Question

The serum protein binding of azithromycin is concentration dependent, ranging from 51% at 0.02mg/mL to 7% at 2.0mg/mL as reported in the literature. Following oral administration, azithromycin is widely distributed throughout the body with an apparent steady-state volume of distribution of 31.1 L/kg. Higher azithromycin concentrations in tissues than in plasma or serum have been observed.

What is the apparent VD for a subject weighing 70 kg ? Is the apparent VD greater or lower than the plasma volume of the body for this subject? Do you think protein binding affect the distribution of this drug? Why? Or Why not?

Answer 1

The apparent volume of distribution (VD) for a subject weighing 70 kg taking azithromycin is 2177 L, which is greatly higher than the body's plasma volume. Protein binding affects drug distribution since only the unbound portion can be effectively distributed to tissues.

Step-by-step explanation:

The apparent volume of distribution (VD) for a subject weighing 70 kg, given the reported steady-state VD of 31.1 L/kg for azithromycin, is calculated by multiplying the VD value by the subject's weight:

Apparent VD = 31.1 L/kg × 70 kg = 2177 L.

This apparent VD is much greater than the estimated plasma volume of the body for this subject which is approximately 5% of the body weight, hence roughly 3.5 L for a 70 kg person. Azithromycin's larger apparent VD indicates extensive distribution into the tissues.

Protein binding does affect the distribution of azithromycin because only the unbound drug is available to cross membrane barriers and be effectively distributed to tissues. Given the concentration-dependent protein binding of azithromycin, interaction with serum albumin could influence tissue concentration levels and pharmacokinetics, including efficacy and clearance of the drug from the body.