Effect of Charge on the Pharmacokinetic Properties

Electric charge is also an important factor in determining the pharmacokinetic properties of macro-molecules.79 Because the glomerular capillary walls also function as a charge-selective barrier having negative charges, positively charged macromolecules exhibit higher glomerular permeability than anionic macromolecules of similar molecular weights. Larger molecular weight cationic macromolecules escaping glomerular filtration mainly accumulate in the liver, kidney, and lung.80-82 This phenomenon can be explained by following factors:

1. Because the cell surface is negatively charged in general, cationic molecules tend to electrostatically bind to it.

2. Because the liver and kidney have fenestrated capillaries, macromolecules can cross the endothelial barrier.

3. Direct interaction with the lung endothelial cells.

4. Embolization of aggregates of the complex with negatively charged blood components.

On the other hand, strongly negatively charged macromolecules are taken up by liver non-parenchymal cells through liver fenestrae by scavenger receptor-mediated endocytosis.78,83 Neutral or weakly anionic macromolecules with a molecular size that does not allow glomerular filtration such as polyethylene glycol and serum albumin posses a very weak affinity for the cell surface, and

they are cleared very slowly. , , , Consequently, such neutral or weakly anionic macromolecules have a longer elimination half-life compared with the approximately same sized macromolecule with a strong negative or positive charge.

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