Blood-Brain Barrier

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Blood-Brain Barrier (BBB)A physical barrier that controls the movement of chemicals from blood into the extracellular fluid of the brain.

In the brain the endothelial cells in blood capillaries form a very tight junction that prevents many chemicals from passive diffusion across the capillary cell membrane into the brain.

Passive Diffusion - Lipid-soluble substances can diffuse across BBB

Active transporters exist in the capillary cell membraneglucoseamino acidshormones

Efflux transporters P-glycoprotein (P-gp)

Multidrug resistance protein (MRP)Organic anion transporters (OAT3)

Transporters can be on blood side or CNS side of membranes.

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Figure 6-27 Barrier systems in and around the brain. Substances can leave extracerebral capillaries but are then blocked by the arachnoid barrier. They can also leave choroidal capillaries but are then blocked by the choroid epithelium. They cannot leave any other capillaries that are inside the arachnoid barrier (except for those in the circumventricular organs). The ventricular and subarachnoid spaces are in free communication with each other, and both communicate with the extracellular spaces of the brain.

Brain Barrier Systems

3 distinct membrane (meninges) layers surround the brain. The meninges are connected to the skull. The brain is mechanically suspended withinthe meninges.

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Figure 6-28 CNS capillaries with and without barrier properties. A, Capillary in a hypothalamic nucleus (supraoptic nucleus) of a rat. The continuous endothelial wall and the lack of pinocytotic vesicles are apparent; tight junctions are also present between endothelial cells but cannot be seen at this magnification. B, Capillary in the subfornical organ, which is a circumventricular organ near the roof of the third ventricle adjacent to the interventricular foramen. The walls of this capillary are quite permeable and are characterized by fenestrations (f), pinocytotic vesicles (v), and substantial spaces (s) around the capillary. (From Gross PM: Brain Res Bull 15:65, 1985.)

BBB in Capillary Cell

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BBB in Capillary Cell

Cerebral endothelial cells are unique in that they form complex tight junctions (TJ) produced by the interaction ofseveral transmembrane proteins that effectively seal the paracellular pathway. These complex molecular junctions make the brain practically inaccessible for polar molecules, unless they are transferred by transport pathways of the BBB that regulate the microenvironment of the brain. There are also adherens junctions (AJ), which stabilize cell–cell interactions in the junctional zone. In addition, the presence of intracellular and extracellular enzymes such as monoamine oxidase (MAO), γ-glutamyl transpeptidase (γ-GT), alkaline phosphatase, peptidases, nucleotidasesand several cytochrome P450 enzymes endow this dynamic interface with metabolic activity. Large molecules such as antibodies, lipoproteins, proteins and peptides can also be transferred to the central compartment by receptor-mediated transcytosis or non-specific adsorptive-mediated transcytosis. The receptors for insulin, low-density lipoprotein (LDL), iron transferrin (Tf) and leptin are all involved in transcytosis. P-gp, P-glycoprotein; MRP, multidrug resistance associated protein family.

Nature Reviews Drug Discovery 2007, 6, 650

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BBB studies

logBB = log of [drug in blood]/[drug in brain]

problems – free unbound drug concentration vs bound drug, only free unbound drug in plasma is available to cross BBB and only free nontissue bound drug is available to bind to the targets

Fuplasma = unbound [drug] in plasmaFubrain = unbound [drug] in brain

in mice Fuplasma ≅ Fubrain even though [drug in blood]/[drug in brain] varied greatly

BBB studies

JPET 2002, 303, 1029

BBB studiesJPET 2002, 303, 1029

passive diffusionCNS faster non-CNS

Pgp substrateCNS poorer non-CNS

if: mw > 400 & large size

non-CNS good PgpCNS poor

Drug Properties Hansch (1972): parapolic relationship between logP and CNS activity in rodents

1988 – linear relationship logBB and ΔlogP (logP(octanol/water) – logP(cyclohexane/water)

hydrogen-bonding & lipophilicity

now – reducing active efflux (“efflux processes have evolved to recognize a wide variety ofsubstrates with immense structural diversity.”)

Structure-Brain Exposure Relationship J Med. Chem. 2006, 49, 7459

Optimizing Passive Diffusion

5-HT6 program

1 potent and selective, good bioavail., but lacked brain penetration

reduce # HBD and make more rigid

Optimizing Passive Diffusion

NAchR α7 program

5: full agonist Ki = 340nM6: Ki = 13 nM but poor bioavail.

“extensive SAR” led to 7Ki = 9.0nMpartial agonistgood bioavail.high B/P lack of HBD, giving lower PSA

Optimizing Passive Diffusion5-HT3 agonist program

For all of the compounds, the average brain concentrations mirrored those in plasma, suggesting that these molecules enter the brain rapidly and reach equilibrium between the brain and blood. Not surprisingly, carboxylic acid 9 displayed the highest blood and lowest brain concentrations; in fact, it was the only compound the brain concentration of which did not exceed the blood concentration (B/P = 0.10:1). Perhaps less predictably, alcohol 10 was more brain-penetrant than the more lipophilic but larger naphthane 11 (B/P = 20:1 and 2.8:1, respectively). This trend was also observed for the less lipophilic but smaller 12 compared to 13 (B/P = 4.9:1 and 2.1:1, respectively).

Optimizing Passive Diffusion

The search for selective antagonists of the D3-subtype of dopamine receptors D1-D5 has been driven by the notion that the extrapyramidal side effects of antipsychotic therapeutics operating through D2- and D3-subtypes arise from the D2 activity. It is therefore hypothesized that an agent selective for D3 might maintain efficacy and possess a better side effectprofile.

inh CYP450

good bioav. but no brain pen

inc PSA?

Optimizing Passive Diffusion

5-HT6 program – cognition

57 potent and selective, 18% brain pen.1 equipotent but poorer brain pen

importance on N-Me, reduce #HBD

59 same potency and selectivity but no Ndealkylation, 24& brain pen.

Optimizing Passive Diffusion

CCK-B antagonist 63 poor water solubilityadded acidic tetrazole – inc water solubility but no brain pen

added R3 and R4 to change conformation and pKa

65 – pKa = 5.1 and logD = 0.8966 – pKa = 5.7 and logD = 1.6

more potent but still not brain avail.

Enhancing Uptakemust bind well to target AND to transporter

A classic example of the power, and potential complexity, of carrier-mediated transport is the permeation of (S)-2-amino-3-(3,4-dihydroxyphenyl)propanoic acid 144 (L-DOPA) into the brain by the type 1 large neutral amino acid transporter (LAT1) Whereas dopamine (145) is a water-soluble catecholamine that does not appreciably cross the BBB, 144 isactively transported across this barrier by LAT1 and then transformed by aromatic amino acid decarboxylase (AAAD) into 145. It is critical that 144 passes both the luminal andabluminal sides of the brain capillary endothelial cells, as premature enzyme metabolism within these cells forms 145, which cannot pass the abluminal membrane and partitions into the blood. This impressive prodrug delivery strategy has enabledthe treatment of Parkinson’s disease for several decades.

Enhancing Uptake

pregabalin (146) and 149 bind well to α2-δ subunit of Ca channel

146 is poor inhibitor of system-L-transporter

149 good inhibitor