Mouse model of uncoupled bone remodeling upon discontinuation of anti-RANKL antibody therapy

The discontinuation of denosumab (anti-RANKL antibody) therapy may increase the risk of multiple vertebral fractures, however, the underlying pathophysiology is largely unknown. Eight patients who underwent discontinuation after multiple injections of denosumab had higher levels of TRAP-5b, the so-called “overshoot” phenomenon, compared to their pre-treatment levels. The rate of decrease in bone mineral density (BMD) during the withdrawal period was higher than in age-matched non-treated patients, suggesting a decrease in BMD independent of age-related physiological bone metabolism. Overshoot and significant bone loss were also observed in mice receiving continuous anti-RANKL antibody administration after treatment was interrupted, resembling the original pathology. In mice long out of overshoot, bone resorption recovered, but osteoblast numbers and bone formation remained markedly reduced. The bone marrow exhibited a significant reduction in Sca-1- and PDGFRα-expressing osteoblast progenitors (PαS cells) and ALP-positive early osteoblasts. Just before the overshoot phase, the osteoclast precursor cell population expands and RANKL-bearing extracellular vesicles (EVs) became abundant in the serum, leading to robust osteoclastogenesis after cessation of anti-RANKL treatment. Thus, accelerated bone resorption due to the accumulation of RANKL-bearing EVs and long-term suppression of bone formation uncoupled from bone resorption leads to the severe bone loss that is characteristic of denosumab discontinuation.