Delpacibart zotadirsen drives exon 44 skipping and dystrophin production in early Duchenne trial
In a first-in-human study for Duchenne muscular dystrophy mutations amenable to exon 44 skipping, delpacibart zotadirsen produced measurable target engagement and biologic activity across a small randomized cohort in the United States. The phase 1/2 EXPLORE44 trial enrolled healthy adult men and boys and men with DMD44, testing whether an antibody-oligonucleotide conjugate could deliver an exon-skipping payload to muscle and restore downstream dystrophin expression.
The concept matters because exon-skipping therapy is only useful when it actually reaches skeletal muscle, enters cells, and changes RNA processing at the right locus. Del-zota was built for that purpose: an antibody-oligonucleotide conjugate designed to ferry a phosphorodiamidate morpholino oligomer to muscle tissue and induce exon 44 skipping. In a disease driven by absence or severe deficiency of dystrophin, even partial restoration can alter the biology of muscle injury, membrane fragility, and the cascade of inflammation and necrosis that follows repeated contraction.
Safety signals emerged early, but most adverse events were mild or moderate
The trial used a two-part, randomized, double-blind, placebo-controlled design across ten U.S. research sites. Part A enrolled healthy adult men aged 18 to 55 years and informed dosing in the Duchenne cohort. Part B then randomized ambulatory and non-ambulatory male participants with DMD44, aged 7 to 27 years, in a 3:1 ratio to del-zota or placebo within two dose cohorts: 5 mg/kg every 6 weeks or 10 mg/kg every 8 weeks, each delivered as three intravenous doses.
Safety was the primary objective, and treatment-emergent adverse events defined the central tolerability readout. In healthy volunteers, adverse events were reported in 21 of 34 participants receiving del-zota and three of 10 receiving placebo. None were severe or serious. In the Duchenne cohort, 16 of 19 participants receiving del-zota and six of seven receiving placebo reported at least one adverse event, with most events classified as mild or moderate.
Two participants in the 5 mg/kg del-zota group discontinued because of adverse events: one anaphylactic reaction and one infusion-related reaction. Those events are clinically consequential in a population that will likely require repeated dosing over time. Recurrent infusion exposure raises the operational stakes for monitoring, premedication strategies, and early recognition of hypersensitivity in routine care if development advances.
RNA correction and protein restoration moved in the same direction
The biologic readout was direct. Mean increases from baseline in exon 44 skipping reached 36.6% at 5 mg/kg and 43.4% at 10 mg/kg, compared with 1.6% in the placebo group. Mean increases from baseline in dystrophin levels were 25.0% at 5 mg/kg and 25.6% at 10 mg/kg, versus -0.7% with placebo.
These paired effects matter because exon skipping alone is only a molecular intermediate. Dystrophin is the structural protein that links the cytoskeleton to the sarcolemma; when expression rises, muscle fibers should be better buffered against contraction-induced injury. The concordance between exon skipping and dystrophin increase suggests the conjugate reached its intended target and triggered the expected splice correction pathway rather than generating a purely biochemical signal with no downstream protein consequence.
Biomarker shifts fit the expected muscle injury pathway
The study also found decreased biomarkers of muscle damage in participants with DMD44, adding another layer of biologic plausibility. In Duchenne muscular dystrophy, membrane instability drives chronic leakage of muscle contents and ongoing tissue damage. A treatment that restores even partial dystrophin expression could blunt that cycle, reducing biomarker evidence of injury before any longer-term functional endpoint becomes visible.
The magnitude of the molecular response was notable even in a small early-phase trial. Exon 44 skipping increased in both active-dose groups, and dystrophin rose by roughly one quarter from baseline in each del-zota arm. Such parallel movement in RNA and protein is exactly the kind of translational signal early drug development seeks, especially in a disease where the therapeutic mechanism is tightly coupled to genotype.
Dose selection now shifts from biology to durability
The trial included nine participants at 5 mg/kg, ten at 10 mg/kg, and seven on placebo in the Duchenne cohort, with a mean age of 13.3 years. Randomization was masked, placebo-controlled, and carried out with a computer-generated permuted-block schedule prepared by an independent statistician. Infusions were prepared by an unmasked pharmacist under supervision of the masked investigator, preserving blinding for participants, site staff, and sponsor team members.
What remains unresolved is not whether the pathway can be engaged, but how durable the molecular effect will be and how safely repeated intravenous exposure can be sustained in children and young adults with a lifelong neuromuscular disorder. Early discontinuations from hypersensitivity-related events place practical pressure on the dose, interval, and mitigation strategy that will carry forward into later testing.
The trial was completed and registered at ClinicalTrials.gov as NCT05670730. Its findings support continued evaluation of delpacibart zotadirsen in a phase 3 study designed to determine whether these molecular gains translate into clinically meaningful efficacy and whether the safety profile remains acceptable over longer treatment exposure.