Phenotypic heterogeneity and tumor immune microenvironment directed therapeutic strategies in pancreatic ductal adenocarcinoma

Abstract

Pancreatic cancer is an aggressive tumor with high metastatic potential which leads to decreased survival rate and resistance to chemotherapy and immunotherapy. Nearly 90% of pancreatic cancer comprises pancreatic ductal adenocarcinoma (PDAC). About 80% of diagnoses takes place at the advanced metastatic stage when it is unresectable, which renders chemotherapy regimens ineffective. There is also a dearth of specific biomarkers for early-stage detection. Advances in next generation sequencing and single cell profiling have identified molecular alterations and signatures that play a role in PDAC progression and subtype plasticity. Most chemotherapy regimens have shown only modest survival benefits, and therefore, translational approaches for immunotherapies and combination therapies are urgently required. In this review, we have examined the immunosuppressive and dense stromal network of tumor immune microenvironment with various metabolic and transcriptional changes that underlie the pro-tumorigenic properties in PDAC in terms of phenotypic heterogeneity, plasticity and subtype co-existence. Moreover, the stromal heterogeneity as well as genetic and epigenetic changes that impact PDAC development is discussed. We also review the PDAC interaction with sequestered cellular and humoral components present in the tumor immune microenvironment that modify the outcome of chemotherapy and radiation therapy. Finally, we discuss different therapeutic interventions targeting the tumor immune microenvironment aimed at better prognosis and improved survival in PDAC.

Keywords: desmoplastic stroma; immunosuppression; immunotherapy; pancreatic cancer; tumor heterogeneity; tumor microenvironment.

Figure 1

Stages in PDAC development. (A) PDAC can be pathologically categorized on the basis of TNM staging system i.e. size and extent of the tumor, spread to lymph nodes and metastasis. Stage 1 is when the tumor is restricted to the pancreas; stage 2 when the tumor has spread to 2 or 3 nearby lymph nodes; stage 3, when the tumor has spread to 4 or more nearby lymph node and may have also reached nearby blood vessel; stage 4, when the cancer has metastasized in other organs (liver, lungs etc.). (B) Acinar to ductal metaplasia is a normal regeneration process happening in pancreas during inflammation. However, due to KRAS hyperactivation, acinar cells fail to redifferentiate and progresses to duct-like cells forming pancreatic intraepithelial neoplasia (PanIN). PanINs are microscopic papillary or flat non-invasive epithelial neoplasms arising in pancreatic ducts characterized by mucin-containing cuboidal to columnar cells. With accumulation of subsequent additional mutations, the cancer progresses to a demoplasia condition causing PDAC.

Figure 2

Epigenetic drivers of PDAC. Epigenetic mechanisms, including aberrant DNA methylation and histone modifications, chromatin remodeling, long non-coding RNAs (IncRNAs) and microRNAs (miRNAs), significantly contribute to inter- and intratumoral heterogeneity, disease progression and metastasis in PDAC. Epigenetic modifiers such as DNA methyltransferases, histone methyltransferases, histone acetyltransferases, histone demethylases, or deacetylases are mutated which contributes to PDAC. Mutations in epigenetic regulators such as TET2, DNMT3A, ASXL1, ARID1A/B, PBRM1, MLL2/3/4, KDM6A, SMARCA2/4 are evident in PDAC. Significant upregulation of various miRNAs such as miR-215-5p, miR-122-5p, and miR-192-5p and decreased levels of miR-30b-5p and miR-320b, were observed in PDAC compared to chronic pancreatitis and hepatocellular carcinoma. Protein arginine methyl transferase is also overexpressed in PDAC. The DKK1-Super Enhancer (DKK1-SE) in PDAC is characterized by aberrantly active histone modifications. Epigenetic modifications correlate with distinct PDAC subtypes in patient-derived xenografts, suggesting that distinct epigenetic states may underpin inter patient PDAC transcriptional heterogeneity.

Figure 3

Various therapeutic options for treatment of PDAC patients. (A) Surgical resection (a. Pancreaticoduodenectomy, b. artery first approach, uncinate process first, triangle operation, c. venous bypass first, d. periarterial divestment, vascular resection, multivisceral resection, f. MIS/Robotic Surgery, g. laparoscopic and robotic distal pancreatectomy/robotic pancreatoduodenectomy. (B) Neoadjuvant Chemotherapy (a. FOLFIRINOX regimen stage IV disease, b. novel combination of nab-paclitaxel, c. oxaliplatin, 5-fluorouracil, and leucovorin in advanced PDAC patients, d. phase III NALIRIFOX trial, e. FOLFIRINOX and losartan); Adjuvant chemotherapy (a. ESPAC-4 and PRODIGE 24, b. modified FOLFIRINOX regimen, c. JASPAC-01, d. CONKO-005, e. combination of nab-paclitaxel and gemcitabine). (C) Radiotherapy in combination with different drugs (durvalumab, rucosopasem manganese, and NBTXR3). (D) Immunotherapy (a. durvalumab with or without tremelimumab, b. IRE (Irreversible Electroporation) + Nivolumab (arm B), c. Lipid nanoparticle embedded IL-12 mRNA, d. NLM-001 + chemotherapy (Gemcitabine and Nab-Paclitaxel) +Zalifrelimab; Antibody therapy (a. durvalumab with tremelimumab, b. nab-paclitaxel and gemcitabine, c. dual antagonist for CCR2 and CCR5 with nivolumab and gemcitabine/ nab-paclitaxel, d. nab-paclitaxel and gemcitabine + camrelizumab and radiotherapy. (E) Cellular therapy (a. chimeric antigen receptor T-cell (CAR-T), b. targeting antigens (CD24, PSCA, CEA, MUC-1, MSLN, FAP-α, Her-2), c. dual targeting MSLN and CEA, d. TnMUC1-targeted CAR-T cells, e. CAR-T cells expressing heparanase. (F) Vaccine therapy (a. dendritic cell vaccine (mDC3/8), b. vaccination against ADAM12).