Exploring novel therapeutic targets in small bowel adenocarcinoma: insights from claudin 18.2, nectin-4, and HER3 expression analysis.

Background: Small bowel adenocarcinoma (SBA) is a rare malignancy with few established chemotherapy options and a dismal prognosis. We investigated the expression of claudin 18.2, nectin-4, human epidermal growth factor receptor 3 (HER3), and programmed death-ligand 1 (PD-L1) in SBA to identify potential antibody drug targets and analyzed associated clinicopathological features and prognosis.

Materials and methods: We retrospectively reviewed patients diagnosed with SBA who underwent adjuvant or palliative chemotherapy at our hospital between July 2010 and July 2023. Pathological samples were immunohistochemically stained for claudin 18.2, nectin-4, HER3, and PD-L1. Overall survival (OS) was assessed in patients receiving palliative chemotherapy to examine its association with the expression of each protein, excluding those with microsatellite instability-high who were treated with immunotherapy.

Results: Pathological samples and clinical data were available for 51 patients. The primary lesion was in the duodenum in 49% of these patients and in the jejunum or ileum in 51%. Positive rates for claudin 18.2, nectin-4, and HER3 were 35%, 82%, and 88%, respectively. All cases expressed at least one of the proteins, and 25% expressed all three proteins. The PD-L1 combined positive score (CPS) was <1, 1-5, and ≥5 in 33%, 32%, and 35%, respectively; nectin-4-positive samples showed higher CPS. Neither claudin 18.2 nor HER3 positivity was associated with OS. However, nectin-4 positivity was associated with significantly shorter OS [12.6 versus 43.2 months, hazard ratio (HR) 5.12, P = 0.006]. Similarly, PD-L1 CPS ≥5 was associated with shorter OS relative to CPS <5 (9.7 versus 18.0 months, HR 2.60, P = 0.028). Multivariate analysis identified nectin-4 positivity (HR 4.55, P = 0.020) as an independent adverse prognostic factor for OS.

Conclusions: Claudin 18.2, nectin-4, and HER3 are potential therapeutic targets in SBA, and nectin-4 positivity is independently associated with an unfavorable prognosis. These proteins may represent new therapeutic targets for SBA.