Phase I Metabolism: Oxidation and Reduction

Phase I metabolism comprises the functionalisation reactions of drug biotransformation — oxidation, reduction, and hydrolysis — that introduce or unmask a reactive chemical group such as a hydroxyl, amino, or carboxyl group. These reactions, dominated by oxidation catalysed by cytochrome P450 enzymes, modestly increase a drug's polarity and often prepare it for the conjugation reactions of Phase II. Phase I can inactivate a drug, activate a prodrug, or generate reactive intermediates.

Definition

Phase I metabolism is the set of functionalisation reactions — oxidation, reduction, and hydrolysis — that add or expose a polar functional group on a drug, typically via cytochrome P450 and other oxidoreductase enzymes, altering its activity and preparing it for excretion or further conjugation.

Scope

The topic covers the chemistry of Phase I reactions, especially cytochrome P450-catalysed oxidations (hydroxylation, dealkylation, epoxidation, heteroatom oxidation) together with reductions and the contributions of non-CYP oxidoreductases such as flavin-containing monooxygenases. It treats Phase I as a chemical and pharmacological topic within drug metabolism, distinct from but feeding into Phase II conjugation; it is not clinical dosing guidance.

Core questions

• What chemical changes define a Phase I functionalisation reaction?
• Why is cytochrome P450-catalysed oxidation the dominant Phase I pathway?
• How do reduction and hydrolysis contribute to Phase I metabolism?
• When does a Phase I reaction inactivate, activate, or bioactivate a drug?
• How does Phase I metabolism relate to subsequent Phase II conjugation?

Key concepts

• Functionalisation reactions
• Cytochrome P450 oxidation
• Hydroxylation and dealkylation
• Epoxidation and heteroatom oxidation
• Reduction reactions
• Hydrolysis (esterases and amidases)
• Flavin-containing monooxygenases
• Reactive intermediates
• Prodrug activation

Mechanisms

Most Phase I metabolism is oxidative and is carried out by the cytochrome P450 monooxygenases, which use molecular oxygen and NADPH to insert a single oxygen atom into the substrate. Characteristic reactions include aliphatic and aromatic hydroxylation, N- and O-dealkylation, epoxidation of double bonds, and oxidation of nitrogen and sulfur heteroatoms; other oxidoreductases such as flavin-containing monooxygenases and monoamine oxidases contribute to particular substrates. Reductive reactions (of nitro, azo, and carbonyl groups) and hydrolytic reactions (cleavage of esters and amides by esterases and amidases) complete the functionalisation repertoire. The net effect is a more polar metabolite bearing a handle for Phase II conjugation; where oxidation produces an electrophilic species such as an arene oxide or quinone, the same chemistry can underlie bioactivation and toxicity.

Clinical relevance

Phase I metabolism governs how quickly many drugs are cleared and is the step most often altered by enzyme induction or inhibition and by genetic differences in CYP enzymes, which helps explain interindividual variability in drug response. It is also the route by which several prodrugs are activated and by which some drugs form reactive metabolites. This entry explains those chemical mechanisms as reference knowledge and does not provide individualised dosing or treatment advice.

Evidence & guidelines

Evidence on Phase I pathways comes chiefly from in vitro studies with recombinant enzymes, liver microsomes, and hepatocytes, complemented by human pharmacokinetic data and structure-activity analysis, as synthesised in drug-metabolism reviews and texts. Regulatory drug-metabolism and drug-interaction guidance (for example from the US FDA and EMA) builds on this evidence, but the topic entry is an educational overview rather than a protocol.

History

Phase I and Phase II were distinguished conceptually in the mid-twentieth century as the functionalisation and conjugation stages of biotransformation. The identification of cytochrome P450 as the oxygen-activating pigment responsible for microsomal drug oxidation in the early 1960s, and the subsequent mechanistic study of its catalytic cycle, established oxidation as the central Phase I process and made the chemistry of these reactions a foundation of modern drug metabolism.

Key figures

• F. Peter Guengerich
• Bernard Testa
• Grant R. Wilkinson

Seminal works

• guengerich-2001
• wilkinson-2005

Frequently asked questions

Why is oxidation the most common Phase I reaction?

Because the cytochrome P450 enzyme family is abundant in the liver and broadly substrate-tolerant, oxidative reactions such as hydroxylation and dealkylation handle a large share of drug functionalisation.

Does every drug undergo Phase I before Phase II?

No. Many drugs already bear a suitable functional group and are conjugated directly in Phase II, while others are eliminated unchanged; Phase I precedes Phase II only when a functional handle must first be added or exposed.