DrugTheGuide
Pharmacokinetics

Intrinsic Clearance CLint

The maximum rate at which the liver can clear unbound drug, independent of blood flow limitations. Fundamental to predicting hepatic clearance, drug-drug interactions, and the effect of enzyme inhibition or induction on systemic drug exposure.

Definition and Formula

Intrinsic clearance (CLint) is the theoretical capacity of metabolic enzymes (typically in the liver) to clear unbound drug from perfusing blood — expressed as the volume of blood completely cleared per unit time if flow and protein binding were not limiting factors.

For a single enzyme following Michaelis-Menten kinetics:

CLint = Vmax / Km

Where Vmax is the maximum metabolic rate (amount per time) and Km is the Michaelis constant (drug concentration at half-maximum rate). In practice, CLint reflects the combined capacity of all enzymes — CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP3A4, UGTs, SULTs, etc. — acting on the drug.

Reference: NCBI StatPearls pharmacokinetics ↗; Rostami-Hodjegan: prediction of CLint (PubMed) ↗.

Well-Stirred Model: Linking CLint to Hepatic Clearance

The well-stirred (venous equilibrium) model — the most widely used pharmacokinetic model of hepatic clearance — relates CLint to hepatic clearance:

CLH = QH × fu × CLint / (QH + fu × CLint)

Where:

  • QH = hepatic blood flow ≈ 1.5 L/min (range 0.9–1.8 L/min)
  • fu = fraction of drug unbound in plasma (1 − protein bound fraction)
  • CLint = intrinsic clearance of unbound drug

The hepatic extraction ratio (ER) = CLH / QH. The well-stirred model predicts: ER = fu × CLint / (QH + fu × CLint).

CLint relative to QH/fu Extraction ratio CLH limited by Examples
Very high (CLint >> QH/fu) High (>0.7) Blood flow (QH) Lidocaine, propranolol, morphine
Moderate Intermediate (0.3–0.7) Both flow and capacity Codeine, nifedipine
Very low (CLint << QH/fu) Low (<0.3) Enzyme capacity (fu × CLint) Warfarin, diazepam, phenytoin

Clinical Relevance: Enzyme Inhibition, Induction, and Liver Disease

  • CYP inhibitors (fluconazole, clarithromycin, amiodarone) reduce CLint for susceptible substrates. For low-extraction drugs (CLH ≈ fu × CLint), this significantly increases plasma AUC and drug exposure. For high-extraction drugs, the effect on oral bioavailability (first-pass) is larger than the effect on systemic clearance. See our CYP450 guide for specific inhibitor-substrate pairs.
  • CYP inducers (rifampicin, carbamazepine, phenytoin, St John's Wort) increase CLint by upregulating enzyme expression, reducing substrate AUC. For warfarin (a low-extraction drug), rifampicin co-administration reduces warfarin AUC by 60–70% — requiring major dose increases.
  • Hepatic disease reduces functional hepatocyte mass and CLint. For low-extraction drugs primarily cleared by the liver, cirrhosis substantially prolongs half-life and increases drug exposure. See our hepatic dosing guide and FDA guidance on hepatic impairment pharmacokinetics ↗.
  • In vitro to in vivo extrapolation (IVIVE) of CLint — measured in liver microsomes during drug development — underpins FDA NDA/IND requirements for predicting human hepatic clearance before first-in-human studies.

FAQ

What is intrinsic clearance?
Intrinsic clearance (CLint) is the theoretical maximum ability of the liver (or other metabolising organ) to clear an unbound drug, in the absence of limitations imposed by blood flow or protein binding. It represents the combined catalytic activity of all metabolic enzymes acting on the drug. Mathematically, CLint = Vmax / Km for a single-enzyme Michaelis-Menten system, where Vmax is the maximum metabolic rate and Km is the enzyme affinity constant.
How does intrinsic clearance relate to hepatic clearance?
Hepatic clearance (CLH) depends on liver blood flow (QH), protein binding (fraction unbound fu), and intrinsic clearance (CLint) according to the well-stirred (venous equilibrium) model: CLH = QH × fu × CLint / (QH + fu × CLint). For high-CLint drugs (high extraction ratio), CLH ≈ QH — clearance is flow-limited, and changes in CLint have little effect. For low-CLint drugs (low extraction ratio), CLH ≈ fu × CLint — clearance is capacity-limited and sensitive to protein binding and enzyme induction/inhibition.
How does enzyme inhibition or induction affect intrinsic clearance?
CYP enzyme inhibitors reduce CLint by competitively or non-competitively blocking metabolic sites. For a low-extraction drug where CLH ≈ fu × CLint, inhibition of a major CYP isoform reduces CLint, increases AUC, and raises plasma concentrations — potentially to toxic levels. For a high-extraction drug, the same inhibition has little systemic effect (though it may affect first-pass extraction). Enzyme inducers (rifampicin, carbamazepine, St John's Wort) increase CLint by increasing CYP enzyme expression, reducing plasma concentrations of co-administered drugs.
What is the difference between in vitro CLint and in vivo CLint?
In vitro CLint is measured in liver microsomes, hepatocytes, or recombinant enzyme systems, and represents the intrinsic metabolic capacity per unit of biological material. In vivo CLint extrapolates from in vitro data using scaling factors (microsomal protein per gram liver, liver weight, hepatocellularity) to predict in vivo hepatic clearance. Discrepancies between in vitro and in vivo CLint (the IVIVE gap) arise from in vitro binding, missing metabolic pathways, transporter contributions, and albumin-mediated uptake not captured in microsomal assays.