Pancreatic cancer: overcoming chemotherapy resistance using synthetic anti-miRs

Pancreatic Cancer: A Tough Nut to Crack!

Pancreatic cancer presents a significant clinical challenge. Currently, the 5-year survival rate for patients diagnosed with this cancer is only 12%, the lowest among all solid malignancies.1

Pancreatic ductal adenocarcinoma (PDAC), arising in the exocrine portion of the pancreas, is the most common histological subtype of pancreatic neoplasm. It accounts for over 90% of cases of pancreatic cancer. PDAC is expected to surpass breast cancer, becoming the third leading cause of cancerrelated death in the European Union.2 It has already reached this status in the United States. A multitude of factors contribute to the extremely poor prognosis of PDAC. Early-stage or localized disease is often asymptomatic. Additionally, the majority of patients present with nonspecific symptoms when the cancer is in a locally advanced or metastatic stage. As a result, only 20% of patients are eligible for curative surgery.3 Consequently, approximately 80% of patients with PDAC undergo treatment with multidrug chemotherapeutic regimens and, in some cases, radiotherapy. However, due to the aggressive tumour biology and both inherent and acquired drug resistance in PDAC cells, many patients respond poorly to current treatments.

MicroRNA Therapeutics: A Promising Anti-Cancer Treatment Approach?

MicroRNAs (miRNAs) are small, endogenous, non-coding RNAs comprising 20–25 nucleotides. They play a crucial role in gene expression regulation by imperfectly pairing with the 3′-untranslated region (3′-UTR) of target mRNA. MiRNAs have been identified as key modulators in numerous cellular processes, including those that contribute to chemotherapy resistance in various tumours.4 Therefore, miRNA-based therapy may offer a new, promising path in cancer treatment, either as a standalone option or in combination with existing standard-of-care regimens.

Overcoming Pancreatic Cancer Chemotherapy Resistance Using Anti-miRs

The multi-drug regimen FOLFIRINOX, comprising folinic acid, 5-FU, irinotecan, and oxaliplatin or cisplatin, is the standard first-line treatment for patients with metastatic PDAC. While FOLFIRINOX often stabilizes the disease, most patients rapidly develop resistance to the therapy.5 Cisplatin, a platinum agent used in treating a variety of solid malignancies, induces DNA damage to promote cell death. However, only 1–4% of the drug reaches the nucleus,6 indicating that drug trafficking and sequestration are crucial in cisplatin resistance.

A small copper (Cu) chaperone known as ATOX1 (Antioxidant 1) plays a role in Cu transport and homeostasis within cells. ATOX1 binds Cu(I) ions in the cytosol and delivers them to the Cu-transporting ATPases in the trans-Golgi network. This process is essential for the biosynthetic maturation of secreted Cudependent enzymes. Studies have shown that cancer cells with higher resistance to cisplatin exhibit significantly elevated levels of ATOX1.7 However, the literature presents conflicting results; some studies indicate that deletion of ATOX1 leads to reduced sensitivity to cisplatin in certain contexts.8,9 Thus, the exact role of ATOX1 in cellular resistance to platinum based chemotherapeutics is not yet fully understood.

Our research group has demonstrated in PDAC, as well as in malignant pleural mesothelioma,10 that the loss of a specific microRNA, miR-31, promotes cellular sensitivity to platinum-based chemotherapies, such as oxaliplatin, cisplatin, and carboplatin. In silico analysis has identified ATOX1 as a putative target of miR-31. We have observed that miR31 regulates the expression levels of ATOX1 in PDAC cells. Suppressing miR-31 in these cells using synthetic anti-miRs leads to increased intracellular levels of ATOX1 and enhanced chemosensitivity (Figure 1A). Furthermore, direct overexpression of ATOX1, independent of miR31, significantly also increases the sensitivity of PDAC cells to cisplatin.

This finding strongly suggests a link between miR-31 and ATOX1, forming a drug resistance axis that warrants further investigation. We hypothesize that miR-31 binds to the 3′-UTR of ATOX1 mRNA, leading to either repression of protein translation or degradation of the mRNA transcript (Figure 1B). Intriguingly, it has been reported that platinum drugs can bind specifically to the ATOX1Cu heterodimeric complex.6

Collectively, our findings imply that suppressing miR-31 may promote the cytoplasmic-to-nuclear trafficking of cisplatin via ATOX1, thereby potentially explaining the restored chemosensitivity observed in PDAC cells.

A. In PDAC cells, low levels of miR-31 enhance cellular sensitivity to cisplatin (CDDP), while high levels of miR-31 lead to increased resistance to CDDP. MiR-31 modulates the levels of the Cu chaperone ATOX1. Specifically, PDAC cells with overexpressed miR-31 show low levels of ATOX1. In contrast, cells with suppressed miR-31 exhibit high levels of ATOX1. CDDP may act as a ‘hitch-hiker,’ increasing in the cytoplasmic-to-nuclear transport of CDDP.

B. ATOX1 mRNA is a predicted target of miR-31. We propose that miR-31 suppresses ATOX1, either through mRNA degradation or inhibition of protein translation. This diagram was created using Biorender.com.

The Future for miRNA-Based Therapeutics in Pancreatic Cancer

The field of miRNA-directed therapeutics is rapidly emerging within oncology. A handful of miRNA-based drugs are currently under clinical trials for various cancers, with some demonstrating promising outcomes.11 A notable figure in this area is Dr. Glen Reid, Associate Professor at the University of Sydney Medical School. Reid developed TargomiRs, a therapeutic based on a miR-16 mimic, delivered via anti-EGFR antibody-targeted bacterial minicells. The TargomiRs underwent a phase I trial in patients with recurrent malignant pleural mesothelioma (MesomiR-1 trial), where it showed both safety and preliminary signs of efficacy.12 Despite the potential of miRNA replacement or inhibition in oncology, significant challenges remain. These include susceptibility of miRNAs and anti-miRs to degradation by ribonucleases in biological systems, poor penetration through cell membranes, risks of off-target toxicities, and potential triggering of unwanted immune responses.13 To advance miRNA-based drugs to phase III clinical trials, these issues must be addressed.

Looking forward, our group aims to study the effect of miR-31 on chemosensitivity in PDAC tumour spheroids and organoids using fresh samples from PDAC patients. We will employ gold nanoparticles, known for their efficiency as inorganic vectors in maintaining the stability of synthetic miRNAs during delivery. This approach will allow us to assess the feasibility of delivering anti-miRs directed against miR-31 to PDAC cells. These models will enable us to determine if co-coupling synthetic anti-miR-31-modulating oligos with chemotherapy-coupled nanoparticles can enhance the sensitivity of PDAC cells to standard chemotherapeutics (Figure 2). The co-administration of miR-31 anti-miRs with conventional platinum-based chemotherapy could potentially convert non-responders to responders, offering a promising avenue for improving treatment outcomes in patients with PDAC.

References on request

Written by David Hackett (Ph.D. student)1, Dr. Jason McGrath (Postdoctoral Research Fellow)2, Dr. Stephen G. Maher (Associate Professor in Translational Oncology)1

1Department of Surgery, Trinity St. James’s Cancer Institute, Trinity Translational Medicine Institute, St. James’s Hospital, D08 W9RT Dublin, Ireland. 2Department of Surgery, Royal College of Surgeons in Ireland (RCSI).

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