If a drug is highly bound to plasma proteins, how does this affect half-life and affinity?

If a drug is highly bound to plasma proteins, how does this affect half-life and affinity?

• A. Shorter half-life and higher affinity to tissue
• B. Shorter half-life and lesser affinity to tissue
• C. Longer half-life and higher affinity to tissue
• D. Longer half-life and lesser affinity to tissue

Answer: (D)

High plasma protein binding means most of the drug in the bloodstream is tied up as a bound form, leaving only a small free fraction to act, be distributed, and be cleared. For half-life, only the unbound drug is readily cleared by metabolism or excretion. When the free portion is small, clearance slows down, so the elimination half-life increases. In other words, the drug stays in the body longer because most of it is bound and not being cleared quickly. For affinity to tissue, only the free drug can cross capillary membranes and reach tissue targets. With most of the drug bound, less free drug is available to distribute into tissues, so tissue exposure and apparent affinity to tissue are reduced. The intrinsic binding to tissue receptors isn’t changed by protein binding, but the amount of drug reaching those tissues is lower due to the reduced free fraction. So, high protein binding leads to a longer half-life and decreased distribution to tissues.

High plasma protein binding means most of the drug in the bloodstream is tied up as a bound form, leaving only a small free fraction to act, be distributed, and be cleared.

For half-life, only the unbound drug is readily cleared by metabolism or excretion. When the free portion is small, clearance slows down, so the elimination half-life increases. In other words, the drug stays in the body longer because most of it is bound and not being cleared quickly.

For affinity to tissue, only the free drug can cross capillary membranes and reach tissue targets. With most of the drug bound, less free drug is available to distribute into tissues, so tissue exposure and apparent affinity to tissue are reduced. The intrinsic binding to tissue receptors isn’t changed by protein binding, but the amount of drug reaching those tissues is lower due to the reduced free fraction.

So, high protein binding leads to a longer half-life and decreased distribution to tissues.