Biotransformation: Phase I Reactions in Drug Metabolism

Biotransformation: Phase I Reactions

Introduction to Drug Metabolism

Drug metabolism is a biochemical process that modifies drugs within the body, primarily to facilitate their elimination. Phase I reactions introduce or expose functional groups on drug molecules, often making them more polar and ready for further metabolism or excretion.

• Phase I reactions include oxidation, reduction, and hydrolysis.

• These reactions are crucial for converting lipophilic drugs into hydrophilic metabolites.

• Enzymes involved are mainly found in the liver, but also in other tissues.

Phase I Reaction Types

Oxidation Reactions

Oxidation is the most common Phase I reaction, involving the addition of oxygen or removal of hydrogen from drug molecules. These reactions are catalyzed by various enzymes, primarily in the liver.

• Microsomal oxidations:

  • Cytochromes P450 (CYP)

  • Flavin-containing mono-oxygenases (FMO)

• Non-microsomal oxidations:

  • Alcohol and aldehyde dehydrogenases

  • Xanthine and aldehyde oxidases

Reduction Reactions

Reduction involves the gain of hydrogen or loss of oxygen, often catalyzed by microsomal and non-microsomal enzymes. These reactions are important for drugs containing nitro, azo, or carbonyl groups.

• Enzymes: Cytochrome P450, NADPH-cytochrome P450 reductases, bacterial reductases.

• Requires NADPH as a cofactor and occurs in low oxygen tension areas.

• Example: Reduction of warfarin produces a mixture of diastereoisomers.

Hydrolysis Reactions

Hydrolysis is the enzymatic addition of water to esters and amides, resulting in their breakdown. This process is catalyzed by esterases and amidases, which are widely distributed in various tissues.

• Esterases: Catalyze hydrolysis of esters (e.g., acetylcholine).

• Amidases: Catalyze hydrolysis of amides (e.g., procainamide).

• Amides are generally hydrolyzed at a slower rate than esters.

Mechanistic Overview of Phase I Reactions

General Reaction Patterns

• Oxidation: Number of C-H bonds decreases, or C-O, C-N, C-X (halogen) bonds increase.

• Reduction: Number of C-H bonds increases, or C-O, C-N, C-X (halogen) bonds decrease.

Microsomal Mono-oxygenases

Cytochrome P450 (CYP)

Cytochrome P450 enzymes are the major catalysts for Phase I metabolism, responsible for metabolizing over 70% of drugs. They are hemoproteins located in the endoplasmic reticulum of liver cells.

• Name origin: Absorbance peak at 450 nm when complexed with CO.

• Functional roles: Inter-individual variability, drug-drug interactions, and specific toxic effects.

Family	Subfamily	Isoforms
CYP1	1A	1A1, 1A2
CYP2	2A, B, C, D, E	2A6, 2B6, 2C8, 2C9, 2C10, 2C18, 2C19, 2D6, 2E1
CYP3	3A	3A3, 3A4, 3A5, 3A7

Additional info: CYP3A4 is particularly important for drug metabolism.

Types of CYP450 Reactions

• Aliphatic & aromatic hydroxylation

• N-, O-, S-dealkylations

• Epoxidation

• Oxidative deamination

• N-, S-oxidations

• Dehalogenation

• Alcohol oxidation

Detailed Mechanisms of Key Phase I Reactions

Aliphatic & Aromatic Hydroxylation

Hydroxylation introduces a hydroxyl group into aliphatic or aromatic side chains, increasing drug polarity and often leading to inactivation or altered activity.

• Example: Hydroxylation of pentobarbitone (sedative/hypnotic) and lignocaine (local anesthetic).

• Metabolites: Hydroxylated or alcoholic products, phenolic compounds.

Dealkylation

Dealkylation removes an alkyl group from a compound adjacent to a heteroatom (N, O, S), often resulting in the formation of an aldehyde.

• Example: O-demethylation of codeine to morphine.

• Occurs readily with drugs containing secondary/tertiary amines, alkoxy groups, or alkyl-substituted thiols.

Epoxidation

Epoxidation involves the addition of oxygen across a double bond to form a cyclic ether (epoxide), typically in aromatic or olefinic drug structures.

• Epoxides are unstable intermediates, further metabolized by epoxide hydrolase.

• Example: Metabolism of benzo(a)pyrene.

Oxidative Deamination

Oxidative deamination converts amines to ketones, releasing ammonium ions. This is common for primary amines.

• Example: Amphetamine to phenylacetone.

• Mechanistically similar to N-dealkylations (carbon-nitrogen cleavage).

N-Oxidation

N-oxidation involves direct oxidation of nitrogen atoms in amines, amides, imines, hydrazines, and heterocyclic compounds.

• Example: Metabolism of phentermine to hydroxyamine and nitroso metabolites.

S-Oxidation

S-oxidation is the sequential oxidation of thioethers to sulfoxide and sulfone metabolites.

• Example: S-oxidation of thioridazine.

Dehalogenation

Dehalogenation involves the removal of halogen atoms from drug molecules, often through oxidative mechanisms.

• Example: Oxidative dehalogenation of halothane to yield alcohol or acid.

Alcohol Oxidation

Alcohol oxidation converts alcohols to aldehydes or ketones, primarily via cytochrome P450 or alcohol dehydrogenase.

• Example: Ethanol to acetaldehyde and then to acetic acid.

Other Phase I Enzymes

Flavin-containing Mono-oxygenases (FMO)

FMOs contain FAD as a prosthetic group and metabolize heteroatom-containing drugs. They transfer electrons from NADPH to FAD and are less likely to be induced or inhibited by other drugs.

• Example: N-oxidation of dimethylaniline.

Alcohol & Aldehyde Dehydrogenases

These non-specific enzymes catalyze the oxidation of alcohols and aldehydes to their corresponding acids, using NAD+ as a cofactor.

• Example: Ethanol to acetaldehyde to acetic acid.

Xanthine and Aldehyde Oxidases

Metalloflavoproteins containing molybdenum and iron, as well as FAD, catalyze the oxidation of purines and other substrates. Oxygen is incorporated from water, not O2.

• Example: Oxidation of xanthine to uric acid (implicated in gout).

• Allopurinol is a xanthine oxidase inhibitor used in gout treatment.

Reduction Reactions: Mechanisms and Examples

Reduction reactions are the reverse of oxidation, often catalyzed by the same or different enzymes. They are important for drugs with nitro, azo, or carbonyl groups.

• Reversible reactions: Same enzyme catalyzes both directions.

• Apparent reversible reactions: Different enzymes for each direction.

• Example: Reduction of warfarin produces diastereoisomers.

Hydrolysis: Mechanisms and Examples

Hydrolysis is the enzymatic cleavage of esters and amides by addition of water, catalyzed by esterases and amidases.

• Esters: Rapidly hydrolyzed (e.g., acetylcholine by acetylcholinesterase).

• Amides: Hydrolyzed at a slower rate (e.g., procainamide).

• Example: Hydrolysis of procaine and procainamide to p-aminobenzoic acid.

Substrate	Enzyme	Product
Procaine	Esterase	p-aminobenzoic acid
Procainamide	Amidase	p-aminobenzoic acid
Acetylcholine	Acetylcholinesterase	Acetate + Choline
Succinylcholine	Pseudocholinesterase	Succinic acid + Choline

Summary Table: Phase I Reactions

Reaction Type	Enzymes	Examples
Oxidation	Cytochrome P450, FMO, Alcohol/Aldehyde Dehydrogenase, Xanthine/Aldehyde Oxidase	Hydroxylation, Dealkylation, Epoxidation, Deamination, N-/S-oxidation, Dehalogenation, Alcohol oxidation
Reduction	Cytochrome P450, NADPH-cytochrome P450 reductase, Bacterial reductases	Reduction of warfarin, nitro, azo, carbonyl compounds
Hydrolysis	Esterases, Amidases	Hydrolysis of procaine, procainamide, acetylcholine, succinylcholine

Key Equations

• General CYP450 oxidation: $\text{RH} + \text{O}_2 + \text{NAD(P)H} \rightarrow \text{ROH} + \text{H}_2\text{O} + \text{NAD(P)}^+$

• Alcohol dehydrogenase: $\text{CH}_3\text{CH}_2\text{OH} + \text{NAD}^+ \rightarrow \text{CH}_3\text{CHO} + \text{NADH} + \text{H}^+$

• Aldehyde dehydrogenase: $\text{CH}_3\text{CHO} + \text{NAD}^+ + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{NADH} + \text{H}^+$

Conclusion

Phase I reactions are essential for the initial metabolism of drugs, increasing their polarity and preparing them for Phase II conjugation and elimination. Understanding the enzymes and mechanisms involved is crucial for predicting drug behavior, interactions, and individual variability in drug response.