Blood-Brain Barrier 강 경 태

Contents 1. BBB Fundamentals 2. Effects of Brain Penetration 3. Structure-BBB Penetration Relationships 4. Structure Modification Strategies to Improve Brain Penetration

BBB is restrictive for some compounds owing to P-glycoprotein efflux, absence of paracelluar permeation, and limited pinocytosis. Brain exposure is assessed in terms of BBB permeability or brain/plasma partition. Brain exposure is enhanced by reducing H-bonds, molecular weight, P-glycoprotein efflux, metabolism, and plasma protein binding, or increasing Log P. Blood-Brain Barrier

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Blood-Brain Barrier

BBB Fundamentals The term BBB permeation is widely used in CNS discovery projects. BBB permeation is a major factor in brain penetration. that it is the sum of multiple mechanisms at the BBB. These depend on the compound properties and target location.

BBB Fundamentals The BBB is associated with the microcapillary blood vessels that run throughout the brain in close proximity to brain cells. These vessels naturally provide the nutrients and oxygen needed by the CNS cells and carry away waste.

BBB Permeation Mechanisms Mechanisms affecting BBB permeation The BBB forms a permeation barrier that is more limiting to compound penetration than are most other membrane barriers in the body. BBB endothelial cells tends to have significant negatively charged polarity head groups, which opposes acids.

Pgp Efflux Efflux is a major limitation to BBB permeation for some compounds. an important strategy for increasing brain exposure of these compounds is to reduce efflux by Pgp. Endothelial cells also express breast cancer resistance protein (BCRP) and multidrug resistance protein 1 (MRP1) through MRP6, which efflux some compounds. ER > 3 (high Pgp efflux) Pgp knockout mice also are used for verification of the in vitro conclusions. A survey of successful CNS drugs indicated ER 3 for 6%. BBB Permeation Mechanisms Efflux ratio(ER) = efflux / infflux

BBB Permeation Mechanisms Paracellular permeation Paracellular permeation is drastically limited in the BBB. because the endothelial cells form tight junctions. Pinocytosis Metabolism Pinocytosis in BBB endothelial cells is limited. Metabolism (phases I and II) has been observed in BBB endothelial cells. This structurally modifies compounds before they can reach brain tissue. The role of metabolism at the BBB likely is small.

Brain Distribution Mechanisms Several mechanisms limit the access of compounds to brain cells by affecting the distribution of compound to or within the brain Metabolic clearance Plasma protein binding Nonspecific binding to proteins and lipids in brain tissue Clearance of compound from the extracellular fluid (ECF) into the blood and cerebrospinal fluid (CSF)

Brain Distribution Mechanisms High hepatic clearance High hepatic clearance limits exposure of compounds to the brain.

Brain Distribution Mechanisms Plasma protein bindng Free unbound drug permeates the BBB. If the compound is highly bound to plasma protein, little free drug is available to penetrate into the brain tissue.

Brain Distribution Mechanisms bound drug can release in brain microvessel circulation in vivo much more than with in vitro assays.

Brain Distribution Mechanisms Compound in ECF is cleared into the blood and the CSF. If a compound has low BBB permeation, this clearance may limit the concentration in the ECF.

Brain-CSF Barrier The BCSFB is not considered an effective route for drug delivery to the brain Because, (1)the surface area of the BBB is 5,000-fold larger than the BCSFB (2)there is little mixing of the CSF components with the ECF (3)the CSF flows very fast away from the brain tissue toward the arachnoid villi (4) CSF is turned over every 5 hours. BBB > BCSFB 5,000-fold BCSFB : blood–cerebrospinal fluid barrier

Interpreting Data for Brain Penetration minimum guideline for CNS discovery projects A ratio of drug in brain to drug in plasma or blood. This value is often termed B/P; Log BB. yet some commercial CNS drugs have B/P < 0.1. B/P = concentration in the brain concentration in the plasma B/P > 0.3 penetrate poorly into the brain B/P < 0.1

Interpreting Data for Brain Penetration Kpfree = Cunbound Brain / Cunbound Plasma The term B/P is calculated from various data: AUC brain / AUC plasma Concentrationbrain / Concentrationplasma at a single time point or at Cmax Concentrationbrain / Concentrationplasma at steady state (also termed Kp)

Effects of Brain Penetration

Structure-BBB Penetration Relationships By Pardridge H-bond (total) <8~10 MW < 400~500 No acids By Sparklin H-bond donors < 2 H-bond acceptors < 6 By Clark and Lobell N + O < 6 PSA < 60~70 Å 2 Log D = 1~3 ClogP – (N+O) >0 BBB rule Hydrogen bonds (acceptors and donors) Lipophilicity Polar surface area (PSA) Molecular weight (MW) Acidity

Structure-BBB Penetration Relationships

75% 19% 6%

Structure Modification Strategies to Improve Brain Penetration Reduce P-glycoprotein efflux Reduce hydrogen bonds Increase lipophilicity Reduce molecular weight Replace carboxylic acid groups Add an intramolecular hydrogen bond Modify or select structures for affinity to uptake transporters

Reduce Hydrogen bonds Series of Steroids Reducing the total number of hydrogen bonds will increase BBB permeation, especially H-bond donors.

Reduce Hydrogen bonds Reduction of one hydrogen bond donor by blocking with a methyl group greatly increased brain penetration.

Increase Lipophilicity Increasing lipophilicity will increase BBB permeation. 10 fold100 fold

Reduce MW If groups on the structure can be removed without greatly impairing activity, then removing them can be beneficial. This reduces molecular size and improves permeation through lipid bilayer membranes.

Replace Carboxylic Acid Groups Elimination of an acidic group will increase BBB permeation.

Add an Intramolecular Hydrogen Bond An intramolecular hydrogen bond will increase BBB permeation. It reduces the total number of hydrogen bonds with water that must be broken for BBB permeation.

Modify or Select Structures for Affinity to Uptake Transporters LAT1 large neutral amino acid transporter enhances the brain uptake of l-dopa and gabapentin GLUT1 Glucose transporters on the luminal side of the BBB endothelial cells MCT1 monocarboxylic acids CAT1 CNT2 cationic amino acids nucleosides

2. Drug concentration is much lower in the blood than in the GI lumen. Pgp can be saturated in the GI but not at the BBB.