Phase I Metabolism (Oxidation, Reduction, Hydrolysis)

Phase I metabolism comprises the functionalisation reactions of biotransformation — oxidation, reduction, and hydrolysis — that introduce or expose a polar functional group on a drug molecule. These reactions, dominated by the cytochrome P450 enzymes, either prepare a compound for Phase II conjugation or directly produce a more water-soluble (and sometimes more reactive) metabolite.

Definition

Phase I metabolism is the set of biotransformation reactions that functionalise a drug — adding or unmasking a reactive group through oxidation, reduction, or hydrolysis — typically yielding a more polar metabolite or a substrate for subsequent Phase II conjugation.

Scope

This topic covers the three Phase I reaction types, the enzyme systems that carry them out, and the consequences of functionalisation for clearance, metabolite activity, and reactive-metabolite formation. It is educational and offers no dosing guidance.

Core questions

• What distinguishes oxidation, reduction, and hydrolysis as Phase I reactions?
• Which enzymes catalyse the major Phase I oxidations?
• How does functionalisation prepare a drug for Phase II conjugation?
• When do Phase I reactions generate active or reactive metabolites?

Key concepts

• Functionalisation reactions
• Oxidation (cytochrome P450-mediated)
• Reduction
• Hydrolysis (esterases, amidases, epoxide hydrolase)
• Reactive and active metabolites
• Flavin-containing monooxygenases
• Substrate exposure of polar groups for conjugation

Mechanisms

Phase I reactions act on the drug molecule itself to create or reveal a functional group. Oxidations are the most numerous and are carried out chiefly by the cytochrome P450 superfamily, with contributions from flavin-containing monooxygenases and other oxidases; these reactions hydroxylate carbon atoms, dealkylate heteroatoms, and oxidise nitrogen or sulphur (Guengerich, 2007). Reductions (for example of azo, nitro, or carbonyl groups) and hydrolyses (of esters and amides by esterases and amidases, or of epoxides by epoxide hydrolase) complete the set. The newly introduced group both increases polarity and provides a chemical handle for Phase II conjugation. Because oxidation can generate electrophilic intermediates, Phase I metabolism is also the step at which reactive metabolites implicated in toxicity may arise (Guengerich, 2007). The overall capacity of these enzymes, especially the P450s, is a principal determinant of metabolic clearance and of between-patient variability in exposure (Wilkinson, 2005; Zanger & Schwab, 2013).

Clinical relevance

Phase I capacity, and its modulation by enzyme induction, inhibition, and genetic variation, contributes to differences in drug exposure between individuals and to reactive-metabolite-related toxicity. This entry describes the chemistry and enzymology as reference material; it is not a basis for individual dosing or interaction decisions.

Evidence & guidelines

The reaction chemistry and the dominant role of cytochrome P450 in oxidative functionalisation are documented in comprehensive reviews (Guengerich, 2007; Zanger & Schwab, 2013), and the link between Phase I metabolic capacity and variability in drug response is summarised in major clinical reviews (Wilkinson, 2005) and standard texts (Rowland & Tozer, 2011).

History

The recognition that drug oxidation is catalysed by a haemoprotein with a characteristic 450 nm absorbance established cytochrome P450 as the central Phase I enzyme system in the latter twentieth century. The two-phase scheme — functionalisation followed by conjugation — became the standard framework for describing biotransformation, with later work emphasising both the diversity of P450 reactions and their role in generating reactive metabolites (Guengerich, 2007).

Key figures

• F. Peter Guengerich
• Ulrich M. Zanger
• Grant R. Wilkinson

Seminal works

• guengerich-2007
• zanger-schwab-2013

Frequently asked questions

What is the difference between Phase I and Phase II metabolism?

Phase I reactions functionalise the drug — adding or exposing a reactive group through oxidation, reduction, or hydrolysis — whereas Phase II reactions conjugate the drug or its Phase I product to an endogenous molecule. Phase I often, but not always, precedes Phase II.

Do Phase I reactions always inactivate a drug?

No. Functionalisation can produce metabolites that are pharmacologically active, inactive, or chemically reactive; prodrugs in particular rely on Phase I (or Phase II) reactions to generate their active form.