A drug has a high first-pass effect. Explain how this affects oral bioavailability and the relationship between hepatic clearance, intrinsic clearance, and hepatic blood flow.

• A. High extraction by the liver reduces F because fu*CLint is large relative to Qh; hepatic clearance (CLh) approaches Qh and oral F decreases.
• B. High extraction increases F and makes CLh much greater than Qh.
• C. Oral F is unchanged but CLint decreases.
• D. Intrinsic clearance alone determines F directly.

Answer: (A)

High first-pass extraction means the liver metabolizes a large portion of the drug during its initial pass through hepatic circulation, so only a small amount reaches systemic circulation.

In the well-stirred model, hepatic clearance is CLh = Qh × (fu × CLint) / (Qh + fu × CLint). If the product fu × CLint is large compared with hepatic blood flow Qh, that fraction dominates and CLh approaches Qh. The hepatic extraction ratio E_h = CLh / Qh then nears 1, so the hepatic bioavailability F_h = 1 − E_h tends toward zero. Consequently, oral bioavailability F, which is influenced by hepatic metabolism on the first pass, decreases.

Intrinsic clearance CLint represents the liver’s metabolic capacity, and fu is the fraction unbound in plasma. A large fu × CLint means high intrinsic metabolic capacity relative to blood flow, driving CLh up toward Qh and pulling F downward. If fu × CLint were small relative to Qh, CLh would be limited by CLint and F would be closer to 1, yielding higher oral bioavailability.