Denosumab (Dmab) treatment can benefit patients with fibrous dysplasia/McCune-Albright syndrome (FD/MAS) by suppressing the receptor activator of nuclear factor κB ligand (RANKL)-mediated increased bone resorption. However, limited data of two pediatric cases indicate that a rebound phenomenon may occur after withdrawal. Therefore we studied the safety of Dmab discontinuation in FD/MAS. Thirty-seven patients using Dmab, mostly after unsuccessful bisphosphonate (BP) treatment, were included. Health records were screened for pain scores, side effects, and bone turnover markers (BTMs) (calcium, alkaline phosphatase [ALP], procollagen 1 N-terminal propeptide [P1NP], and β-crosslaps [B-CTX, also termed β-C-terminal telopeptide]) during treatment, and for BTMs and clinical rebound effects after withdrawal. BTM levels after withdrawal were compared to pretreatment values. Data were calculated as median (interquartile range [IQR]). BTMs normalized in two-thirds of patients and pain scores decreased significantly during treatment (p = 0.002). One patient (2.7%) developed osteonecrosis of the jaw. Sixteen patients discontinued Dmab treatment after a median of 1.6 years (IQR 1.0 years) because of insufficient effect on pain (n = 10, 63%), side effects (n = 4, 25%), or other reasons (n = 4, 25%). Follow-up posttreatment was 3.2 (2.8) years, wherein no fractures, pain flares, or lesion progression occurred. Calcium remained normal in all but one patient, who had a mild asymptomatic hypercalcemia (2.73 mmol/L) 5 months after discontinuation. ALP passed pretreatment levels in five of 11 patients (46%), increased most after 6 months by 18 (43) U/L, and returned to baseline levels thereafter. P1NP exceeded pretreatment levels in four of nine patients (44%), CTX in eight of nine patients (89%). P1NP rose most after 3 months and stabilized thereafter. CTX showed the highest relative elevation. Patients with high pretreatment levels responding well to Dmab seemed to have the highest rebound. These results suggest beneficial effects of Dmab on pain and BTMs, and show a biochemical but asymptomatic rebound phenomenon after withdrawal in adults with FD/MAS, mainly in case of high pretreatment levels, good response, and multiple injections. Further studies on the safety of Dmab and withdrawal are needed and ongoing.

Fibrous dysplasia/McCune‐Albright syndrome (FD/MAS) is a rare bone and endocrine disorder arising along a broad spectrum. Long‐bone fractures are a common, painful, and potentially disabling complication. However, fracture prevalence and risk factors have not been well‐established, making it difficult to predict which patients are at risk for a severe course. Clinical and imaging data were reviewed from two large, well‐phenotyped cohorts (National Institutes of Health [NIH] in the United States and the Leiden University Medical Center [LUMC] in the Netherlands) to identify long‐bone fractures at FD sites. Skeletal burden score was quantified using bone scintigraphy. Multiple linear regressions were performed to identify clinical associations with fractures. A total of 419 patients were included (186 NIH, 233 LUMC); 194 (46%) had MAS endocrinopathies. Median age at last follow‐up was 30.2 years (range 3.2–84.6, interquartile range [IQR] 25.5), and median skeletal burden score was 16.6 (range 0–75, IQR 33). A total of 48 (59%) patients suffered one or more lifetime fracture (median 1, range 0–70, IQR 4). Median age at first fracture was 8 years (range 1–76, IQR 10). Fracture rates peaked between 6 and 10 years of age and decreased thereafter. Lifetime fracture rate was associated with skeletal burden score (β = 0.40, p < 0.01) and MAS hyperthyroidism (β = 0.22, p = 0.01). Younger age at first fracture was associated with skeletal burden score (β = −0.26, p = 0.01) and male sex (β = −0.23, p = 0.01). Both skeletal burden score >25 and age at first fracture ≤7 years were associated with a higher total number of lifetime fractures (median 4, range 1–70, IQR 5 versus median 1, range 1–13, IQR 1) (p < 0.01). In conclusion, higher skeletal burden score and MAS hyperthyroidism are associated with long‐bone fractures in FD/MAS. Both skeletal burden score ≥25 and age at first fracture ≤7 years are associated with a higher lifetime long‐bone fracture risk and may predict a more severe clinical course. These results may allow clinicians to identify FD/MAS patients at risk for severe disease who may be candidates for early therapeutic interventions. © 2021 American Society for Bone and Mineral Research (ASBMR). This article has been contributed to by US Government employees and their work is in the public domain in the USA.

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Context The correlation between Fibrous Dysplasia/McCune-Albright syndrome (FD/MAS) skeletal disease burden on Na[18F]F-PET-CT and serum bone turnover markers (BTMs) was recently described. The effect of treatment on lesional fluoride burden in FD/MAS is unknown. Objective To investigate treatment response measurements in FD/MAS patients who underwent Na[18F]F-PET-CT and treatment with antiresorptives. Design Observational case series. Setting Academic center of expertise for rare bone diseases. Patients 15 consecutive patients with FD/MAS with baseline and follow-up Na[18F]F-PET-CT-parameters of healthy bone and FD lesions, BTMs and pain scores at start of denosumab (n=8) and non-denosumab patients (n=7). Main outcome(s) On Na[18F]F-PET-CT the Volumetric measures of FD-burden (FTV) and ‘Fraction affected skeleton’ (FAS), represented the portion of the skeleton affected. This was correlated with BTMs and pain. Results Disease activity decreased significantly, with FTV 361cm 3 to 97cm 3, p=0.018 in denosumab-treated patients, not in non-denosumab patients (p=0.249). Serum P1NP and Alkaline Phosphatase (ALP) decreased significantly: 82ng/mL vs 55ng/mL, p=0.023 and 119 IU/L vs 84 IU/L, p=0.020, respectively. In denosumab-treated patients pain scores improved leading to pain medication reduction. This correlated with lesional uptake whilst healthy bone activity did not change. BTMs and FTV correlated positively, P1NP r=0.730; p&lt;0.001 and ALP r=0.406; p=0.006, as dis change in BTMs and FTV: P1NP (p=0.032) and ALP (p=0.024). FAS strongly correlated with treatment-induced decrease in ALP (p=0.027) and P1NP (p=0.009). Conclusion Na[18F]F-PET-CT captured treatment-induced lesional changes which correlated with BTMs and pain reduction. Therefore Na[18F]F-PET-CT can be used as an objective local parameter of response to denosumab treatment in FD/MAS.