Tumor Vascular Permeabilization by Vascular-Targeting Photosensitization: Effects, Mechanism, and Therapeutic Implications

Abstract

Purpose: Loss of vascular barrier function has been observed shortly following vasculartargeting photodynamic therapy. However, the mechanism involved in this event is still not clear, and the therapeutic implications associated with this pathophysiologic change have not been fully explored. Experimental Design: The effect of vascular-targeting photodynamic therapy on vascular barrier function was examined in both s.c. and orthotopic MatLyLu rat prostate tumor models and endothelial cells in vitro, using photosensitizer verteporfin. Vascular permeability to macromolecules (Evans blue-albumin and high molecular weight dextran) was assessed with dye extraction (ex vivo) and intravital microscopy (in vivo) methods. Intravital microscopy was also used to monitor tumor vascular functional changes after vascular-targeting photodynamic therapy. The effects of photosensitization on monolayer endothelial cell morphology and cytoskeleton structures were studied with immunofluorescence staining. Results: Vascular-targeting photodynamic therapy induced vascular barrier dysfunction in the MatLyLu tumors. Thus, tumor uptake of macromolecules was significantly increased following photodynamic therapy treatments. In addition to vascular permeability increase, blood cell adherence to vessel wall was observed shortly after treatment, further suggesting the loss of endothelial integrity. Blood cell adhesion led to the formation of thrombi that can occlude blood vessels, causing vascular shutdown. However, viable tumor cells were often detected at tumor periphery after vascular-targeting photodynamic therapy. Endothelial cell barrier dysfunction following photodynamic therapy treatment was also observed in vitro by culturing monolayer endothelial cells on Transwell inserts. Immunofluorescence study revealed microtubule depolymerization shortly after photosensitization treatment and stress actin fiber formation thereafter. Consequently, endothelial cells were found to retract, and this endothelial morphologic change led to the formation of intercellular gaps. Conclusions: Vascular-targeting photodynamic therapy permeabilizes blood vessels through the formation of endothelial intercellular gaps, which are likely induced via endothelial cell microtubule depolymerization following vascular photosensitization. Loss of endothelial barrier function can ultimately lead to tumor vascular shutdown and has significant implications in drug transport and tumor cell metastasis.

Chemophototherapy: An Emerging Treatment Option for Solid Tumors

“…With a 24 h drug–light interval, PDT regimens that used low fluence rates were demonstrated to be more successful, with the highest accumulation of fluorescent microspheres occurred within the range of 48–88 J cm –2 and 14–28 mW cm –2 . PDT‐enhanced EPR effects have been shown to be more effective for nanoparticles and large molecules such as dextrans compared to small molecules189, 190, 197 and in this study PDT enhanced accumulation of 0.1 to 2 μm particles while small 0.02 μm particles did not show enhanced accumulation after PDT ( Figure 3 A). PDT improved the tumor uptake of Doxil 2–3‐fold (Figure 3B).…”

“…PDT has been demonstrated to permeabilize the tumor vasculature and enhance the delivery of nanoparticles 189, 190, 191, 192, 193. The mechanism of PDT‐induced vascular permeabilization is believed to be the direct attack of cytotoxic singlet oxygen generated by PDT, which leads to damage of the vascular endothelial cells and formation of endothelial intercellular gaps 190, 193.…”

“…The mechanism of PDT‐induced vascular permeabilization is believed to be the direct attack of cytotoxic singlet oxygen generated by PDT, which leads to damage of the vascular endothelial cells and formation of endothelial intercellular gaps 190, 193. Other mechanisms such as leukocyte recruitment also play important roles for the PDT‐induced permeabilization, as inhibition of leukocyte interaction with endothelial cells led to significantly reduced extravasation of macromolecules 194.…”

Nanoparticle-Based Photodynamic Therapy: Current Status and Future Application to Improve Outcomes of Cancer Treatment

“…Another proposed mechanism is that ROS generated in tumor tissues would form or enlarge the inter-cellular gaps between endothelial cells, leading to enhanced permeability of tumor vasculature. 39) However, no cor-relation was obtained between the size of inter-cellular gaps and the vascular permeability of nanoparticles with different sizes ranging from 100 to 300 nm. 40) Thus, further studies are necessary to elucidate the mechanism(s) behind the enhancement of vascular permeability by PVT and to optimize the treatment for more efficient tumor delivery of subsequently injected nanoparticles.…”

Enhanced antitumor effects of BPD-MA-mediated photodynamic therapy combined with adriamycin on breast cancer in mice

“…In addition to directly killing cancer cells, PDT appears to shrink or destroy tumors in other ways. The photosensitizer can damage blood vessels in the tumor, thereby preventing the cancer from receiving necessary nutrients [17][18][19] . Gomer et al [20] showed that bovine endothelial cells were significantly more sensitive to PDT with porfimer sodium than smooth muscle cells or fibroblasts from the same species.…”