Question

Drugs are distributed where the blood is distributed, however certain barriers can limit drug distribution to the target site to take effect, which of the following can act as barriers to drug distribution to target site?

*Hint more than one can be correct

Select all that apply:

1. lipid solubility
2. ionization of drug
3. blood brain barrier
4. placenta
5. median eminence
6. epithelial cells on capillaries
7. serum/depot binding

Answer 1

• Lipid solubility - The lipid solubility is one of the factors which influence the drug solubility to the areas of distribution. Lipid solubility of the drug helps in determining if it would allow the diffusion and distribution of the drugs across the membranes. When the drug is more lipid soluble, it is more easily distributed across the target site.
• Degree of ionization – This is another way how the drug distribution can vary. The drugs in order for them to be distributed need to be present in either weak acid or basic form. When they are present in their ionized form, they are more readily distributed across the blood.
• Blood brain barrier – This is a barrier which is present in the brain and hence it limits the movement of drug and its diffusion into the brain across the bloodstream. Due to the presence of endothelial cells and different glial cells, they tend to form a barrier and hence it limits the movement of the drug leading to its selective movement.
• Epithelial cells on capillaries – These cells act as barrier and they bind to each other forming tight junctions and hence limits the movement of the molecules. It only allows selective movement of the drug molecule and limits its distribution to target site.
• Serum/Depot binding – when the drug binds to depot sites or certain inactive receptors, it tends to decrease the bioavailability and distribution of the drug to the target site. It is most common in people with high fatty acid and when there is weight loss, the drug tends to be release from such sites leading to increased distribution.

The median eminence does not impact the drug bioavailabilty and distribution as it is a part within the hypothalamus which regulates the formation and release of the hormones. The placental barrier is a barrier between blood of mother and fetus and does not necessarily lead to impacting the distribution of the drug across the target parts.