Transcript
0:00Kp and Kp,uu
Lesson 3.3 of our distribution module: the blood-brain barrier. Distribution is the movement of a drug from the blood into the tissues and back. The blood-brain barrier is the wall of the brain capillaries, the smallest blood vessels. Endothelial cells form this wall. In the blood and in the brain, part of the drug binds to proteins and tissue. Kp is the total brain concentration, bound plus unbound, divided by the total concentration in plasma, the liquid part of the blood. Kp,uu is the same ratio for unbound drug. Among drugs that act in the brain, Kp,uu differs by up to 150-fold, from 0.02 to 3. Kp differs by at least 2,000-fold. Because only unbound drug binds to its target, Kp,uu probably best predicts whether a drug acts in the brain. Two parts of the barrier limit drug entry.
1:16The physical barrier
The first is the physical barrier. Tight junctions close the gaps between the endothelial cells, so a drug must cross the cells. It can cross by passive diffusion, movement from a higher to a lower concentration without energy use. Lipophilicity, the tendency to dissolve in lipids, mainly sets the rate, because cell membranes are made of lipids. For this, a drug needs a molecular weight below 400 daltons and fewer than 8 hydrogen bonds. Most small-molecule drugs lack these properties.
1:56The transport barrier
The second is the transport barrier. Efflux transporters are proteins that carry their substrates, the molecules they transport, out of a cell. P-glycoprotein, or P-gp, and breast cancer resistance protein, or BCRP, lie in the membrane that faces the blood. They use energy from ATP, adenosine triphosphate, the molecule that stores energy in cells. In human brain capillaries, BCRP is the most abundant drug transporter, at 8.14 femtomoles per microgram of protein. Mouse brain capillaries hold 2.33 times as much P-gp as human capillaries, so the effect of P-gp can differ in size between mice and humans.
2:53Kp,uu and clearance
At steady state, when concentrations stay constant, Kp,uu equals the clearance into the brain divided by the clearance out of the brain. Each clearance is the volume of fluid from which all unbound drug is removed per unit time. This fluid is plasma for entry and brain fluid, the fluid between brain cells, for exit. The clearance out of the brain includes efflux across the barrier, metabolism in the brain and the bulk flow of brain fluid. If a drug crosses by passive diffusion alone and has little metabolism in the brain, the clearances are similar, and Kp,uu is close to 1. Kp,uu is below 1 when active efflux dominates and above 1 when active entry dominates.
3:50Binding to brain tissue
Binding to brain tissue raises the total brain concentration and Kp. At steady state, it leaves the unbound brain concentration and Kp,uu unchanged.
4:06P-gp in humans
A study in 37 healthy volunteers measured how much P-gp limits the brain uptake of carbon-11 N-desmethyl-loperamide, a radioactive P-gp substrate. It made 42 scans with positron emission tomography, or PET. In 12 volunteers, the substrate was injected during an infusion of tariquidar, a drug that blocks P-gp, into a vein. Its brain uptake was then about 5 times the uptake without tariquidar. Oral tariquidar gave a plasma concentration of 13 percent of the intravenous value and no increase in brain uptake.
4:55Loperamide in mice
Loperamide is an opioid, a drug that acts on the same receptors as morphine. It treats diarrhea and acts outside the brain, and it is a P-gp substrate. In mice without the mdr1a gene for P-gp, the brain uptake of loperamide and its metabolites rose 7 times. In these mice, oral loperamide had opioid effects in the brain and spinal cord.
5:28Summary
In summary, tight junctions close the gaps between the endothelial cells, and lipophilicity mainly sets the rate of passive diffusion. At steady state, Kp,uu equals the clearance into the brain divided by the clearance out of the brain. When efflux by transporters such as P-gp and BCRP dominates, Kp,uu is below 1. At steady state, binding to brain tissue raises Kp but leaves Kp,uu unchanged. In healthy volunteers, an infusion of the P-gp inhibitor tariquidar into a vein raised the brain uptake of a P-gp substrate about 5 times. In mice without P-gp, the brain uptake of loperamide rose 7 times, and oral loperamide had opioid effects in the brain and spinal cord. This concludes lesson 3.3 on the blood-brain barrier.
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Illustrations adapted from Servier Medical Art (smart.servier.com), CC BY 4.0. Structures drawn with RDKit.
