In silico, in vitro and in vivo methods to analyse drug permeation across the blood–brain barrier: A critical review

Introduction The existence of the blood–brain barrier in the human body leads to the insufficiency in delivering therapeutic compounds into the brain for the effective treatment of various neurological disorders. In order to determine the possibility of such agents to penetrate through the blood–brain barrier, different in silico, in vitro and in vivo methods may be implemented. Some of them are often provided with unreliable results while others are not feasible in high-throughput screening environment. The goal of this review was to characterise the latest state-of-the-art methods that have been developed and used in the pharmaceutical research in the last few decades to assess the permeation of novel therapeutic entities across the blood–brain barrier. We carried out a literature research and study selection by searching for published biomedical articles in the PubMed archive. Conclusion Overall, the combination of in silico, in vitro and in vivo methods in the blood– brain barrier research may lead to the discovery of promising drug compounds and more accurate information of brain uptake mechanisms. Introduction In the drug discovery process, drug permeation across the blood–brain barrier (BBB) is fundamental for neuropharmaceuticals to reach their site of action within the central nervous system (CNS). This BBB consists of highly specialised microvascular endothelial cells together with pericytes, astrocytes, microglia, neurons and basement membrane1. The capillary endothelial cells are connected by proteins (occludin, claudins and junctional adhesion molecules) forming tight junctions (TJs), which seal the intercellular space, thereby restricting the permeability for the CNS-active substances2,3. In addition, these cells contain numerous active membrane transporters to regulate transcellular transport of drug-like molecules and their metabolites between the blood–brain interface. Over 98% of all known therapeutics are unable to penetrate the BBB due to their molecular properties and physicochemical factors, including hydrophilicity, hydrophobicity, polar surface area, molecular size and charge (Figure 1). On the contrary, the permeation of the CNS-inactive compounds would generate various undesired side effects. Receptormediated and non-specific adsorption-mediated transcytosis can also contribute to the translocation of peptides, antibodies and lipoproteins across the BBB4. To minimise this risk, the healthy BBB itself imposes a highly efficient impediment for most of the clinically administered neuropharmaceuticals. On the other hand, the BBB dysfunction is highly implicated in auto-immune, neuropathological processes (Alzheimer and Parkinson’s diseases), neuroinfections (meningitis and encephalitis), haemorrhagic and ischemic stroke and traumatic brain injury5–7. In this regard, the assessment of the BBB permeation for drug candidates at physiological and pathological conditions would be a primary concern for rational drug design and development through various in silico, in vitro and in vivo methods. The goal of this review is to describe the state-of-the-art techniques and methods that have been used so far in pharmaceutical research to evaluate the BBB function and assess the ability of drug-like molecules to permeate the BBB. We performed a literature search and study selection by seeking published biomedical research papers in PubMed. The criteria for search were as follows: • article type: review, research article • publication date: various • species: mammals • language: English • key words: blood–brain barrier, in silico, in vitro, in vivo methods, drug-like compounds, rational drug design We also used monographs dedicated to the BBB research and drug design strategies to bring readers the state-of the-art information in regard to describing issues. Discussion The authors have referenced some of their own studies in this review. These referenced studies have been conducted in accordance with the Declaration of Helsinki (1964) and the protocols of these studies have been approved by the relevant ethics committees related to the institution * Corresponding author Email: [email protected]; E_Shityako_S@ ukw.de Department of Anaesthesia and Critical Care, University of Würzburg, 97080 Würzburg, Germany