Furosemide (Frusemide) - Padmaja Udaykumar

Chemistry

Mechanism of Action

1. Furosemide is secreted into the tubular lumen by the proximal tubule (organic anion secretory pathway).
2. It reaches the luminal surface of the thick ascending limb.
3. It blocks the Na⁺-K⁺-2Cl⁻ symporter, preventing reabsorption of these ions.
4. Because a large fraction of NaCl is normally absorbed in this segment, the drug is highly efficacious.
5. The diuretic response increases with dose - there is no "ceiling" on the response at higher doses (hence "high-ceiling" diuretic).

• IV furosemide causes immediate venodilation (within minutes) even before diuresis begins - this is mediated by prostaglandin E₂ and prostacyclin release.
• This accounts for its rapid benefit in acute pulmonary oedema and acute LVF.
• It also causes renal vasodilation and increases renal blood flow.

Uses (Diuretic Uses)

1. Oedema - Cardiac, hepatic (cirrhotic ascites), renal oedema; reduces venous and pulmonary congestion in CCF.
2. Acute pulmonary oedema / Acute LVF - IV furosemide gives rapid relief due to its immediate venodilator effect, followed by diuretic action.
3. Acute renal failure - Enhances urine output and K⁺ excretion; useful in impending acute renal failure; large doses needed in CRF.
4. Cerebral oedema - Used as an alternative to or in combination with osmotic diuretics (mannitol).
5. Hypertension - Though less favoured than thiazides, used when diuresis is needed.

Non-Diuretic Uses

Adverse Drug Reactions

1. Electrolyte Disturbances

• Hypokalaemia - Most important ADR. Results from increased Na⁺ delivery to the distal tubule, stimulating aldosterone-mediated K⁺ secretion. Can precipitate cardiac arrhythmias, muscle weakness, and digitalis toxicity.
• Hyponatraemia - Due to Na⁺ loss with water retention.
• Hypomagnesaemia - Increased Mg²⁺ excretion.
• Hypocalcaemia - Unlike thiazides, furosemide increases Ca²⁺ excretion (used therapeutically in hypercalcaemia).
• Hypochloraemia + Metabolic alkalosis - Cl⁻ loss with compensatory HCO₃⁻ retention.

2. Volume/Haemodynamic Effects

• Hypovolaemia - Excess fluid loss; can progress to dehydration.
• Hypotension - Postural/orthostatic.
• Haemoconcentration - Risk of thromboembolism in elderly.

3. Ototoxicity (Unique ADR)

• Sensorineural hearing loss, tinnitus, vertigo - dose-related, usually reversible but may be permanent with high doses or rapid IV injection.
• Risk is greatly increased when furosemide is combined with aminoglycoside antibiotics (synergistic ototoxicity).
• Prevention: use slow IV infusion (<4 mg/min) rather than rapid bolus.

4. Hyperuricaemia

• Furosemide competes with uric acid for tubular secretion and promotes uric acid retention.
• Can precipitate gout in susceptible individuals.

5. Hyperglycaemia

• Less marked than with thiazides, but can impair glucose tolerance.

6. Hyperlipidaemia

• Minor and reversible elevation in serum triglycerides and LDL.

7. Allergic Reactions

• Rash, photosensitivity - Cross-sensitivity with sulfonamides possible.
• Rare: Interstitial nephritis, thrombocytopenia, haemolytic anaemia.

8. Miscellaneous

• Nausea, vomiting, diarrhoea - GI disturbances.
• Weakness and fatigue - Often due to electrolyte imbalance.

Contraindications

• Allergy to sulfonamides (cross-sensitivity)
• Anuria/severe renal failure (unless very high doses in CRF)
• Hepatic coma (can worsen by precipitating electrolyte imbalance)
• Hypovolaemia and dehydration
• Pregnancy (Category C - teratogenic in animals)

Quick-Reference Summary Table

Clopidogrel - Padmaja Udaykumar's Medical Pharmacology

Classification

Mechanism of Action

Step-by-step mechanism:

1. Absorption & Activation: After oral ingestion, clopidogrel is absorbed from the GI tract and transported to the liver.
2. Hepatic activation: It undergoes two-step oxidation by CYP450 enzymes - principally CYP2C19 - to form a short-lived active thiol metabolite.
3. Irreversible receptor blockade: The active metabolite selectively and irreversibly binds to the P2Y12 ADP receptor on the platelet surface via a disulfide bond.
4. Downstream effects: Blockade of P2Y12 prevents ADP-mediated:
   • Activation of the GP IIb/IIIa receptor (fibrinogen receptor)
   • Platelet shape change, degranulation, and aggregation
   • Platelet-platelet cross-linking via fibrinogen
5. Duration: Because binding is irreversible, the platelet remains inhibited for its entire lifespan (~7-10 days). Normal platelet function returns only as new platelets are generated.
6. Onset: Full antiplatelet effect takes 3-5 days with standard dosing; a loading dose (300-600 mg) shortens this to 1-2 hours.

Key pharmacokinetic point - CYP2C19 polymorphism:

• Patients who are "poor metabolizers" of CYP2C19 (genetic polymorphism) produce less active metabolite, resulting in reduced antiplatelet effect and increased cardiovascular risk (clopidogrel resistance).
• Drugs that inhibit CYP2C19 (e.g., omeprazole, esomeprazole) reduce clopidogrel efficacy - a clinically significant drug interaction.

Uses

1. Acute Coronary Syndromes (ACS)

• Unstable angina and NSTEMI: Clopidogrel + aspirin (dual antiplatelet therapy, DAPT) significantly reduces the risk of cardiovascular death, MI, and stroke compared to aspirin alone.
• STEMI: Used in combination with aspirin and thrombolytics or PCI.

2. Percutaneous Coronary Intervention (PCI) / Coronary Stenting

• Bare metal stent: DAPT (aspirin + clopidogrel) for at least 4 weeks post-implantation.
• Drug-eluting stent: DAPT for at least 12 months (to prevent late in-stent thrombosis).

3. Ischaemic Stroke / TIA Prevention

• Patients with a history of ischaemic stroke or TIA. Clopidogrel alone is more effective than aspirin (8.7% relative risk reduction over aspirin for cardiovascular death, MI, stroke).

4. Peripheral Arterial Disease (PAD)

• Prevention of atherosclerotic events (cardiovascular death, MI, stroke) in patients with established peripheral arterial disease.

5. Atrial Fibrillation (selected patients)

• In patients who cannot receive oral anticoagulants, clopidogrel + aspirin is an alternative for stroke prevention.

Adverse Drug Reactions

1. Bleeding (Most important ADR)

• Prolonged bleeding - There is no specific antidote.
• GI bleeding, bruising, epistaxis, and intracranial haemorrhage.
• Risk is substantially increased when combined with aspirin (DAPT) or other anticoagulants.
• Pre-operative management: Clopidogrel should be stopped 5-7 days before elective surgery to allow recovery of platelet function.

2. Thrombotic Thrombocytopenic Purpura (TTP)

• Rare but life-threatening complication.
• Characterised by microangiopathic haemolytic anaemia, thrombocytopenia, fever, renal failure, and neurological symptoms (the classic pentad).
• Requires urgent plasmapheresis.
• Less common with clopidogrel than with ticlopidine (its predecessor), but the risk still exists.

3. Neutropenia / Agranulocytosis

• Incidence is lower with clopidogrel compared to ticlopidine.
• Periodic blood counts may be advised in long-term users.

4. GI Disturbances

• Nausea, vomiting, dyspepsia, abdominal pain, diarrhoea.
• Peptic ulceration and GI haemorrhage (less than aspirin in terms of GI mucosal damage, but bleeding risk is additive with aspirin).

5. Skin Reactions

• Rash, urticaria, pruritus.
• In mild hypersensitivity, a steroid burst may allow continuation; alternatively switch to ticagrelor (different chemical structure).

6. Hepatotoxicity

• Elevated liver enzymes (rare); interstitial liver disease reported.

7. Headache and Dizziness

• Relatively common, usually mild.

Contraindications

Important Drug Interactions

Quick Comparison: Clopidogrel vs Ticlopidine (as per Padmaja Udaykumar)