Cytochrome P450 & Phase I Oxidation

• Cytochrome P450 (CYP) enzyme binds with carbon monoxide, forming a complex with a spectroscopic absorption maximum at 450 nm.
• The hepatic CYP mixed function oxidase system can metabolize many substrates through various oxidative transformations.
• CYP's versatility stems from its substrate nonspecificity and the presence of multiple enzyme forms, some inducible by chemicals.

Phase I Oxidation Reaction

• This reaction involves the oxidation of aromatic moieties.
• Aromatic hydroxylation is a mixed-function oxidation of aromatic compounds (arenes) into corresponding phenolic metabolites (arenols).
• Aromatic hydroxylation reactions proceed through an arene oxide intermediate, which rapidly rearranges into the arenol product.
• Aromatic hydroxylation is a major route of drug metabolism in humans, especially for drugs with phenyl groups.
• Propranolol, phenobarbital, phenytoin, and atorvastatin undergo aromatic oxidation, typically at the para position.
• Substituents on the aromatic ring influence the ease of hydroxylation.

Types of Substitutions

• Activating groups (hydroxyl, amine, alkyl) are electron-donating and lead to rapid metabolism, with hydroxylation at the para position.
• Deactivating groups (halogens, NO2, ammonium ion, COOH, SO2NHR) are electron-withdrawing and cause slow or resistant hydroxylation.
• Diazepam hydroxylation occurs in the more electron-rich ring when it contains two aromatic rings.
• The presence of deactivating groups (Cl, -N+H=C) explains why clonidine undergoes limited aromatic hydroxylation.
• Arene oxide intermediates are formed when a double bond in aromatic moieties is epoxidized.
• These intermediates are electrophilic and chemically reactive because of their strained three-membered epoxide ring, causing toxicologic concerns.

Detoxification Pathways for Arene Oxides:

• Spontaneous rearrangement into arenols may occur, often accompanied by a novel intramolecular hydride migration called the NIH shift.
• Enzymatic hydration to trans-dihydrodiols is catalyzed by microsomal epoxide hydrases.
• Enzymatic conjugation with glutathione (GSH), in the presence of glutathione S-transferase enzyme, leads to glutathione derivatives and further metabolism into mercapturic acid derivatives.
• If not detoxified, arene oxides can bind covalently with nucleophilic groups on proteins, DNA, and RNA, causing cellular damage and contributing to benzene toxicity.

Oxidation of Olefin

• The metabolic oxidation of olefinic carbon-carbon double bonds results in the corresponding epoxide (or oxirane).
• Epoxides are susceptible to enzymatic hydration by epoxide hydrase, which forms trans-dihydrodiols and GSH conjugation.
• Carbamazepine (Tegretol) and Alcofenac are examples of drugs that undergo this process.
• Compounds like vinyl chloride and stilbene undergo metabolic epoxidation, and the resulting epoxide metabolites can be reactive species responsible for cellular toxicity.
• The styrene molecule produces non toxic mercapturic acid derivative

Compounds Causing CYP Destruction

• Secobarbital and fluroxene, can cause this
• A reactive intermediate covalently binds to the heme portion of CYP.
• Long-term administration can inhibit oxidative drug metabolism, leading to drug interactions and prolonged pharmacological effects.

Oxidation at Benzyl Carbon Atoms

• Carbon atoms attached to aromatic rings (benzylic position) are oxidized, forming alcohol (or carbinol) metabolites.
• Primary alcohol metabolites are further oxidized to aldehydes and carboxylic acids.
• Secondary alcohols are converted to ketones by soluble alcohol and aldehyde dehydrogenases.
• Alternatively, the alcohol may be conjugated directly with glucuronic acid.
• Tolbutamide can be oxidized to form an alcohol and carboxylic acid, both found in human urine

Oxidation at Allylic Carbon Atoms

• Allylic carbon atoms commonly undergo Microsomal hydroxylation during drug metabolism.
• Hexobarbital is an example of a drug that undergoes allylic oxidation.
• The 3′-hydroxylated metabolite from hexobarbital can undergo glucuronide conjugation and further oxidation to the 3′-oxo compound.

Oxidation at Carbon Atoms α to Carbonyls and Imines

• The mixed-function oxidase system oxidizes carbon atoms adjacent to carbonyl and imino functionalities.
• Benzodiazepines, such as diazepam (Valium), are an example of drugs undergoing this type of oxidation.

Oxidation of Aliphatic and Alicyclic Carbon Atoms

• Straight or branched-chain compounds undergo ω-oxidation (terminal methyl group) and ω-1 oxidation (carbon atom before the last carbon.)
• The resulting alcohol metabolites can be further oxidized to form aldehyde, ketones, or carboxylic acids or they can undergo glucuronide conjugation.
• Valproic acid (Depakene) undergoes both ω and ω-1 oxidation to produce the 5-hydroxy and 4-hydroxy metabolites.
• The cyclohexyl group is also susceptible to mixed-function oxidation resulting in alicyclic hydroxylation.
• This process typically occurs at C-3 or C-4, leading to cis and trans conformational stereoisomers. Acetohexamide is and example of an oral hypoglycemic

Oxidation Involving Carbon–Heteroatom Systems

• Nitrogen and oxygen functionalities are oxidation targets in most drugs and foreign compounds.
• Sulfur functionalities are less common targets.
• The systems involved are carbon-nitrogen, carbon-oxygen, and carbon-sulfur.
• Two basic biotransformation processes exist

Two Basic Biotransformation Processes

• Hydroxylation of the α-carbon atom attached directly to the heteroatom (N, O, S) results in an unstable intermediate. This decomposes involving cleavage of the carbon-heteroatom bond. Oxidative N-, O-, and S-dealkylation and oxidative deamination reactions fall under this mechanistic pathway.
• Hydroxylation or oxidation of the heteroatom (N, S only). For example N-hydroxylation involves N-oxide formation, sulfoxide, and sulfone formation
• Oxidative deamination (through the carbinolamine pathway) by CYP leads to the formation of carbonyl metabolites and ammonia.
• N-oxidation leads to the formation of N-hydroxyl amine metabolites, which are susceptible to further oxidation leading to nitroso (N=O) and nitro (nitrogen dioxide)
• tertiary aromatic amines (such as N,N-dimethylaniline) and secondary aromatic amines can undergo oxidative N-dealkylation as well as N-oxide formation that takes place. Further oxidation of the N-hydroxylamine leads to nitrone products, which in turn can be hydrolyzed to primary hydroxylamines.
• Examples include secondary and primary amines

Amides

• They undergo through oxidative carbon–nitrogen bond cleavage (via α -carbon hydroxylation) and or N-hydroxylation reactions. Involving oxidative dealkylation
• These proceed via an unstable carbinolamide, which then fragments
• Diazepam transforms to desmethyldiazepam by this mechanism
• Also in cyclic amides or lactams, hydroxylation of the alicyclic carbon to the nitrogen atom will also turn compounds into carbinolamides such as in conversion of cotinine to 5-hydroxycotinine.

Oxidation Involving Carbon-Oxygen (ethers)

• This involves α-carbon hydroxylation to form either hemiacetal or hemiketal
• which causes a spontaneous carbon-oxygen bond cleavage which then forms dealkylated oxygen species (phenol or alcohol) and a carbonyl moiety (aldehyde or ketone).

Oxidation Involving Carbon-Sulfur

• Several drugs with a Carbon-Sulfur functional group undergo this

Dealkylation Mechanisms

• This is similar to N- and O-dealkylation, involving oxidative carbon-sulfur bond cleavage.

Desulfuration Mechanisms

• Conversions by oxidation of the carbon-sulfur double bonds (C=S) (thiono) to produce corresponding carbon-oxygen double bond (C=O) (desulfuration).

S-oxidation Mechanisms

• Yields corresponding sulfoxide derivatives which then may be oxidized to sulfones (-SO2).

Oxidation of Alcohols and Aldehydes

• This processes generates alcohol or carbinol metabolites as intermediate products (e.g., benzylic, allylic, , or aliphatic hydroxylation).
• If not conjugated, alcohols products are further oxidized to aldehydes (if primary alcohols) or to ketones (if secondary alcohols).
• Aldehyde metabolites coming from primary alcohol oxidation or oxidative deamination of primary aliphatic amines can then also turn to polar carboxylic acid derivatives.
• This bioconversion of alcohols to aldehydes and ketones is catalyzed by soluble alcohol dehydrogenases in the liver and other tissues.
• Oxidation of secondary alcohol to ketones does not often occur

Oxidative Biotransformation Pathways

• There are some pathways: For example with Oxidative aromatization reactions ex, the progesterone derivative norgestrel may be made
• Also there is Oxidative dehalogenation reactions ex, volatile anesthetic agent halothane, may be made