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Updated from 5:48 am EST with additional information.

Amgen (AMGN) - Get Free Report announced Monday that postmenopausal women with osteoporosis reported fewer new spinal fractures following monthly injections of an experimental bone-forming drug compared to similar women treated with a placebo.

The results achieved the primary endpoint of a phase III study conducted by Amgen and its partner, Belgium-based UCB. The companies intend to seek regulatory approval for the new osteoporosis drug, romosozumab, later this year.

If approved, "romo" could generate sales of $1 billion or more split evenly between Amgen and UCB, analysts predict, although the drug faces competition from Eli Lilly's (LLY) - Get Free Report Forteo, which is going generic soon, and an another experimental osteoporosis drug under development by Radius Health (RDUS) - Get Free Report .

Importantly, monthly injections of romo failed to reduce the incidence of fractures outside the spine, a key secondary efficacy endpoint of the phase III study. This is a disappointing outcome because reduction in "non-vertebral fractures" was seen as a way romo could potentially differentiate itself from the osteoporosis drug competition.

Overall, results from the phase III "FRAME" study showed that women receiving a monthly injection of romo experienced a statistically significant 73% reduction in the relative risk of a spine fracture through 12 months compared to placebo.

The efficacy of romo persisted through the second year of the study after Prolia, Amgen's currently approved osteoporosis drug, was added. At two years, women treated with romo and Prolia showed a statistically significant, 75% reduction in the risk of spinal fractures compared to women treated with placebo and Prolia.

While romo achieved a secondary efficacy endpoint of reduction of clinical fractures (spine and non-spine fractures combined) through one year, the drug did not reduce the incidence of non-vertebral (non-spine) fractures between one and two years of the study, Amgen said.

"In order for romo to significantly differentiate itself from other agents, a large benefit (40-50%) in non-vertebral fracture, where there is a larger medical need, may need to be seen," said J.P. Morgan biotech analyst Cory Kasimov, in an Amgen research note which previewed Monday's romo phase III trial.

Radius shares are up 15% to $31 in early Monday trading, primarily because the company's experimental osteoporosis drug abaloparatide demonstrated a higher, 86% reduction in spine fracture risk in its own phase III study, previously disclosed. Abaloparatide must be injected every day, however, while romo is injected once per month.

On the safety side, the incidence of adverse events reported in the study were balanced between the romo and placebo arms, Amgen said. Two romo-treated patients reported osteonecrosis of the jaw, while another romo patient suffered an atypical leg fracture after three months on treatment.

Romo is a monoclonal antibody designed to inhibit the protein sclerostin. Blocking sclerostin increases bone formation and reduces bone breakdown.

Amgen shares are up less than 1% to $151 in early Monday trading.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.