Biotransformation

Biotransformation is also known as drug metabolism. It occurs in 2 phases: Phase 1 and phase 2.

Phase I

Phase I biotransformation reactions involve the modification of drugs by introducing or exposing a functional group, which often increases the polarity of the drug and can make it more amenable to further metabolism in Phase II. The primary reactions in Phase I biotransformation include oxidation, reduction, and hydrolysis. Here’s a detailed look at each of these reactions:

1. Oxidation

Oxidation is the most common type of Phase I reaction and is primarily catalyzed by the cytochrome P450 (CYP450) enzyme system. These reactions involve the addition of oxygen or the removal of hydrogen atoms. Key oxidative reactions include:

• Hydroxylation: Addition of a hydroxyl group (-OH) to an aromatic or aliphatic carbon.
• Dealkylation: Removal of an alkyl group from nitrogen (N-dealkylation), oxygen (O-dealkylation), or sulfur (S-dealkylation).
• Deamination: Removal of an amine group.
• Sulfoxidation: Addition of an oxygen to a sulfur atom.

Examples:

• Hydroxylation: AAcetanilide to acetaminophen
• N-dealkylation: Conversion of codeine to morphine.

2. Reduction

Reduction reactions involve the gain of electrons or hydrogen, often occurring in environments with low oxygen tension. These reactions are less common than oxidation and can be catalyzed by enzymes other than CYP450, such as reductases. Reduction reactions include:

• Azo reduction: Reduction of azo compounds to amines. Azo compounds contain -N=N- functional group.
• Nitro reduction: Reduction of nitro groups to amines.
• Carbonyl reduction: Reduction of ketones or aldehydes to alcohols.

Examples:

• Azo reduction: Sulfasalazine undergoes azo reduction. Azo reduction of sulfasalazine gives 5 ASA (5 – aminosalicylic acid).
  • Additional point – side effect of sulfasalazine is megalobalstic anemia and infertility in men. Avoid sulfasalazine in sulfa allergy.
• Nitro reduction: Reduction of chloramphenicol and clonazepam.

3. Hydrolysis

Hydrolysis reactions involve the cleavage of bonds by the addition of water. These reactions are commonly catalyzed by esterases, amidases, and other hydrolases. Hydrolysis is important for the metabolism of esters and amides.

• Ester hydrolysis: Conversion of esters to alcohols and acids. Enzyme involved – esterases
• Amide hydrolysis: Conversion of amides to amines and acids. Enzyme involved – amidases

Examples:

• Ester hydrolysis: Conversion of aspirin (acetylsalicylic acid) to salicylic acid and acetic acid.
• Amide hydrolysis: Conversion of lidocaine to 2,6-dimethylaniline and acetic acid.

Phase 2

Phase 2 metabolism, also known as conjugation, is the body’s second act in transforming and eliminating foreign chemicals, including medications. Here, unlike Phase 1’s focus on introducing functional groups, Phase 2 reactions specialize in attaching a hydrophilic (water-loving) molecule to the already modified drug molecule. This creates a “conjugated” molecule, significantly more water-soluble and easier for the body to excrete through urine or bile.

Conjugation Reaction	Donor Molecule	Enzyme	Effect	Examples
Glucuronidation (most common)	Glucuronic Acid	UDP-Glucuronosyltransferases (UGTs)	Increases water solubility for excretion	Tamoxifen (breast cancer), Morphine (pain medication), Acetaminophen (pain reliever)
Sulfation	Sulfate Group (SO₄²⁻)	Sulfotransferases	Increases water solubility for excretion	Salbutamol (asthma), Estrone (estrogen hormone), Minoxidil (hair loss treatment)
Glutathione Conjugation	Glutathione (tripeptide)	Glutathione S-transferases (GSTs)	Increases water solubility for excretion, may detoxify harmful metabolites	Ibuprofen (pain reliever), Diclofenac (pain medication), Cisplatin (cancer chemotherapy)
Acetylation	Acetyl Group (CH₃CO-)	N-acetyltransferases (NATs)	May affect drug activity or excretion	Isoniazid (tuberculosis), Sulfanilamide (antibiotic), Hydralazine (blood pressure medication)
Methylation	Methyl Group (CH₃)	Methyltransferases	May alter drug activity or influence metabolism	Caffeine, Theophylline (bronchodilator), Nicotine (from tobacco)
Amino Acid Conjugation	Amino Acid (Glycine, Glutamine)	Specific enzymes	Impacts water solubility and potentially drug activity	Valproic acid (anticonvulsant), Phenytoin (anticonvulsant), Nalidixic acid (antibiotic)

Two most important examples

Factors affecting drug metabolism

Internal factors:

• Genetics: Variations can lead to rapid or slow metabolizers, affecting drug response.
• Age: Declining liver and kidney function in older adults can slow metabolism.
• Organ health: Liver and kidney disease can impair metabolism and lead to drug buildup.
• Hormones: Sex hormones and pregnancy can influence enzyme activity and drug metabolism.

External factors:

• Diet: Certain foods and drinks can interact with enzymes, altering drug breakdown.
• Smoking: Can speed up drug breakdown, requiring higher doses for some medications.
• Alcohol: Can damage the liver and interact with medications, increasing side effects.
• Medications: Some medications can interact, affecting the metabolism of others.
• Environment: Exposure to toxins can compete with drugs for enzyme binding sites.

CYP 450 enzymes

CYP450 enzymes are protein masters of drug metabolism, located in the endoplasmic reticulum.

• Core: Heme prosthetic group (iron) is the heart of the action, where drug oxidation happens.
• Scaffold: A large protein scaffold surrounds and stabilizes the heme group, creating a pocket for the drug molecule to bind.
• Anchor: A transmembrane domain anchors the enzyme in the membrane, allowing interaction with other metabolism proteins.

Cytochrome enzymes are located primarily in the endoplasmic reticulum of liver cells. It is also found in other tissues like the intestines, lungs, and kidneys, contributing to overall drug metabolism.

Order of CYP 450 enzymes on the basis of their prevalence in drug metabolism: CYP3A4 > CYP2D6 > CYP2C9 > CYP1A2 > CYP2C19

Mnemonic to remember this is – 4 – 69 smokes 19 cigarettes

CYP Enzyme	Substrates	Inhibitors	Inducers
CYP3A4	Statins (atorvastatin, simvastatin, lovastatin), Antivirals (ritonavir, saquinavir), Immunosuppressants (cyclosporine, tacrolimus), Benzodiazepines (midazolam, diazepam, alprazolam), Antifungals (ketoconazole, itraconazole, voriconazole), Macrolide antibiotics (erythromycin, clarithromycin), Calcium channel blockers (amlodipine, verapamil)	Ketoconazole, Clarithromycin, Grapefruit juice, Ritonavir, Fluconazole, Cimetidine, Diltiazem, Amiodarone, Atazanavir	Rifampin, Carbamazepine, St. John’s wort, Efavirenz, Phenytoin, Nevirapine
CYP2D6	Antidepressants (fluoxetine, paroxetine), Antipsychotics (haloperidol, risperidone), Opioids (codeine, tramadol), Beta-blockers (metoprolol, propranolol), Tamoxifen	Fluoxetine, Paroxetine, Quinidine, Bupropion, Duloxetine, Sertraline, Diphenhydramine	Rifampin, Carbamazepine, Phenytoin, St. John’s wort, Smoking
CYP2C9	NSAIDs (ibuprofen, celecoxib), Anticoagulants (warfarin), Antidiabetic drugs (tolbutamide, glimepiride), Antiepileptic drugs (phenytoin)	Fluconazole, Amiodarone, Sulfamethoxazole, Metronidazole, Voriconazole, Fluvoxamine	Rifampin, Carbamazepine, Phenobarbital, St. John’s wort, Alcohol
CYP1A2	Caffeine, Theophylline, Clozapine, Olanzapine, Ropinirole	Fluvoxamine, Ciprofloxacin, Cimetidine, Grapefruit juice, Amiodarone, Zileuton	Smoking, Omeprazole (at lower doses), Cruciferous vegetables, Charbroiled meats
CYP2C19	Proton pump inhibitors (omeprazole, esomeprazole), Clopidogrel, Diazepam, Escitalopram, Phenytoin	Omeprazole, Esomeprazole, Fluconazole, Fluoxetine, Ticlopidine, Voriconazole	Rifampin, Carbamazepine, Phenytoin, St. John’s wort, Smoking
CYP 20	Ethanol, Acetaminophen, Isoniazid, Chlorzoxazone	Disulfiram, Isoniazid, Cimetidine, Ketoconazole, Chlorzoxazone	Ethanol (chronic use), Fasting, Diabetes Mellitus, Protein-deficient diet
CYP2B6	Efavirenz, Bupropion, Cyclophosphamide, Methadone	Ticlopidine, Clopidogrel, Voriconazole, Etravirine, Piperine	Rifampin, Phenobarbital, Nevirapine, Efavirenz, Phenytoin
CYP2A6	Nicotine, Coumarin, Tacrine, Letrozole, Halothane	Tranylcypromine, Clomethiazole, Methoxsalen, Tegafur, Tramadol	Rifampin, Phenobarbital, Fasting, Diabetes Mellitus, Charcoal-broiled meat

CYP3A4 Mnemonic

Inhibitors	Inducers
“GRAPEFRUIT TEA”	“CARBS PINE”
Grapefruit juice Ritonavir Amiodarone Posaconazole Erythromycin Fluconazole Ritonavir (again, to emphasize its potent inhibition) Umbralisib Itraconazole Ticagrelor Elbasvir (added to complete the mnemonic)	Carbamazepine Amprenavir Rifampin Bosentan St. John’s wort Phenytoin Ifosfamide Nevirapine Efavirenz

Mnemocis for other enzymes

CYP2D6	CYP2C9
“NICE MAP”	“PICK ME”
Nicotine (Inducer) Isoniazid (Inhibitor) Cimetidine (Inhibitor) Esomeprazole (Inhibitor) Mexiletine (Inhibitor) Amitriptyline (Inhibitor) Phenytoin (Inducer)	Phenytoin (Inducer) Ibuprofen (Inhibitor) Celecoxib (Inhibitor) Ketoconazole (Inhibitor) Miconazole (Inhibitor) Etoricoxib (Inhibitor)
1st and last are inducers, rest are all inhibitors.	Only 1st is inducer. Rest all inhibitors.