Will Attruby maintain or increase its ATTR-CM NBRx market share above 30% through H1 2026?

Attruby crossed >25% NBRx share in December 2025, roughly 12 months post-launch. Getting to 30% requires an additional 5+ percentage points of share gain over 6 months. The second prescriber adoption wave and near-complete TTR stabilization data support continued share gains. However, the rate of share gain typically decelerates after the initial adoption surge — going from 25% to 30% is harder than 15% to 25% because the remaining market is more entrenched with existing therapies. The FDA warning to Alnylam on vutrisiran advertising is a tailwind.

The 30% threshold is specific to NBRx (new prescriptions), which is more volatile than TRx (total prescriptions). NBRx share can fluctuate with seasonal patterns and competitive promotional efforts. The current >25% was measured in December — a single data point. If the pending tafamidis IP proceedings create uncertainty among prescribers about the TTR stabilizer class generally, it could temporarily slow adoption decisions. The clinical data advantage (0.41 HR in variant population, near-complete stabilization) is the strongest argument for continued share growth.

The question asks if share reaches 30% 'at any point' during H1 2026. This is more favorable than requiring sustained 30% — a single monthly data point above 30% would resolve YES. Given the trajectory from >25% in December 2025 with accelerating prescriber adoption, reaching 30% in one of the next 6 months is more likely than not. The combination of clinical superiority, broadening prescriber base, and competitive headwinds facing Alnylam creates favorable conditions. The main risk is whether the tafamidis IP proceedings create market-wide uncertainty.

Going from 25% to 30% NBRx share is a 5pp gain that took roughly 4-5 months to achieve in the prior trajectory (15% to 25%). Extrapolating the adoption curve, 30% is plausible but not certain within 6 months. The tafamidis IP proceedings in April could actually boost Attruby NBRx if prescribers begin positioning patients on the superior therapy before potential generic disruption to the tafamidis class. Conversely, the IP uncertainty could create a 'wait and see' dynamic that slows all ATTR-CM prescribing decisions temporarily.

NBRx share data is less frequently disclosed than total revenue. The >25% figure comes from management claims in December 2025 — not independently verifiable IQVIA data that is publicly available. Resolution depends on whether credible data sources confirm 30%+ at any point during H1 2026. If BridgeBio continues to reduce disclosure granularity (as with patient starts), the data to confirm resolution may not be available. Setting aside data availability, the clinical trajectory supports continued gains but rate of gain typically slows as share approaches the 30-40% range for a second-in-class drug.

The COMPETITIVE_POSITION = DEFENSIBLE assessment with the specific evidence of 0.41 HR in variants, near-complete stabilization, and 15% mortality reduction for switchers creates a compelling physician narrative. The 1,856 prescribers with a second adoption wave suggests the clinical message is landing. The Moat Mapper specifically noted the clinical moat is strongest in the near-term (2-4 years). Given we are still in the strongest moat period and prescriber momentum is accelerating, 30% NBRx is more likely than not but subject to IP-related uncertainty.

Trajectory from >25% in December with accelerating prescriber adoption supports reaching 30% within 6 months. Clinical superiority is the key driver. 'At any point' resolution criteria is favorable. IP uncertainty is the main risk factor.

Share gain deceleration is normal after initial surge. 25% to 30% may take longer than 15% to 25%. Data availability could be a constraint for resolution. Probability slightly above coin-flip.

The combination of clinical differentiation, second prescriber wave, and competitive tailwind from Alnylam FDA warning supports continued share growth. The 30% threshold is challenging but achievable given the 'at any point' criteria over 6 months. Near coin-flip slightly favoring YES.