Photodynamic Stromal Depletion in Pancreatic Ductal Adenocarcinoma

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is one of the deadliest solid malignancies, with a five-year survival of less than 10%. The resistance of the disease and the associated lack of therapeutic response is attributed primarily to its dense, fibrotic stroma, which acts as a barrier to drug perfusion and permits tumour survival and invasion. As clinical trials of chemotherapy (CT), radiotherapy (RT), and targeted agents have not been successful, improving the survival rate in unresectable PDAC remains an urgent clinical need. Photodynamic stromal depletion (PSD) is a recent approach that uses visible or near-infrared light to destroy the desmoplastic tissue. Preclinical evidence suggests this can resensitise tumour cells to subsequent therapies whilst averting the tumorigenic effects of tumour-stromal cell interactions. So far, the pre-clinical studies have suggested that PDT can successfully mediate the destruction of various stromal elements without increasing the aggressiveness of the tumour. However, the complexity of this interplay, including the combined tumour promoting and suppressing effects, poses unknowns for the clinical application of photodynamic stromal depletion in PDAC.

Keywords: cancer therapeutics; extracellular matrix; fibrosis; pancreatic ductal adenocarcinoma; photodynamic therapy; stroma; tumour microenvironment.

Figure 1

An outline of the standard treatment pathways for pancreatic ductal adenocarcinoma. At initial presentation, just 15–20% of patients are suitable for upfront resection. Adjuvant CT (AC) is the standard of care (SOC) following resection in PDAC. A greater proportion of patients present with borderline resectable (BR) disease (20–30%) and typically receive neoadjuvant CT (NACT) before surgical evaluation. Half of patients present with initially non-resectable (NR) disease, though if locally advanced (LA) surgical evaluation will be considered following NACT. As there is little evidence supporting surgical resection for metastatic PDAC, palliative CT is adopted. CT: chemotherapy.

Figure 2

The main PDAC TME components and tumourigenic signalling pathways. The TME is a complex mixture of structural (ECM) and cellular components that play a reciprocal role in the progression of the stroma and the cancer itself. Facilitating many of the interactions between the tumour and stromal components are integrins. Via connecting proteins FN-1/VN-1, collagens interact with the integrins on the PDAC cell surface, which activates FAK and converts actin polymerisation into traction force, driving tumour cell invasion. Collagen also interacts with DDR-1, which binds and activates several signalling proteins, including PYK2, leading to the upregulation of pro-survival genes such as Hes1 and migratory pathways via p130Cas/JNK. The result is the upregulation of N-cad, which promotes the EMT and acquisition of a motile phenotype. Upon Shh pathway activation, PSCs form CAFs, which are the greatest producer of the ECM components, including collagen, PG, and HA. Secreted proteins SPARC, LOX, and TG2 mediate collagen crosslinking, which leads to tissue stiffness, the production of further collagen, and the activation of YAP/TAZ, which promotes cell proliferation. MMPs, which are upregulated by integrin signalling, also modulate the proteolytic activity of the matrices, leading to the release of individual PDAC cells from the TME. Cytokines such as TGF-β and IL-1 released from the PDAC secretome have many functions, including promotion of further collagen production, activation of CAFs, and the adhesion of PDAC cells to capillaries. Activated fibroblasts have several functions: MFs are crucial for the continued deposition of ECM components, whereas ICAFs release various tumour-promoting inflammatory molecules such as IL-6, which enhance the formation of the immunosuppressive environment by regulating immune cell activity. They also support the metabolism of PDAC cells, such as by providing them with labile iron for survival or acting as a cellular reservoir for vitamin A and lipids. BV, blood vessel; RBC, red blood cell; PSC, pancreatic stellate cell; CAF, cancer-associated fibroblast; MDSC, myeloid-derived suppressor cell; TGF-β, transforming growth factor beta; HA, hyaluronan; FN-1, fibronectin 1; VN, vitronectin; DDR-1/2, discoidin receptor ½; PYK2, FAK-related protein tyrosine kinase; FAK, focal adhesion kinase; MMP, matrix metalloproteinase; TG2, tissue transglutimase; LOX, lysyl oxidase; Shh, sonic hedgehog; FGF2, fibroblast growth factor 2; YAP, yes-associated protein; TAZ, transcriptional co-activator with PDZ-binding motif; ICAF, inflammatory cancer-associated fibroblast; MF, myofibroblast; A-PSC, activated pancreatic stellate cell; PG, proteoglycan; SPARC, secreted protein acidic and rich in cysteine; EMT, epithelial to mesenchymal transition; IL-6, interleukin 6; IL-1, interleukin 1; Fe, iron; PDAC, pancreatic ductal adenocarcinoma; JNK, c-Jun N-terminal kinase; N-cad, n-cadherin; TME, tumour microenvironment; ECM, extracellular matrix; SMAD, mothers against decapentaplegic; Ptch1, patched-1 receptor. The figure was created on Biorender.com (accessed on 5 January 2023).

Figure 3

The mechanism of action PDT. After the photosensitiser (PS) is taken up by the cancer cell, light is delivered to the treatment area and absorbed by the PS, which adopts an excited singlet state (¹PS). The PS is transferred to an excited triplet state (³PS) through the process of intersystem crossing and transfers an electron to biological molecules to form free radicals, including the hydroxyl radical (OH), hydroperoxyl radical (HO₂), or the superoxide anion radical (O2⁻) (Type 1 reaction), or transfers energy directly to tissue oxygen, generating unstable singlet oxygen (¹O₂) (Type 2 reaction). Inside the cell, these highly reactive molecules lead to oxidative stress and DNA damage, rendering the cell susceptible to several forms of death, including apoptosis, necrosis, or autophagy. This figure was created on Biorender.com (accessed on 5 January 2023).

Figure 4

The overall physical and biophysical effects of the PDAC stroma on the tumour and its TME. The unique stroma in PDAC has a broad range of implications that prevent the delivery of therapeutic agents, mainly via their effects on blood vessels. This includes high levels of IFP, solid stress, and tissue stiffness. Adding to this effect is the unusual morphology of the vessels, owing to abnormal angiogenesis characteristic of the disease. The result of these features creates a harsh environment where PDAC cells are subject to nutrient deprivation, severe hypoxia, and immune suppression. This contributes to tumourigenesis and is further enhanced by the production of cytokines which interfere with PDAC cell–cell junctions and their mechanotransducive properties. As a result, PDAC cells gain a more migratory and invasive phenotype. IFP, interstitial fluid pressure. This figure was created on Biorender.com (accessed on 5 January 2023).

Figure 5

Proposed destruction of PSCs by PDT and subsequent effects on the PDAC stroma. The PDT-mediated production of highly reactive singlet oxygen mediates PSC death via the disruption of its organelles and various cellular constituents, such as lipids and proteins. The cellular death that results from this damage, as well as direct damage to the DNA material in the nucleus, causes the halting of expression and deposition of PSC products. This includes the ECM molecules collagen, HA, laminin, and fibronectin. As a result, the associated desmoplastic and pro-tumourigenic signalling routes are restricted, which reduces the proliferation, migration, and survival of the PDAC cells through altered nuclear gene expression. Destruction of PSCs also reduces their production of various signalling molecules and cytokines such as TGF-β. In turn, CAF activation is prevented, as is their ability to produce further ECM components and cytokines. This enhances the dampening of the desmoplastic reaction whilst also restricting adhesion to vessels and the formation of an immunosuppressive microenvironment. The figure was created on Biorender.com (accessed on 5 January 2023). PDAC, pancreatic ductal adenocarcinoma; PDT, photodynamic therapy; PSC, pancreatic stellate cell; FN-1, fibronectin 1; HAS2, hyaluronan synthase 2; αSMA, alpha smooth muscle actin; DNA, deoxyribonucleic acid; COL1A1, collagen type I alpha 1 chain; TGF-β, transforming growth factor beta; PGF2, prostaglandin F2; PDGF, platelet-derived growth factor; CAF, cancer-associated fibroblast; MF, myofibroblast; ICAF, inflammatory cancer-associated fibroblast; IL-6, interleukin-6; MMP, matrix metalloproteinase; CTGF, connective tissue growth factor.