Purpose: This article characterizes the vascular effects following vascular-targeted photodynamic therapy with a photosensitizer which actively targets endothelial cells.
Methods: This strategy was considered by coupling a chlorin to a heptapeptide targeting neuropilin-1 in human malignant glioma-bearing nude mice. A laser Doppler microvascular perfusion monitor was used to monitor microvascular blood perfusion in tumor tissue. Endothelial cells' ultra structural integrity was observed by transmission electron microscopy. The consequences of photosensitization on tumor vessels, tissue factor expression, fibrinogen consumption, and thrombogenic effects were studied by immunohistochemical staining.
Results: Treatment of glioma-bearing mice with the conjugate showed a statistically significant tumor growth delay. Vascular effect was characterized by a decrease in tumor tissue blood flow at about 50% baseline during treatment not related to variations in temperature. This vascular shutdown was mediated by tumor blood vessels' congestion. A pro-thrombotic behavior of targeted endothelial cells in the absence of ultra structural changes led to the induction of tissue factor expression from the earliest times post-treatment. Expression of tissue factor-initiated thrombi formation was also related to an increase in fibrinogen consumption.
Conclusion: Using a peptide-conjugated photosensitizer targeting neuropilin-1, induction of tissue factor expression immediately post-treatment, led to the establishment of thrombogenic effects within the vessel lumen.