# Modeling rebound bone loss following denosumab discontinuation and sequential zoledronate therapy in TgRANKL osteoporotic mice

## TL;DR

This study uses a mouse model to investigate bone loss after stopping denosumab and explores how zoledronate can help prevent this effect.

## Contribution

The study introduces a transgenic mouse model expressing human RANKL to investigate denosumab rebound and sequential zoledronate therapy.

## Key findings

- Denosumab discontinuation in TgRANKL mice caused rebound bone loss and increased bone turnover markers.
- Sequential zoledronate therapy after denosumab prevented rebound bone loss but its effects were not persistent after discontinuation.
- Denosumab reduced marrow adipose tissue formation in TgRANKL mice.

## Abstract

Receptor activator of nuclear factor-κB ligand (RANKL) plays a central role in regulating osteoclast formation and bone resorption, while its inhibition by the monoclonal antibody denosumab serves as an effective antiresorptive treatment for postmenopausal osteoporosis. However, denosumab discontinuation triggers a severe rebound effect involving rapid bone mineral density (BMD) loss, accompanied by an overshooting of bone turnover markers (BTMs), and increased risk of multiple fractures. Preclinical studies investigating this rebound phenomenon after denosumab discontinuation have been limited, mainly because denosumab does not cross-react with murine RANKL. This study explores the rebound phenomenon in a transgenic mouse model of osteoporosis expressing human RANKL (TgRANKL) and evaluates the impact of sequential zoledronate therapy.

TgRANKL mice were divided into four experimental groups: vehicle control, continuous denosumab treatment, denosumab withdrawal, and sequential denosumab followed by zoledronate, including an additional follow-up phase after zoledronate discontinuation. Skeletal alterations were characterized using microCT, histomorphometric assessments, serum bone turnover markers (BTMs), and bone gene expression analyses.

Denosumab therapy rescued the osteoporotic phenotype of TgRANKL mice, whereas its discontinuation resulted in a rebound bone loss accompanied by elevated bone turnover markers. Denosumab also inhibited bone marrow adipose tissue formation in TgRANKL mice, while its discontinuation led to moderate reformation of marrow adiposity. Sequential administration of zoledronate effectively prevented the rebound bone loss response. However, discontinued therapy after denosumab-zoledronate sequence, showed that the protective effects of zoledronate were not persistent.

Our findings establish TgRANKL mice as a unique osteoporotic model for investigating the mechanisms driving denosumab rebound and testing sequential antiresorptive strategies.