Final answer:
The biologic activity of a drug when it is distributed into systemic circulation is related to plasma protein binding and the drug's ability to penetrate physiological barriers.
Step-by-step explanation:
The biologic activity of a drug when it is distributed into systemic circulation is related to several factors. One important factor is the plasma protein binding (PPB) of the drug. High PPB can reduce the clearance of a compound and increase its half-life. Another factor to consider is the drug's ability to penetrate physiological barriers, such as the blood-brain barrier (BBB) and cell membranes. The affinity of a compound to plasma proteins and its physiochemical properties can influence its ability to cross these barriers and reach the intended target.