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 central idea. 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. Three threads run through everything that follows: 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.

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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. The wider context of who to treat and when sits in osteoporosis, and the RANK/RANKL/OPG axis is part of normal bone remodelling.

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

Two practical points the exam loves:

• The oncology dose is a different safety proposition, and the gap is large enough to quantify. At the osteoporosis dose (60 mg six-monthly) FREEDOM reported no cases of osteonecrosis of the jaw at all across 7,868 women over three years and no excess hypocalcaemia. At the oncology dose (120 mg every 4 weeks) ONJ ran at 2.0% in Stopeck's 2,046-patient breast-cancer trial and at 1% (3 of 281) in Chawla's giant cell tumour series, which also recorded hypocalcaemia in 5% and grade 3-4 hypophosphataemia in 3%. Same molecule, twelve times the annual dose, and a categorically different consent conversation.
• Osteonecrosis of the jaw is not random. Risk rises with dental extraction or other dentoalveolar surgery, denture trauma, pre-existing periodontal disease or abscess, poor oral hygiene, smoking, diabetes, corticosteroids and concurrent antiangiogenic agents. Get dental assessment and any needed extractions done before starting where the clinical situation allows. At osteoporosis doses, guidance does not support interrupting therapy for routine dentistry — the fracture risk of a lapse outweighs the small ONJ risk. At oncology doses the balance shifts and treatment is planned jointly with the oral surgeons.
• Because denosumab is not renally cleared, it is an option in patients with poor renal function where intravenous zoledronate would be avoided - but hypocalcaemia is then the dominant hazard and calcium and vitamin D must be corrected and rechecked. It is also not removed by dialysis, and no dose adjustment is required for renal function.

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Osteoporosis is increasingly treated as a sequence of drugs rather than one drug, and the order matters more than most candidates expect. The DATA-Switch study randomised 94 postmenopausal women to 24 months of teriparatide, denosumab, or both, then switched them.

• Teriparatide → denosumab: bone density keeps climbing. Total hip rose 6.6% and femoral neck 8.3% over the four years.
• Denosumab → teriparatide: bone is lost, at least transiently. Total hip rose only 2.8% (P=0.0002 against the other order), femoral neck 4.9%, and the radius actually fell by 1.8%.
• Combination then denosumab gave the largest hip gain of all (8.6%).

The rule to carry into the exam: put the anabolic first and the antiresorptive after it. Giving teriparatide to a patient who has just stopped denosumab is the worst of both worlds — an anabolic agent cannot restrain the resorptive rebound, and bone is lost while you wait for it to work. If an anabolic is needed after denosumab, it is generally given with antiresorptive cover rather than instead of it.

Before denosumab, prior bisphosphonate exposure appears to blunt the biochemical rebound when denosumab is later stopped — one reason a bisphosphonate is the logical follow-on agent.

How long to continue? The ECTS advises a formal re-evaluation at 5 years: patients still at high fracture risk should continue (efficacy and safety are maintained to 10 years in the FREEDOM Extension), while a lower-risk patient may stop — but only onto bisphosphonate cover. There is no "drug holiday" for denosumab in the sense that exists for bisphosphonates.

• Hypocalcaemia is an absolute contraindication. Correct it before the first dose and before every subsequent dose. Risk is concentrated in chronic kidney disease, vitamin D deficiency, malabsorption (including after bariatric surgery), and hypoparathyroidism.
• Pregnancy and women of childbearing potential. RANKL signalling is essential to fetal skeletal and mammary development, and denosumab crosses the placenta, so it is contraindicated in pregnancy. Effective contraception is advised during treatment and for at least 5 months after the last dose. This is not a theoretical concern in orthopaedics: giant cell tumour of bone typically affects patients in their twenties and thirties, and the oncology dose is the one used in that group.
• Children and the growing skeleton. Denosumab is not licensed for skeletally immature patients outside specialist use; RANKL blockade in a growing skeleton risks growth-plate effects, and severe rebound hypercalcaemia after stopping is most characteristic in children and young adults.
• Dental disease — see the osteonecrosis guidance above; assess before starting.
• Adherence is a safety issue, not a convenience issue. A patient who will not reliably attend for six-monthly injections is a poor candidate, because a lapsed dose carries the rebound risk.

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 markers climb within 3 months of an omitted dose, exceed the pre-treatment level by 6 months, and bone mineral density is back to baseline by 12 months. A subset of patients sustain multiple spontaneous vertebral fractures.

Get the numbers right, because the finding is more specific than "fractures go up". In the post hoc analysis of FREEDOM and its Extension (1,001 patients who stopped denosumab), the vertebral fracture rate rose from 1.2 to 7.1 per 100 participant-years — which is essentially the rate in patients who stopped placebo (8.5). So the overall fracture rate returns to the untreated level rather than overshooting it. What is distinctive is the pattern: of those who fractured off treatment, 60.7% sustained more than one vertebral fracture after denosumab versus 38.7% after placebo (P=0.049) — an absolute risk of multiple vertebral fractures of 3.4% versus 2.2%. Non-vertebral fracture rates were no different (2.8 vs 3.8 per 100 participant-years).

Who is most at risk, from the same analysis:

• Prior vertebral fracture — odds of multiple vertebral fractures 3.9× higher (95% CI 2.1-7.2)
• Each additional year off treatment — 1.6× higher (1.3-1.9)
• Each 1% annualised loss of total hip BMD off treatment — 1.2× higher (1.1-1.3)

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.

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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.

• 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.