# Enhancement of EPR Effect for Passive Tumor Targeting: Current Status and Future Perspectives
## 3 Morphological Characteristics of the Vascular Network
### Functional Characteristics of the Tumor Vascular System
#### 3.4.5 Vascular Mediators Related to the EPR Phenomenon  
The enhanced permeability and retention (EPR) phenomenon in solid tumors is influenced by the increased production of vascular mediators that aid in the extravasation of macromolecules [113; 114]. Specifically, vascular mediators facilitate the opening of gaps between endothelial cells from 20 nm to 2 \(\upmu\)m, compared to gaps below 5 nm in normal vessels, allowing the extravasation of macromolecules or nanoparticles larger than 10 nm in size [115]. Vascular mediators involved in the EPR phenomenon include: (a) bradykinin, produced through the activation of the kallikrein-kinin system via a proteolytic cascade; (b) nitric oxide (NO), produced by the inducible form of NO synthase (iNOS) in leukocytes and cancer cells, as well as by the oxidative by-product of NO, peroxynitrite (ONOO-); (c) prostaglandandins (PCs); (d) angiotensin-converting enzyme (ACE) inhibitors; and (e) vascular permeability factor (VPF)/VEGF and other cytokines [8; 116]. Bradykinin (quinine), a major inflammatory mediator, is present at high levels in tumors, causing extravasation, increased vascular permeability, and fluid accumulation in inflammatory or cancerous tissues through vasodilatation, resulting in swelling and pain [8; 116; 117]. Due to its very short half-life and pain-causing action, ACE inhibitors are used. ACE inhibitors block the conversion of angiotensin I to angiotensin II and enhance the vascular actions of bradykinin, increasing permeability and enhancing the EPR phenomenon [8; 118]. Increased levels of bradykinin lead to the accumulation of anticancer drugs in tumors with more selective delivery and reduced side effects in normal tissues, especially under angiotensin II-induced hypertensive conditions [116]. In addition, bradykinin stimulates nitric oxide (NO) synthesis, increasing tumor permeability in parallel with prostaglandandins, which contributes to the accumulation of macromolecules in tumors [8; 117; 119]. High concentrations of bradykinin derivatives are found in the plasma and biological fluids of patients with advanced stages of cancer [8; 120; 121]. Nitric oxide (NO), a critical signaling molecule in the body, is produced in large quantities by infiltrating leukocytes via inducible NO synthase (iNOS) [116]. Increased levels of NO correlate with tumor weight and increase vascular permeability, facilitating better drug delivery [116; 117; 118]. NO derivatives, such as peroxynitrite (ONOO-), resulting from the reaction of NO with carbon dioxide, specifically target tumors and enhance the EPR effect by activating matrix metalloproteinases (MMPs), such as collagenase, leading to extracellular matrix disruption and increased vascular permeability [8; 122]. MMP inhibitors can reduce fluid accumulation in tumors [116]. Prostaglandins (PGs), lipid molecules derived from arachidonic acid via cyclooxygenase-nases (COXs), are important inflammatory mediators and can be increased by inflammatory cytokines. PGE1 and PGI2 enhance blood vessel permeability and the EPR effect, like nitric oxide (NO) [6; 73; 74]. Administration of the PGI2 analog, beraprost, increases extravasation in tumors without affecting blood pressure or flow in normal tissues, while inhibiting tumor growth [8]. COX inhibitors again reduce both permeability and tumor growth [116; 117; 118]. Vascular endothelial growth factor (VEGF), as previously mentioned, is a key angiogenesis factor critical for the growth of solid tumors and is significantly increased in most tumors in hypoxic environments. VEGF action leads to increased extravasation and vascular permeability and involves increased nitric oxide (NO) production, thus contributing to the EPR effect, like bradykinin [8; 116; 117]. VEGF inhibitors have been developed as therapeutic tools, targeting the reduction in angiogenesis and tumor growth [8]. The enhanced permeability and retention (EPR) phenomenon results from multifactorial interactions between vascular factors, which influence each other through cross-mediated interactions.