Drug metabolism

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Need to add stuffs about slow/fast acetylators

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Role of metabolism

• .. to convert lipid-soluble drug into water-soluble metabolites
• By increasing water solubility
  --> Decrease Vd, and enhance renal excretion and sometimes GIT elimination
• Metabolism does not always lead to pharmacologically inactive metabolite
  * e.g. diazepam and propranolol may be metabolised into active compounds

Rate of metabolism

Rate of metabolism depends on

• Concentration of drug at the site of metabolism
• Intrinsic rate of metabolism process

First-order kinetics

• Amount of drug eliminated is proportional to the plasma concentration
  * e.g. Most drugs

Zero-order kinetics

• Constant amount of drug is eliminated, regardless of plasma concentration
  * Occurs when the plasma concentration exceeds enzyme capacity
  * e.g. alcohol, aspirin, phenytoin

Pathways of metabolism

4 basic pathways

1. Oxidation
   * e.g. hydroxylation, deamination, desulfuration, dealkylation, dehalogenation
   * Reactive intermediates may cause organ damage
2. Reduction
   * More common when oxygen partial pressure is low
3. Hydrolysis
   * Do not involve cytochrome P-450 enzymes
   * Nonmicrosomal enzymes are involved
4. Conjugation
   * Conjugation with glucuronic acid involves cytochrome P-450 enzymes
   * Decreased in neonates and during pregnancy

2 phases

1. Phase I
   * Oxidation, reduction, and hydrolysis
   * Increase drug's polarity, preparing it for phase II
2. Phase II
   * Conjugation reactions
   * Increase water solubility by linking with a highly polar molecule

Site of metabolism

• Liver
• Plasma (Hofmann elimination, ester hydrolysis)
• Lungs
• Kidneys
• GIT
  * Especially small intestine
  * Cytochrome P-450 enzymes

Enzymes involved

• Microsomal enzymes
  * Mainly located in the hepatic smooth endoplasmic reticulum
  * Can also be present in kidney, GIT, adrenal cortex

Phase I enzymes

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Enzymes involved in phase I reactions include:

1. Cytochrome P-450 enzymes
2. Non-cytochrome P-450 Enzymes (Esters)
3. Flavin-containing monooxygenase enzymes

1. Cytochrome P-450 enzymes

aka mixed function oxidase system, or mono-oxygenases

• Superfamily of membrane-bound heme proteins
  * Predominantly hepatic microsomal enzymes
  * Some mitochondrial P-450 enzymes
• When combined with carbon monoxide
  --> peak absorption at 450nm
• Involved in both oxidation and reduction
• Functions as the terminal oxidase in the electron transport scheme

Grouping of cytochrome P-450 enzymes

• Enzymes that share more than 40% of sequence homology
  --> Grouped into a family
  * e.g. CYP2
• Enzymes that share more than 55% of sequence homology
  --> Grouped into a subfamily
  * e.g. CYP2A
• Individual enzymes are then assigned an individual number
  * e.g. CYP2A6

Actions of cytochrome P-450 enzymes

• 10 isoforms of cytochrome P-450 are responsible for the oxidative metabolism of most drugs
• Most CYP activities are generated by CYP2D6, CYP3A4, CYP3A5

CYP3A4

• Most abundant
• Comprising 20% to 60% of total P-450 activity
• CYP3A4 + CYP3A5
  --> Responsible for metabolism of more than half of the currently available drugs
  * e.g. opoids (alfentanil, sufentanil, fentanyl)
  * e.g. benzodiazepines, local anaesthetics (lidocaine, ropivacaine)
  * e.g. immunosuppresants (cyclosporine), antihistamines (terfenadine)
• Gender dependent
  --> Clearance of some drugs are 20-40% higher in women
  * Due to stimulation by steroid hormone

CYP2D6

• Responsible for 25% of drugs
• e.g. analgesics, antidysrhythmics, amide local anesthetics, ketamine, propofol, antiemetics, and betablocker

 

For example,

• Codeine is converted to morphine and morphine-6-glucuronide
  * By CYP2D6-mediated O-demethylation
• Quinidine inhibits CYP2D6
  --> Markedly diminishes effect of codeine

 

Cytochrome P-450 and neonates

• Hepatic microsomal enzyme activity is low in neonates
  * especially premature infants
  --> Interferes with conjugation
  --> Hyperbilirubinaemia of the neonate and risk of bilirubin encephalopathy

Cytochrome P-450 and enzyme induction

• Hepatic microsomal enzyme activity can be stimulated by drugs/chemicals
• Also occurs (to limited extent) in lungs, kidney, and GIT

 

For example,

• Phenobarbital
• Polycyclic hydrocarbons (e.g. in cigarette smoke)

NB:

• May increase risk of halothane hepatotoxicity

2. Noncytochrome P-450 enzymes (esters)

aka nonmicrosomal enzymes

• Involved in conjugation, hydrolysis
  * All conjugation reactions (except for conjugation of glucuronic acid)
• Also involved (to less extent) in oxidation and reduction
• Present mainly in liver, also plasma and GIT
• (Plasma cholinesterase and acetylating enzymes) Do NOT undergo enzyme induction
  * Activity level determined genetically

 

For example,

• Hydrolysis of drugs containing ester bonds
  * Includes succinylcholine, atracurium, mivacurium, esmolol, ester local anaesthetics

Atypical cholinesterase

• Genetic mutation
• Glycine is substituted for an aspartate in anionic binding site
  --> Loss of electrostatic interaction
  --> Less drug-substrate binding
  --> Dramatically increased duration of action for drugs such as succinylcholine and mivacurium

3. Falvin-containing monooxygenase enzymes

• Nicotinamide-adenine dinucleotide (NAD)-dependent microsomal enzymes
• Oxidise nitrogen, sulfur, and phosphorus-containing compounds

 

For example,

• FMO3 catalyzes oxidation of tricyclic antidepressants and H2 antagonists

Phase II enzymes

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Enzymes involved in phase II reactions include:

1. Glucuronosyltransferase
2. Glutathione-S-transferase
3. N-acetyl-transferase
4. Sulfotransferase

Glucuronosyltransferase

Uridine diphosphate glucuronosyltransferase

• A family of hepatic microsomal enzymes
• Catalyses covalent addition of glucuronic acid to compounds
  --> More water soluble

 

Intestine contains beta-glucuronidase
--> Hydrolyse glucuronides back to parent compound
--> Reabsorbed and transport to liver for reconjugation
* Enterohepatic recirculation

 

For example,

• Propofol
  --> Glucuronidation in liver and kidney
• Opioids
  --> Undergoes glucuronidation
  * Morphine-3-glucuronide and morphine-6-glucuronide
• Midazolam
  --> Glucuronidation product 1-hydroxymidazolam (active)
  * Contribute to prolonged effect in renal insufficiency

Glutathione-S-transferase

• Primarily a defensive system for detoxification and oxidative stress
• GST-dependent activation is responsible for compound A nephrotoxicity in rats
  * Absence of toxicity in human is probably a species difference

N-Acetyl-Transferase

• Catalyse N-acetylation
• Common phase II reaction for heterocyclic aromatic amines and arylamine hydrazine
• Responsible for inactivation of isoniazid and hydralazine
• Genetic polymorphism
  --> Fast and slow acetylators