7740 Adult-Onset Hypophosphatasia Presenting As Low Alkaline Phosphatase Level Following Denosumab Use For Osteoporosis

Abstract Disclosure: J. Ocampo Mascaro: None. P. Wax: None. M. Al Mukaddam: Grant Recipient; Self; Ipsen, Incyte Research Funding. Background: Hypophosphatasia (HPP) is a genetic disease characterized by low activity of the tissue non-specific alkaline phosphatase. It has a broad range of clinical phenotypes, with adults usually presenting with an incidental low alkaline phosphatase (ALP) finding. Differentiating this from other causes of low ALP is challenging, especially when treating osteoporosis, as antiresorptive therapy such as denosumab and bisphosphonates may cause low ALP and carry the potential of increasing risk of atypical femoral fractures in patients with undiagnosed HPP. Clinical Case: A 64-year-old woman with history of nephrolithiasis, hypercalciuria, and osteoporosis (screening DXA lowest T-score -3.3 in femoral neck (FN)) but no prior fractures or dental issues was seen in clinic. Initial bloodwork showed normal range calcium, 25-OH vitamin D, and intact PTH. Pre-treatment ALP was 42 IU/L (N 39-117). Work up for secondary causes of osteoporosis was negative. Three yearly doses of IV zoledronic acid resulted in +4.2% BMD increase in lumbar spine (LS) but no changes in FN. She had no side effects but was found to have a decrease in ALP to 33 IU/L. This normalized back to 42 IU/L during a three-year drug holiday. Follow up DXA noted significant decline in BMD, hence denosumab was initiated. ALP decreased back to the 30-35 IU/L range. Fasting Vitamin B6 level (off B6 supplements for 1 week) was 23.4 ng/ml (N 2.1-21). Urine phosphoethanolamine was mildly elevated at 5 mmol/mol creatinine (N ≤4). Genetic testing evidenced a pathogenic ALPL variant (Exon 6, c.575T>C (p.Met192Thr), heterozygous) compatible with HPP. She had tolerated three doses of denosumab by then with BMD improvement of 5.0% in LS but no changes in FN and no atypical femoral fracture. Denosumab was stopped. Abaloparatide was started for osteoporosis with close monitoring for worsening hypercalciuria. Conclusion: This case demonstrates that adult HPP patients may present with subtle findings and lower range of normal ALP and may tolerate short courses of antiresorptive agents. There is a need for better screening and diagnostic criteria for adult HPP patients whose ALP levels may not be initially low before initiating antiresorptive drugs for osteoporosis. The risk of adverse effects from therapy will likely vary significantly based on the clinical severity/presentation of HPP as some adults with lower ALP ranges might be more susceptible to developing atypical femur fractures (1). Balancing atypical femur fracture risk with the benefits of antiresorptive treatment in decreasing risk of osteoporotic fractures highlights the need for better risk stratification to prevent undertreatment of osteoporosis. Reference: 1. Warren AM, Ebeling PR, Grill V, Seeman E, Sztal-Mazer S. Bilateral atypical femoral fractures during denosumab therapy in a patient with adult-onset hypophosphatasia. Endocrinol Diabetes Metab Case Rep. 2021 Sep 1;2021:21-0096. Presentation: 6/2/2024