title: EPR: Evidence and fallacy.
abstract: The enhanced permeability and retention (EPR) of nanoparticles in tumors has long stood as one of the fundamental principles of cancer drug delivery, holding the promise of safe, simple and effective therapy. By allowing particles preferential access to tumors by virtue of size and longevity in circulation, EPR provided a neat rationale for the trend toward nano-sized drug carriers. Following the discovery of the phenomenon by Maeda in the mid-1980s, this rationale appeared to be well justified by the flood of evidence from preclinical studies and by the clinical success of Doxil. Clinical outcomes from nano-sized drug delivery systems, however, have indicated that EPR is not as reliable as previously thought. Drug carriers generally fail to provide superior efficacy to free drug systems when tested in clinical trials. A closer look reveals that EPR-dependent drug delivery is complicated by high tumor interstitial fluid pressure (IFP), irregular vascular distribution, and poor blood flow inside tumors. Furthermore, the animal tumor models used to study EPR differ from clinical tumors in several key aspects that seem to make EPR more pronounced than in human patients. On the basis of this evidence, we believe that EPR should only be invoked on a case-by-case basis, when clinical evidence suggests the tumor type is susceptible.
authors.affiliation: University of Utah, Department of Bioengineering, College of Engineering, Salt Lake City, 84112, USA.
authors.forename: Joseph W
authors.initials: JW
authors.lastname: Nichols
authors.affiliation: University of Utah, Department of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy, Salt Lake City, 84112, USA. Electronic address: you.bae@utah.edu.
authors.forename: You Han
authors.initials: YH
authors.lastname: Bae
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000008:administration & dosage* / Q000266:history; D004337:Drug Carriers / Q000737:chemistry* / Q000266:history; D049673:History, 20th Century; D006801:Humans; D008299:Maleic Anhydrides / Q000008:administration & dosage / Q000737:chemistry / Q000266:history; D053758:Nanoparticles / Q000737:chemistry; D009369:Neoplasms / Q000188:drug therapy* / Q000473:pathology; D010539:Permeability; D011137:Polystyrenes / Q000008:administration & dosage / Q000737:chemistry / Q000266:history; D009353:Zinostatin / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000737:chemistry / Q000266:history
chemical_list: D000970:Antineoplastic Agents; D004337:Drug Carriers; D008299:Maleic Anhydrides; D011137:Polystyrenes; C035755:poly(maleic acid-styrene)neocarzinostatin; D009353:Zinostatin
keywords: Clinical efficacy; Drug delivery; Enhanced permeability and retention; Intratumoral distribution; Tumor models