Participants in Phase 2 will be randomized 1:1 to receive low dose or high dose setrusumab. Participants will continue receiving their assigned dose until the selected dosing strategy is determined for the remainder of the study. Phase 2 participants will receive setrusumab at the selected dosing strategy in the open-label treatment extension period. Phase 3 participants will be randomized 2:1 to receive setrusumab or placebo. Phase 3 participants will transition to an open-label treatment extension period after the Phase 3 primary analysis is complete, or once a participant has completed 24 months of treatment in the double-blind period, whichever is sooner. Participants in the Phase 2 and Phase 3 treatment extension periods will receive open-label setrusumab treatment for at least 12 months, with the ability to extend until commercial availability in their respective country.
The primary objectives of the study are to identify a setrusumab dosing strategy in participants with OI and to evaluate the effect of setrusumab vs placebo on reduction in fracture rate.