The Amgen drug Romosozumab, or romo, achieved its primary endpoint of a significantly reduced risk of spinal fracture, but did not reduce breaks outside the spine area over longer period, its secondary endpoint.
Even so, Amgen and its Belgian partner UCB intend to file for regulatory approval later this year.
Analysts suggest Romo may generate sales of $1bn but faces competition from Radius’s experimental drug abaloparatide and also from an Eli Lily drug, Forteo, which comes off patent soon.
Abaloparatide reduced the risk of spinal fracture by 86% in its phase III trial compared to romo’s 73%, though it has to be injected daily compared to once a month with Amgen/UCB’s drug.
Amgen added that when combined over two years with Prolia, its current osteoporosis drug, the reduction rose to 75%.
Amgen said although Romo, designed to inhibit the protein sclerostin, reduced both spine and non-spine fractures in the first year, it did not repeat the performance in non-vertebral fractures in the second year.
Blocking sclerostin increases bone formation.
Shares in Amgen fell 2% to US$147.5 while Radius climbed US$7.24 to US$34.24.