Tight albumin binding reduces diffusion to the target organ. What is the consequence?

• A. Increases diffusion
• B. Has no effect
• C. Reduces diffusion/transport to the target organ
• D. Accelerates target receptor binding

Answer: (C)

Tight albumin binding keeps most of the drug in a bound state, and the bound form cannot cross capillary walls easily. Only the unbound fraction can diffuse into tissues to reach the target. So when binding is strong, the free drug concentration in plasma is low and diffusion/transport into the target organ is reduced. In other words, the drug’s ability to reach the target is limited by how much remains unbound. This binding also slows clearance, since the bound portion is not readily filtered or metabolized, but the immediate consequence for diffusion to the target is a decrease.