Dec 29 (Reuters) - Ultragenyx Pharmaceutical said on Monday late-stage studies showed its experimental drug for a type of genetic bone disease failed to reduce the number of fractures that occurred per year.

Shares of the company were down 42% in morning trading.

The drug developer was testing the treatment called setrusumab in pediatric and young adult patients with osteogenesis imperfecta in two-late stage studies.

The condition, a group of genetic disorders impacting bone metabolism, can result in increased bone brittleness, which contributes to a high rate of fractures. There are several types of the disorder, with symptoms that range from mild to severe.

The company said setrusumab failed to meet the main goal of significantly reducing annualized clinical fracture rate, compared to placebo or bisphosphonates - a class of medications primarily used to treat conditions that affect bone density.

The drug, however, achieved the secondary goals of improvements in bone mineral density in both the late-stage studies, Ultragenyx said.

"We are surprised and disappointed by these results given the promising data from our Phase 2 study and the lack of approved treatment options available to patients with osteogenesis imperfecta," said CEO Emil Kakkis.

Ultragenyx is conducting additional analyses on the data across both studies, including on other bone health and clinical endpoints beyond fractures, to assess the next steps, it said.

It is also evaluating its planned operations to implement significant expense reductions.

Setrusumab is designed to block sclerostin, a negative regulator of bone formation. Blocking sclerostin is expected to increase new bone formation, bone mineral density and bone strength in patients with the disoder.