Biotech stock Ultragenyx Pharmaceutical Inc (RARE) plunged on Monday after the company said late-stage trials of its bone disease drug failed to meet the main study goal.

The stock fell 43.5% to a record low following the disclosure. The move came after the company reported results from two pivotal studies.

Ultragenyx said the trials showed its experimental treatment did not significantly reduce the number of fractures per year. The outcome compared unfavorably with placebo or existing therapies.

The drug, called setrusumab, was tested in children, adolescents and young adults. Patients in the studies had osteogenesis imperfecta, a rare inherited disorder that affects bone metabolism.

Ultragenyx conducted the late-stage trials with U.K.-based partner Mereo Biopharma Group PLC. Both companies had previously reported encouraging mid-stage data.

Osteogenesis imperfecta causes bones to be fragile and prone to breaking. Patients can experience frequent fractures, with severity varying widely.

Ultragenyx said setrusumab missed the primary endpoint of reducing the annualized clinical fracture rate. The comparison included placebo or bisphosphonates, which are commonly used to treat bone density disorders.

The company said the drug met secondary endpoints in both trials. These included improvements in bone mineral density.

Chief executive Emil Kakkis said the company was surprised by the outcome. He cited earlier data and the absence of approved treatments for the condition.

An analyst reaction added pressure to the shares. Joon Lee of Truist said the firm had remained cautious on the drug despite earlier results.

Ultragenyx said it will conduct further analyses across both trials. The review will include additional bone health measures and other clinical endpoints beyond fractures.

The company also said it is reviewing planned operations to significantly reduce costs. The analyst said the move suggests winding down the osteogenesis imperfecta program.

Setrusumab is designed to block sclerostin. Sclerostin is a protein that inhibits bone formation, and blocking it is intended to increase bone strength and density.