Abstract 6546: Peritoneal microenvironment promotes appendiceal adenocarcinoma tumor formation in PDX models
Authors: Pattalachinti Vinay K. , Ito Ichiaki , Chowdhury Saikat , Yousef Abdelrahman , Gu Yue , Fournier Keith F. , Shen John P. 
Published: April 2023

Abstract
Background: Few preclinical models of appendiceal adenocarcinomas (AAs) exist. Orthotopic patient-derived xenografts (PDXs) better recapitulate the original tumor microenvironment. Therefore, we hypothesized that AA patient tumors should grow more successfully in orthotopic PDXs compared to standard flank implanted PDXs.
Methods: After obtaining informed consent, tumor samples were obtained from 16 patients with AA undergoing surgical resection at MD Anderson Cancer Center. Tumors were implanted in both the flank and peritoneum of NSG mice; from each patient, at least 6 mice (n≥3 IP, n≥3 flank) were implanted with an approximately 0.5 cm3 tumor. Mice were assessed after 6 months for tumor formation.
Whole transcriptomic RNAseq was performed on tumor samples taken from either the flank (n=3) or peritoneum (n=3) of two poorly differentiated (grade 3) patient tumors. The R package DESeq2 was used to conduct differential gene expression analysis comparing IP to Flank PDXs, controlling for the specific patient tumor. Finally, Gene Set Enrichment Analysis (GSEA) with the hallmark gene sets from MSigDB was implemented to study the relative up/down-regulation patterns of gene sets in each PDX cohort.
Results: Tumor formation rate, regardless of grade, was greater in IP vs. flank implantation (31.4% vs 11.7%). Higher grade was associated with higher average tumor formation rates (IP—G1: 13.3%, G2: 33.8%, G3: 44.5%; Flank—G1: 0%, G2: 0%, G3: 31.2%).
GSEA identified that cell proliferation and energy metabolism gene sets were significantly enriched in IP tumors relative to flank: E2F Targets, G2M Checkpoint, Oxidative Phosphorylation, MYC Targets V1, MYC Targets V2, and DNA Repair (NES = 2.91, 2.62, 2.60, 2.31, 1.61, 1.82, respectively; FDR q-val ≤ 0.01 for all). Meanwhile, gene sets associated with tumor environmental stress were enriched in flank tumors: Angiogenesis, Hypoxia, and TGF-β Signaling (NES = -2.00, -1.73, -1.62, respectively; FDR q-val ≤ 0.01 for all). Epithelial Mesenchymal Transition and TNF-α signaling via NF-κB were also enriched in flank tumors (NES = -2.28, -2.20; FDR q-val &lt; 0.01).
Conclusions: The peritoneal microenvironment promotes growth of appendiceal tumors in PDXs, with higher-grade being associated with greater tumor formation rate. GSEA also suggests that AAs preferentially grow in the peritoneal microenvironment in association with greater activation of cellular proliferation and energy metabolism pathways and lower activation of tumor environmental stress pathways.
Citation Format: Vinay K. Pattalachinti, Ichiaki Ito, Saikat Chowdhury, Abdelrahman Yousef, Yue Gu, Keith F. Fournier, John P. Shen. Peritoneal microenvironment promotes appendiceal adenocarcinoma tumor formation in PDX models. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6546.