PGN-EDO51

What is PGN-EDO51?

PGN-EDO51 was PepGen's investigational conjugate of a phosphorodiamidate morpholino oligomer (PMO) and the company's Enhanced Delivery Oligonucleotide (EDO) cell-penetrating peptide, designed to induce exon 51 skipping in dystrophin pre-mRNA for the roughly 13% of Duchenne muscular dystrophy patients with exon 51-amenable mutations. The PMO portion targeted the same splice junction as eteplirsen (Exondys 51, approved 2016); the EDO peptide was the differentiator, intended to solve naked PMO's central weakness — poor uptake into skeletal and cardiac muscle. It did not work well enough. On 28 May 2025 PepGen announced, under the headline 'PepGen to Focus on Development of Promising DM1 Program Following 10 mg/kg PGN-EDO51 Update,' that 'the Company is voluntarily discontinuing development of PGN-EDO51' and 'intends to wind down all DMD-related research and development activities.' The data behind that decision are specific and worth stating precisely, because the usual summary of this programme is imprecise: in the 10 mg/kg cohort of the Phase 2 CONNECT1 trial, PGN-EDO51 'increased exon 51 skipped transcripts to 4.26% (a mean increase of 3.5%)' while 'total dystrophin only increased to 0.59% of normal levels (a mean increase of 0.36%).' The delivery peptide did produce exon skipping. What failed was the translation of that skipping into protein. Two corrections to the record this page previously carried. The terminating trial was **Phase 2**, not Phase 1 — CONNECT1-EDO51 (NCT06079736) is registered as Phase 2 and is now TERMINATED, with the registry reason 'PGN-EDO51 development terminated by Sponsor'; the companion CONNECT2-EDO51 (NCT06833931) was WITHDRAWN with zero participants enrolled. And the discontinuation was in **2025**, not 2024. A separate Phase 1 healthy-volunteer study did exist in 2022, but it is not the study that ended the programme. The entry is retained because the failure is informative: it is one of two clinical peptide-PMO exon-51 programmes to die within seven months of each other, for opposite reasons.

What PGN-EDO51 Is Investigated For

PGN-EDO51 has no current use. Its value now is as the clearest available data point on where peptide-conjugated oligonucleotide delivery hits its ceiling in human muscle. The numbers are the argument. Exon skipping rose to 4.26% of transcripts at 10 mg/kg — a real pharmacodynamic effect, and more than naked PMO achieves. Dystrophin reached 0.59% of normal. The gap between those two figures is the whole story of the programme: getting the oligonucleotide into the cell and onto the splice junction turned out to be a solvable problem, and converting skipped transcripts into enough protein to matter did not follow. Preclinical work using the same PPMO chemistry had already hinted at this ceiling and at its age-dependence — in mdx mice, adult animals reached 79% exon skipping and 35% dystrophin restoration while aged animals fell to 44% and 8%. The competitive context is unusually clean. Sarepta discontinued its own peptide-conjugated PMO for exon 51, SRP-5051 (vesleteplirsen), on 6 November 2024, citing long-term safety and tolerability — persistent hypomagnesemia in some participants and declines in a kidney-function measure, after an earlier 2022 FDA clinical hold over a serious hypomagnesemia event. Sarepta dropped its broader PPMO work at the same time, since those programmes shared the cell-penetrating peptide. PepGen then stopped its programme on efficacy six months later. Both clinical peptide-PMO exon-51 programmes are therefore dead, one on safety and one on potency. What replaced them is the antibody-oligonucleotide conjugate. Avidity's delpacibart zotadirsen reported Phase 1/2 EXPLORE44 results in DMD exon 44 in Lancet Neurology in 2026, and its delpacibart etedesiran produced New England Journal of Medicine Phase 1/2 data in myotonic dystrophy type 1. Separately, a 2026 Cell paper demonstrated AAV-delivered circular arRNA-guided exon skipping. For patients with exon 51-amenable mutations, the approved options remain eteplirsen, with golodirsen and viltolarsen for exon 53 and casimersen for exon 45.

History & Discovery

PepGen was founded on the EDO cell-penetrating peptide platform, licensed out of Oxford, with the aim of improving oligonucleotide delivery to skeletal and cardiac muscle — the bottleneck that has limited every antisense approach to Duchenne muscular dystrophy. PGN-EDO51 was the lead DMD candidate and PGN-EDODM1 the parallel myotonic dystrophy type 1 candidate. A Phase 1 study in 32 healthy adult males read out in September 2022, reporting exon 51 skipping of 1.4% at 10 mg/kg and 2.0% at 15 mg/kg at day 28. The same study produced the programme's only notable safety signal: at 15 mg/kg, one non-life-threatening serious adverse event involving kidney biomarkers and two participants with mild-to-moderate hypomagnesemia — the same class signal that would later end Sarepta's competing compound. CONNECT1-EDO51, a Phase 2 study in DMD patients, began in January 2024, with CONNECT2-EDO51 registered in December 2024 and first posted in February 2025 — PepGen was still opening new PGN-EDO51 trials months after some accounts claim the programme had been halted. On 28 May 2025 the 10 mg/kg CONNECT1 cohort reported exon skipping of 4.26% and dystrophin of 0.59% of normal, and PepGen discontinued the compound and wound down all DMD activity. CONNECT1 is recorded as terminated and CONNECT2 as withdrawn with zero enrolment. The timing against the rest of the field is what makes this a useful case study. Sarepta had discontinued SRP-5051 (vesleteplirsen), its own peptide-conjugated PMO for exon 51, on 6 November 2024, citing long-term safety and tolerability — persistent hypomagnesemia and kidney-function decline in some participants, after a 2022 FDA clinical hold over a serious hypomagnesemia event — and dropped its wider PPMO work at the same time because those programmes shared the cell-penetrating peptide. Within seven months, both clinical peptide-PMO exon-51 programmes were gone: one on safety, one on potency. What moved in to replace them did so quickly. Avidity's antibody-oligonucleotide conjugates produced Phase 1/2 data in myotonic dystrophy type 1 in the New England Journal of Medicine and in DMD exon 44 skipping in Lancet Neurology, and a 2026 Cell paper demonstrated AAV-delivered circular arRNA-guided exon skipping with durable dystrophin restoration and sustained motor improvement for at least 1.5 years in nonhuman primates, alongside a first-in-human study showing safe, dose-dependent exon skipping in three patients. An earlier version of this page attributed that 1.5-year durability to the human patients; it belongs to the primate arm. PepGen itself did not fold. It remains Nasdaq-listed with PGN-EDODM1 in Phase 2 for myotonic dystrophy type 1 — the programme it explicitly pivoted toward, and which an earlier version of this page incorrectly described as also discontinued.

How It Works

Duchenne muscular dystrophy is caused by mutations that stop muscle cells making working dystrophin protein. Exon-skipping drugs make the cell ignore the faulty exon when processing the gene's RNA, producing a shorter but partly functional protein. PGN-EDO51 attached a peptide to that drug to help it get inside muscle cells. The peptide worked — more RNA was skipped than with the older drugs — but the cells still made only about half a percent of normal dystrophin, far too little to change the disease.

PGN-EDO51 consisted of a phosphorodiamidate morpholino oligomer targeting the dystrophin exon 51 splice junction, conjugated to PepGen's EDO cell-penetrating peptide. The PMO component is mechanistically identical in intent to eteplirsen: hybridise to the splice signal, force exclusion of exon 51 during pre-mRNA processing, restore the reading frame, and produce an internally truncated but partially functional dystrophin in patients whose mutations are exon 51-amenable. The EDO peptide addressed the well-known limitation that naked PMOs are taken up poorly by skeletal and cardiac muscle. The CONNECT1 readout separates those two steps in an unusually clean way. At 10 mg/kg, exon 51 skipped transcripts reached 4.26%, against 1.4% at 10 mg/kg and 2.0% at 15 mg/kg in the earlier healthy-volunteer study. Delivery and splice modulation improved. Dystrophin reached 0.59% of normal. The conversion from skipped transcript to measurable protein is where the therapeutic case collapsed, and it is a different problem from the one the EDO platform was built to solve. Preclinical work in the same chemical class had already mapped this ceiling and shown it worsens with disease progression. A 2026 mdx-mouse study using PPMO chemistry reported 79% exon skipping and 35% dystrophin restoration in adult animals, falling to 44% and 8% in aged animals — so the gap between RNA-level and protein-level effect widens exactly as muscle is lost and replaced by fibrotic and adipose tissue. Human trials necessarily enrol patients further along that trajectory than a young adult mouse. There is a complication in interpreting what dystrophin percentage would even have been sufficient. A 2026 synthesis comparing six approved DMD therapies — four exon-skipping antisense oligonucleotides, the AAV micro-dystrophin gene therapy delandistrogene moxeparvovec, and the nonsense-readthrough agent ataluren — reported that larger biological dystrophin restoration did not correspond to longer preservation of walking, describing this as the absence of an expected positive association rather than as evidence against one. If the relationship between dystrophin percentage and ambulation is that loose across the approved agents, then the pharmacodynamic bar a peptide-PMO would need to clear was never well defined.

Evidence Snapshot

Research Gaps & Open Questions

What the current literature has not yet settled about PGN-EDO51:

• 01Why exon skipping and dystrophin restoration decouple so sharply in human muscle. CONNECT1 produced 4.26% skipped transcripts but only 0.59% of normal dystrophin; understanding that conversion step matters for every oligonucleotide approach to DMD, not just peptide conjugates.
• 02Publication of any PepGen clinical data. No peer-reviewed paper on PGN-EDO51, CONNECT1, CONNECT2 or the EDO platform exists, and no results have been posted to ClinicalTrials.gov. The entire clinical record is press releases and SEC filings, which is a weaker evidentiary basis than anything else cited on this page.
• 03Whether PGN-EDO51 shared the class hypomagnesemia and renal liability. PepGen's 2022 Phase 1 produced a serious adverse event involving kidney biomarkers and two cases of hypomagnesemia at 15 mg/kg, and the same signal ended Sarepta's SRP-5051 — but with 7 participants in CONNECT1 and no published data, this cannot be resolved.
• 04How the age-widening dystrophin-restoration ceiling seen in mdx mice (35% restoration in adults, 8% in aged animals) maps onto the ages and disease stages that human trials actually enrol.
• 05What pharmacodynamic bar a peptide-PMO revival would need to clear, given that larger dystrophin restoration has not corresponded to longer preserved ambulation across the approved DMD therapies.
• 06Whether the EDO platform retains value in indications where the protein-conversion step is less demanding — PepGen's own bet, since PGN-EDODM1 for myotonic dystrophy type 1 works by reducing a toxic transcript rather than restoring a large structural protein.

Forms & Administration

Intravenous infusion in the Phase 1 and Phase 2 trials. Not commercially available and permanently discontinued — there is no trial, no expanded access, and no supply. Patients with exon 51-amenable mutations should discuss eteplirsen and the current trial landscape with their neuromuscular specialist.

Dosing & Protocols

The ranges below reflect protocols commonly discussed in the literature and by clinicians — not a prescription. Actual dosing for any individual should be determined by a qualified healthcare provider who knows the patient.

Programme voluntarily discontinued 28 May 2025; not available through any route.

Timeline of Effects

Common Questions

Who PGN-EDO51 Is NOT For

Drug & Supplement Interactions

Not characterised; the programme was discontinued before interaction studies were reported, and no clinical pharmacology data from either Phase 2 trial has been published.

Safety Profile

Legal Status

Regulatory status changes over time. Verify current local rules with a qualified professional.

Myths & Misconceptions