Novartis shares tumbled after del-desiran (delpacibart etedesiran), the lead asset it gained by acquiring Avidity Biosciences for $12 billion in February 2026, failed its Phase III trial — a costly reminder that even a richly valued platform must ultimately prove itself in patients.

The disease

Myotonic dystrophy type 1 (DM1) is the most common form of muscular dystrophy in adults. It is a multi-system genetic disease whose hallmark is myotonia — muscles that contract but struggle to relax — along with progressive weakness. A classic sign is difficulty releasing one’s grip after a handshake. There is currently no approved therapy that modifies the underlying disease, so the field has watched del-desiran closely.

Why the drug was scientifically interesting

DM1 is caused by a genetic defect that produces a toxic form of RNA from the DMPK gene, which gums up the cell’s machinery. Del-desiran is an antibody-oligonucleotide conjugate (AOC) — a clever design that links a muscle-targeting antibody (aimed at a receptor called transferrin receptor 1) to a small interfering RNA that degrades the toxic DMPK RNA. The antibody acts as a homing device, ferrying the RNA payload into muscle tissue that is otherwise hard to reach. This AOC platform was the crown jewel of Novartis’s Avidity purchase.

What the trial showed

The Phase III HARBOR trial missed its primary endpoint: a statistically significant improvement over placebo in video Hand Opening Time (vHOT) — a measure of the grip-release problem central to DM1 — through week 54. In plain terms, on the trial’s main yardstick, the drug could not be distinguished from placebo.

The market reaction, and what’s next

Investors reacted sharply. Novartis fell roughly 11% in Zurich and 14% on the NYSE — its worst single-day decline since March 2020. “Developing therapies for a complex disease like DM1 remains challenging, and setbacks are part of scientific progress,” said chief medical officer Shreeram Aradhye. Novartis says it will review the full HARBOR dataset and engage regulators about a path forward; a primary-endpoint miss does not automatically end a program, but it is a serious blow. The episode underscores a broader truth: paying a premium for an elegant scientific platform does not guarantee the biology will hold up in a rigorous trial. Business and clinical news, not investment or medical advice.