Enhanced Permeability and Retention (EPR) Effect Based Tumor Targeting: The Concept, Application and Prospect

Conventional chemotherapy with small molecule drugs has been used for many types of cancer for decades. However, the therapeutic efficacy remains less successful, mostly because of poor tumor selectivity and severe adverse side effects which hampered the use of high drug doses. The development of tumor-targeted chemotherapy is thus critically important for more successful treatment. The enhanced permeability and retention (EPR) effect is a unique phenomenon of solid tumors based on their anatomical and pathophysiological differences from normal tissues, namely macromolecular drugs could accumulate and retain in solid tumor tissues selectively but they will not distribute much in normal tissue. EPR based chemotherapy is thus becoming an important strategy to improve the delivery of therapeutic agents to tumors for anticancer drug development, and macromolecular agents are potentially usefully for not only cancer therapy, but for cancer diagnosis and imaging. In this commentary, the concept and application of the EPR effect, as well as methods to further augment EPR effect by using factors associated with EPR effect such as nitric oxide, carbon monoxide, are briefly discussed. We believe that understanding EPR effect and EPR based tumor targeting will greatly promote the development of new therapeutic strategies and anticancer drugs for anticancer therapeutics. Central Fang et al. (2014) Email: JSM Clin Oncol Res 2(1): 1010 (2014) 2/5 Furthermore, most solid tumors have blood vessels with defective architecture, such as large gap between endothelial cells (e.g., ~ 1μm), lack of smooth muscle layers, so that macromolecules will have the opportunity to escape from tumor blood vessels and accumulate selectively in tumor tissues, whereas they could not cross normal blood vessels which will result in less side effects. The enhanced permeability of tumor blood vessels is also partly attributed to the over-produced vascular mediators, such as bradykinin, nitric oxide (NO), vascular endothelial growth factor (VEGF), carbon monoxide (CO) etc. [2,6,7]. These factors further enhance the permeability of tumor vasculature, thus being useful for the augmentation of EPR based tumor targeting which is discussed below. In addition, defected lymphatic function that is important for the recovery of macromolecules in tissues, is always found in tumor tissues [6-10]. Consequently, once macromolecules accumulate in tumor tissues, they will not cleared from tumor tissues but retain there for long time. Above-mentioned anatomic and pathophysiological characteristics together lead to the unique phenomenon of EPR effect, the principal mechanism of EPR effect is diagrammatically represented in Figure 1. APPLICATION OF EPR EFFECT EPR-based tumor targeting requires macromolecular drugs to have longer half-life time in order to provide a sufficient effective pharmacodynamic level. The usual way to obtain macromolecular drugs is to “mask” conventional small molecular drugs by modifying their surface with certain water-soluble polymers with a well-solvated and flexible main chain, such as polyethylene glycol (PEG), styrene maleic acid (SMA), N-(2hydroxypropyl)methacrylamide (HPMA), and so on [1,13,14]. Regarding the tumor targeting strategy, two kinds of targeting are always mentioned in literatures, i.e., “passive targeting” and “active targeting”. Passive targeting is so far EPR-based tumor targeting. To date, many macromolecular drugs were developed based on EPR effect, some of which are used in clinic and more are in preclinical stages, including liposomes, polymers, micelles, and nanoparticles [3-6, 8-10]. Examples of macromolecular drugs approved for clinical use are DaunoXome® (nonPEGylated liposomal daunorubicin), Abraxane® (albumin-based paclitaxel), DepoCyt® (non-PEGylated liposomal cytarabine), Doxil® (Caelyx in Europe; PEGylated liposomal doxorubicin), Myocet® (non-PEGylated liposomal doxorubicin), Oncaspar® (PEGylated L-asparaginase), and Genexol-PM® (paclitaxelcontaining polymeric micelles; approved in Korea). Many other ones are in earlyand late-stage preclinical development [15-19], which may be approved for certain cancer patients in few years. Another widely used tumor-targeting strategy is aiming at the special molecules or receptors in tumor cells using, for example, transferrin, folate, integrin receptors, epidermal growth factor, antibodies, glycoprotein etc., namely active targeting [20-22]. However, it is critically important and should be noted and emphasized that for targeted tumor delivery, no matter “passive targeting” or “active targeting”, EPR effect is the first and necessary step, namely, the drugs need to accumulate first in tumor by EPR effect, and then active targeting using ligands, antibodies could be achieved. Tumor specific Fab with molecular weight less than 40 kDa (no EPR effect) could not accumulate efficiently in tumor, but most were cleared out from circulation [5]. On the other hand, it may be a more effective strategy when active targeting techniques are designed based on EPR effect. In fact, many clinically-approved active targeting drugs are antibody-based nanomedicines which can be taking advantage of EPR effect. For example, Zevalin (CD20-targeted 90yttriumibritumomab tiuxetan), Bexxar (CD20-targeted iodine131-tositumomab), Ontak (CD25-targeted diphteria toxin-IL2 fusion protein), and Mylotarg (CD33-targeted ozogamycingemtuzumab) have been successfully used for non-Hodgkin`s lymphoma, T-cell lymphoma and acute myeloid leukemia [23]. AUGMENTATION OF EPR EFFECT Because EPR effect is mostly due to the high permeability of tumor vasculature, it is reasonable to use vascular mediators such as NO, CO, bradykinin, VEGF to further enhance EPR effect thus achieving more tumor accumulation of macromolecular drugs. In this section, we discuss this issue by focusing on the effect of NO and CO, using our recent findings. NO and its Derivatives NO is a vital molecule in living creatures and is produced from L-arginine by NO synthase (NOS) in the presence of oxygen, which has multiple roles, directly or indirectly as a signaling messenger. It is important to note that tumors tissue also showed extensive iNOS expression, mostly in extensively infiltrated leukocytes and macrophages, indicating that tumors produce significantly more NO compared with normal tissues [24,25] (Figure 2A). The amount of NO produced in tumors has a positive association with tumor weight-up to 1.75 g in AH136B tumor-bearing rats and 250 mg in mice bearing sarcoma 180 (S-180) tumors [26]. iNOS knockout mice evidenced clearly delayed tumor growth [27]. Tumor tissue Lymphatic system EPR effect Normal tissue Figure 1 Diagrammatic representation of EPR effect. Normal blood vessels have surrounding smooth muscle-cell layer with tighten the cell-cell junctions which macromolecular agents are difficult to extravasate. In contrast, in tumor tissues, blood vessels are always with loose cell-cell junctions, via which macromolecular agents can escape to tumor tissue. In addition, the defected lymphatic system in tumors leads to the retention of macromolecular agents in