Key Clinical Message: Appendiceal mucoceles (AMs) infrequently arise from an underlying malignancy. Treatment has progressed toward a less aggressive approach over time; they can be managed by appendectomy‐only unless pathology reveals malignancy. The ultimate goal of management is to prevent AM rupture, avoiding the syndrome of pseudomyxoma peritonei.

Peritoneal metastasis (PM) is an advanced stage malignancy largely refractory to modern therapy. Intraperitoneal (IP) immunotherapy offers a novel approach for the control of regional disease of the peritoneal cavity by breaking immune tolerance. These strategies include heightening T-cell response and vaccine induction of anti-cancer memory against tumor-associated antigens. Early investigations with chimeric antigen receptor T cells (CAR-T cells), vaccine-based therapies, dendritic cells (DCs) in combination with pro-inflammatory cytokines and natural killer cells (NKs), adoptive cell transfer, and immune checkpoint inhibitors represent significant advances in the treatment of PM. IP delivery of CAR-T cells has shown demonstrable suppression of tumors expressing carcinoembryonic antigen. This response was enhanced when IP injected CAR-T cells were combined with anti-PD-L1 or anti-Gr1. Similarly, CAR-T cells against folate receptor α expressing tumors improved T-cell tumor localization and survival when combined with CD137 co-stimulatory signaling. Moreover, IP immunotherapy with catumaxomab, a trifunctional antibody approved in Europe, targets epithelial cell adhesion molecule (EpCAM) and has shown considerable promise with control of malignant ascites. Herein, we discuss immunologic approaches under investigation for treatment of PM.

Objectives: Pseudomyxoma peritonei (PMP) is a rare malignancy originating from the appendix, characterized by disseminated mucinous tumor implants on peritoneal surfaces. We examined the role of multiplatform molecular profiling to study biomarker-guided treatment strategies for this rare malignancy. Methods: A total of 54 patients with appendix-derived PMP were included in the study. Tests included one or more of the following: gene sequencing (Sanger or next generation sequencing), protein expression (immunohistochemistry), and gene amplification (C/fluorescent in situ hybridization). Results: Targeted sequencing of 47 genes detected variants in KRAS (81%), GNAS (74%), SMAD4 (16%), and ATM (16%). Mutations were found at low frequencies (n=1 to 2) in APC, BRAF, PIK3CA, MLH1, and TP53. GNAS and KRAS co-occurrence was found in 87%. Protein overexpression was found in epidermal growth factor receptor (83%), cyclooxygenase-2 (73%), cMET (63%), cKIT (58%), and platelet-derived growth factor receptor alpha (58%). Immune checkpoint expression was found in 36% (programmed cell death protein 1) and 18% (programmed death-ligand 1). Surrogate markers of cell proliferation were found at low rates (TLE3 23%, TOP2A 22%), consistent with the slow-growing biology of PMP. Phosophatase and tensin homolog was intact (wild type [100%]) and positive (immunohistochemistry [80%]). Patients exhibited stable microsatellite status and mismatch repair proficiency (93%). Importantly, multidrug resistance protein expression was elevated (100% BCRP, 94% MRP1, 88% PGP). Markers for gemcitabine (RRM1), fluorouracil (TS), oxaliplatin (ERCC1), and irinotecan (TOPO1) chemosensitivities were detected at favorable rates: 93%, 87%, 77% and 65%, respectively. Conclusions: Molecular profiling by multiple platforms identified potential therapies for the nontargetable KRAS-mutated population. The role of cMET-targeted therapeutics and immune checkpoint inhibitors merits further investigation. Biomarker-guided selection of cytotoxic chemotherapies may facilitate efficacy to systemic treatment.