Eyes, fat, and bone have a longer time to reach equilibrium due to which combination of factors?

Eyes, fat, and bone have a longer time to reach equilibrium due to which combination of factors?

• A. Longer rate to equilibrium; low perfusion
• B. Shorter rate to equilibrium; high perfusion
• C. Longer rate to equilibrium; high perfusion
• D. Shorter rate to equilibrium; low perfusion

Answer: (A)

The key idea is that how fast a drug’s concentration in a tissue reaches equilibrium with plasma depends on how much blood flows to that tissue (perfusion) and how much capacity or affinity the tissue has for the drug. Eyes, fat, and bone receive relatively little blood flow, so the delivery of drug from the plasma to these tissues is slow. That low perfusion means the rate constant for transfer into the tissue is small, leading to a longer time to reach equilibrium. These tissues also tend to have high capacity to hold drug (fat for lipophilic drugs, bone matrix binding, etc.), which can further slow the return of drug to plasma and extend the equilibration period. So the combination of a slower rate of distribution and low perfusion best explains why eyes, fat, and bone take longer to equilibrate.

The key idea is that how fast a drug’s concentration in a tissue reaches equilibrium with plasma depends on how much blood flows to that tissue (perfusion) and how much capacity or affinity the tissue has for the drug. Eyes, fat, and bone receive relatively little blood flow, so the delivery of drug from the plasma to these tissues is slow. That low perfusion means the rate constant for transfer into the tissue is small, leading to a longer time to reach equilibrium. These tissues also tend to have high capacity to hold drug (fat for lipophilic drugs, bone matrix binding, etc.), which can further slow the return of drug to plasma and extend the equilibration period. So the combination of a slower rate of distribution and low perfusion best explains why eyes, fat, and bone take longer to equilibrate.