8-9

• Enzyme-carbon monoxide complex


  Has a distinguishing spectroscopic absorption maximum at 450 nm
• Hepatic CYP mixed function oxidase system

  • Ability to metabolize an almost unlimited number of diverse substrates by various oxidative transformations
  • Substrate nonspecificity of CYP
  • Presence of multiple forms of the enzyme
  • Some P450 enzymes are selectively inducible by various chemicals

  
• Phase I Oxidation reaction

  1. Oxidation of Aromatic Moieties

  2. Oxidation of Olefin

  3. Oxidation at Benzyl Carbon Atoms

  4. Oxidation at Allylic Carbon Atoms

  5. Oxidation at Carbon Atoms α to Carbonyls and Imines

  6. Oxidation of Aliphatic and Alicyclic Carbon Atom

  7. Oxidation Involving Carbon–Heteroatom Systems
• Aromatic hydroxylation


  Mixed-function oxidation of aromatic compounds (arenes) to their corresponding phenolic metabolites (arenols)
• Aromatic hydroxylation reactions are believed to proceed initially through an epoxide intermediate called an "arene oxide", which rearranges rapidly and spontaneously to the arenol product in most instances

  
• Most foreign compounds containing aromatic moieties are susceptible to aromatic oxidation
• In humans, aromatic hydroxylation is a major route of metabolism for many drugs containing phenyl groups

  
• In most of these drugs hydroxylation occurs at the para position
• Substituents attached to the aromatic ring

  • Activated group (like hydroxyl, amine, alkyl, etc.) cause ring activation, in this case oxidation is characterized by rapid metabolism and position of OH group at para position
  • Deactivation group (like halogens, NO2, ammonium ion, COOH, SO2NHR, etc.) are generally slow or resistant to hydroxylation
• Diazepam


  • Compounds with two aromatic rings, hydroxylation occurs preferentially in the more electron-rich ring
• Clonidine

  • The deactivating groups (Cl, -N+H=C) present may explain why this drug undergoes little aromatic hydroxylation in humans

  
• Arene oxide intermediates


  • Formed when a double bond in aromatic moieties is epoxidized
  • Electrophilic and chemically reactive (because of the strained three-membered epoxide ring)
• Detoxification of arene oxides

  1. Spontaneous rearrangement to corresponding arenols (often accompanied by a novel intramolecular hydride migration called the "NIH shift")

  2. Enzymatic hydration to trans-dihydrodiols (catalyzed by microsomal enzymes called epoxide hydrases)

  3. Enzymatic conjugation with glutathione (GSH) in the presence of glutathione S-transferase enzyme to give glutathione derivatives, which undergo further metabolism to give mercapturic derivative

  
• If not effectively detoxified, arene oxides will bind covalently with nucleophilic groups present on proteins, (DNA), and (RNA), thereby leading to serious cellular damage

  
• Oxidation of Olefin


  1. Metabolic oxidation of olefinic carbon-carbon double bonds leads to the corresponding epoxide (or oxirane)

  2. Epoxides are susceptible to enzymatic hydration by epoxide hydrase to form trans-dihydrodiols

  3. Several epoxides undergo GSH conjugation

  
• Carbamazepine (Tegretol) and Alcofenac


  • Anticonvulsant drug and other olefinic compounds that undergo metabolic epoxidation

  
• The corresponding epoxide metabolites may be the reactive species responsible for the cellular toxicity seen with these compounds
• Some olefin-containing compounds causes the destruction of CYP
• Secobarbital and the volatile anesthetic agent fluroxene

  • Compounds with olefinic moiety that is activated metabolically by CYP to form a very reactive intermediate that covalently binds to the heme portion of CYP

  
• Long-term administration of the above-mentioned agents is expected to lead to inhibition of oxidative drug metabolism, potential drug interactions, and prolonged pharmacological effects
• Oxidation at Benzyl Carbon Atoms

  1. Carbon atoms attached to aromatic rings (benzylic position) are susceptible to oxidation, thereby forming the corresponding alcohol (or carbinol) metabolite

  2. Primary alcohol metabolites are often oxidized further to aldehydes and carboxylic acids (-CH2OH → -CHO → -COOH)

  3. Secondary alcohols are converted to ketones by soluble alcohol and aldehyde dehydrogenases

  4. Alternatively, the alcohol may be conjugated directly with glucuronic acid
• Tolbutamide


  • The benzylic carbon atom is oxidized extensively to the corresponding alcohol and carboxylic acid metabolites
• Oxidation at Allylic Carbon Atoms

  Microsomal hydroxylation at allylic carbon atoms is commonly observed in drug metabolism

  
• Hexobarbital


  • The 3'-hydroxylated metabolite formed is susceptible to glucuronide conjugation as well as further oxidation to the 3'-oxo compound

  
• Oxidation at Carbon Atoms α to Carbonyls and Imines

  The mixed-function oxidase system also oxidizes carbon atoms adjacent (i.e., α) to carbonyl and imino functionalities

  
• Oxidation of Aliphatic and Alicyclic Carbon Atom

  1. Oxidation takes place at terminal methyl group (ω-oxidation)

  2. Oxidation takes place at carbon atom before the last carbon (ω-1 oxidation)

  3. The initial alcohol metabolites are susceptible to further oxidation to yield aldehyde, ketones, or carboxylic acids

  4. Alternatively, the alcohol metabolites may undergo glucuronide conjugation

  
• Valproic acid (Depakene)


  • Undergoes both ω and ω-1 oxidation to the 5-hydroxy and 4-hydroxy metabolites, respectively
  • Further oxidation of the 5-hydroxy metabolite yields 2-n-propylglutaric acid

  
• The cyclohexyl group is commonly found in many medicinal agents, and is also susceptible to mixed-function oxidation (alicyclic hydroxylation)
• Acetohexamide


  • Enzymatic introduction of a hydroxyl group into a monosubstituted cyclohexane ring generally occurs at C-3 or C-4 and can lead to cis and trans conformational stereoisomers
• Oxidation Involving Carbon–Heteroatom Systems


  1. Hydroxylation of the α-carbon atom attached directly to the heteroatom (N, O, S)

  2. Hydroxylation or oxidation of the heteroatom (N, S only, e.g., N-hydroxylation, N-oxide formation, sulfoxide, and sulfone formation)

  
• Oxidation involving carbon–nitrogen systems


  Metabolism of nitrogen functionalities (e.g., amines, amides) is important because such functional groups are found in many natural products and in numerous important drugs
• Nitrogen-containing compounds

  • Aliphatic (primary, secondary, and tertiary) and alicyclic (secondary and tertiary) amines
  • Aromatic and heterocyclic nitrogen compounds
  • Amides
• Oxidative N-dealkylation


  1. Oxidative removal of alkyl groups (particularly methyl groups) from tertiary aliphatic and alicyclic amines carried out by hepatic CYP mixed-function oxidase enzymes

  2. Initial step involves α-carbon hydroxylation to form a carbinolamine intermediate, which is unstable and undergoes spontaneous heterolytic cleavage of the C–N bond to give a secondary amine and a carbonyl moiety (aldehyde or ketone)

  
• Small alkyl groups, such as methyl, ethyl, and isopropyl, are removed rapidly by N-dealkylation
• 1. dealkylation of the t-butyl group is not possible by the carbinolamine pathway because α-carbon hydroxylation cannot occur

  
• The first alkyl group from a tertiary amine is removed more rapidly than the second alkyl group
• Imipramine (Tofranil®) and lidocaine

  • Drugs that undergo bisdealkylation of the tertiary aliphatic amine to the corresponding primary aliphatic amine, which occurs very slowly

  
• Meperidine


  • Alicyclic tertiary amine drug that is susceptible to oxidative N-dealkylation reactions
• Formation of lactam metabolites

  Alicyclic tertiary amines often generate lactam metabolites by α-carbon hydroxylation reactions at the ring carbon atom α to the nitrogen which further oxidized to lactam metabolites
• Nicotine


  • Undergoes formation of lactam metabolites