# Reconstruction of remodeling units reveals positive effects after 2 and 12 months of romosozumab treatment

Erik Eriksen, Rogely Boyce, Yifei Shi, Jacques Brown, Donald Betah, Cesar

Libanati, Mary Oates, Roland Chapurlat, Pascale Chavassieux

► To cite this version:

Erik Eriksen, Rogely Boyce, Yifei Shi, Jacques Brown, Donald Betah, et al. Reconstruction of remodeling units reveals positive effects after 2 and 12 months of romosozumab treatment. Journal of Bone and Mineral Research, 2024, 39 (6), pp.729-736. 10.1093/jbmr/zjae055. inserm-05028874

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# Reconstruction of remodeling units reveals positive effects after 2 and 12 months of romosozumab treatment

Erik F. Eriksen<fnref n="1" />,<fnref n="2" />,* Rogely W. Boyce<fnref n="3" />, Yifei Shi<fnref n="3" />, Jacques P. Brown<fnref n="4" /> ID, Donald Betah<fnref n="3" />, Cesar Libanati<fnref n="5" />, Mary Oates<fnref n="3" />, Roland Chapurlat<fnref n="6" />, Pascale Chavassieux<fnref n="6" /> ID

## Abstract

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

## Lay Summary

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.

## Introduction

Romosozumab is a monoclonal antibody that inhibits sclerostin, resulting in an early transient increase in bone formation markers and a sustained decrease in bone resorption markers.<fnref n="1" />,<fnref n="2" /> Histomorphometric analysis of bone biopsies in a substudy of the phase 3 FRAME clinical trial (NCT01575834) evaluating the effects of romosozumab vs placebo in postmenopausal women with osteoporosis<fnref n="2" /> confirmed that these effects on bone resorption and formation

occur at the tissue level on both cancellous and endocortical surfaces.<fnref n="3" /> Additional histomorphometric analyses of mo 2 (M2) biopsies revealed that the early increase in bone formation was due to a significant increase in a modeling-based bone formation on cancellous and endocortical surfaces in the romosozumab arm compared with the placebo arm, with no significant effect on remodeling-based formation.<fnref n="4" />

The effects of romosozumab on surface-based bone resorption and formation have been well documented in preclinical studies.<fnref n="5" /> In addition, the effects of romosozumab on

<footnote n="1">¹Institute of Clinical Dentistry, University of Oslo, Oslo 0176, Norway</footnote>

<footnote n="2">²SpesialistSenteret Pilestredet Park, Oslo, Norway</footnote>

<footnote n="3">³Amgen Inc., Thousand Oaks, CA 91230, United States</footnote>

<footnote n="4">⁴CHU de Québec Research Centre and Laval University, Québec, H3A 1B9, Canada</footnote>

<footnote n="5">⁵UCB Pharma, 1070 Brussels, Belgium</footnote>

<footnote n="6">⁶INSERM UMR 1033, Université de Lyon, Hospices Civils de Lyon, 69437, Lyon Cedex 03, France</footnote>

<footnote>*Corresponding author: Erik F. Eriksen, SpesialistSenteret Pilestredet Park, Pilestredet Park 12A, Oslo 0176, Norway (efinkeriksen@gmail.com).</footnote>
bone resorption and formation at the level of the remodeling unit have been reported.⁶ Kinetic reconstruction of the cancellous remodeling unit in cynomolgus monkeys treated with romosozumab 30 mg/kg every 2 wk for up to 28 wk revealed a significant increase in positive bone balance due to a decrease in the pre-osteoblast erosion depth (the final depth of the resorptive site) and an increase in wall thickness in bone packets forming and near completion at the end of the treatment period (reconstructed wall thickness). To determine if romosozumab affects bone resorption and formation at the level of the remodeling unit in postmenopausal women with osteoporosis at high risk of fracture in the clinical setting, bone biopsies from patients in FRAME were analyzed at M2 and mo 12 (M12) using remodeling unit reconstruction methods to assess whether changes at the level of the remodeling unit occur early and/or late in treatment.

# Materials and methods

## Study design

This study utilized the bone biopsies from a subset of patients from the FRAME trial who provided consent for bone biopsy.²,³ Briefly, the FRAME trial enrolled 7180 postmenopausal women (55–90 years of age) with osteoporosis, and a T-score of −2.5 to −3.5 at the total hip or femoral neck. Patients were randomized 1:1 to receive romosozumab (subcutaneous, 210 mg) or placebo monthly for 12 mo, thereafter both groups received denosumab (subcutaneous, 60 mg) once every 6 mo for 12 mo.

## Bone biopsy and histomorphometric analyses

Transiliac bone biopsy at M12 and M2 were performed as described previously.³ From biopsy blocks, 10-µm unstained and 8-µm Goldner's stained sections were collected from three section levels separated by 150 µm (Charles River Laboratories Montreal ULC, Senneville, Quebec, Canada) and were used in the analyses. Remodeling unit reconstruction techniques were based on Eriksen's method modified by Steiniche et al.⁷ For M12 biopsies, full kinetic reconstruction of the remodeling unit was performed⁸ because steady state was considered to have been achieved by 12 mo of treatment.

Histomorphometric analyses of the cancellous and endocortical surfaces were performed on stained and unstained histologic sections at 200× magnification by light microscopy. Endocortical surfaces were defined as surfaces on the inner cortical surface and surfaces within 1 trabecular width from the inner cortical surface. For unbiased sampling of bone surfaces, an ocular linear test system was randomly rotated between fields of view.

For M12 biopsies, parameters assessed by orthogonally sampled lamellar counts at the test line intersection with the bone surface included erosion depth (classified by cell type [osteoclasts, mononuclear cells, pre-osteoblasts/early osteoid seams both denoting final erosion depth]; Figure 1A) and completed wall thickness. Using identical sampling, osteoid/mineralized bone thickness ratio at osteoid-covered surfaces was assessed (Figure 1B). The latter measurements capture active formative sites classified by a 4-sector system (Figure 1C) as described by Eriksen et al.⁸ The sector system classifies evolving formative sites by stage of development (early [sector 1] to late [sector 4]) by binning the osteoid/mineralized wall thickness ratios, thus correcting

for section obliquity. Lamellar counts were converted to microns by multiplying the counts by 3.2 µm, the average thickness of lamellae in human bone.⁸ Reconstructed wall thickness was calculated by adding sector 4 osteoid thickness and mineralized wall thickness, which projects the final completed wall thickness of formative sites that developed during the treatment period. Bone balance was calculated by subtracting the final erosion depth from both wall thickness and reconstructed wall thickness. Mineralizing surface was estimated by the intercept counts on unstained sections using fluorescence microscopy, and mineralization apposition rate values from Chavassieux et al³ were used to calculate the surface-based bone formation for kinetic analyses.

For M2 biopsies, analyses were limited to static parameters related to bone balance due to the short duration of treatment and transient, non-steady state of remodeling kinetics. Parameters assessed on cancellous and endocortical surfaces by lamellar counts included final erosion depth and osteoid/mineralized bone thickness ratio at osteoid-covered surfaces. Reconstructed wall thickness and bone balance were calculated as described for 12-mo analyses.

## Statistical analysis

Data were expressed as median (quartiles 1 and 3). The effect of 2 mo and 12 mo of romosozumab vs placebo on bone multicellular unit (BMU) parameters at each envelope was compared using the Wilcoxon rank-sum test, without multiplicity adjustment.

## Results

Of patients who provided consent for bone biopsy from the FRAME trial, bone biopsies evaluable for histology were available from 40 romosozumab and 33 placebo patients at M12 and from 14 romosozumab and 10 placebo patients at M2. Clinical characteristics of these patients are described in Table 1. Of these patients, bone biopsies from 38 romosozumab and 31 placebo patients at M12 and bone biopsies from all patients at M2 were evaluable for histomorphometric analyses.

At M12 on the cancellous bone surfaces, romosozumab compared with placebo resulted in a net positive bone balance (+6.1 vs +1.5 µm; *P* = .012) by its effects on the resorptive site, with lower final erosion depth (40.7 vs 43.7 µm; *P* = .05). Romosozumab also resulted in lower osteoclast erosion depth (15.4 vs 18.1 µm; *P* = .027) and a slightly lower mononuclear erosion depth (29.3 vs 33.6 µm; *P* = .067). Romosozumab did not affect the completed (45.4 vs 45.4 µm; *P* = .57) or reconstructed wall thickness (51.2 vs 51.2 µm; *P* = .64; Figure 2).

At M12 on the endocortical bone surfaces, romosozumab vs placebo resulted in a net positive balance (+5.2 vs −1.7 µm; *P* = .02) due to a higher completed wall thickness (50.8 vs 47.5 µm; *P* = .037). Reconstructed wall thickness was not higher (56.5 µm vs 56.0 µm; *P* = .84). Although osteoclastic (12.8 µm vs 19.2 µm; *P* < .001) and mononuclear cell (26.7 µm vs 33.1 µm; *P* < .001) erosion depths were lower with romosozumab, the final erosion depth was similar to that with placebo (45.1 µm vs 46.5 µm; *P* = .33) and hence did not contribute to the net positive bone balance (Figure 3).

Kinetic reconstruction of the remodeling site on the cancellous and endocortical bone surfaces demonstrated limited
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Methods used in BMU reconstruction. (A) Illustration of the linear test system sampling a resorptive site. Erosion depth is estimated by counting the number of lamellae eroded at the line intersections with the surface, starting the count from the adjacent intact quiescent surface. (B) Illustration of a linear test system sampling a formative site. Osteoid thickness and underlying mineralized wall thickness are estimated by counting the number of lamellae at the line intersections from the surface to the osteoid/mineralized bone boundary and from the boundary to the underlying cement line, respectively. (C) Example of the sector analysis of the paired osteoid/mineralized wall thickness measurement allowing for the classification of formative sites by stage of development. Sampling points in sector 1 represent walls that are less mature than walls represented by points in sector 2, etc. Figures adapted from Eriksen⁸ and Steiniche et al.⁷ BMU, bone multicellular unit; O.Th = osteoid thickness; uW.Th = uncompleted wall thickness; µm = micrometers.</figcaption></figure>

<table><caption>**Table 1.** Baseline patient characteristics.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Mo 12 cohort</th><th colspan="2">Mo 2 cohort</th></tr><tr><th>Placebo (N = 33)</th><th>Romosozumab, 210 mg QM (N = 40)</th><th>Placebo (N = 10)</th><th>Romosozumab, 210 mg QM (N = 14)</th></tr></thead><tbody><tr><td>Age, years</td><td>70.0 (66.0, 77.0)</td><td>72.5 (68.0, 76.0)</td><td>73.0 (63.0, 75.0)</td><td>70.0 (65.0, 72.0)</td></tr><tr><td>BMI, kg/m<sup>2</sup></td><td>23.60 (21.90, 25.70)</td><td>23.90 (21.75, 26.30)</td><td>25.75 (23.00, 28.70)</td><td>24.65 (21.70, 26.20)</td></tr><tr><td>Prior osteoporotic fracture, n (%)</td><td>14 (42.4)</td><td>11 (27.5)</td><td>3 (30.0)</td><td>0 (0.0)</td></tr><tr><td>Prevalent vertebral fracture, n (%)</td><td>3 (9.1)</td><td>9 (22.5)</td><td>1 (10.0)</td><td>0 (0.0)</td></tr><tr><td>BMD T-score</td><td></td><td></td><td></td><td></td></tr><tr><td>Lumbar L1-L4</td><td>-3.00 (-3.60, -2.07)</td><td>-2.88 (-3.46, -1.82)</td><td>-3.53 (-3.65, -3.02)</td><td>-2.77 (-3.16, -1.62)</td></tr><tr><td>Total hip</td><td>-2.56 (-2.90, -2.13)</td><td>-2.52 (-2.78, -2.21)</td><td>-2.39 (-2.70, -2.21)</td><td>-2.47 (-2.60, -2.01)</td></tr><tr><td>Femoral neck</td><td>-2.81 (-2.94, -2.64)</td><td>-2.74 (-2.93, -2.60)</td><td>-2.56 (-2.59, -2.50)</td><td>-2.72 (-2.83, -2.62)</td></tr></tbody></table>

Abbreviations: N, number of randomized patients who enrolled in the bone biopsy substudy and provided consent, received at least 1 dose of investigational product, and had an biopsy evaluable for histology. Q1, Q3, quartiles 1 and 3; QM, once monthly. Values are median (Q1, Q3) unless otherwise specified.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Effects of romosozumab at the cancellous remodeling site at mo 12 (M12) on (A) erosion depth, (B) wall thickness, and (C) bone balance. Box plots show Q1 and Q3 (lower and upper edges of the box), median (horizontal bar inside the box), mean values (+), individual data-points (○), and minimum to maximum ranges (error bars). Nominal P-values are the treatment difference (romosozumab vs placebo). N, number of patients with available biopsies at M12; n, number of patients with evaluable data; Q, quartile.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Effects of romosozumab at the endocortical remodeling site at mo 12 (M12) on (A) erosion depth, (B) wall thickness, and (C) bone balance. Box plots show Q1 and Q3 (lower and upper edges of the box), median (horizontal bar inside the box), mean values (+), individual data-points (○), and minimum to maximum ranges (error bars). Nominal *P*-values are the treatment difference (romosozumab vs placebo). *N*, number of patients with available biopsies at M12; *n*, number of patients with evaluable data; *Q*, quartile.</figcaption></figure>

effects of romosozumab on remodeling cycle duration. The formative period was slightly longer in both cancellous and endocortical surfaces with a slightly shorter resorptive period on the endocortical surfaces (Figure 4).

Because completed wall thickness captures bone packets formed over many remodeling cycles, including those formed or completed during romosozumab treatment, analyses were also conducted on 2-mo biopsies to further explore the early effects of romosozumab on the remodeling unit. At M2, on cancellous surfaces, romosozumab resulted in a slightly lower final erosion depth (38.5 vs 44.6 µm; *P* = .11) and higher bone balance (6.0 vs 1.0 µm; *P* = .061) compared with placebo. Reconstructed wall thickness was similar to that with placebo (45.2 vs 45.2 µm; *P* = .64; Figure 5). On the endocortical surface, the final erosion depth was lower (42.7 vs 50.7 µm; *P* = .021) with reconstructed wall thickness similar to that with placebo (55.1 vs 52.6 µm; *P* = .98). Because of the effects on erosion depth, bone balance was slightly higher with romosozumab compared with placebo (10.6 vs 2.1 µm; *P* = .11; Figure 6).

Examination of osteoid thickness and mineralized wall thickness for each sector at M2 revealed the positive effects of romosozumab on the formative site restricted to sector 2. Positive effects on endocortical bone formation were observed with a higher osteoid thickness (29.9 vs 19.2 µm; *P* = .005) and mineralized wall thickness (18.3 vs 11.9 µm; *P* = .004) compared with placebo. On the cancellous surfaces, the mean mineralized wall thickness was higher with romosozumab compared with placebo (median: 12.8 vs 12.8 µm, mean: 14.2 vs 11.0 µm; *P* = .042) without an effect on osteoid thickness (19.2 vs 19.2 µm; *P* = .17; Figure 7).

## Discussion

Bone turnover marker and histomorphometric analyses of the FRAME clinical trial have demonstrated that the major mechanisms contributing to the increase in bone mass,

improvement in bone microstructure, and reduction in fractures observed in postmenopausal women with osteoporosis at high risk of fracture treated with romosozumab, are a consequence of a transient increase in modeling-based bone formation early in treatment with a sustained decrease in surface-based bone resorption.²⁻⁴ In addition to these major effects, reconstruction analyses conducted in the present study demonstrate that romosozumab can affect bone resorption and formation at the level of the remodeling unit early in treatment as well as later, resulting in a continued net positive bone balance without meaningfully affecting the lifespan of the remodeling cycle.

At M12, reconstruction of the remodeling unit revealed a positive bone balance on the cancellous surfaces due to a reduction in final resorption depth, the final depth achieved in the resorption cavity. This aligns with the sustained decrease on surface-based bone resorption at M12, indicating that romosozumab exerts an effect by decreasing resorptive cell activity.³ There was no effect on completed or reconstructed wall thickness, the latter capturing formative sites initiated after ~100 d of treatment. On the endocortical surfaces, a net positive bone balance was observed due to a higher final wall thickness, an estimate that includes bone packets completed over many remodeling cycles, including bone packets actively formed and completed during treatment. Based on the estimated formation period for endocortical formative sites for romosozumab (~200 d), bone packets that were at some stage of active formation during romosozumab treatment that could contribute to completed wall thickness would have initiated less than ~200 d prior to treatment and before ~d 55 of treatment. After 55 d, formative sites would have been captured in the formative site reconstruction at M12 and would be reflected in reconstructed wall thickness, which was similar to that with placebo.

At M2, a trend toward lower final erosion depth and higher bone balance was observed on the cancellous surfaces, aligning with effects observed at M12. On endocortical
A

Cancellous

Placebo

<figure><img src="image_5.png" /></figure>

Endocortical

<figure><img src="image_6.png" /></figure>

B

Cancellous

Romosozumab

Endocortical

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Graphical representation of the remodeling sequence on cancellous and endocortical envelopes in postmenopausal women with osteoporosis treated with (A) placebo or (B) romosozumab for 12 mo. Nonlinear one phase decay curve fitting was performed on raw data in GraphPad Prism (v 7.05) to smooth the curves. BB, bone balance; FP, formative period; Md.W.Th, mineralized wall thickness; O.th, osteoid thickness; Final E.De, pre-osteoblast (final erosion depth; Rs.P, resorptive period; W.Th, wall thickness.</figcaption></figure>

A

Final Erosion Depth

<figure><img src="image_9.png" /></figure>

B

Reconstructed Wall Thickness

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

C

Bone Balance

<figure><img src="image_12.png" /><figcaption>**Figure 5.** Effects of romosozumab at the cancellous remodeling site at mo 2 (M2) on (A) final erosion depth, (B) reconstructed wall thickness, and (C) bone balance. Box plots show Q1 and Q3 (lower and upper edges of the box), median (horizontal bar inside the box), mean values (+), individual data-points (○), and minimum to maximum ranges (error bars). Nominal *P*-values are the treatment difference (romosozumab vs placebo). *N*, number of patients with available biopsies at M2; *n*, number of patients with evaluable data; *Q*, quartile.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure 6.** Effects of romosozumab at the endocortical remodeling site at mo 2 (M2) on (A) final erosion depth, (B) reconstructed wall thickness, and (C) bone balance. Box plots show Q1 and Q3 (lower and upper edges of the box), median (horizontal bar inside the box), mean values (+), individual data-points (○), and minimum to maximum ranges (error bars). Nominal *P*-values are the treatment difference (romosozumab vs placebo). *N*, number of patients with available biopsies at M2; *n*, number of patients with evaluable data; *Q*, quartile.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 7.** Osteoid thickness and wall thickness for sector 2 formative sites on (A) cancellous and (B) endocortical surfaces following 2 mo of treatment with romosozumab or placebo. Box plots show Q1 and Q3 (lower and upper edges of the box), median (horizontal bar inside the box), mean values (+), individual data-points (○), and minimum to maximum ranges (error bars). Nominal *P*-values are the treatment difference (romosozumab vs placebo). *N*, number of patients with available biopsies at mo 2; *n*, number of patients with evaluable data; *Q*, quartile.</figcaption></figure>

surfaces, a significantly lower final erosion depth with a trend toward a higher bone balance was observed. On both surfaces, there was no effect on reconstructed wall thickness.

The sector analysis of formative sites at M2 provides a real-time assessment of the effects of treatment on various stages of formative sites early in treatment. The sector analysis revealed positive effects on the formative sites in sector 2, most notably, a significantly higher osteoid thickness and mineralized wall thickness on the endocortical surfaces. Based on the endocortical formation and resorption periods that we observed with romosozumab treatment by kinetic reconstruction at M12, these formative sites would be estimated to be approximately 40 d into the formative period at M2. Hence, these evolving packets would have initiated in the first 2–3 wk of romosozumab treatment, with the terminal phases

of the resorptive cycle occurring around the time of initiation of treatment. Osteoblast recruitment to the terminal resorptive and early formative sites would occur in this window of time. Enhanced osteoblast recruitment may explain the positive formation effects on sector 2 packets. Bone formation markers peak in the first 2 wk of romosozumab treatment,² reflective of high modeling-based bone formation and likely high canonical Wnt signaling. Studies in rats have demonstrated increased osteoprogenitor proliferation and recruitment within the first wk of treatment with an antisclerostin antibody that temporally aligns with high bone formation and transcriptional changes, which is consistent with canonical Wnt signaling. After 4 wk of treatment, progenitor proliferation and recruitment attenuate, which is temporally associated with the onset of signaling pathways in osteocytes that
negatively regulate canonical Wnt signaling.⁹,¹⁰ These sector 2 endocortical formative sites would have completed formation prior to 12 mo and may represent a portion of completed bone packets incorporated into completed wall thickness estimates that resulted in the observed increase at M12.

Similar effects on sector 2 formative sites were not observed on the cancellous surfaces. Mineralized wall thickness was significantly higher, but not osteoid thickness. Sector 2 sites were approximately 70 d into the formation period, which would translate to these sites initiating just prior to treatment with the terminal phases of the resorption period occurring a few wk prior. If osteoblast recruitment early in treatment is responsible for the effects on endocortical surfaces at M2, the effects on cancellous surfaces may suggest some limited effects on the evolving sector 2 sites early in the formative period on this surface, perhaps a transient increase in osteoblast vigor. These limited effects did not result in higher completed wall thickness at M12, suggesting that the positive effects on a portion of the evolving formative sites was primarily on the endocortical surfaces. Although romosozumab has a robust effect on bone formation on cancellous surfaces, the magnitude of the effect is even greater on endocortical surfaces.⁴ The preponderant increase in bone remodeling at the endocortical surfaces compared with cancellous surfaces has been reported in treatment-naive postmenopausal women with osteoporosis.¹¹ These differential bone envelope effects on bone formation may explain why the positive effect of romosozumab on bone balance due to its effects on formation was more apparent on the endocortical surfaces.

A limitation to this study was the low number of patients in these analyses. This was particularly the case at M2, resulting in limited power. In addition, weight-bearing bone samples were not attainable; hence, analyses were limited to non-weight bearing bone (iliac crest).

In conclusion, the major effects of romosozumab that contribute to higher bone mass, improved bone microstructure, and reduced fracture risk are twofold: 1) robust early transient increases in modeling-based bone formation and 2) sustained decreases in surface-based bone resorption on cancellous and endocortical surfaces.³,⁴ The present analyses that examined the effects of romosozumab on bone resorption and formation at the level of the remodeling unit suggest that reduction in the final erosion depth is the mechanism resulting in a positive bone balance on cancellous bone. On endocortical surfaces, collective data suggest that a combination of reduced final erosion depth and a positive effect on formative sites evolving early in treatment contribute to a positive bone balance. Taken together, these effects on bone balance on cancellous and endocortical bone envelopes may contribute to the progressive increase in mass, improved structure, and fracture reduction observed in the FRAME clinical trial.

## Acknowledgments

This study was sponsored by Amgen Inc., Astellas Pharma Inc., and UCB Pharma. Lisa Humphries, PhD (Amgen Inc.) and Varsha Jain, PhD (Cactus Life Sciences on behalf of Amgen Inc.) provided editorial support.

## Author contributions

Erik F. Eriksen (Conceptualization, Methodology, Data curation, Formal analysis, Supervision, Writing—original draft, Writing—review & editing), Rogely W. Boyce (Writing—original draft, Writing—review & editing), Yifei Shi (Data curation, Formal analysis, Writing—review & editing), Jacques P. Brown (Writing—review & editing), Donald Betah (Writing—review & editing), Cesar Libanati (Writing—review & editing), Mary Oates (Writing—review & editing), Roland Chapurlat (Writing—review & editing), and Pascale Chavassieux (Writing—review & editing). All authors approved the final version of the manuscript and agreed to submit it to *Journal of Bone and Mineral Research* for publication.

## Funding

Amgen Inc., Astellas Pharma Inc., and UCB Pharma sponsored this study.

## Conflicts of interest

E.F.E. has received grant support from Amgen and has served as an advisor for Amgen, Novartis, Lilly, UCB, EffRx, Ascendis, and Takeda. R.W.B. is a former employee of Amgen. Y.S. and M.O. are employees and stockholders of Amgen. J.P.B. has received research support from Mereo BioPharma, Radius Health, and Servier; has served as a consultant for Amgen, Gilead, Paladin, Pfizer, and Servier; has served on an Advisory Board for Amgen; has served on a Data Safety Monitoring Board for Ultragenyx; and has served on speakers' bureaus for Amgen. D.B. is a former employee and remains a stockholder of Amgen. C.L. is an employee and stockholder of UCB Pharma. R.C. has received speaker fees and consulted for Amgen, UCB, Lilly, Arrow, Kyowa Kirin, MSD, BMS, Janssen-Cilag, Sanofi, Chugai, Pfizer, Mylan, Medac, Novartis, and AbbVie. P.C. has received travel grants from Amgen and UCB.

## Data availability

Qualified researchers may request data from Amgen clinical studies. Complete details are available at https://wwwext.amgen.com/science/clinical-trials/clinical-data-transparency-practices/clinical-trial-data-sharing-request/.

## Target journal

JBMR.

## References

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