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Primary, secondary, and tertiary metabolite kinetics
M V St-Pierre1, X Xu, K S Pang
1Faculty of Pharmacy, University of Toronto, Ontario, Canada.
Journal of Pharmacokinetics and Biopharmaceutics
|October 1, 1988
Summary
This study models drug metabolism, revealing how drug dose, route, and absorption influence metabolite levels. Comparing oral and intravenous dosing helps determine drug absorption and liver availability, crucial for understanding drug disposition.
Area of Science:
- Pharmacokinetics and Drug Metabolism
- Quantitative Systems Pharmacology
- Biopharmaceutical Sciences
Background:
- Sequential metabolism of drugs generates primary, secondary, and tertiary metabolites.
- Accurate mass balance requires accounting for all metabolite formation sources.
- Understanding drug and metabolite disposition is vital for accurate pharmacokinetic modeling.
Purpose of the Study:
- To develop a theoretical framework for sequential drug metabolism.
- To illustrate metabolite concentration-time profiles following oral and intravenous administration.
- To establish relationships between pharmacokinetic parameters and metabolite exposure.
Main Methods:
- Utilized a one-compartment open model with first-order kinetics.
- Simulated drug and metabolite concentration-time profiles.
- Integrated mass balance equations considering various absorption and conversion rates.
Main Results:
- Derived explicit solutions for metabolite concentrations.
- Identified a simple relationship between metabolite area under the curve (AUC), clearance, and route of administration.
- Demonstrated that AUC(m) is influenced by drug dose, absorption fraction (Fabs), and metabolite clearance (CL(m)) and availability (F(m)).
Conclusions:
- The ratio of metabolite AUC after oral versus intravenous dosing can determine the fraction absorbed (Fabs).
- Systemic availability (Fsys) and Fabs can be used to calculate drug availability from first-pass organs (F).
- This model provides a quantitative approach to understanding sequential metabolism and drug disposition.