Blood-Brain Barrier

1. Blood-Brain Barrier

2. 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 membrane glucose amino acids hormones 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. CHEM E-120

3. Brain Barrier Systems 3 distinct membrane (meninges) layers surround the brain. The meninges are connected to the skull. The brain is mechanically suspended within the meninges. 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. CHEM E-120

4. BBB in Capillary Cell 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.) CHEM E-120

5. BBB in Capillary Cell Cerebral endothelial cells are unique in that they form complex tight junctions (TJ) produced by the interaction of several 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, nucleotidases and 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 CHEM E-120

6. 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 plasma Fubrain = unbound [drug] in brain in mice Fuplasma≅Fubrain even though [drug in blood]/[drug in brain] varied greatly

7. BBB studies JPET 2002, 303, 1029

8. JPET 2002, 303, 1029 BBB studies passive diffusion CNS faster non-CNS Pgp substrate CNS poorer non-CNS if: mw > 400 & large size non-CNS good Pgp CNS poor

9. 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 of substrates with immense structural diversity.”) Structure-Brain Exposure Relationship J Med. Chem. 2006, 49, 7459

10. Optimizing Passive Diffusion 5-HT6 program 1 potent and selective, good bioavail., but lacked brain penetration reduce # HBD and make more rigid

11. Optimizing Passive Diffusion NAchR α7 program 5: full agonist Ki = 340nM 6: Ki = 13 nM but poor bioavail. “extensive SAR” led to 7 Ki = 9.0nM partial agonist good bioavail. high B/P lack of HBD, giving lower PSA

12. Optimizing Passive Diffusion 5-HT3 agonist program Forall of the compounds, the average brain concentrations mirroredthose in plasma, suggesting that these molecules enter the brainrapidly and reach equilibrium between the brain and blood. Notsurprisingly, carboxylic acid 9 displayed the highest blood and lowest brain concentrations; in fact, it was the only compoundthe brain concentration of which did not exceed the bloodconcentration (B/P = 0.10:1). Perhaps less predictably, alcohol 10 was more brain-penetrant than the more lipophilic but largernaphthane 11 (B/P = 20:1 and 2.8:1, respectively). This trend was also observed for the less lipophilic but smaller 12 comparedto 13 (B/P = 4.9:1 and 2.1:1, respectively).

13. Optimizing Passive Diffusion inh CYP450 inc PSA? good bioav. but no brain pen The search for selective antagonists of the D3-subtype ofdopamine receptors D1-D5 has been driven by the notion that the extrapyramidal side effects of antipsychotic therapeuticsoperating through D2- and D3-subtypes arise from the D2activity. It is therefore hypothesized that an agent selectivefor D3 might maintain efficacy and possess a better side effect profile.

14. 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 N dealkylation, 24& brain pen.

15. Optimizing Passive Diffusion CCK-B antagonist 63 poor water solubility added acidic tetrazole – inc water solubility but no brain pen added R3 and R4 to change conformation and pKa 65 – pKa = 5.1 and logD = 0.89 66 – pKa = 5.7 and logD = 1.6 more potent but still not brain avail.

16. Enhancing Uptake must 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 thebrain by the type 1 large neutral amino acid transporter (LAT1) Whereas dopamine (145) is a water-solublecatecholamine that does not appreciably cross the BBB, 144 is actively transported across this barrier by LAT1 and thentransformed by aromatic amino acid decarboxylase (AAAD)into 145. It is critical that 144 passes both the luminal and abluminal sides of the brain capillary endothelial cells, aspremature enzyme metabolism within these cells forms 145,which cannot pass the abluminal membrane and partitions intothe blood. This impressive prodrug delivery strategy has enabled the treatment of Parkinson’s disease for several decades.

17. 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