Inotersen's ATTR-CM Failure Shatters Silencer Class Hopes for Cardiac Indication

AstraZeneca and Ionis' Wainua Fails Phase 3 in ATTR-CM

AstraZeneca and Ionis Pharmaceuticals' antisense therapy, Wainua (Inotersen), failed to demonstrate significant cardiovascular benefit in a Phase 3 trial for transthyretin amyloid cardiomyopathy (ATTR-CM). This unexpected outcome, despite Wainua's existing approval for transthyretin amyloid polyneuropathy (ATTR-PN) in December 2023, has significantly impacted the ATTR treatment landscape. The failure has led to a re-evaluation of therapeutic strategies, casting doubt on similar transthyretin silencers while potentially benefiting drugs with alternative mechanisms, such as stabilizers or amyloid depleters, in this rare and progressive condition.

• Wainua's Unexpected Phase 3 Failure in ATTR-CM: AstraZeneca and Ionis Pharmaceuticals' antisense therapy, Wainua (Inotersen), failed its Phase 3 CARDIO-TTRansform trial for transthyretin amyloid cardiomyopathy (ATTR-CM). The drug did not demonstrate significant cardiovascular benefit, a surprising outcome given its prior approval for transthyretin amyloid polyneuropathy (ATTR-PN). This highlights the complex nature of ATTR, where the same underlying biology can manifest with vastly different clinical results depending on the affected organ and disease stage, necessitating distinct therapeutic approaches for each subtype.
• Reshaping the ATTR Treatment Landscape: The late-stage failure of Wainua has sent ripples through the ATTR treatment space, prompting a re-evaluation of development strategies. This event has created an overhang for other transthyretin silencers, raising questions about their efficacy in ATTR-CM, while simultaneously boosting the prospects of therapies utilizing alternative mechanisms, such as amyloid depleters like cliramitug and coramitug, or transthyretin stabilizers like acoramidis, which may now face reduced competition in this significant market.
• Clinical Heterogeneity of Transthyretin Amyloidosis: The article underscores the critical distinction between ATTR-CM and ATTR-PN, despite both stemming from transthyretin misfolding. Experts note that while the mechanism is shared, clinical outcomes can differ significantly based on the primary organ affected, baseline disease stage, and endpoint sensitivity. This explains why a drug effective in ATTR-PN might not translate to success in ATTR-CM, emphasizing the need for targeted development strategies that account for these clinical nuances.

Diverse Mechanisms Driving the Evolving ATTR-CM Therapeutic Pipeline

While TTR stabilizers represent the current approved standard of care for ATTR-CM, the clinical pipeline is diversifying with novel mechanisms that target different points in the disease cascade. These emerging therapies aim to reduce transthyretin production at the genetic level or clear existing amyloid deposits, offering new therapeutic paradigms. The three most common mechanisms of action for unapproved drugs in clinical trials are TTR gene silencing, CRISPR-based gene editing, and amyloid fibril disruption.

• TTR Gene Silencing: This approach uses RNA-based therapeutics to reduce the hepatic production of both wild-type and variant TTR protein. Two main classes are under investigation for ATTR-CM: antisense oligonucleotides (ASOs) like eplontersen and small interfering RNAs (siRNAs) like patisiran and vutrisiran. These therapies target TTR messenger RNA (mRNA) to prevent its translation into protein, and large-scale Phase 3 trials, such as CARDIO-TTRansform with eplontersen, are assessing their efficacy and safety in this patient population.

• CRISPR-Cas9 Gene Editing: Representing a potential one-time, permanent treatment, CRISPR-Cas9 therapies aim to silence the TTR gene itself. The investigational therapy nexiguran ziclumeran targets the gene encoding transthyretin to permanently halt its production. In a Phase 1 study, a single infusion resulted in rapid and durable reductions in serum TTR levels, with a mean reduction of 90% at 12 months, alongside stabilization of cardiac biomarkers like NT-proBNP and troponin T.

• Amyloid Fibril Disruption and Degradation: Acting downstream in the amyloidogenic cascade, this strategy aims to clear existing TTR amyloid deposits from tissues. This mechanism is being explored through two primary modalities. The first involves investigational monoclonal antibodies designed to bind to amyloid deposits and facilitate their degradation. The second approach uses small molecule combinations, such as doxycycline with tauroursodeoxycholic acid, to disrupt fibril integrity.

Navigating ATTR-CM Trial Design, Endpoints, and Safety Challenges

The landscape of clinical trials for transthyretin amyloid cardiomyopathy (ATTR-CM) has evolved significantly, moving from foundational studies to more complex, contemporary designs. Early pivotal trials like ATTR-ACT established key efficacy and safety benchmarks, primarily focusing on all-cause mortality and cardiovascular hospitalizations. Subsequent trials, including ATTRibute-CM and CARDIO-TTRansform, have adopted more sophisticated hierarchical and composite endpoints to capture a broader spectrum of clinical benefits in a patient population that is increasingly being diagnosed at an earlier stage of disease.

A Setback for Gene Silencers in ATTR-CM

The recent announcement regarding Wainua (inotersen)'s failure to achieve significant cardiovascular benefit in its Phase 3 trial for transthyretin amyloid cardiomyopathy (ATTR-CM) sends ripples through the rare disease landscape. This outcome is particularly notable given inotersen's existing approval and demonstrated efficacy in transthyretin amyloid polyneuropathy (ATTR-PN), underscoring a critical distinction: what works for neurological manifestations may not directly translate to the complex cardiac pathology of ATTR-CM.

While earlier, smaller, open-label studies had offered some positive signals for inotersen in ATTR-CM, an expert panel had previously expressed reservations, not recommending its use for ATTR-CM due to a lack of robust supporting evidence and potential toxicity risks. This Phase 3 result now validates those concerns, suggesting that simply reducing transthyretin (TTR) production, while a powerful mechanism, might not be sufficient to significantly impact advanced cardiac amyloid burden.

For the broader class of gene-silencing therapies, including other antisense oligonucleotides (ASOs) and small interfering RNAs (siRNAs) in development for ATTR-CM, this creates a heightened bar. While some, like eplontersen, have shown improved tolerability and TTR reduction compared to inotersen, they will now face increased scrutiny to demonstrate clear, robust cardiac efficacy. The known safety profile of ASOs, including risks of thrombocytopenia, glomerulonephritis, and injection-site reactions, remains a consideration, necessitating careful monitoring.

In this evolving landscape, tafamidis, a TTR stabilizer, solidifies its position as the sole FDA-approved therapy for ATTR-CM, reinforcing its market dominance. Its oral administration also offers a significant advantage over the higher cost and administrative burden associated with injectable therapies requiring hospital visits for monitoring. Looking ahead, the field will likely see increased focus on combination therapies, earlier diagnosis, and alternative mechanisms such as amyloid depleters or fibril disrupters, as researchers continue to navigate the complexities of this life-threatening condition.