Denosumab

Denosumab is a fully human monoclonal antibody that has become one of the most important anti-resorptive drugs in metabolic bone disease and orthopaedic oncology. Like bisphosphonates, it inhibits osteoclasts and so reduces bone resorption, raises bone mineral density, and lowers fracture risk. But it does this in a completely different way - by neutralising a single signalling protein, RANKL - and, crucially, it is not stored in the skeleton, so everything it does is reversible.

That reversibility is the heart of the topic. It gives denosumab some advantages (it works in renal impairment, and the effect can be measured cleanly) but also its single most dangerous property: when the drug is stopped or even just delayed, bone turnover rebounds and a cluster of vertebral fractures can follow. For the exam, three threads recur throughout: how it works (the RANK / RANKL / OPG system), what it treats (osteoporosis and giant cell tumour of bone, with strong evidence), and why you must never simply stop it (the rebound phenomenon).

To understand denosumab you need the RANK / RANKL / OPG system - the master switch for osteoclast activity and a favourite basic-science viva.

• RANKL (receptor activator of nuclear factor-kappa-B ligand) is produced by osteoblasts and bone marrow stromal cells.
• RANK is the receptor on osteoclast precursors (and mature osteoclasts).
• When RANKL binds RANK, the precursors mature into active osteoclasts, survive longer, and resorb bone.
• Osteoprotegerin (OPG), also made by osteoblasts, is a natural decoy receptor: it binds RANKL and stops it reaching RANK, putting the brakes on resorption.

The balance of RANKL versus OPG therefore sets the rate of bone resorption. Denosumab works exactly like OPG - it is an antibody that binds RANKL and prevents it activating RANK. With RANKL neutralised, osteoclasts do not form, do not function, and do not survive, so resorption falls quickly and bone mineral density rises.

The line examiners want to hear: denosumab and bisphosphonates reach the same end (less osteoclast resorption) by different means, and they behave very differently when stopped - which is why denosumab must never simply be discontinued.

Denosumab is given by subcutaneous injection, with the dose and frequency depending on the indication.

Two practical points the exam loves:

• The higher, more frequent oncology dose (120mg monthly) carries a much greater risk of osteonecrosis of the jaw and hypocalcaemia than the osteoporosis dose (60mg every 6 months).
• Because denosumab is not renally cleared, it is an option in patients with poor renal function where intravenous zoledronate would be avoided - but you must watch even more carefully for hypocalcaemia.

Denosumab turns up across the whole exam and in daily orthopaedic practice. In fracture liaison and bone-health clinics it is a key agent for secondary fracture prevention, especially when bisphosphonates are not tolerated or renal function is poor. In basic-science vivas the RANK / RANKL / OPG axis and the contrast with bisphosphonates are classic asks. In orthopaedic oncology denosumab is central to the modern management of giant cell tumour of bone, both to downstage tumours before surgery and to treat unresectable disease. And the rebound fracture problem - recognising it, preventing it, and never stopping the drug without cover - is a safety-critical theme examiners reward. Knowing when to start, how to dose, what to check before each injection, and how to stop safely is the practical core.

The Rebound Phenomenon - the must-know safety point

Because denosumab is not stored in bone, stopping it removes the brake on osteoclasts all at once. Bone turnover then rebounds above the pre-treatment level, bone density is lost rapidly over months, and a subset of patients sustain multiple spontaneous vertebral fractures. The risk is greatest in patients who already have vertebral fractures, who have been off therapy longer, and who gained a lot of hip bone density on treatment.

Osteonecrosis of the Jaw and Atypical Femoral Fracture

These rare class effects of potent antiresorptives also apply to denosumab. Arrange dental assessment before starting where feasible (the risk is much higher at the oncology dose), and counsel patients to report new thigh pain, which warrants radiographs of both femurs to look for an atypical femoral fracture.

• Osteoporosis guidelines (for example AAOS/AOA bone-health initiatives, NICE/NOGG in the UK, and endocrine society guidance) position denosumab as an effective antiresorptive, often second-line or for patients in whom bisphosphonates are unsuitable (for example renal impairment or intolerance), given its efficacy and non-renal clearance.
• The rebound phenomenon is now formally recognised across major bodies: the European Calcified Tissue Society and others advise never stopping denosumab without follow-on antiresorptive cover, usually a bisphosphonate started about 6 months after the last dose.
• In orthopaedic oncology, denosumab is established for giant cell tumour of bone - to downstage unresectable or morbid lesions and to control disease - with specialist guidance emphasising close imaging follow-up because the tumour can recur after cessation.
• Global practice variation largely reflects cost, access, and whether a bisphosphonate or denosumab is preferred first-line, rather than disagreement on the underlying biology - but the safety message about not stopping denosumab is universal.