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Other (genetic syndromes, new innovation, etc)

(11.04) X-linked Hypophosphatemia Treated With Anti-FGF23 Monoclonal Antibody

Friday, May 10, 2024
12:10 PM – 12:25 PM CDT
Location: ePoster Showcase, Empire Ballroom Foyer, Level 2
Introduction : X-linked Hypophosphatemia (XLH) is a rare inherited disease characterized by low serum phosphate levels. In XLH, inactivating mutations in the phosphate regulating endopeptidase on the X chromosome gene (PHEX) leads to elevated serum fibroblast growth factor 23 (FGF23) levels that cause renal phosphorus wasting and subsequent hypophosphatemia. Most often diagnosed in childhood, XLH typically manifests as rickets, growth failure, and osteomalacia. We present a patient with recurrent low serum phosphorus diagnosed with XLH in adulthood and discuss the patient’s clinical manifestations and treatment response to burosumab.

Case(s) Description :

A 59 year old woman presented in November 2019 with chronic bone and muscle pains in her hands, thighs, and calves. Her family history was significant for multiple members with a bone disorder causing bowing of their legs during childhood, dental abscesses, and hearing problems. Her youngest son is also diagnosed with XLH. Her history was significant for chronic bilateral leg pain as a child, bowed legs which were corrected with two years of leg braces, and short stature. Lab work showed low-normal phosphorus 2.6 mg/dL (normal 2.5-4.5), elevated parathyroid hormone (PTH) 85.2 pg/mL (normal 15-65), and normal calcium, 1, 25 dihydroxy Vitamin D, and renal function. Genetic testing showed two likely pathogenic variants in the PHEX gene, c.*231A >G and Gain, indicating XLH.

Due to the persistence of diffuse musculoskeletal pains, the patient initiated burosumab, an anti-FGF23 monoclonal antibody injection, in March of 2020. The patient tolerated the treatment well and reported significant improvement in clinical symptoms. She used to take daily non-steroidal anti-inflammatory drugs (NSAIDs) to relieve her chronic widespread pains. After treatment was initiated, her frequency of NSAID use reduced significantly. Her serum phosphorus level increased following the first injection, therefore, dosing required titration. Since then, levels have been within normal limits with her most recent serum phosphorus being 3.0 mg/dL in August 2023 and other pertinent lab values including serum calcium, vitamin D and PTH levels are within normal limits as well.

Discussion : XLH is a rare inherited disease of the bone that typically presents with musculoskeletal deformities which include rickets, short stature, and osteomalacia. Patients presenting with recurrent low serum phosphorus levels should undergo evaluation for hypophosphatemia since patients diagnosed with XLH can benefit from burosumab therapy. Patients should be encouraged to have family members tested and treated appropriately to avoid the clinical consequences of XLH.