What determines the ultimate brain concentration of a drug after crossing the BBB?

• A. Only rate of systemic clearance determines brain concentration.
• B. Balance of influx and efflux plus distribution into brain tissue.
• C. Only lipophilicity determines brain concentration.
• D. Once it crosses, brain concentration equals plasma concentration.

Answer: (B)

The amount of drug that ends up in brain tissue after crossing the BBB is governed by the balance of entry into the brain (influx) and exit from the brain (efflux), together with how the drug distributes within brain tissue. Influx includes both passive diffusion across the barrier and any carrier-mediated uptake that helps the drug get into the brain. Efflux comes from active transporters, such as P‑glycoprotein and other efflux pumps, that move the drug back into blood or CSF, which can dramatically limit brain exposure even if entry is favorable. Once inside, the drug then partitions into the brain interstitial fluid and binds to tissue components, determining how much remains in the brain versus how much is distributed or cleared. So the steady-state brain concentration reflects the net result of how fast it enters, how fast it is removed, and how it distributes within brain tissue, rather than being determined solely by systemic clearance, lipophilicity, or plasma levels alone.