Role of urine ph in drug excretion

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Role of Urine pH in Drug Excretion

Overview

The kidney excretes drugs through three processes: glomerular filtration, active tubular secretion, and passive tubular reabsorption. Urine pH influences the last of these - passive reabsorption - and therefore significantly affects the net excretion of drugs that are weak acids or weak bases. - Goodman & Gilman's The Pharmacological Basis of Therapeutics, p. 51

Renal drug handling diagram showing filtration, active secretion at proximal tubule, and passive reabsorption at distal tubule

The Core Principle: Non-Ionic Diffusion and Ion Trapping

Weak acids and weak bases exist in two forms - ionized (charged) and non-ionized (uncharged). The ratio of each form depends on the urine pH and the drug's pKa, following the Henderson-Hasselbalch equation.

Key rule: Only the uncharged (non-ionic) form can passively diffuse back across the tubular cell membrane from urine into blood. The charged form cannot cross and is "trapped" in the urine, forcing its excretion.

This is called ion trapping. - Costanzo Physiology 7th Edition, p. 279; Brenner and Rector's The Kidney, p. 2850

Effect on Weak Acids (e.g., salicylate, phenobarbital, aspirin)

Urine pH	Predominant Form	Back-Diffusion	Net Excretion
Acidic (low pH)	HA (uncharged)	High	Decreased
Alkaline (high pH)	A⁻ (charged)	Low	Increased

Rule: Alkaline urine increases excretion of weak acids.

Example - salicylate (pKa = 3): When urine pH rises from 3 to 7.4, the ratio of ionized to non-ionized form shifts from 1:1 to 20,000:1 in favor of the ionized form, which cannot be reabsorbed. This reduces the elimination half-life of salicylate from ~19 hours to ~5 hours. - Brenner and Rector's The Kidney, p. 2850

Effect on Weak Bases (e.g., amphetamine, quinine, morphine)

Urine pH	Predominant Form	Back-Diffusion	Net Excretion
Alkaline (high pH)	B (uncharged)	High	Decreased
Acidic (low pH)	BH⁺ (charged)	Low	Increased

Rule: Acidic urine increases excretion of weak bases. - Costanzo Physiology 7th Edition, p. 279

This is the mirror image of the effect on weak acids.

Clinical Applications

1. Urine Alkalinization (IV sodium bicarbonate)

Used to enhance excretion of weak acid drugs in overdose. Criteria for a drug to benefit:

Eliminated unchanged by kidneys
Not strongly protein-bound
Distributed primarily in extracellular fluid
Weakly acidic (pKa 3.0-7.0) - Brenner and Rector's The Kidney, p. 2850

Drugs where alkalinization is used:

Salicylates (aspirin overdose) - most important clinical use
Phenobarbital overdose
Chlorpropamide, diflunisal, methotrexate, fluoride, 2,4-dichlorophenoxyacetic acid

2. Urine Acidification

Theoretically enhances excretion of weak bases (e.g., amphetamine, phencyclidine)
However, acidification is no longer clinically recommended due to risks of worsening rhabdomyolysis-induced acute kidney injury and systemic acidosis - Brenner and Rector's The Kidney, p. 2850

3. Forced Diuresis - Historical Context

Increasing urine flow (forced diuresis) was historically used, but studies show no significant benefit and carries risks such as volume overload, pulmonary edema, cerebral edema, and electrolyte disturbances (hyponatremia, hypokalemia). Forced diuresis is no longer recommended. - Brenner and Rector's The Kidney, p. 2849

Summary Table

Drug Type	pKa Range	Alkaline Urine Effect	Acidic Urine Effect
Weak acid (e.g., salicylate)	3-7	↑ Excretion	↓ Excretion
Weak base (e.g., amphetamine)	8-10	↓ Excretion	↑ Excretion

Important Limitations

Urine pH manipulation only works if the kidney is the primary route of elimination (renal clearance must be significant relative to total body clearance)
Drugs with extensive protein binding or large volumes of distribution (e.g., tricyclic antidepressants) do not respond well to pH manipulation
Active tubular secretion mechanisms (organic acid and base transporters: OAT1, OAT3, OCT) are not pH-dependent and operate independently - Goodman & Gilman's, p. 50-51

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Here's the complete list arranged topic-wise for easier revision. --- General Pharmacology Pharmacokinetics Definition of pharmacokinetics Drug absorption: definition Factors affecting absorption Drug-related factors affecting absorption Patient-related factors affecting absorption Effect of pH on drug absorption Examples of all processes of drug absorption Drug distribution: definition Factors affecting distribution Plasma protein binding Difference between protein-bound drug and free drug Drug excretion: definition Tubular secretion Role of urine pH in drug excretion Substances excreted through feces Substances excreted through saliva Receptors Definition of receptor Regulation of receptors Types of neurotransmitters Types of adrenergic receptors and their sites Cholinergic receptors Muscarinic receptors Agonist Partial agonist Examples of agonists and partial agonists Non-receptor mediated drug mechanisms with examples Drug Development & Pharmacogenetics Drug development Recombinant DNA technology with examples Succinylcholine and pharmacogenetics/pharmacogenomics Dosage & Routes Types of doses (definition, examples, indications) Four parenteral routes with examples Advantages and disadvantages of rectal route Solitary dosage forms (likely "solid dosage forms") ADR WHO definition of ADR --- Autonomic Nervous System Cholinergic Drugs Indirect cholinergic drugs classification Neostigmine mechanism Non-depolarizing neuromuscular blockers Succinylcholine Differences between neuromuscular blockers Organophosphorus (OPC) Poisoning Antidote Dose Duration Mechanism Complete management Cross questions Atropine Signs of atropine poisoning Atropine-like drugs Why atropine is used as pre-anaesthetic medication Ophthalmic use Mydriatics Which mydriatic is preferred in elderly and why --- Adrenergic Drugs Adrenergic Agonists Classification of indirect adrenergic agonists Examples Beta agonists Why adrenaline is used in BP Beta Blockers Classification Atenolol vs Propranolol Clinical PBQ --- Antihypertensive Drugs General Drugs used in hypertension with diabetes Why ACE inhibitors are first-line ACE inhibitor vs ARB Hypertension with bronchial asthma Hypertension with obesity and nephropathy Hypertension with diabetic neuropathy Individual Drugs ACE inhibitors Mechanism Adverse effects Why not used in bronchial asthma (as asked in viva) ARBs Comparison with ACE inhibitors Prazosin Adverse effects First-dose phenomenon Mechanism Prevention Methyldopa Mechanism Adverse effects Drug of choice in pregnancy-induced hypertension Hydralazine Mechanism Thiazides Mechanism Why they cause hyperuricemia Adverse effects --- Diuretics Diuretics acting on DCT Mechanism Diuretics acting on collecting duct Frusemide vs Thiazide Frusemide vs Spironolactone Drug for leg swelling --- Diabetes Mellitus Insulin Euglycemic agents with examples Drugs used in pregnancy Insulin preparations Classification according to duration Mechanism Adverse effects Why insulin is given during trauma/accident in diabetic patients Glycogen vs insulin-related questions Oral Antidiabetics Metformin Indications Mechanism Adverse effects GLP-1 agonists Linagliptin mechanism --- NSAIDs & Steroids Anti-inflammatory mechanism of glucocorticoids Anti-inflammatory action (other drugs) --- Antianginal Drugs Classification Calcium channel blockers Effect on hypertension Effect on angina GTN adverse effects --- Hormonal Contraceptives Classification Mechanism of oral contraceptives Adverse effects Contraindications Why minipill is preferred in lactating mothers --- Obstetrics Drugs used in diabetes during pregnancy Pregnancy-induced hypertension Primigravida definition Drug of choice Mechanism Adverse effects Types of uterine bleeding --- Endocrine Animal sources of drugs with examples --- Ophthalmology (Clinical Pharmacology) Mydriatic drugs Drug used for fundus examination Other mydriatics Best mydriatic in elderly and why --- Frequently Repeated High-Yield Topics (asked by multiple teachers) Organophosphorus poisoning Atropine Beta blockers ACE inhibitors & ARBs Hypertension in DM Hypertension in pregnancy Methyldopa Prazosin Thiazide diuretics Collecting duct diuretics Metformin Insulin Oral contraceptive pills Calcium channel blockers Drug absorption Drug distribution Receptors Agonist & partial agonist Pharmacokinetics Recombinant DNA technology Mydriatics Pre-anaesthetic atropine Neuromuscular blockers Pharmacogenetics (Succinylcholine) This organized list removes duplicates and groups all the viva questions into their respective pharmacology topics, making it much easier to revise before exams.

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Here is your complete, topic-wise viva preparation guide. Each answer is written in the concise, direct style examiners expect, followed by likely cross-questions.

MBBS 2nd Year Pharmacology - Complete Viva Preparation

SECTION 1: GENERAL PHARMACOLOGY

PHARMACOKINETICS

Q: What is pharmacokinetics?

A: Pharmacokinetics is "what the body does to the drug." It involves four processes - Absorption, Distribution, Metabolism, and Excretion (ADME).

Q: Define drug absorption.

A: Absorption is the movement of a drug from its site of administration into the systemic circulation.

Q: What are the factors affecting drug absorption?

Drug-related factors:

Physicochemical properties: lipid solubility, molecular weight, ionization (pKa)
Drug formulation: tablet vs liquid, coating, particle size
Route of administration

Patient-related factors:

GI motility (faster motility = less absorption)
Gastric pH
Gut surface area (reduced in bowel disease)
Presence of food in stomach
Blood flow to absorption site
Age (infants/elderly have altered GI function)

Cross Q: How does food affect drug absorption? Food generally delays absorption (slows gastric emptying). Some drugs need food (e.g., griseofulvin - fat-soluble); others are better on empty stomach (e.g., ampicillin).

Q: Effect of pH on drug absorption?

A: Follows Henderson-Hasselbalch principle - non-ionized form is lipid-soluble and better absorbed.

Weak acids (e.g., aspirin, pKa 3.5): absorbed better in acidic stomach (non-ionized here)
Weak bases (e.g., morphine): absorbed better in alkaline small intestine (non-ionized here)

Key Rule: Weak acids absorbed in stomach; weak bases absorbed in intestine.

Q: Examples of different processes of absorption?

Process	Example
Passive diffusion	Most drugs (aspirin, barbiturates)
Active transport	Levodopa, methyldopa, 5-FU
Facilitated diffusion	Vitamin B12
Pinocytosis	Proteins, vaccines
Filtration	Water, small ions

Q: Define drug distribution.

A: Distribution is the reversible transfer of drug from systemic circulation to tissues and organs.

Q: Factors affecting distribution?

Lipid solubility (higher = wider distribution)
Plasma protein binding
Tissue binding
Blood-brain barrier (lipid-soluble drugs cross)
Placental barrier
Volume of distribution (Vd)

Q: What is plasma protein binding? Difference between protein-bound and free drug?

Feature	Protein-Bound Drug	Free Drug
Pharmacologically active?	No	Yes (active)
Filtered at glomerulus?	No	Yes
Metabolized?	No	Yes
Crosses membranes?	No	Yes
Acts as reservoir?	Yes	No

Cross Q: Which plasma protein do acidic drugs bind to? Albumin. Basics drugs? Alpha-1 acid glycoprotein.

Cross Q: Name two highly protein-bound drugs. Warfarin (99%), Diazepam (98%).

Cross Q: What is drug displacement interaction? When two drugs compete for same binding site, the displaced drug has increased free fraction - risk of toxicity (e.g., warfarin + aspirin).

Q: Define drug excretion.

A: Excretion is the irreversible removal of the drug or its metabolites from the body.

Q: What is tubular secretion?

A: Active, carrier-mediated transport of drugs from peritubular capillaries into the proximal tubular lumen. Involves two systems:

Organic anion transporters (OAT1, OAT3) - for acidic drugs (penicillin, furosemide, methotrexate)
Organic cation transporters (OCT) - for basic drugs (dopamine, histamine)

Cross Q: Give a clinical example of tubular secretion interaction. Probenecid competes with penicillin at OAT, blocking its secretion - prolongs penicillin's action.

Q: Role of urine pH in drug excretion?

(Covered in detail in your previous session)

Alkaline urine: increases excretion of weak acids (salicylate, phenobarbital) - used in aspirin overdose
Acidic urine: increases excretion of weak bases (amphetamine) - rarely used clinically due to risk of rhabdomyolysis

Q: Substances excreted through feces?

Unabsorbed orally ingested drugs
Drugs excreted in bile (enterohepatic circulation - digoxin, rifampicin, oral contraceptives)
Drugs secreted directly into intestine

Q: Substances excreted through saliva?

Heavy metals (lead, mercury, bismuth)
Metronidazole (causes metallic taste)
Urea (in renal failure - uremic frost equivalent in saliva)
Iodides, bromides

RECEPTORS

Q: Define receptor.

A: A receptor is a macromolecule (usually a protein) that specifically binds a drug or endogenous ligand and initiates a pharmacological response. It has two domains: ligand-binding domain and effector domain.

Q: Regulation of receptors?

Up-regulation (supersensitivity): Chronic antagonist use or denervation increases receptor number (e.g., beta-blockers increase beta-receptor density)
Down-regulation (desensitization): Chronic agonist use decreases receptor number (e.g., chronic beta-agonist use)

Cross Q: Clinical example of up-regulation? Sudden stopping of beta-blockers causes rebound tachycardia due to supersensitive beta-receptors.

Q: Types of neurotransmitters?

Type	Examples
Cholinergic	Acetylcholine
Adrenergic	Norepinephrine, Epinephrine
Amino acid	GABA, Glutamate, Glycine
Peptide	Substance P, Enkephalins, Endorphins
Monoamine	Dopamine, Serotonin, Histamine

Q: Types of adrenergic receptors and their sites?

Receptor	Location	Response
α1	Blood vessels, eye, bladder neck	Vasoconstriction, mydriasis, urinary retention
α2	Presynaptic nerve terminals, CNS	Inhibits NE release; CNS: reduces BP
β1	Heart, kidney (JGA)	↑ HR, ↑ contractility; renin release
β2	Bronchi, blood vessels, uterus, liver	Bronchodilation, vasodilation, uterine relaxation
β3	Adipose tissue	Lipolysis

Q: Cholinergic receptors and muscarinic receptors?

Cholinergic receptors:

Nicotinic (NM): Neuromuscular junction → muscle contraction
Nicotinic (NN): Autonomic ganglia, adrenal medulla
Muscarinic (M1-M5): Post-ganglionic parasympathetic synapses

Muscarinic receptor subtypes:

Subtype	Location	Effect
M1	Gastric glands, CNS, ganglia	↑ gastric acid, CNS excitation
M2	Heart (SA node, AV node)	↓ HR, ↓ conduction
M3	Smooth muscle, glands, eye	Contraction, secretion, miosis, lacrimation

Memory: M2 = heart (2 chambers pumping), M3 = secretion, smooth muscle.

Q: Define agonist and partial agonist. Examples?

Term	Definition	Example
Agonist (full)	Binds receptor + produces maximal response (efficacy = 1)	Morphine, adrenaline, salbutamol
Partial agonist	Binds receptor but produces submaximal response even at full occupancy (efficacy <1)	Buprenorphine, aripiprazole, pindolol
Antagonist	Binds but produces no response (efficacy = 0)	Naloxone, atropine, propranolol

Cross Q: What happens if a partial agonist is given with a full agonist? The partial agonist acts as an antagonist by competing for receptors while producing less effect - NET EFFECT: reduced response (e.g., buprenorphine reduces morphine's effect).

Q: Non-receptor mediated drug mechanisms with examples?

Mechanism	Example
Physical action (osmosis)	Mannitol (osmotic diuretic), antacids
Chemical neutralization	Antacids (NaHCO₃ + HCl → NaCl + H₂O)
Metal chelation	Dimercaprol, EDTA in heavy metal poisoning
Enzyme inhibition (non-receptor)	Acetazolamide (inhibits carbonic anhydrase)
Membrane stabilization	Local anesthetics (non-specific Na⁺ channel block)
DNA intercalation	Chloroquine, doxorubicin

DRUG DEVELOPMENT & PHARMACOGENETICS

Q: Steps in drug development?

Drug discovery - natural sources, synthesis, screening
Preclinical testing - in vitro, animal studies (safety, toxicity)
Phase I trials - healthy volunteers, safety, dose-finding, pharmacokinetics (20-100 subjects)
Phase II trials - patients, efficacy and safety (100-300)
Phase III trials - large multicenter RCTs (1000-3000 patients), comparison with placebo/standard
Regulatory approval (CDSCO in India, FDA in USA)
Phase IV (post-marketing surveillance) - long-term safety, rare ADRs

Cross Q: What is Phase 0? Microdosing studies in humans - sub-therapeutic doses to study PK/PD.

Q: Recombinant DNA technology with examples?

A: Genetic engineering technique that inserts human DNA coding for a protein into a vector (plasmid/virus) → expressed in bacteria/yeast/CHO cells → purified protein used as drug.

Examples:

Human insulin (produced in E. coli) - replaced animal insulin
Human growth hormone (somatropin)
Erythropoietin (EPO) - for anemia in CKD
Tissue plasminogen activator (tPA/alteplase) - thrombolysis
Interferon-alpha, beta, gamma
Hepatitis B vaccine (recombinant HBsAg)
Monoclonal antibodies (trastuzumab, rituximab)

Q: Succinylcholine and pharmacogenetics/pharmacogenomics?

A: Succinylcholine is normally hydrolyzed by plasma pseudocholinesterase (butyrylcholinesterase) within 5-10 minutes. Some individuals have a genetic variant of pseudocholinesterase with reduced dibucaine number (normal = 80, atypical = 20).

In these patients, succinylcholine is NOT hydrolyzed normally
Results in prolonged neuromuscular blockade - "scoline apnea" lasting 2-3 hours
Management: mechanical ventilation until drug wears off; fresh frozen plasma (contains pseudocholinesterase)

This is the classic pharmacogenetics example in MBBS viva - always mention dibucaine number.

DOSAGE & ROUTES

Q: Types of doses?

Type	Definition	Example
Therapeutic dose	Dose producing desired effect without toxicity	Paracetamol 500 mg
Minimum effective dose	Smallest dose producing desired effect	-
Maximum tolerated dose	Largest dose without toxic effects	-
Lethal dose (LD50)	Dose killing 50% of experimental animals	-
Loading dose	Initial large dose to rapidly achieve therapeutic level	Digoxin, amiodarone
Maintenance dose	Dose to maintain steady-state plasma level	Daily digoxin
Ceiling dose	Dose beyond which no additional effect occurs	NSAIDs, diuretics

Q: Four parenteral routes with examples?

Route	Example Drugs	Key Points
Intravenous (IV)	Morphine, diazepam, heparin	Fastest onset; 100% bioavailability; risk of embolism
Intramuscular (IM)	Penicillin G, vaccines, diclofenac	Depot preparations possible
Subcutaneous (SC)	Insulin, heparin, adrenaline	Slow absorption, suitable for self-injection
Intrathecal	Morphine, bupivacaine, methotrexate	Bypasses blood-brain barrier

Others: intra-arterial (streptokinase), intra-articular (steroids), epidural.

Q: Advantages and disadvantages of rectal route?

Advantages	Disadvantages
Avoids first-pass metabolism (50%)	Irregular, unpredictable absorption
Useful when oral route unavailable (vomiting, unconscious)	Patient non-compliance, discomfort
Useful in pediatrics	Cannot use if anorectal disease
Local action possible (enemas)	Irritation of rectal mucosa

Q: Solid dosage forms?

Tablets - compressed powder; most common
Capsules - gelatin shell; mask unpleasant taste
Enteric-coated tablets - coating resists gastric acid (aspirin EC, diclofenac EC)
Sustained-release (SR/XL/ER) - slow release over time; reduces dosing frequency
Lozenges - dissolve in mouth (for local throat action)
Suppositories - rectal/vaginal solid form

Q: WHO definition of ADR?

A: "A response to a drug which is noxious and unintended, and which occurs at doses normally used in man for prophylaxis, diagnosis, or therapy of disease, or for modification of physiological function."

Key phrase: "noxious and unintended" + "at normal doses" (excludes overdose/poisoning).

SECTION 2: AUTONOMIC NERVOUS SYSTEM

CHOLINERGIC DRUGS

Q: Classification of indirect cholinergic drugs?

Indirect cholinergic drugs work by inhibiting acetylcholinesterase (anticholinesterase agents):

Reversible:

Short-acting: Edrophonium (diagnosis of myasthenia)
Medium-acting: Neostigmine, Pyridostigmine (quaternary ammonium - does NOT cross BBB)
Lipid-soluble (crosses BBB): Physostigmine, Donepezil, Rivastigmine, Galantamine

Irreversible:

Organophosphates: Echothiophate (glaucoma), Parathion, Malathion (insecticides), Nerve gases (Sarin, VX)

Q: Neostigmine - mechanism?

A: Neostigmine is a reversible, competitive inhibitor of acetylcholinesterase. It binds the anionic and esteratic sites of AChE, preventing breakdown of acetylcholine. This accumulates ACh at all cholinergic synapses (NMJ, autonomic ganglia, parasympathetic effectors).

Uses: Myasthenia gravis, reversal of non-depolarizing NMB, postoperative urinary retention/paralytic ileus.

Does NOT cross BBB (quaternary ammonium compound).

Q: Non-depolarizing neuromuscular blockers (NDNMBs)?

Short-acting: Mivacurium
Intermediate: Vecuronium, Rocuronium, Atracurium, Cisatracurium
Long-acting: Pancuronium, d-Tubocurarine

Mechanism: Competitive antagonism at nicotinic NM receptors - block ACh binding without depolarizing end-plate.

Reversal: Neostigmine + atropine (to block muscarinic side effects of neostigmine); OR Sugammadex (for rocuronium/vecuronium - forms inclusion complex).

Q: Succinylcholine (Suxamethonium)?

Class: Depolarizing neuromuscular blocker
Mechanism: Acts as ACh mimic at NMJ → persistent depolarization → initial fasciculations → flaccid paralysis (receptor cannot repolarize)
Duration: Ultra-short (5-10 min) - hydrolyzed by plasma pseudocholinesterase
Uses: Rapid sequence intubation (RSI), short procedures
Side effects: Hyperkalemia (contraindicated in burns, crush injury, denervation), malignant hyperthermia (with halothane), raised IOP, bradycardia, prolonged block (genetic - see pharmacogenetics above)
NOT reversed by neostigmine (no competitive antagonism)

Q: Differences between depolarizing and non-depolarizing NMBs?

Feature	Depolarizing (Succinylcholine)	Non-depolarizing (Vecuronium)
Mechanism	ACh mimic - persistent depolarization	Competitive antagonist
Initial fasciculations	Yes	No
Reversal	Not reversible (wait for metabolism)	Neostigmine or Sugammadex
Effect of AChE inhibitors	Potentiated (more block)	Antagonized (reversed)
Tetanic stimulation	Sustained response	Fade
Post-tetanic facilitation	No	Yes
Duration	Ultra-short	Variable (short to long)

ORGANOPHOSPHATE (OPC) POISONING

Q: Mechanism of OPC poisoning?

A: Organophosphates irreversibly phosphorylate the esteratic site of acetylcholinesterase → AChE permanently inactivated → ACh accumulates at all cholinergic synapses.

Features (SLUDGE + Nicotinic + CNS):

Muscarinic (SLUDGE): Salivation, Lacrimation, Urination, Defecation, GI cramps, Emesis + Bradycardia, Bronchoconstriction, Miosis
Nicotinic: Muscle fasciculations → weakness → paralysis; hypertension, tachycardia
CNS: Anxiety, seizures, coma, respiratory failure

Mnemonic for muscarinic effects: DUMBELS - Diarrhea, Urination, Miosis, Bradycardia, Emesis, Lacrimation, Salivation

Q: Antidote, dose, and mechanism for OPC poisoning?

Atropine:

Antidote for muscarinic effects only
Dose: 2-4 mg IV every 5-10 min until muscarinic features dry up (secretions dry, bronchospasm resolves) - no upper limit; massive doses may be needed
Mechanism: Competitive muscarinic antagonist - blocks ACh at muscarinic receptors

Pralidoxime (2-PAM):

Antidote for both muscarinic AND nicotinic effects
Dose: 1-2 g IV over 15-30 min, then infusion
Mechanism: Reactivates acetylcholinesterase by cleaving the phosphate-AChE bond (if given BEFORE "aging" - irreversible conformational change, usually within 24-48 hours)
Must be given early - becomes ineffective after aging

Cross Q: Which effects does atropine NOT treat in OPC? Nicotinic effects (muscle fasciculations, paralysis) - only pralidoxime helps these.

Q: Complete management of OPC poisoning?

Remove from exposure - decontamination, remove clothing, wash skin
ABC - airway (intubation if needed), suction secretions
Atropine - large doses IV until secretions dry
Pralidoxime - early administration
Benzodiazepines (diazepam) - for seizures
Avoid: succinylcholine (potentiated), organophosphate-class insecticide exposure
Supportive care - mechanical ventilation, fluids

ATROPINE

Q: Signs of atropine poisoning ("Hot as a hare, Dry as a bone, Red as a beet, Mad as a hatter, Blind as a bat")?

Hyperthermia (↓ sweating) - "Hot as a hare"
Dry skin and mouth - "Dry as a bone"
Flushing (vasodilation) - "Red as a beet"
Delirium, hallucinations - "Mad as a hatter"
Mydriasis, cycloplegia (blurred vision) - "Blind as a bat"
Also: Tachycardia, urinary retention, constipation

Treatment: Physostigmine (crosses BBB - reverses CNS effects too)

Q: Atropine-like drugs (antimuscarinics)?

Drug	Special Feature
Hyoscine (scopolamine)	Better for motion sickness; CNS depression (unlike atropine which causes excitation)
Ipratropium	Inhaled; bronchodilation in COPD/asthma; no systemic effects
Tiotropium	Long-acting; once-daily; COPD
Oxybutynin, Solifenacin	Overactive bladder
Benztropine, Trihexyphenidyl	Parkinson's disease
Dicyclomine	Irritable bowel syndrome
Homatropine, Tropicamide	Mydriatic/cycloplegic (eye)

Q: Why is atropine used as pre-anaesthetic medication?

A: To reduce secretions (anti-sialagogue effect) caused by inhalational anesthetics and airway manipulation. Additional benefits:

Prevents bradycardia from vagal stimulation during intubation/surgery
Prevents bronchospasm
Reduces gastric acid secretion (slight)
Causes sedation with hyoscine (if hyoscine used instead)

Modern anesthetics are less irritating, so atropine as pre-med is now selective, not routine.

Q: Ophthalmic use of atropine?

Cycloplegia (ciliary muscle paralysis) - refraction testing in children
Mydriasis - fundus examination, pre/post intraocular surgery
Treatment of uveitis/iritis - prevents synechiae formation
Amblyopia treatment - penalizing the better eye

Q: Mydriatic drugs. Which is preferred in elderly and why?

Mydriatics:

Drug	Mechanism	Duration	Cycloplegia?
Atropine	Muscarinic block	7-10 days	Yes
Homatropine	Muscarinic block	1-3 days	Yes (mild)
Tropicamide	Muscarinic block	4-6 hours	Mild
Cyclopentolate	Muscarinic block	24 hours	Yes
Phenylephrine	α1 agonist (dilates dilator pupillae)	4-6 hours	No

Preferred in elderly: Tropicamide (short-acting) or Phenylephrine

Why? Elderly patients are at high risk of acute angle-closure glaucoma with long-acting mydriatics (atropine, homatropine), because:

Elderly have shallow anterior chambers (lens thickens with age)
Pupil dilation obstructs trabecular drainage → ↑ IOP
Tropicamide wears off quickly (4-6 hrs), reducing this risk

SECTION 3: ADRENERGIC DRUGS

Q: Classification of indirect adrenergic agonists?

Indirect adrenergic agonists work by increasing norepinephrine at the synapse:

Releasing agents: Amphetamine, tyramine, ephedrine
Uptake inhibitors: Cocaine, tricyclic antidepressants (block reuptake of NE)
MAO inhibitors: Phenelzine, tranylcypromine (block degradation of NE)

Q: Beta agonists - classification and examples?

Selectivity	Drug	Use
Non-selective β1+β2	Isoprenaline	Bradycardia (historical)
Selective β1	Dobutamine	Acute heart failure (inotrope)
Selective β2	Salbutamol, Terbutaline	Asthma bronchodilation
Long-acting β2	Salmeterol, Formoterol	Maintenance asthma/COPD

Q: Why is adrenaline used in anaphylaxis and shock?

A: Adrenaline (epinephrine) acts on both alpha and beta receptors:

α1 effects: Vasoconstriction → ↑ BP, reduces angioedema and urticaria
β1 effects: ↑ HR and contractility → ↑ cardiac output
β2 effects: Bronchodilation → relieves bronchospasm; inhibits mast cell mediator release
It is the only drug that addresses ALL components of anaphylaxis simultaneously.

Cross Q: Route in anaphylaxis? IM into anterolateral thigh (NOT IV unless cardiac arrest).

Q: Beta blocker classification?

Generation	Selectivity	Examples
1st gen (non-selective)	β1 + β2	Propranolol, Timolol, Nadolol
2nd gen (cardioselective)	β1 selective	Atenolol, Metoprolol, Bisoprolol
3rd gen (vasodilating)	β1 + α1 block OR β3/NO	Carvedilol (α+β), Labetalol (α+β), Nebivolol (β1 + ↑NO)

Q: Atenolol vs Propranolol?

Feature	Atenolol	Propranolol
Receptor selectivity	β1 selective	Non-selective (β1 + β2)
Lipid solubility	Low (hydrophilic)	High (lipophilic)
CNS penetration	Poor	Good (nightmares, depression)
Route	Oral (poor IV use)	Oral + IV
Renal excretion	Mainly renal	Mainly hepatic
Use in asthma	Safer (relative)	Contraindicated
First-pass effect	Minimal	Extensive (large first-pass)

SECTION 4: ANTIHYPERTENSIVE DRUGS

Q: Drugs used in hypertension with diabetes?

First-line: ACE inhibitors or ARBs (nephroprotective - reduce proteinuria, slow diabetic nephropathy)
If still not controlled: Add thiazide or CCB (amlodipine)
Avoid: Beta-blockers (mask hypoglycemia symptoms, worsen insulin resistance)

Q: Why are ACE inhibitors first-line in hypertension with DM?

Reduce blood pressure
Reduce intraglomerular pressure (dilate efferent arteriole) → reduce proteinuria
Slow progression of diabetic nephropathy
Cardioprotective
Reduce microalbuminuria even before frank proteinuria

Q: ACE inhibitor vs ARB comparison?

Feature	ACE Inhibitor	ARB
Mechanism	Blocks conversion of Ang I → Ang II	Blocks AT1 receptor
Cough	Yes (↑ bradykinin)	No
Angioedema	More common	Less common but possible
Effect on bradykinin	↑ bradykinin (beneficial for BP)	No effect
Use if cough	Switch to ARB	First choice
Examples	Enalapril, Ramipril, Lisinopril	Losartan, Valsartan, Telmisartan

Cross Q: Why do ACE inhibitors cause cough? ACE also breaks down bradykinin. When ACE is inhibited, bradykinin accumulates → irritates bronchi → dry, persistent cough in ~10-15% patients.

Q: Why ACE inhibitors are NOT used in bronchial asthma?

Bradykinin accumulation → airway inflammation, bronchoconstriction → cough and potential bronchospasm
Use ARBs instead (do not affect bradykinin)

Q: Hypertension with bronchial asthma?

Use: CCB (amlodipine), ARBs
Avoid: Beta-blockers (β2 block → bronchospasm), ACE inhibitors (cough/bronchospasm)

Q: Hypertension with obesity and nephropathy?

ACE inhibitor or ARB (nephroprotection)
Add CCB or thiazide if needed

Q: Hypertension with diabetic neuropathy?

Duloxetine or pregabalin (for neuropathic pain)
Antihypertensive: ACE inhibitor/ARB

Q: Prazosin - mechanism, adverse effects, first-dose phenomenon?

Mechanism: Selective α1 antagonist → vasodilation (arteriolar + venous) → ↓ BP
Adverse effects: First-dose phenomenon, postural hypotension, reflex tachycardia, nasal stuffiness, headache, fluid retention
First-dose phenomenon: Sudden severe hypotension and syncope after the first dose due to loss of α1-mediated vascular tone without reflex compensation
Prevention: Start with very low dose (0.5 mg) at bedtime, with the patient lying down; titrate gradually

Q: Methyldopa - mechanism, adverse effects, use in PIH?

Mechanism: Centrally acting antihypertensive. Converted to alpha-methylnorepinephrine in CNS → stimulates α2 receptors (presynaptic) → reduces sympathetic outflow → ↓ BP
Adverse effects: Sedation (most common), dry mouth, positive Coombs test (hemolytic anemia - rare), lupus-like syndrome, hepatotoxicity
Drug of choice in pregnancy-induced hypertension (PIH): Safe for fetus; crosses placenta without teratogenicity; long safety track record

Cross Q: What is the drug of choice for severe/acute PIH? Labetalol or Hydralazine IV. For maintenance: Methyldopa, Nifedipine.

Q: Hydralazine - mechanism?

Direct vasodilator → relaxes arteriolar smooth muscle (mainly) → ↓ peripheral resistance
Mechanism: Increases cGMP in vascular smooth muscle; may open K⁺ channels
Reflex effects: Tachycardia, fluid retention, ↑ renin release
Uses: PIH (IV), heart failure with nitrates (combination), hypertensive crisis
Side effect: Lupus-like syndrome (dose-dependent, in slow acetylators)

Q: Thiazide diuretics - mechanism, hyperuricemia, adverse effects?

Mechanism: Inhibit Na⁺/Cl⁻ cotransporter in distal convoluted tubule (DCT) → ↑ Na⁺ and water excretion → ↓ plasma volume → ↓ BP
Why hyperuricemia? Thiazides compete with uric acid for proximal tubule secretion via OAT transporters. Also volume contraction → ↑ urate reabsorption → hyperuricemia → can precipitate gout
Adverse effects: Hypokalemia, hyponatremia, hyperuricemia, hyperglycemia, hyperlipidemia, hypercalcemia (remember: thiazides retain calcium - used in hypercalciuria/nephrolithiasis), erectile dysfunction

SECTION 5: DIURETICS

Q: Diuretics acting on DCT and collecting duct?

DCT (Distal Convoluted Tubule):

Thiazides (hydrochlorothiazide, chlorthalidone) - block Na/Cl cotransporter

Collecting Duct:

Potassium-sparing diuretics:
- Aldosterone antagonists: Spironolactone, Eplerenone (block mineralocorticoid receptor → ↓ Na⁺ reabsorption, ↓ K⁺ secretion)
- Na⁺ channel blockers: Amiloride, Triamterene (block epithelial Na⁺ channel - ENaC)

Q: Furosemide vs Thiazide?

Feature	Furosemide	Thiazide
Site of action	Loop of Henle (thick ascending)	DCT
Transporter blocked	NKCC2 (Na-K-2Cl)	NCC (Na-Cl)
Potency	High (ceiling diuretic)	Moderate
Calcium excretion	Increases Ca²⁺ excretion	Decreases Ca²⁺ excretion
Glucose effect	Can cause hyperglycemia	More pronounced hyperglycemia
Use in renal failure	Effective (GFR <30)	Ineffective if GFR <30
Use in hypercalcemia	Yes (↑Ca excretion)	No
Use in nephrolithiasis	No	Yes (↓Ca excretion)

Q: Furosemide vs Spironolactone?

Feature	Furosemide	Spironolactone
Site	Loop of Henle	Collecting duct
Potassium effect	Hypokalemia	Hyperkalemia (K-sparing)
Mechanism	NKCC2 block	Aldosterone antagonist
Use in cirrhotic ascites	Yes (with spironolactone)	Yes (preferred as aldosterone is elevated)
Anti-androgen effect	No	Yes (gynecomastia, menstrual irregularity)

Classic combination: Furosemide + Spironolactone in hepatic ascites (ratio 40:100 mg to maintain normokalemia).

Q: Drug for leg swelling (edema)?

Cardiac edema: Furosemide ± spironolactone
Cirrhotic ascites/edema: Spironolactone (preferred) + furosemide
Hypertensive: Thiazide
Nephrotic: Furosemide

SECTION 6: DIABETES MELLITUS

Q: Euglycemic agents (drugs that don't cause hypoglycemia) with examples?

Metformin (biguanide)
Acarbose (alpha-glucosidase inhibitor)
GLP-1 agonists (exenatide, liraglutide) - very low risk of hypoglycemia alone
DPP-4 inhibitors (sitagliptin, linagliptin) - weight neutral, low hypoglycemia risk
SGLT-2 inhibitors (empagliflozin, dapagliflozin)
Thiazolidinediones (pioglitazone) - very low hypoglycemia risk

Q: Drugs used for diabetes in pregnancy?

Insulin is the drug of choice (does not cross placenta; safest)
Metformin is used in some countries (some evidence for safety, used for GDM)
Sulfonylureas, GLP-1 agonists, SGLT-2 inhibitors - AVOID (teratogenic potential or insufficient data)

Q: Insulin preparations - classification by duration?

Type	Onset	Peak	Duration	Examples
Ultra-rapid	10-15 min	1 hr	2-4 hrs	Lispro, Aspart, Glulisine
Short-acting (Regular)	30 min	2-3 hrs	6-8 hrs	Regular/Soluble insulin
Intermediate	1-2 hrs	4-8 hrs	12-18 hrs	NPH (Isophane)
Long-acting	1-2 hrs	Peakless	20-24 hrs	Glargine, Detemir
Ultra-long	6 hrs	Peakless	42+ hrs	Degludec

Q: Mechanism of insulin?

Binds insulin receptor (tyrosine kinase receptor) on cell membrane
Autophosphorylation → activates IRS proteins
PI3K pathway → GLUT-4 translocation to membrane → ↑ glucose uptake
Promotes glycogen synthesis (liver, muscle), protein synthesis, fat storage
Inhibits gluconeogenesis, glycogenolysis, lipolysis

Q: Adverse effects of insulin?

Hypoglycemia (most common and dangerous)
Lipodystrophy at injection site (lipoatrophy or lipohypertrophy)
Weight gain
Hypokalemia (insulin shifts K⁺ into cells)
Edema (sodium retention)
Insulin resistance (rare)

Cross Q: Why is insulin given in trauma/accident in diabetic patients? Trauma → stress response → counter-regulatory hormones (cortisol, glucagon, adrenaline) → hyperglycemia. Insulin is given to control blood glucose and prevent DKA. Also, insulin has anabolic effects promoting wound healing.

Q: Metformin - indications, mechanism, adverse effects?

Mechanism:

Primary: Activates AMPK → inhibits hepatic gluconeogenesis (main effect)
Improves peripheral insulin sensitivity
Delays GI glucose absorption
Does NOT stimulate insulin secretion → no hypoglycemia

Indications:

First-line in Type 2 DM (especially obese patients)
PCOS (improves insulin resistance)
Pre-diabetes prevention

Adverse effects:

GI: nausea, diarrhea, abdominal pain (most common - take with food)
Lactic acidosis (rare but serious - accumulation in hypoxic states)
Vitamin B12 deficiency (long-term use)
Metallic taste

Contraindications: eGFR <30, acute heart failure, acute liver failure, severe infection, iodinated contrast dye (hold for 48 hrs), surgery

Q: GLP-1 agonists mechanism?

Mimic glucagon-like peptide-1 (GLP-1) - incretin hormone
Mechanism:
1. Stimulate insulin secretion in a glucose-dependent manner (only when glucose is high → no hypoglycemia)
2. Suppress glucagon secretion
3. Delay gastric emptying → reduce postprandial glucose
4. Central satiety → weight loss
Examples: Exenatide, Liraglutide, Semaglutide
Benefits: Weight loss, cardiovascular protection (liraglutide, semaglutide), nephroprotection

Q: Linagliptin mechanism?

DPP-4 inhibitor (gliptin class)
Dipeptidyl peptidase-4 is the enzyme that degrades GLP-1 and GIP (incretin hormones)
Linagliptin blocks DPP-4 → GLP-1 levels increase → ↑ glucose-dependent insulin secretion + ↓ glucagon
Advantage over other gliptins: Excreted mainly via bile (not kidney) → safe in renal impairment without dose adjustment

SECTION 7: ANTIANGINAL DRUGS

Q: Classification of antianginal drugs?

Nitrates: GTN (sublingual, patch), Isosorbide mononitrate/dinitrate
Beta-blockers: Propranolol, Atenolol, Metoprolol
Calcium channel blockers:
- Non-dihydropyridines (heart rate-lowering): Verapamil, Diltiazem
- Dihydropyridines (vasodilating): Amlodipine, Nifedipine
Other: Ranolazine (late Na⁺ channel blocker), Ivabradine (If channel blocker)

Q: Calcium channel blockers - effect on hypertension and angina?

Hypertension:

Dihydropyridines (amlodipine, nifedipine): Vasodilate arterioles → ↓ peripheral resistance → ↓ BP
Verapamil/Diltiazem: Also lower BP but mainly via cardiac effects

Angina:

Dilate coronary arteries → ↑ oxygen supply
Reduce afterload → ↓ cardiac work → ↓ oxygen demand
Verapamil/Diltiazem: Also ↓ HR → ↑ diastolic filling time → ↑ coronary perfusion

Cross Q: Which CCB is preferred in angina with tachycardia? Verapamil or Diltiazem (rate-limiting CCBs). NOT amlodipine (causes reflex tachycardia).

Q: GTN (glyceryl trinitrate) adverse effects?

Headache (most common - vasodilation of meningeal vessels)
Flushing, postural hypotension, syncope
Tolerance (tachyphylaxis) - with continuous use; prevent by nitrate-free interval (8-12 hrs)
Reflex tachycardia
Methemoglobinemia (high doses)

Mechanism of GTN: → Releases NO → activates guanylate cyclase → ↑ cGMP → smooth muscle relaxation → venodilation (primarily) + arteriolar dilation at higher doses → ↓ preload (and afterload) → ↓ cardiac work.

SECTION 8: HORMONAL CONTRACEPTIVES

Q: Classification of oral contraceptives?

Combined oral contraceptives (COC): Estrogen + Progestin
- Monophasic (fixed dose throughout cycle)
- Biphasic, Triphasic (varying doses)
Progestin-only pill (POP / Minipill): Norethindrone, Levonorgestrel
Emergency contraception: Levonorgestrel (Plan B), Ulipristal acetate
Long-acting: DMPA injection (Depo-Provera), Implant (Implanon - etonogestrel)

Q: Mechanism of oral contraceptives?

Primary (estrogen + progestin): Inhibit GnRH pulsatility → suppress FSH and LH → inhibit ovulation (most important)
Progestin effect: Thickens cervical mucus → prevents sperm penetration
Alters endometrium → unfavorable for implantation
Reduces tubal motility

Q: Adverse effects of COC?

Nausea, vomiting, breakthrough bleeding (common, usually transient)
Thromboembolic events (DVT, PE, stroke) - due to estrogen → ↑ clotting factors
Hypertension
Weight gain
Decreased libido
Breast tenderness
Chloasma (facial pigmentation)
Drug interactions (rifampicin, phenytoin reduce efficacy via CYP450 induction)

Q: Contraindications to COC?

History of DVT, PE, or stroke
Hypertension
Smoker >35 years old
Breast cancer or estrogen-sensitive cancer
Migraine with aura
Severe liver disease
Pregnancy
Breastfeeding (estrogen reduces milk production)

Q: Why is the minipill (progestin-only) preferred in lactating mothers?

COC contains estrogen → suppresses prolactin → reduces breast milk production
Minipill has no estrogen → does not affect lactation
Progestin-only pill primarily acts by thickening cervical mucus; works without suppressing lactation
Also excreted in small amounts in breast milk but considered safe

SECTION 9: OBSTETRICS - ANTIHYPERTENSIVES IN PREGNANCY

Q: Drug of choice for pregnancy-induced hypertension (PIH)?

Mild-moderate PIH: Methyldopa (first-line, safest)
Alternatives: Labetalol, Nifedipine (long-acting)
Acute/severe PIH: Labetalol IV or Hydralazine IV
Magnesium sulfate for eclampsia prevention (not antihypertensive, but prevents seizures)
Avoid: ACE inhibitors, ARBs (fetotoxic - renal agenesis, oligohydramnios), thiazides

Q: Types of uterine bleeding?

Type	Description
Menorrhagia	Heavy, regular menstrual bleeding
Metrorrhagia	Irregular, between periods
Menometrorrhagia	Heavy + irregular
Polymenorrhea	Frequent periods (<21 days)
Oligomenorrhea	Infrequent periods (>35 days)
Dysfunctional uterine bleeding (DUB)	Abnormal bleeding without structural cause
Postpartum hemorrhage	Bleeding after delivery

SECTION 10: OPHTHALMOLOGY

(Covered under Atropine section above)

Fundus examination: Tropicamide (preferred - short-acting, 4-6 hrs)
Elderly: Tropicamide or phenylephrine (to avoid angle-closure glaucoma)
Uveitis: Atropine (long-acting, prevents synechiae)
Refraction in children: Cyclopentolate (balances duration with cycloplegia needed)

RAPID REVISION: HIGH-YIELD ONE-LINERS

Topic	Key Point
Succinylcholine + genetics	Atypical pseudocholinesterase → prolonged apnea; dibucaine number
ACE inhibitor cough	↑ bradykinin → use ARB if cough develops
Thiazide + uric acid	Compete with urate secretion + volume contraction → hyperuricemia
Prazosin 1st dose	Take at bedtime; start 0.5 mg
Methyldopa in pregnancy	Drug of choice - central α2 agonist, fetal safe
Atropine poisoning	Physostigmine antidote
OPC poisoning	Atropine (muscarinic) + Pralidoxime (reactivates AChE) - give early
Furosemide vs thiazide Ca	Furo = ↑Ca excretion; Thiazide = ↓Ca excretion
Metformin contraindications	GFR <30, acute HF, hypoxic states (lactic acidosis risk)
Minipill in lactation	No estrogen → does not reduce milk supply
Partial agonist + full agonist	Partial agonist acts as antagonist in presence of full agonist
Down-regulation	Chronic agonist → receptor decrease
Up-regulation	Chronic antagonist (e.g., beta-blocker) → receptor increase → rebound if stopped suddenly
WHO ADR definition	Noxious + unintended + at normal therapeutic doses
GLP-1 agonists	Glucose-dependent insulin secretion = no hypoglycemia alone
Linagliptin	Only DPP-4 inhibitor safe in severe renal failure without dose adjustment
Mydriatic in elderly	Tropicamide (short-acting) - avoids angle-closure glaucoma

This covers your entire viva list. Let me know which topic you want to drill deeper, practice in Q&A format, or get examiner-style grilling on - and we'll go topic by topic from here.

Procesd of biotransformation

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Excellent - I now have comprehensive, textbook-sourced content with diagrams. Here is the complete answer.

Biotransformation (Drug Metabolism)

Definition

Biotransformation is the chemical alteration of a drug in the body, primarily in the liver, that converts lipid-soluble drugs into more water-soluble (polar) metabolites so they can be excreted by the kidneys.

Why needed? Lipophilic drugs readily cross cell membranes but are reabsorbed in the distal tubule and cannot be efficiently excreted. Biotransformation makes them polar enough for renal excretion. - Lippincott Pharmacology, p. 53

Overview Diagram

Biotransformation of drugs - Phase I and Phase II

Key point from the diagram:

Lipophilic drug → Phase I (oxidation/reduction/hydrolysis) → polar metabolite → Phase II (conjugation) → water-soluble conjugate → excreted
Some drugs skip Phase I and go directly to Phase II (if they already have -OH, -NH₂, or -COOH groups)
After Phase I, the drug may be activated, unchanged, or inactivated
After Phase II, the conjugated product is usually inactive (exception: morphine-6-glucuronide is MORE potent than morphine)

Site of Biotransformation

Site	Details
Liver	Primary site - smooth ER (microsomes) and cytosol
GI tract (enterocytes)	Phase I + II during absorption (contributes to first-pass effect)
Kidney	Minor
Lungs	Minor
Plasma	Esterases (e.g., succinylcholine, aspirin)
Brain, skin	Minor

Kinetics of Biotransformation

1. First-Order Kinetics (Linear Kinetics)

When drug concentration << Km (Michaelis constant)
Rate of metabolism is proportional to drug concentration
A constant fraction is metabolized per unit time (50% per half-life)
Most drugs at therapeutic doses follow this
Examples: Most standard drugs at normal doses

2. Zero-Order Kinetics (Non-linear / Saturation Kinetics)

When drug concentration >> Km (enzyme is saturated)
Rate of metabolism is constant regardless of concentration
A constant amount (not fraction) is metabolized per unit time
Small dose increase → disproportionately large plasma level rise → toxicity
Examples: Aspirin (high dose), Ethanol, Phenytoin

Viva tip: Phenytoin and ethanol show zero-order kinetics → reason for their narrow therapeutic window and toxicity at slightly higher doses.

PHASE I REACTIONS (Functionalization)

Goal: Introduce or unmask a polar functional group (-OH, -NH₂, -SH, -COOH)

Types: Oxidation, Reduction, Hydrolysis

A. Reactions Using Cytochrome P450 (CYP) System

The most important Phase I reactions are catalyzed by CYP enzymes.

CYP system:

Located in smooth ER (microsomes) of liver cells and GI tract
Heme-containing enzymes (monooxygenases)
Require NADPH and O₂

Reaction:

Drug + O₂ + NADPH → Oxidized drug + H₂O + NADP⁺

CYP Isoforms and their contributions:

Relative contribution of CYP isoforms to drug metabolism

Isoform	% Contribution	Key Substrates
CYP3A4/5	36% (largest)	Simvastatin, nifedipine, cyclosporine, erythromycin, verapamil, carbamazepine
CYP2D6	19%	Propranolol, fluoxetine, haloperidol, codeine (→ morphine)
CYP2C8/9	16%	Warfarin, ibuprofen, phenytoin, glimepiride
CYP1A2	11%	Theophylline, caffeine, clozapine
CYP2C19	8%	Omeprazole, clopidogrel (prodrug activation)
CYP2E1	4%	Ethanol, paracetamol (toxic metabolite NAPQI)

Viva critical: CYP3A4 is found in large quantities in intestinal mucosa → first-pass metabolism of cyclosporine, midazolam.

B. Phase I Reactions NOT Using CYP System

Reaction	Enzyme	Example
Amine oxidation	MAO	Oxidation of catecholamines, histamine
Alcohol dehydrogenation	Alcohol dehydrogenase	Ethanol → acetaldehyde
Esterase hydrolysis	Plasma/liver esterases	Aspirin → salicylic acid; succinylcholine
Hydrolysis	Various	Methylphenidate

PHASE II REACTIONS (Conjugation / Synthetic Reactions)

Goal: Attach (conjugate) an endogenous polar molecule to the drug or Phase I metabolite → highly water-soluble, usually inactive compound → excreted by kidney or bile

Conjugation Type	Enzyme	Endogenous Substrate	Example
Glucuronidation	UDP-glucuronosyltransferase (UGT)	Glucuronic acid	Morphine → M-6-G and M-3-G; paracetamol
Sulfation	Sulfotransferase	Sulfate	Paracetamol, steroids
Acetylation	N-acetyltransferase (NAT)	Acetyl CoA	Isoniazid, hydralazine, sulfonamides
Methylation	Methyltransferase	SAM (methyl donor)	Dopamine, norepinephrine
Glutathione conjugation	GSH-S-transferase	Glutathione	Paracetamol (NAPQI detoxification)
Glycine conjugation	-	Glycine	Salicylic acid
Amino acid conjugation	-	Glutamine, glycine	Bile acids

Most important conjugation reaction: Glucuronidation - most common, highest capacity, occurs in liver, kidney, intestine.

Enzyme Induction

Definition: Increased synthesis of CYP enzymes due to prolonged exposure to certain drugs/chemicals → faster metabolism of co-administered drugs → reduced plasma levels → therapeutic failure.

Mechanism: Inducers bind nuclear receptors (PXR, CAR, AhR) → increase CYP gene transcription → more enzyme produced (takes days to weeks to develop)

Classic inducers (mnemonic: PC BRAS):

Phenytoin
Carbamazepine
Barbiturates (phenobarbitone)
Rifampicin (most potent inducer)
Alcohol (chronic)
St. John's Wort (herbal)

Consequences:

Reduced efficacy of co-administered drugs (e.g., rifampicin reduces OCP efficacy → breakthrough bleeding, pregnancy)
Toxicity: induction may generate more toxic metabolites (e.g., paracetamol + rifampicin → more NAPQI → hepatotoxicity)

Enzyme Inhibition

Definition: Drug inhibits CYP enzyme → slower metabolism of co-administered drugs → accumulation → toxicity

Mechanism: Usually competitive inhibition (competition for same CYP active site); onset is rapid (within hours, unlike induction)

Classic inhibitors (mnemonic: KECGIV):

Ketoconazole (all azole antifungals)
Erythromycin (and clarithromycin - macrolides)
Cimetidine
Grapefruit juice (CYP3A4 in gut)
Isoniazid
Valporate, Verapamil, ritonavir

Clinical example: Ketoconazole + simvastatin → ↑ simvastatin levels → rhabdomyolysis

Note: CYP2D6 is not inducible - unlike most other CYPs.

Factors Affecting Biotransformation

Factor	Effect
Age	Neonates (immature enzymes → e.g., chloramphenicol → Gray baby syndrome); Elderly (↓ CYP activity)
Liver disease	↓ First-pass metabolism → ↑ bioavailability of high extraction drugs
Genetic polymorphism	Poor metabolizers vs ultra-rapid metabolizers (e.g., CYP2D6 - codeine, CYP2C19 - clopidogrel)
Sex	Minor differences; CYP3A4 slightly higher in women
Drug interactions	Induction or inhibition of CYP enzymes
Nutritional status	Malnutrition → ↓ enzyme activity
Route of administration	Oral drugs subject to first-pass; IV bypasses it

First-Pass Effect

When an oral drug is absorbed from the gut, it passes via the portal vein → liver before entering systemic circulation
The liver metabolizes a portion of the drug → reduced bioavailability
High first-pass drugs (oral bioavailability very low): GTN (~1%), Lidocaine, Propranolol, Morphine, Aspirin, Isosorbide dinitrate, Testosterone

Clinical implication: GTN is given sublingually (bypasses portal circulation) to avoid near-complete first-pass destruction.

Prodrugs (Bioactivation)

Some drugs are administered as inactive prodrugs and converted to active form by biotransformation:

Prodrug	Active Form	Enzyme
Enalapril	Enalaprilat	Esterases
Codeine	Morphine	CYP2D6
Clopidogrel	Active thiol metabolite	CYP2C19
Cyclophosphamide	Phosphoramide mustard	CYP2B6
Levodopa	Dopamine	DOPA decarboxylase
Prednisone	Prednisolone	Liver

Viva tip: Poor metabolizers of CYP2C19 get less benefit from clopidogrel (antiplatelet effect reduced). CYP2D6 ultra-rapid metabolizers of codeine get morphine toxicity.

Quick Viva Summary

Point	Answer
Primary site of biotransformation	Liver (smooth ER)
Purpose	Convert lipophilic → hydrophilic for excretion
Phase I reactions	Oxidation, Reduction, Hydrolysis
Phase II reactions	Conjugation (glucuronidation most important)
Most important CYP enzyme	CYP3A4 (36% of all drug metabolism)
Most potent enzyme inducer	Rifampicin
Drug showing zero-order kinetics	Phenytoin, Ethanol, Aspirin (high dose)
Phase II product usually	Inactive (exception: morphine-6-glucuronide)
CYP2D6	Not inducible; genetic polymorphism important (codeine)
First-pass high drugs	GTN, propranolol, morphine, lidocaine

Process of biotransformation

I just gave you a comprehensive answer on biotransformation in our previous message! Here's a quick recap of the full structure covered:

What Was Covered

Definition - Chemical alteration of lipophilic drug → water-soluble metabolite for excretion, primarily in liver

Phase I - Functionalization

Introduces/unmasks polar group (-OH, -NH₂)

Reaction	Example
Oxidation (CYP450)	Most drugs via CYP3A4, CYP2D6, etc.
Reduction	Chloramphenicol, warfarin
Hydrolysis	Aspirin, succinylcholine (plasma esterases)

Phase II - Conjugation (Synthetic)

Attaches endogenous molecule → highly polar, usually inactive

Type	Example Drug
Glucuronidation (most important)	Morphine, paracetamol
Sulfation	Paracetamol, steroids
Acetylation	Isoniazid, hydralazine
Methylation	Dopamine, NE
Glutathione conjugation	Paracetamol (NAPQI detox)

Kinetics

First-order - constant fraction metabolized (most drugs)
Zero-order - constant amount metabolized (Phenytoin, Ethanol, Aspirin at high dose - enzyme saturated)

Enzyme Induction vs Inhibition

	Induction	Inhibition
Onset	Days to weeks	Rapid (hours)
Effect	↓ drug levels	↑ drug levels
Examples	Rifampicin, phenytoin, carbamazepine, alcohol (chronic)	Ketoconazole, erythromycin, cimetidine, grapefruit juice

Key Viva Points

CYP3A4 = most important isoform (36% of all drug metabolism)
CYP2D6 = not inducible; genetic polymorphism (codeine → morphine)
CYP2C19 = clopidogrel activation (poor metabolizers get less antiplatelet effect)
Prodrugs activated by biotransformation: enalapril, codeine, clopidogrel, levodopa
Exception to Phase II inactivation: Morphine-6-glucuronide is MORE potent than morphine
First-pass high drugs: GTN, propranolol, morphine, lidocaine

Scroll up to see the full detailed answer with diagrams including the Phase I/II overview chart and the CYP isoform pie chart. Let me know if you want to go deeper on any specific part - enzyme induction/inhibition interactions, prodrugs, or zero-order kinetics!

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