Antibodies, Antibody Fragments, and Peptides: Targeting Muscular Diseases with RNA

Two Natural Capital examines the competitive landscape of RNA therapeutics targeting muscular diseases, revealing significant technical hurdles and potentially inflated claims from companies attempting to replicate Alnylam's RNAi success beyond the liver. While Alnylam has expanded from one liver-targeted drug in 2018 to six approvals today, competitors pursuing Duchenne Muscular Dystrophy (DMD) and Myotonic Dystrophy Type 1 (DM1) therapies face unresolved challenges in muscle delivery and nuclear targeting, challenges that several companies may be understating in their public positioning.

Ticker Symbols: ALNY (Alnylam), AVDX (Avidity), DYN (Dyne), PEPG (PepGen), ARWR (Arrowhead), SRPT (Sarepta)
Research Firm: Two Natural Capital
Report URL: https://www.twonaturalcap.com/p/antibodies-antibody-fragments-and
Position Disclosure: The original research does not explicitly disclose whether Two Natural Capital or the author holds long or short positions in the companies discussed. Please refer to the original report for any disclosures.

Core Thesis

Delivery remains the fundamental obstacle: Reaching muscle tissue and achieving nuclear entry for pre-mRNA splicing modification presents a dual challenge that liver-targeted therapies like Alnylam's avoid entirely.
TfR1-targeting carries anemia risk: While transferrin receptor 1 (TfR1) antibodies enable muscle delivery, they risk disrupting iron transport. Avidity's approach has already reported anemic patients despite engineering designed to avoid transferrin interference.
Antibody fragment trade-offs underplayed: Dyne's Fab antibody strategy improves tissue penetration and reduces immune activation but sacrifices circulation half-life, a limitation the company downplays in its public communications.
Cell-penetrating peptide improvements questioned: PepGen and Entrada claim dramatically improved endosomal escape and report phase 1 splicing corrections of 53.7%, figures that appear unrealistic given historical CPP toxicity issues (Sarepta halted a peptide program due to kidney dysfunction and hypomagnesemia).
Integrin-targeting dose advantages overstated: Arrowhead argues its avβ6 integrin approach enables significantly lower dosing than antibody conjugates, but off-target deposition risks remain substantial since avβ6 expresses outside muscle tissue.
Nuclear delivery remains unproven: DM1 pathology requires therapies to reach the nucleus to affect pre-mRNA splicing, not just cytoplasmic mRNA, a hurdle that applies equally to DMD exon-skipping approaches, yet companies provide limited evidence of consistent nuclear access.

Clinical Development Reality Check

Avidity leads but timeline appears optimistic: Plans to submit a BLA for an FSHD candidate in 2H26 assume clinical and regulatory progress faster than typically seen in this complex therapeutic space.
Efficacy variations raise questions: Dyne reports 5.46% dystrophin increase versus higher expectations from comparable approaches; PepGen's 53.7% DM1 splicing correction dramatically exceeds Avidity's 16% and Dyne's 27%, suggesting either breakthrough performance or measurement inconsistencies.
Safety profiles vary significantly by modality: Anemia with antibodies, kidney effects with peptides, immune responses, and receptor-function disruption create different risk profiles that may not be adequately weighted in company communications.

Notable Research Insights

Alnylam's liver-focused success exploits unique advantages: The liver's fenestrated endothelium and GalNAc receptor targeting provide delivery mechanisms unavailable for muscle tissue, making extra-hepatic expansion fundamentally more difficult.
Dosing comparisons reveal payload differences: Arrowhead's data suggest 12 mg/kg siRNA dose roughly equals 100 mg/kg antibody-oligo conjugate, indicating antibody-delivered modalities may offer payload and cost-of-goods advantages despite their other limitations.
Strategic pivots signal clinical setbacks: PepGen's transition to single-product strategy after halting its exon 51 DMD candidate reflects the difficulty of translating promising technology into multiple viable products.
DM1 approaches fundamentally diverge: Avidity, Dyne, and Arrowhead pursue broad DMPK RNA reduction while PepGen and Entrada target expanded CTG repeats specifically to preserve normal gene function, representing different therapeutic philosophies with distinct risk-reward profiles.
First-generation failures inform current attempts: Sarepta's terminated peptide-conjugated program due to prolonged kidney issues demonstrates that even established players struggle with delivery innovation, contradicting implications that technical tweaks easily overcome these barriers.
Nuclear activity from cytoplasmic drugs raises questions: Arrowhead's and Avidity's siRNA showing splicing correction despite siRNA's expected cytoplasmic activity suggests either unexpected nuclear penetration or indirect mechanisms not fully explained.

FAQs

What are the main obstacles preventing RNA therapeutics from successfully treating muscular diseases like DMD and DM1?

RNA therapeutics for muscular diseases face dual challenges: achieving robust muscle cell penetration and ensuring therapeutic entry into the nucleus to affect pre-mRNA splicing. Unlike liver-targeted therapies that benefit from the liver's fenestrated endothelium and established GalNAc delivery, muscle targeting requires overcoming tight vascular barriers and cellular membranes. For DM1 specifically, the research notes that effective therapy must reach the nucleus to disrupt pathological DMPK transcripts that interfere with splicing, not merely address mature mRNA in the cytoplasm.

How do the different delivery approaches (antibodies, antibody fragments, peptides, and integrin-targeting) compare in effectiveness and safety?

Distinct trade-offs across delivery modalities. Antibody-based approaches (like Avidity's) target TfR1 to enable muscle delivery but risk anemia through potential transferrin transport disruption, with some anemic cases already reported. Dyne's antibody fragment (Fab) strategy is presented as improving tissue penetration and reducing immune activation, but the research notes that removing the Fc domain shortens circulation half-life, a limitation Dyne allegedly downplays. Cell-penetrating peptides from PepGen and Entrada claim improved endosomal escape with reduced toxicity, though questions whether improvements are as robust as stated given historical CPP challenges and Sarepta's program halt due to kidney dysfunction. Arrowhead's integrin-targeting approach potentially enables lower dosing but raises off-target concerns since avβ6 expresses beyond muscle tissue.

Are the clinical efficacy claims from companies like PepGen, Avidity, and Dyne realistic given industry experience?

Two Natural Capital expresses skepticism about several efficacy claims. PepGen's reported 53.7% mean splicing correction in DM1 Phase 1 trials dramatically exceeds Avidity's 16% and Dyne's 27%, which the research characterizes as appearing "unrealistic against prior industry experience with cell-penetrating peptides." For DMD therapies, Dyne's reported 5.46% dystrophin increase falls below expectations from comparable approaches. These variations may indicate either genuine breakthrough performance, measurement inconsistencies, or overstated claims that don't account for the complexity of translating dose-sparing and delivery improvements into consistent clinical practice.

What safety concerns should investors monitor across these muscular disease therapy programs?

Safety profiles vary significantly by delivery modality. Antibody approaches present anemia risk through TfR1 interaction and potential immune activation (though antibody fragments may reduce the latter). Peptide-conjugated therapies have demonstrated kidney-related adverse effects including hypomagnesemia and kidney function decline, as evidenced by Sarepta's program termination. The immune responses, receptor-function disruption, and off-target deposition (particularly with integrin-targeted approaches) create distinct risk profiles that may not receive adequate emphasis in company communications. These safety trade-offs will prove critical for regulatory approval regardless of efficacy signals.

Is Avidity's timeline to file a BLA for its FSHD candidate in 2H26 achievable?

The Two Natural Capital analysis characterizes Avidity's BLA filing timeline as relying on "optimistic assumptions about clinical and regulatory progress that are not fully substantiated when compared to the timeframes typically seen within this complex therapeutic space." While Avidity is described as the furthest along toward a platform approach, with multiple candidates including a Phase 1/2 FSHD program, the 2H26 target may not adequately account for the technical challenges, regulatory scrutiny, and potential clinical setbacks common in novel delivery mechanism development for extra-hepatic RNA therapeutics.

What advantages does Alnylam's established RNAi platform have over these emerging muscular disease therapies?

Alnylam as "the RNAi touchstone," having expanded from one liver-targeted drug in 2018 to six approvals by leveraging GalNAc delivery technology and the liver's inherent advantages for drug delivery. Liver targeting exploits the organ's fenestrated endothelium and established receptor mechanisms unavailable for muscle tissue, making Alnylam's expansion fundamentally easier than extra-hepatic approaches. Companies pursuing muscular indications face unresolved technical barriers that Alnylam's liver-focused strategy sidesteps entirely, setting a high bar that may be difficult to replicate in other tissue types.

How do the different approaches to treating DM1 compare strategically?

Two Natural Capital identifies a fundamental strategic divergence in DM1 treatment approaches. Avidity, Dyne, and Arrowhead pursue broad reduction of overall DMPK RNA expression, which addresses pathological transcripts but also eliminates normal gene function. In contrast, PepGen and Entrada employ allele-specific binding to expanded CTG repeats, aiming to selectively target disease-causing RNA while preserving healthy DMPK function. The research presents these as representing different therapeutic philosophies with distinct risk-reward profiles, broad knockdown offers simpler mechanistic certainty but risks eliminating beneficial protein, while selective targeting preserves normal function but requires more sophisticated delivery and binding specificity.

What does the nuclear activity observed with cytoplasmic siRNA drugs suggest about these therapies?

Both Arrowhead's and Avidity's siRNA-based approaches have shown splicing correction despite siRNA's classic cytoplasmic activity, a finding the research characterizes as unexpected. This "indicates nuclear activity despite cytoplasmic expectations" but suggests the mechanism remains incompletely explained. This observation raises questions about whether the therapies achieve unexpected nuclear penetration or work through indirect mechanisms not fully disclosed by the companies, representing either a positive surprise in delivery capability or an incompletely understood aspect of the therapeutic mechanism that could affect reproducibility and regulatory evaluation.

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