Aldosterone Synthase Inhibitors

Definition

Aldosterone synthase inhibitors (ASIs) are a class of agents that directly block CYP11B2 (aldosterone synthase), the enzyme catalysing the final three steps of aldosterone biosynthesis in the adrenal cortex. Unlike mineralocorticoid receptor antagonists (MRAs), which block aldosterone's downstream receptor-mediated effects, ASIs reduce circulating aldosterone production itself. This mechanism avoids the counter-regulatory rise in renin and aldosterone induced by MRAs, which may stimulate MR-independent aldosterone effects.

Key Concepts

Mechanism of Action

• CYP11B2 (aldosterone synthase) catalyses the final three steps of aldosterone biosynthesis (11β-hydroxylation, 18-hydroxylation, 18-oxidation). Selective ASI blockade reduces circulating aldosterone without affecting cortisol synthesis (catalysed by the closely homologous CYP11B1). (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)
• Aldosterone dysregulation (elevated relative to sodium status) is a key driver of hard-to-control hypertension and hypertension-mediated organ damage, including cardiac, vascular, and renal injury.
• ASIs reduce serum aldosterone and increase plasma renin activity (reactive), suggesting aldosterone breakthrough contributes to hard-to-control BP even in patients already receiving RAASi therapy. (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)
• High selectivity for CYP11B2 over CYP11B1 is critical: cross-inhibition of cortisol synthesis could cause adrenal insufficiency. No adrenal insufficiency was reported in BaxHTN (n=794) or launch-HTN. (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)

Advantage Over Mineralocorticoid Receptor Antagonists

• MRAs (spironolactone, eplerenone, finerenone) block the mineralocorticoid receptor but induce counter-regulatory increases in renin and circulating aldosterone, which may stimulate MR-independent aldosterone effects on the vasculature and sympathetic nervous system.
• ASIs reduce aldosterone production directly, avoiding this counter-regulatory escape and the MR-independent effects. (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)
• MRAs carry dose-dependent hyperkalemia risk and, for steroidal MRAs (spironolactone), antiandrogenic side effects (gynecomastia, sexual dysfunction); ASIs carry similar hyperkalemia risk but no antiandrogenic effects.

Baxdrostat (AstraZeneca)

• Highly selective ASI; plasma half-life ~30 hours; once-daily oral dosing. (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)
• BrigHTN (phase 2, resistant HT): Significant SBP reduction vs placebo — established proof-of-concept in resistant HT.
• HALO (phase 2, uncontrolled HT, 8 weeks): Neutral — did not show between-group SBP difference at 8 weeks; later superseded by BaxHTN (longer duration, broader population).
• BaxHTN (phase 3, n=794, 12 weeks, uncontrolled + resistant HT):
  • 1 mg: placebo-corrected SBP −8.7 mmHg (95% CI −11.5 to −5.8; P<0.001)
  • 2 mg: placebo-corrected SBP −9.8 mmHg (95% CI −12.6 to −7.0; P<0.001)
  • BP control (SBP <130 mmHg): ~40% vs 18.7% placebo; OR ~2.9 (P<0.001)
  • Resistant HT subgroup: −9.1 mmHg (1 mg) and −9.8 mmHg (2 mg); P<0.001
  • DBP placebo-corrected: −3.3 mmHg (1 mg) and −3.9 mmHg (2 mg)
  • Randomised withdrawal (part 3): BP rose only +1.4 mmHg 8 weeks after stopping baxdrostat (vs −3.7 mmHg in continuing arm); estimated difference −5.1 mmHg (P=0.002)
  • Hyperkalemia (K >5.5): 6.1% (1 mg), 11.1% (2 mg) vs 0.4% placebo
  • Hyponatremia (Na <135): ~19–23% vs 7%; mostly asymptomatic
  • eGFR: −7 ml/min/1.73m² (reversible; consistent with hemodynamic BP-lowering effect)
  • No adrenal insufficiency (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)

Lorundrostat (Mineralys Therapeutics)

• Another selective ASI with once-daily oral dosing.
• ADVANCE-HTN (phase 2b, n=285, 12 weeks, uncontrolled + resistant HT, NEJM 2025):
  • 285 participants on standardized regimen (olmesartan ± indapamide/HCTZ ± amlodipine); 53% Black; 103 US sites
  • Primary: 24-hour ambulatory SBP placebo-adjusted change at 12 weeks (distinguishes this trial from BaxHTN which used office BP):
    • 50 mg stable dose: −7.9 mmHg (97.5% CI −13.3 to −2.6; P=0.001)
    • 50→100 mg dose-adjustment: −6.5 mmHg (97.5% CI −11.8 to −1.2; P=0.006)
  • Week 4 (combined groups): −5.3 mmHg (P<0.001); SBP <125 at week 4: 41% vs 18% (OR 3.3)
  • 50 mg is the optimal dose: 100 mg adds no meaningful BP reduction but increases adverse event burden
  • Hyperkalemia K>6.0: 5% (stable) and 7% (dose-adjustment) vs 0% placebo; exacerbated by concomitant ARB
  • eGFR (cystatin C): −13–15% at 12 weeks, fully reversible on washout — hemodynamic mechanism
  • MATE1 transporter caveat: lorundrostat competitively inhibits the MATE1 renal transporter → serum creatinine rises without true GFR decline (same mechanism as cimetidine or trimethoprim); cystatin C should be used to monitor renal function during lorundrostat therapy
  • No adrenal insufficiency (sources/lorundrostat-advancehtn-nejm-2025, rating: high)
• Launch-HTN (phase 3, uncontrolled + resistant HT, JAMA 2025): Placebo-corrected SBP −9.1 mmHg (95% CI −13.3 to −4.9) at 6 weeks (primary endpoint) — consistent class effect with baxdrostat. (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)

Comparison: ASIs vs MRAs

Slow BP Offset After Withdrawal

• During the 8-week randomised withdrawal in BaxHTN (Part 3), BP rose only +1.4 mmHg in the group switched to placebo, despite expected drug clearance within ~1 week; aldosterone and renin did not fully normalise. (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)
• Proposed mechanisms: persistent effects on sodium homeostasis; reversal of aldosterone-mediated vascular dysfunction or sympathetic overactivation; possibly structural arterial effects.

Contradictions / Open Questions

• ADVANCE-HTN (ambulatory BP) vs BaxHTN (office BP) — cross-trial comparison indirect: ADVANCE-HTN used 24-hour ambulatory SBP as endpoint; BaxHTN used office BP. Ambulatory BP is typically 5–10 mmHg lower than office BP and less susceptible to white-coat effect. Direct quantitative comparison of lorundrostat vs baxdrostat effect sizes is therefore not valid from these two trials. (sources/lorundrostat-advancehtn-nejm-2025, rating: high; sources/baxdrostat-baxhtn-nejm-2025, rating: very high)
• No direct MRA comparator in BaxHTN, ADVANCE-HTN, or Launch-HTN: All ASI trials used placebo comparators; no head-to-head with spironolactone (current COR 1 guideline 4th agent in resistant HT). Whether ASIs are superior, equivalent, or inferior to MRAs for resistant HT remains unknown. (sources/baxdrostat-baxhtn-nejm-2025, rating: very high)
• No cardiovascular outcomes data: Both BaxHTN and Launch-HTN demonstrate BP reduction only; hard cardiovascular outcomes (MI, stroke, mortality) not assessed. Historical evidence supports ~20% CVD risk reduction per 5–10 mmHg SBP reduction, but this has not been proven for ASIs specifically.
• HALO neutral vs BaxHTN positive in uncontrolled HT: Phase 2 HALO (8 weeks) was neutral; BaxHTN (12 weeks, broader population including resistant HT) was positive. Discrepancy may reflect: duration (8 vs 12 weeks), population enrichment, or sample size. The 12-week timeframe now appears important for demonstrating full ASI effect.
• Optimal dose: 1 mg and 2 mg show statistically similar BP reduction (~1 mmHg difference) but 2 mg has meaningfully higher hyperkalemia rates (K>5.5: 11.1% vs 6.1%; intervention for hyperkalemia: 7.9% vs 2.7%). Clinical dose selection needs further guidance, particularly in patients with CKD or concurrent RAASi.
• Long-term eGFR trajectory in CKD: The early ~7 ml/min eGFR dip is described as hemodynamically reversible, but long-term renal safety in patients with CKD or proteinuria requires dedicated investigation.

Connections

• Related to entities/Hypertension — primary clinical application: resistant and uncontrolled hypertension
• Related to concepts/Renal-Denervation — alternative add-on strategy for resistant hypertension (COR 2b, AHA 2025)
• Related to concepts/Blood-Pressure-Target-T2DM — ASI therapy may be relevant for T2DM patients with resistant HT pursuing SBP <120 mmHg targets

Sources

• sources/baxdrostat-baxhtn-nejm-2025
• sources/lorundrostat-advancehtn-nejm-2025