ stem-like CD8 T cells with proliferative capacity shows that the cellular machinery to respond to PD-1 blockade exists in HPV-positive head and neck cancer, supporting the further investigation of PD-1 targeted therapies in this malignancy. Furthermore, HPV therapeutic vaccination efforts have focused on E6 and E7 proteins; our results suggest that E2 and E5 should also be considered for inclusion as vaccine antigens to elicit tumour-reactive CD8 T cell responses of maximal breadth.

The Proteomic Landscape of Growth Factor Signaling Networks Associated with FAT1 Mutations in Head and Neck Cancers.

FAT1 is frequently mutated in head and neck squamous cell carcinoma (HNSCC), but the biological and clinical effects of FAT1 mutations in HNSCC remain to be fully elucidated. We investigated the landscape of altered protein and gene expression associated with FAT1 mutations and clinical outcomes of patients with HNSCC. FAT1 mutation was stratified with clinical information from The Cancer Genome Atlas HNSCC databases with more than 200 proteins or phosphorylated sites. FAT1 mutation was significantly more prevalent among HPV(-), female, and older patients and was enriched in oral, larynx, and hypopharynx primary tumors. FAT1 mutation was also significantly associated with lower FAT1 gene expression and increased protein expression of HER3_pY1289, IRS1, and CAVEOLIN1. From an independent International Cancer Genome Consortium dataset, FAT1 mutation in oral cancer co-occurred with top mutated genes TP53 and CASP8. Poorer overall survival or progression-free survival was observed in patients with FAT1 mutation or altered HER3_pY1289, IRS1, or CAVEOLIN1. Pathway analysis revealed dominant ERBB/neuregulin pathways linked to FAT1 mutations in HNSCC, and protein signature panels uncovered the heterogeneity of patient subgroups. Decreased pEGFR, pHER2, and pERK and upregulated pHER3 and HER3 proteins were observed in two FAT1 knockout HNSCC cell lines, supporting that FAT1 alterations lead to altered EGFR/ERBB signaling. In squamous cancers of the lung and cervix, a strong association of FAT1 and EGFR gene expressions was identified. Collectively, these results suggest that alteration of FAT1 appears to involve mostly HPV(-) HNSCC and may contribute to resistance to EGFR-targeted therapy. SIGNIFICANCE: Integrative bioinformatics and statistical analyses reveal a panel of genes and proteins associated with FAT1 mutation in HNSCC, providing important insights into prospective clinical investigations with targeted therapies.

Co-expression of fibroblast growth factor receptor 3 with mutant p53, and its association with worse outcome in oropharyngeal squamous cell carcinoma.

Fibroblast growth factor receptor 3 (FGFR3) is expressed in squamous cell carcinoma of the head and neck (SCCHN) including oropharyngeal squamous cell carcinoma (OPSCC) and is a potential therapeutic target. However, information on its correlation with other relevant cancer related proteins stratified by p16 status and its prognostic significance in OPSCC is limited. We examined FGFR3 expression and its correlation with clinical characteristics, p16 status, and mutant p53 (mp53) among 220 retrospectively collected OPSCC cases and 40 prospectively collected SCCHN cases, including a majority of OPSCC. Correlations of FGFR3 Weighted Index (WI) with p16 status and mp53 WI as well as its association with disease-free survival (DFS) and overall survival (OS) were evaluated. FGFR3 expression was detected in 61% and 70% of cases in cohorts 1 and 2, respectively. FGFR3 level was significantly higher in p16-negative tumors in both cohorts (p<0.001 and 0.006). FGFR3 expression was highly correlated with mp53 expression in both p16 + and p16- OPSCC (p<0.0001 and p = 0.0006, respectively). In cohort 1, univariate analysis showed that FGFR3 was associated with DFS but not OS. Kaplan-Meier analysis showed that higher FGFR3 and mp53 level correlated with worse DFS (p = 0.025) and OS (p = 0.009). As expected, p16 positive status was associated with improved OS and DFS (p<0.001 for both). Our results suggest that high FGFR3 expression is associated with p16 negative status and mp53 expression in OPSCC and correlates with a worse clinical outcome. The biological relationship between FGFR3 and mp53 in OPSCC deserves further investigation.

Defining HPV-specific B cell responses in patients with head and neck cancer.

Tumours often contain B cells and plasma cells but the antigen specificity of these intratumoral B cells is not well understood1-8. Here we show that human papillomavirus (HPV)-specific B cell responses are detectable in samples from patients with HPV-positive head and neck cancers, with active production of HPV-specific IgG antibodies in situ. HPV-specific antibody secreting cells (ASCs) were present in the tumour microenvironment, with minimal bystander recruitment of influenza-specific cells, suggesting a localized and antigen-specific ASC response. HPV-specific ASC responses correlated with titres of plasma IgG and were directed against the HPV proteins E2, E6 and E7, with the most dominant response against E2. Using intratumoral B cells and plasma cells, we generated several HPV-specific human monoclonal antibodies, which exhibited a high degree of somatic hypermutation, consistent with chronic antigen exposure. Single-cell RNA sequencing analyses detected activated B cells, germinal centre B cells and ASCs within the tumour microenvironment. Compared with the tumour parenchyma, B cells and ASCs were preferentially localized in the tumour stroma, with well-formed clusters of activated B cells indicating ongoing germinal centre reactions. Overall, we show that antigen-specific activated and germinal centre B cells as well as plasma cells can be found in the tumour microenvironment. Our findings provide a better understanding of humoral immune responses in human cancer and suggest that tumour-infiltrating B cells could be harnessed for the development of therapeutic agents.

Combinatorial approaches targeting the EGFR family and c-Met in SCCHN.

OBJECTIVE: We aimed to develop novel combinations of inhibitors targeting EGFR family members and c-Met for the treatment of recurrent SCCHN. MATERIALS AND METHODS: Three different c-Met inhibitors in combination with a pan-HER inhibitor (crizotinib/afatinib, tivantinib/afatinib and cabozantinib/afatinib) were investigated for their anti-tumor effects on SCCHN cell lines in vitro. In vivo activity of the combinations was tested in SCCHN cell line xenografts and patient-derived xenograft (PDX) animal models generated from patients with recurrent SCCHN. RESULTS: Western blot assay indicated that activation of EGFR, HER2, HER3, and c-Met was blocked by all three combinations and the downstream PI3K/AKT and ERK signaling pathways were inhibited. Sulforhodamine B colorimetric assay revealed SCCHN cell growth was more effectively inhibited by the combinations than by single agents, particularly in cell lines with high c-Met expression. Furthermore, the combinations were more potent in inducing apoptosis than each of the single agents. In the PDX models, the combination treatments exhibited significantly better efficacy in tumor growth inhibition compared to the respective single agents. CONCLUSION: In conclusion, we demonstrated that the simultaneous targeting of EGFR, HER2, and c-Met is more effective than the individual inhibition of these targets in vitro and in SCCHN cell line xenograft and PDX models. Our findings pave the way for further clinical investigation of such combinations in SCCHN.

Phase Ib Study of Chemoprevention with Green Tea Polyphenon E and Erlotinib in Patients with Advanced Premalignant Lesions (APL) of the Head and Neck.

PURPOSE: On the basis of synergistic effects between green tea polyphenon E (PPE) and EGFR-tyrosine kinase inhibitor in preclinical studies, we conducted a phase Ib study of the PPE and erlotinib combination in patients with advanced premalignant lesions (APL) of the oral cavity and larynx. PATIENTS AND METHODS: Patients were treated with a fixed dose of PPE (200 mg three times a day) and dose escalation of erlotinib (50, 75, 100 mg daily) for 6 months with tissue biopsy at baseline and 6 months. Primary endpoints were safety and toxicity; secondary endpoints were evaluation of pathologic response, cancer-free survival (CFS), overall survival (OS), and biomarker modulation. RESULTS: Among 21 enrolled patients, 19 began treatment and 17 completed 6 months of treatment with PPE and erlotinib. Main characteristics of treated patients: 15 severe dysplasia or carcinoma in situ and 17 oral cavity. Only skin rash was associated with dose-limiting toxicity and MTD. Recommended doses for phase II studies are PPE 600 mg daily plus erlotinib 100 mg daily for 6 months. Pathologic responses in 17 evaluable patients: pathologic complete response (47%) and pathologic partial response (18%). The 5-year CFS and OS were 66.3% and 93%, respectively. Among tested biomarkers, only phosphorylated ERK was correlated with response to treatment. CONCLUSIONS: Treatment with PPE and erlotinib combination was well tolerated in patients with APLs of the head and neck, and showed a high rate of pathologic response with excellent CFS. This combination deserves further investigation for the chemoprevention and/or prevention of second primary tumors in early-stage head and neck cancer.

A Historic Perspective and Overview of H-Ras Structure, Oncogenicity, and Targeting.

H-Ras is a unique isoform of the Ras GTPase family, one of the most prominently mutated oncogene families across the cancer landscape. Relative to other isoforms, though, mutations of H-Ras account for the smallest proportion of mutant Ras cancers. Yet, in recent years, there have been renewed efforts to study this isoform, especially as certain H-Ras-driven cancers, like those of the head and neck, have become more prominent. Important advances have therefore been made not only in the understanding of H-Ras structural biology but also in approaches designed to inhibit and impair its signaling activity. In this review, we outline historic and present initiatives to elucidate the mechanisms of H-Ras-dependent tumorigenesis as well as highlight ongoing developments in the quest to target this critical oncogene.

Tumor Membrane Vesicle Vaccine Augments the Efficacy of Anti-PD1 Antibody in Immune Checkpoint Inhibitor-Resistant Squamous Cell Carcinoma Models of Head and Neck Cancer.

Immune checkpoint inhibitor (ICI) immunotherapy improved the survival of head and neck squamous cell carcinoma (HNSCC) patients. However, more than 80% of the patients are still resistant to this therapy. To test whether the efficacy of ICI therapy can be improved by vaccine-induced immunity, we investigated the efficacy of a tumor membrane-based vaccine immunotherapy in murine models of HNSCC. The tumors, grown subcutaneously, are used to prepare tumor membrane vesicles (TMVs). TMVs are then incorporated with glycolipid-anchored immunostimulatory molecules GPI-B7-1 and GPI-IL-12 by protein transfer to generate the TMV vaccine. This TMV vaccine inhibited tumor growth and improved the survival of mice challenged with SCCVII tumor cells. The tumor-free mice survived for several months, remained tumor-free, and were protected following a secondary tumor cell challenge, suggesting that the TMV vaccine induced an anti-tumor immune memory response. However, no synergy with anti-PD1 mAb was observed in this model. In contrast, the TMV vaccine was effective in inhibiting MOC1 and MOC2 murine oral cancer models and synergized with anti-PD1 mAb in extending the survival of tumor-bearing mice. These observations suggest that tumor tissue based TMV vaccines can be harnessed to develop an effective personalized immunotherapy for HNSCC that can enhance the efficacy of immune checkpoint inhibitors.

Hsp90B enhances MAST1-mediated cisplatin resistance by protecting MAST1 from proteosomal degradation.

Microtubule-associated serine/threonine kinase 1 (MAST1) is a central driver of cisplatin resistance in human cancers. However, the molecular mechanism regulating MAST1 levels in cisplatin-resistant tumors is unknown. Through a proteomics screen, we identified the heat shock protein 90 B (hsp90B) chaperone as a direct MAST1 binding partner essential for its stabilization. Targeting hsp90B sensitized cancer cells to cisplatin predominantly through MAST1 destabilization. Mechanistically, interaction of hsp90B with MAST1 blocked ubiquitination of MAST1 at lysines 317 and 545 by the E3 ubiquitin ligase CHIP and prevented proteasomal degradation. The hsp90B-MAST1-CHIP signaling axis and its relationship with cisplatin response were clinically validated in cancer patients. Furthermore, combined treatment with a hsp90 inhibitor and the MAST1 inhibitor lestaurtinib further abrogated MAST1 activity and consequently enhanced cisplatin-induced tumor growth arrest in a patient-derived xenograft model. Our study not only uncovers the regulatory mechanism of MAST1 in tumors but also suggests a promising combinatorial therapy to overcome cisplatin resistance in human cancers.

mTORC2 Suppresses GSK3-Dependent Snail Degradation to Positively Regulate Cancer Cell Invasion and Metastasis.

mTOR complex 1 (mTORC1) positively regulates cell invasion and metastasis by enhancing translation of Snail. A connection between mTOR complex 2 (mTORC2) and cell invasion and metastasis has also been suggested, yet the underlying biology or mechanism is largely unknown and thus is the focus of this study. Inhibition of mTOR with both mTOR inhibitors and knockdown of key components of mTORC, including rictor, Sin1, and raptor, decreased Snail protein levels. Inhibition of mTOR enhanced the rate of Snail degradation, which could be rescued by inhibition of the proteasome. Critically, inhibition of mTORC2 (by knocking down rictor) but not mTORC1 (by knocking down raptor) enhanced Snail degradation. Therefore, only mTORC2 inhibition induces Snail proteasomal degradation, resulting in eventual Snail reduction. Interestingly, inhibition of GSK3 but not SCF/ß-TrCP rescued the Snail reduction induced by mTOR inhibitors, suggesting GSK3-dependent, but SCF/ß-TrCP-independent proteasomal degradation of Snail. Accordingly, mTOR inhibitors elevated E-cadherin levels and suppressed cancer cell migration and invasion in vitro and metastasis in vivo. Collectively, this study reveals that mTORC2 positively regulates Snail stability to control cell invasion and metastasis. SIGNIFICANCE: These findings delineate a new regulation mechanism of Snail, an important master regulator of epithelial-mesenchymal transition and invasion in cancers.

A continuous-flow acoustofluidic cytometer for single-cell mechanotyping.

The biophysical properties of cells such as their compressibility have been found to be closely related to disease progression such as cancer development and metastasis. As cancer cells are heterogeneous, rapid and high-throughput evaluation of cell biophysical properties at single-cell resolution is needed to assess their potential as biomarkers for cancer staging and prognosis. Acoustofluidics has shown promise as a contactless method for accurately measuring cell biophysical properties; however, previously reported methods had relatively low throughput due to their requirement of no-flow conditions. This work presents a high-throughput continuous flow-based acoustofluidic cell mechanotyping method at single-cell resolution that retains the advantage of simplicity and low-cost.

Interleukin-6/STAT3 Signaling is Prominent and Associated with Reduced Overall Survival in p16 Negative Oropharyngeal Squamous Cell Carcinoma.

This study addresses the hypothesis that IL-6/STAT3 signaling is of clinical relevance in oropharyngeal squamous cell carcinoma (OPSCC). We evaluated relationships between key components of this pathway in tumors from a unique cohort of n = 59 fully annotated, treatment-naïve patients with OPSCC. The multiplex Opal platform was utilized for immunofluorescence (IF) analysis of tissues to detect IL-6 and phosphorylated STAT3 (pSTAT3), taking into consideration its nuclear versus cytoplasmic localization. Abundant staining for both IL-6 and pSTAT3 was evident in tumor-rich regions of each specimen. IL-6 correlated with cytoplasmic pSTAT3 but not nuclear or total pSTAT3 in this cohort of OPSCC tumors, regardless of p16 status (r = 0.682, p < 0.0001). There was a significant association between increased total pSTAT3, nuclear pSTAT3, cytoplasmic pSTAT3 and IL-6 in p16 negative tumors. Our data indicate STAT3 phosphorylation was a key feature in p16-negative OPSCC tumors. When IL-6 data was stratified by median expression in tumors, there was no association with overall survival. In contrast, both total and nuclear pSTAT3 were significant predictors of poor overall and disease free survival. This strong inverse relationship with overall survival was present in p16 negative tumors for both total and nuclear pSTAT3, but not in p16 positive OPSCC tumors. Together these data indicate that activation of the STAT3 signaling pathway is a marker of p16 negative tumors and relevant to OPSCC prognosis and a potential target for treatment of this more aggressive OPSCC sub-population.

Prognostic implications of peritumoral vasculature in head and neck cancer.

BACKGROUND: There is conflicting evidence regarding the role of peritumoral lymphatic vessel density (LVD) and blood microvessel density (MVD) in the metastasis and prognosis of head and neck squamous cell carcinoma (HNSCC). Existing studies are limited to one or two head and neck subsites and/or small sample sizes. A larger study incorporating multiple sub-sites is needed to address the role of peritumoral LVD and MVD in HNSCC metastasis and prognosis. METHODS: Tissue samples from 200 HNSCC cases were stained simultaneously using immunohistochemistry (IHC) for markers of peritumoral LVD (lymphatic vessel marker D240) and MVD (blood vessel marker CD31). Of the stained slides, 166 and 167 were evaluable for LVD and MVD, respectively. The results were then correlated with clinicopathologic features and patient outcomes. RESULTS: Patients with metastatic disease were more likely to have high peritumoral MVD. Through multivariable analyses, MVD was not significantly related to DFS and OS, while low LVD was related to higher risk of disease progression and poor survival. CONCLUSIONS: Peritumoral MVD was found to be positively associated with metastasis, while LVD was found to be inversely related to both metastasis and progression of HNSCC. These findings may suggest a prognostic role of both peritumoral LVD and MVD in patients with HNSCC.

A Correlative Analysis of PD-L1, PD-1, PD-L2, EGFR, HER2, and HER3 Expression in Oropharyngeal Squamous Cell Carcinoma.

We explored potential associations of the PD-1/PD-L1/PD-L2 pathway with clinical characteristics, outcome, and expression of EGFR, HER2, HER3 in oropharyngeal squamous cell carcinoma (OPSCC) using an institutional database. Protein expression was assessed by IHC on tissue microarray sections (EGFR, HER2, HER3) or whole tissue sections (PD-1/PD-L1/PD-L2). Expression of EGFR, HER2, HER3, PD-L1, and PD-L2 was quantified on tumor cells. Maximum density of PD-1 positive lymphocytes was measured on a scale of 0 to 4 within the tumor mass and peritumoral stroma. Associations between biomarkers and patient outcomes were tested using descriptive and inferential statistics, logistic regression, and Cox proportional hazards models. We analyzed tissue samples from 97 OPSCC cases: median age 59 years, p16+ (71%), male (83.5%), never smokers (18%), stage 3 to 4 disease (77%). Twenty-five percent of cases were PD-L1 positive. The proportion of PD-L1+ tumors was higher in p16+ (29%) than p16- OPSCC (11%, P = 0.047). There was no correlation between PD-L1, PD-L2, PD-1, EGFR, HER2, or HER3 expression. Positive PD-L1 status correlated with advanced nodal disease on multivariate analysis (OR 5.53; 95% CI, 1.06-28.77; P = 0.042). Negative PD-L2 expression was associated with worse survival (HR 3.99; 95% CI, 1.37-11.58; P = 0.011) in p16- OPSCC. Lower density of PD-1 positive lymphocytes in peritumoral stroma was associated with significantly increased risk of death on multivariate analysis (HR 3.17; 95% CI, 1.03-9.78; P = 0.045) after controlling for prognostic factors such as stage and p16 status. PD-L1 expression on tumor cells correlates with p16 status and advanced nodal status in OPSCC. PD-1 positive lymphocytes in peritumoral stroma serve as an independent prognostic factor for overall survival. Mol Cancer Ther; 17(3); 710-6. ©2018 AACR.

Visualization of the Cellular Uptake and Trafficking of DNA Origami Nanostructures in Cancer Cells.

DNA origami is a promising molecular delivery system for a variety of therapeutic applications including cancer therapy, given its capability to fabricate homogeneous nanostructures whose physicochemical properties (size, shape, surface chemistry) can be precisely tailored. However, the correlation between DNA-origami design and internalization efficiency in different cancer cell lines remains elusive. We investigated the cellular uptake of four DNA-origami nanostructures (DONs) with programmed sizes and shapes in multiple human cancer cell lines. The cellular uptake efficiency of DONs was influenced by size, shape, and cell line. Scavenger receptors were responsible for the internalization of DONs into cancer cells. We observed distinct stages of the internalization process of a gold nanoparticle (AuNP)-tagged rod-shape DON, using high-resolution transmission electron microscopy. This study provides detailed understanding of cellular uptake and intracellular trafficking of DONs in cancer cells, and offers new insights for future optimization of DON-based drug delivery systems for cancer treatment.