What is the effect of albumin's tight binding with many drugs on the drug's availability for diffusion into its target organ?

• A. Increases diffusion
• B. Reduces availability for diffusion/transport
• C. Has no effect on diffusion
• D. Increases drug-target interaction

Answer: (B)

Protein binding controls how much drug is free to move. Only the unbound (free) drug can cross capillary walls and diffuse into a target organ. When a drug is tightly bound to albumin, most of it is in the bound form and not available to diffuse, so the rate and extent of diffusion into the target tissue are reduced. The bound portion acts like a reservoir, releasing drug as the unbound fraction is used or as equilibrium shifts, but it does not directly contribute to diffusion unless it dissociates. This is why high albumin binding decreases availability for diffusion/transport into the target organ.