Pharamcokinetic Characteristics of Biologics

Compared to small molecule drugs, biologics have unique characteristics in absorption, distribution, metabolism, and elimination. Pharmacokinetics of a biologics is typically characterized by a two-compartment model, with three key parameters, half-life, volume of distribution and clearance. The volume of distribution reflects the relationship between the amount of drug in the body at steady-state and plasma drug concentration. It is a mathematical concept, which does not necessarily reflect a physiological or “real” distribution space, although for biologics, they are more closely related. The model dependent estimation of the volume of distribution will be discussed.

PK of biologics is generally known to be characterized by a two-compartment model, with three key parameters, half-life, volume of distribution and clearance. The volume of distribution is reflective of the extracellular space, which is about 15-20 liters. It’s considered to be small (<1 l/kg) compared to small molecules. It is to be noted that model dependent volume of distribution is likely underestimated, due to the invalid assumption that clearance occurs only from the central compartment. Therefore, imaging study to actually measure biologics distribution in specific tissues or organs is more meaningful.