How does red blood cell binding affect distribution for drugs that bind to hemoglobin?

How does red blood cell binding affect distribution for drugs that bind to hemoglobin?

• A. It increases free distribution into tissues
• B. It sequesters drug in blood cells, potentially reducing free distribution but increasing apparent Vd if RBCs act as reservoir
• C. It has no impact on distribution
• D. It enhances penetration into CNS

Answer: (B)

When a drug binds to hemoglobin in red blood cells, a portion of the drug becomes trapped inside circulating cells. This means there is less free drug in the plasma to diffuse into tissues, so the immediate distribution to non-blood tissues is reduced. At the same time, because a sizeable amount of drug is held within the blood cells, the total amount of drug in the body relative to the plasma concentration can make the apparent volume of distribution look larger. The RBC-bound portion can also slowly release drug back into plasma, acting as a reservoir and potentially prolonging its presence in the body. This binding does not inherently enhance penetration into the CNS, since crossing the blood–brain barrier depends on the free, unbound drug.

When a drug binds to hemoglobin in red blood cells, a portion of the drug becomes trapped inside circulating cells. This means there is less free drug in the plasma to diffuse into tissues, so the immediate distribution to non-blood tissues is reduced. At the same time, because a sizeable amount of drug is held within the blood cells, the total amount of drug in the body relative to the plasma concentration can make the apparent volume of distribution look larger. The RBC-bound portion can also slowly release drug back into plasma, acting as a reservoir and potentially prolonging its presence in the body. This binding does not inherently enhance penetration into the CNS, since crossing the blood–brain barrier depends on the free, unbound drug.