Tolvaptan

💧 Tolvaptan is a vasopressin (ADH) V2-receptor antagonist that causes “aquaresis” (free-water loss without sodium loss) → serum sodium rises.
🧠 Safety headline: in chronic SIADH the brain adapts to low sodium; correcting too fast can cause osmotic demyelination. Aim for a controlled rise (often ~4–6 mmol/L/day) and keep within recommended limits: ≤10 mmol/L in the first 24 h and ≤8 mmol/L per 24 h thereafter.

📘 About: Always check the BNF link here

• 🇬🇧 In UK practice, tolvaptan for SIADH is generally specialist-initiated with close sodium monitoring.
• ⚠️ MHRA warns about over-rapid rises in sodium and advises close monitoring, especially if baseline Na <120 mmol/L or high ODS risk (malnutrition, alcoholism, hypoxia).
• 🧬 Separate indication: ADPKD (Jinarc®) to slow cyst growth in rapidly progressive disease (requires strict LFT monitoring).

⚙️ Mode of action

• 🚰 Selective V2-receptor antagonist in the renal collecting duct → blocks ADH → ↓ aquaporin insertion → ↑ free-water excretion (“aquaresis”).
• 📈 Serum sodium rises because water is lost more than sodium (not because sodium is “added”).

✅ Indications

• 🧪 Hyponatraemia due to SIADH (euvolaemic hyponatraemia) in adults - specialist use.
• 🫘 ADPKD in adults with CKD stage 1–4 at initiation and evidence of rapidly progressive disease (Jinarc®).

💊 Dose range (check local policy + SmPC)

🧪 Monitoring (SIADH / hyponatraemia use)

• 🩸 Check serum sodium frequently after first dose (common practice: baseline, then ~4–6 h, 6–12 h, and 24 h; then daily until stable).
• ⚠️ MHRA: a rise of >6 mmol/L in the first 6 h or >8 mmol/L in the first 6–12 h may be too rapid → intensify monitoring and consider hypotonic fluid.
• 🧠 If Na rise exceeds 12 mmol/L in 24 h (or 18 mmol/L in 48 h), interrupt/stop and actively manage overcorrection (e.g. hypotonic fluid ± desmopressin per local guideline).
• 🚰 Ensure patient has free access to water and can drink to thirst; monitor urine output, weight, BP, volume status.
• 🧂 Avoid combining with hypertonic/normal saline or high-sodium medicines unless specialist-directed (overcorrection risk).

🧪 Monitoring (ADPKD / Jinarc®)

• 🧫 Liver safety: check ALT/AST and bilirubin before start, then monthly for 18 months, then every 3 months thereafter (and if symptoms).
• 🧠 Counsel on hepatotoxicity symptoms: fatigue, anorexia, RUQ pain, dark urine, jaundice → stop and investigate urgently.
• 🚰 Expect polyuria/nocturia and thirst; counsel hydration and sick-day planning.

🔗 Interactions

• 🧬 Tolvaptan is metabolised by CYP3A4:
  • ⛔ Avoid strong CYP3A4 inhibitors (e.g. ketoconazole, clarithromycin, protease inhibitors) - marked exposure increase.
  • ⚠️ Moderate inhibitors (e.g. diltiazem, verapamil, erythromycin, grapefruit juice) may require dose adjustment/avoidance.
  • ⬇️ CYP3A4 inducers (e.g. rifampicin, carbamazepine, phenytoin, St John’s wort) can reduce effect.
• 🩸 Vasopressin analogues (e.g. desmopressin) may have reduced effect; MHRA highlights interaction potential.
• 🧂 Do not combine routinely with other hyponatraemia treatments (saline, demeclocycline, etc.) unless specialist-directed (overcorrection risk).

⚠️ Cautions

• 💧 Dehydration / hypovolaemia: high urine output can cause hypotension/AKI-monitor closely, especially in frail older adults.
• 🧠 High ODS risk: Na ≤105, malnutrition, alcoholism, hypokalaemia, advanced liver disease → use stricter correction targets and consider “re-lowering” strategy if overshoots.
• 🧫 Abnormal LFTs or underlying liver disease (especially for longer-term ADPKD treatment).
• 🍬 Hyperglycaemia can occur-monitor in diabetes and acute illness.

⛔ Contraindications (high-yield)

• 🚱 Inability to sense/respond to thirst (cannot safely compensate water loss).
• 📉 Hypovolaemic hyponatraemia (treat volume first; tolvaptan can worsen).
• 🚫 Anuria.
• 🧂 Hypernatraemia or urgent need for rapid controlled correction with hypertonic saline (e.g. seizures) - tolvaptan is not the emergency drug.
• 💊 Concomitant strong CYP3A4 inhibitors.

😵 Side effects

• 🚰 Thirst, dry mouth, polyuria, pollakiuria, nocturia.
• 📉 Hypotension, dizziness; dehydration/AKI if intake inadequate.
• 🍬 Hyperglycaemia (occasionally).
• 🧫 Liver injury risk (primarily emphasised in ADPKD long-term use).

📖 References

• MHRA Drug Safety Update - Tolvaptan (Samsca): over-rapid sodium correction
• Society for Endocrinology (2022) - Emergency management of symptomatic hyponatraemia
• EMA product information - Samsca (tolvaptan)
• EMA product information - Jinarc (tolvaptan)
• UKKA (2024) - Tolvaptan for ADPKD commentary