4) Assume that a researcher has finally produced two different drugs that increa

Question

4) Assume that a researcher has finally produced two different drugs that increased survival of neurons in cell culture to be used for treating Alzheimer's disease: one is a small uncharged molecule and another is a small but charged molecule. Please explain a) how each of these two molecules would be transported into the brain if administered in blood, and which one you would chose for further testing in animals and why; b) if these molecules were found to increase the amount of Acetylcholine in the synaptic cleft, what might be their mode of action, please discuss?

Explanation / Answer

4a) Two drugs of differing nature are being discussed here. One is small, uncharged while the other is small but charged molecule. We have to consider 1) transport in blood and 2) transport across the blood-brain barrier. Once a drug is administered, it is pumped by the heart and carried by the carotid and vertebral arteries to the brain. During this transport, the solubility of the drug in the blood plasma is the determining factor. Usually structure and charge of the drug molecule decides its solubility. If the drug molecule is small and charged, then it can interact with the water molecules through its polar groups and form hydrogen bonds. The polarity of the water molecules dissociates polar molecules. On the contrary, the uncharged nature of the drug molecules restricts its solubility even if small. In such cases, these drug molecules are transported with the help of blood proteins. The polar amino acids on the exterior of these proteins interact with water via their charged groups increasing water solubility. While the non-polar amino acids in the interior shield the uncharged drug molecules from the hydrophilic exterior. Transport across the blood brain barrier (BBB) can occur primarily by transmembrane diffusion and by saturable transport, but also by adsorptive endocytosis, and other extracellular pathways. Transmembrane diffusion is a non-saturable mechanism and depends on the molecular weight, hydrophobicity of the drug and the ability of the drug to partition from the blood into the BBB. A low molecular weight (small) and lipid soluble (uncharged, low hydrophilicity) drug molecule is therefore greatly favored during transmembrane diffusion. On the contrary, saturable transport systems are exploited by charged, polar or hydrophilic drug molecules. These transporters are selective and usually have high uptake rates, making this transport process specific and faster than transmembrane diffusion.

4b) Both the drug molecules increase the amount of Acetylcholine in the synaptic cleft.

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