Introduction
Denosumab is a fully human monoclonal antibody against the receptor activator of nuclear factor kappa B ligand (RANKL), commonly used to treat osteoporosis in males and postmenopausal females.1 Rare but serious adverse effects include hypersensitivity reactions, atypical femoral fractures, severe infections and hypocalcaemia.1,2 Relatively common side effects include mild infections, arthralgia and musculoskeletal pain­–with a reported incidence of 7.7% in the FREEDOM trial.2,3
Denosumab-induced arthralgia and musculoskeletal pain is typically transient and mild, and not associated with an acute phase response.4 In contrast, an acute phase response manifesting as a flu-like syndrome is a relatively common adverse effect of bisphosphonates.5 In a Phase III denosumab study,6 2% of patients had a flare of underlying polymyalgia rheumatica (PMR)–an inflammatory disorder typically presenting with shoulder and pelvic girdle musculoskeletal pain. However, there are no previous reports of new-onset PMR or other inflammatory arthritis following denosumab use. Furthermore, denosumab has also been studied in patients with rheumatoid arthritis (RA)–a systemic autoimmune disease characterised by synovitis and localised joint destruction–with no reports of flares secondary to denosumab.8-10
We present a case of an elderly patient who developed a flare of previously undiagnosed seropositive inflammatory arthritis with overlapping features of PMR and RA, after treatment with denosumab.