Define intrinsic clearance (CLint) and describe how it is measured in vitro.

• A. The liver's enzymatic metabolism capacity independent of blood flow; measured with liver microsomes or hepatocytes.
• B. The total body clearance considering cardiac output; measured in vivo.
• C. The rate of renal excretion of the drug; measured with renal cells.
• D. The capacity of the gut to absorb drug from the lumen; measured with intestinal cells.

Answer: (B)

Intrinsic clearance is the liver’s inherent enzymatic capacity to metabolize a drug, independent of blood flow or transport limitations. In vitro, it’s measured by incubating the drug with liver-derived preparations such as microsomes or hepatocytes and tracking the rate at which the parent drug disappears (or the rate at which metabolites form). From this rate, you derive CLint (often tied to enzyme kinetics as Vmax/Km or simply rate per unit drug concentration). This in vitro CLint can then be scaled up to a whole liver value using factors like the amount of microsomal protein per gram of liver and total liver weight, enabling prediction of in vivo hepatic clearance with models like the well-stirred model. The description that fits this concept is the liver’s enzymatic metabolism capacity independent of blood flow measured with liver microsomes or hepatocytes. The other options describe total body clearance in vivo, renal excretion, or intestinal absorption—processes that are not intrinsic hepatic clearance.