RANKL/Bone Remodelling

Bone remodelling is maintained by the tightly coupled activity of osteoblasts (bone formation) and osteoclasts (bone resorption), coordinated by the RANK/RANKL/OPG signalling axis. RANKL (receptor activator of NF-kappaB ligand), expressed on osteoblasts and bone marrow stromal cells, binds to RANK on osteoclast precursors and mature osteoclasts, driving osteoclastogenesis, osteoclast activation, and prolonged osteoclast survival. Osteoprotegerin (OPG), a decoy receptor secreted by osteoblasts, normally neutralises RANKL and limits osteoclast activity. In postmenopausal osteoporosis, oestrogen deficiency reduces OPG expression and increases RANKL, shifting the balance toward excessive bone resorption and progressive bone mineral density loss.

Denosumab, a fully human anti-RANKL monoclonal antibody, mimics OPG by binding RANKL with high affinity and preventing RANKL-RANK interaction, thereby reducing osteoclast formation, function, and survival. Unlike bisphosphonates, denosumab is not incorporated into bone matrix and has no chemical affinity for hydroxyapatite, meaning its anti-resorptive effect is fully reversible upon discontinuation. This reversibility is clinically important: abrupt discontinuation of denosumab without transitioning to antiresorptive therapy is associated with a rebound increase in bone turnover markers and multiple vertebral fractures within 12-18 months — a pharmacovigilance concern requiring active management at the point of denosumab cessation.

Anabolic agents targeting the Wnt signalling pathway represent a complementary approach: teriparatide (PTH 1-34) and abaloparatide (PTHrP analogue) stimulate osteoblast activity via PTH1R signalling, while romosozumab, an anti-sclerostin antibody, removes the sclerostin-mediated brake on Wnt/beta-catenin osteoblast stimulation. Romosozumab uniquely combines anabolic (bone formation) and anti-catabolic (reduced bone resorption) effects within a single agent, administered as monthly subcutaneous injections for 12 months, after which transition to antiresorptive therapy is required to maintain gains.