Romosozumab treatment results in a transient early increase in bone formation and sustained decrease in bone resorption. Histomorphometric analyses revealed that the primary bone-forming effect of romosozumab is a transient early stimulation of modeling-based bone formation on cancellous and endocortical surfaces. Furthermore, preclinical studies have demonstrated that romosozumab may affect changes in the remodeling unit, resulting in positive bone balance. To further investigate the effects of romosozumab on bone balance, mo 12 (M12) and mo 2 (M2) (to analyze early effects) unpaired bone biopsies from the FRAME clinical trial were analyzed using remodeling site reconstruction to assess whether positive changes in bone balance on cancellous/endocortical surfaces may contribute to the progressive improvement in bone mass/structure and reduced fracture risk in osteoporotic women at high fracture risk. At M12, bone balance was higher with romosozumab vs placebo on cancellous (+6.1 vs +1.5 μm; P = .012) and endocortical (+5.2 vs -1.7 μm; P = .02) surfaces; higher bone balance was due to lower final erosion depth (40.7 vs 43.7 μm; P = .05) on cancellous surfaces and higher completed wall thickness (50.8 vs 47.5 μm; P = .037) on endocortical surfaces. At M2, the final erosion depth was lower on the endocortical surfaces (42.7 vs 50.7 μm; P = .021) and was slightly lower on the cancellous surfaces (38.5 vs 44.6 μm; P = .11) with romosozumab vs placebo. Sector analysis of early endocortical formative sites revealed higher osteoid thickness (29.9 vs 19.2 μm; P = .005) and mineralized wall thickness (18.3 vs 11.9 μm; P = .004) with romosozumab vs placebo. These evolving bone packets may reflect the early stimulation of bone formation that contributes to the increase in completed wall thickness at M12. These data suggest that romosozumab induces a positive bone balance due to its effects on bone resorption and formation at the level of the remodeling unit, contributing to the positive effects on bone mass, structure, and fracture risk.
Keywords: bone histomorphometry; bone modeling and remodeling; cell/tissue signaling - Wnt/β-catenin/LRPs; osteoporosis; therapeutics.
Romosozumab treatment has a dual effect on bone, adding new bone and reducing bone loss. In the FRAME clinical trial, romosozumab increased the bone mass and strength and reduced fracture risk in postmenopausal women with osteoporosis. Addition of new bone occurs early in treatment and rapidly on cancellous and endocortical bone surfaces where bone resorption is not ongoing. However, it remains unclear if romosozumab affects bone loss or gain in areas where bone resorption is ongoing (remodeling units), contributing to a further positive bone balance. Here, we examined whether changes at the remodeling unit occur early (2 mo) and/or late (12 mo) in treatment by using bone biopsies from patients treated with romosozumab or placebo in FRAME. At M2, a combination of lower bone resorption and higher bone gain on endocortical surfaces resulted in a positive bone balance with romosozumab vs placebo. At M12, the bone balance was positive with romosozumab vs placebo due to lower bone resorption on cancellous surfaces and greater bone gain on endocortical surfaces. This demonstrates that romosozumab induces a positive bone balance at remodeling units early in treatment, leading to overall gains observed later, contributing to the positive effects of romosozumab on bone mass and structure.
© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society for Bone and Mineral Research.