Epicrispr’s EPI-321 Shows Early Muscle Gain and Favorable Safety in First Human Trial for FSHD

Epicrispr Biotechnologies has reported positive interim results from its ongoing Phase 1/2 first-in-human trial (NCT06907875) evaluating EPI-321 in adults with facioscapulohumeral muscular dystrophy (FSHD). The single-dose investigational therapy demonstrated a favorable safety profile and produced measurable increases in lean muscle volume across the first three evaluable patients after six months, marking the first reported clinical evidence of muscle growth following treatment in FSHD.

The data cutoff of May 12, 2026, included nine treated patients across two dose cohorts. Six patients received a single intravenous infusion of EPI-321 at 2×10¹³ vg/kg, while three patients received a higher dose of 4×10¹³ vg/kg. No serious adverse events have been reported to date.

EPI-321 Targets the Genetic Driver of FSHD

FSHD is a progressive genetic muscle disorder caused by abnormal activation of the DUX4 gene, leading to chronic muscle damage, inflammation, and gradual loss of muscle strength. There are currently no approved disease-modifying treatments for the disease.

EPI-321 uses Epicrispr’s Gene Expression Modulation System (GEMS), an epigenetic platform that suppresses DUX4 activity without altering the underlying DNA sequence. By durably silencing the disease-causing gene after a single infusion, the therapy aims to protect muscle tissue from ongoing degeneration.

Early Trial Data Show Increased Muscle Volume

Among the first three evaluable participants treated at the target dose, all experienced increases in lean muscle volume six months after treatment. Patients gained an average of approximately 370 mL of lean muscle, equivalent to about 0.8 pounds, with individual gains ranging from approximately 0.5 to 1.3 pounds. Some individual muscles demonstrated increases of up to 15%.

These findings contrast with the progressive muscle loss typically observed in FSHD natural history studies and recent late-stage clinical trials. The same patients had previously shown favorable improvements in strength and functional assessments at the three-month evaluation.

MRI findings were supported by reductions in a novel circulating cell-free DNA biomarker associated with DUX4 pathway activity, providing additional evidence that the therapy is suppressing the underlying disease mechanism.

Imaging and Biomarker Data Strengthen Early Evidence

Chief Executive Officer Amber Salzman, Ph.D., said the consistent improvement across imaging, biomarker, and functional assessments strengthens confidence that EPI-321 may alter the course of FSHD rather than simply relieve symptoms.

Principal investigator Russell Butterfield, M.D., of the University of Utah noted that although the study remains in its early stages, the observed increase in lean muscle volume is encouraging because patients with FSHD typically experience continuous muscle deterioration over time.

The MRI analyses were performed in collaboration with Springbok Analytics, whose AI-powered whole-body imaging platform quantified changes across as many as 140 individual muscles, enabling detailed assessment of treatment-related changes.

Clinical Path Forward

The ongoing open-label Phase 1/2 study continues to evaluate the safety, tolerability, biological activity, and preliminary efficacy of EPI-321 in adults with FSHD. Epicrispr plans to present additional clinical data at the World Muscle Society Annual Congress in September 2026 and expects to complete the primary portion of the study by mid-2027.

If future studies confirm these early findings, EPI-321 could become the first therapy to directly suppress DUX4 while preserving or restoring muscle tissue in patients with FSHD.

Reference

Epicrispr Reports First Clinical Evidence of Increased Lean Muscle Volume in Patients with FSHD Following Treatment with EPI-321 – Epicrispr Biotechnologies