Novartis HARBOR Phase III Trial Misses Primary Endpoint

Regulatory strategy teams at Novartis face a consequential dataset review after the global Phase III HARBOR study evaluating del-desiran (delpacibart etedesiran) in myotonic dystrophy type 1 failed to demonstrate statistically significant improvement over placebo on its primary endpoint, video hand opening time (vHOT), a novel measure of hand myotonia.

The 54-week, randomized, double-blind, placebo-controlled study enrolled approximately 150 participants who received del-desiran or placebo every eight weeks. Secondary endpoints and exploratory analyses showed evidence of clinical activity, and the safety profile was consistent with previously reported data, two data points that will likely anchor Novartis's forthcoming discussions with health authorities on the most appropriate development path forward.

For rare neuromuscular disease programs, a missed primary endpoint against a novel instrument like vHOT introduces a layered regulatory question: whether the endpoint itself captured the disease construct adequately, or whether the compound's mechanism did not translate to functional benefit at the doses and intervals studied. Novartis has not yet disclosed which interpretation the full dataset supports, and that determination will shape the agency engagement strategy.

Del-desiran carries Orphan Drug, Fast Track, and Breakthrough Therapy Designations from the FDA, as well as Orphan Medicinal Product Designation in the EU, designations that preserve certain procedural advantages even after a Phase III miss, including expedited meetings with FDA reviewers. DM1 remains a disease with no approved treatments, a fact that sustains regulatory interest in viable development paths despite the setback.

The compound is one of three antibody oligonucleotide conjugate (AOC) therapies Novartis added through its acquisition of Avidity Biosciences. The broader AOC portfolio remains active: del-zota (delpacibart zotadirsen) for Duchenne muscular dystrophy with exon 44-amenable mutations has been filed for accelerated approval and received FDA priority review designation, while del-brax (delpacibart braxlosiran) for facioscapulohumeral muscular dystrophy is advancing toward an FDA meeting following positive Phase I/II biomarker data.

The outcome of Novartis's health authority consultations on del-desiran, and whether the secondary endpoint dataset supports a revised registration strategy or a protocol amendment, will be a reference point for regulatory affairs teams navigating rare neuromuscular disease programs with novel functional endpoints.

Source: Novartis via GlobeNewswire, September 8, 2026.