AB104. SOH26AB_0390. HDAC8 inhibition in oesophageal adenocarcinoma: translational insights into radiosensitisation, T-cell cytotoxicity, and ex vivo therapeutic response

Background: Oesophageal adenocarcinoma (OAC) remains associated with poor survival despite multimodal therapy. Histone deacetylase inhibitors (HDACi) may enhance radiosensitivity and tumour immunogenicity, but their relevance in OAC and interaction with immune checkpoint inhibition (ICI) are incompletely defined. This study aimed to address these deficiencies.

Methods: Radiosensitive (OE33P) and radioresistant (OE33R) OAC cell lines were exposed to the HDAC8-selective inhibitor PCI-34051, 1.8 Gy radiation, and pembrolizumab in two peripheral blood mononuclear cell (PBMC) co-culture models, assessing direct effects of HDACi/ICI on T-cell cytotoxicity and HDACi effects on tumour radiosensitivity/immunogenicity. Cell viability was quantified by the cell counting kit-8 (CCK-8) assay. Ex vivo, treatment-naïve OAC tumour biopsies (n=4) underwent pre-treatment with PCI-34051 or mock treated prior to treatment with 1.8 Gy radiation and pembrolizumab. Flow cytometry assessed tumour and T-cell viability, damage-associated molecular pattern (DAMP) expression (calreticulin and HMGB1), and T-cell phenotype (CD27 and CD62L).

Results: Radiation alone significantly reduced OE33P, but not OE33R, viability. PCI-34051 pre-treatment markedly enhanced radiosensitivity in both lines (OE33P: 17.0%±0.7%; OE33R: 42.4%±2.8% viable; P<0.0001), with additive cytotoxicity following PBMC co-culture. Pre-treatment of PBMCs with PCI-34051 and/or pembrolizumab did not significantly augment T-cell-mediated killing. In ex-vivo biopsies, addition of PCI-34051 to radiation and pembrolizumab did not significantly reduce tumour or T-cell viability, but modest, non-significant increases in calreticulin and HMGB1 expression were observed, without clear changes in CD27/CD62L.

Conclusions: HDAC8 inhibition robustly radiosensitises radiosensitive and radioresistant OAC models and cooperates additively with cytotoxic lymphocytes in vitro, while ex vivo data suggest preserved intra-tumoural T-cell viability and a trend towards increased immunogenicity. These findings support further evaluation of HDACi in the neoadjuvant treatment of OAC.

Keywords: Immunotherapy; oesophageal cancer; immune checkpoint inhibition (ICI); oncology; histone deacetylase inhibitors (HDACi)