THU464 An Uncommon Case Of Adult-Onset Hypophosphatemic Osteomalacia

Disclosure: P.M. Lockhart Pastor: None. N.M. Maalouf: None. Introduction: Hypophosphatemic conditions that interfere with bone mineralization and cause osteomalacia comprise many hereditary and acquired diseases. We describe a case of acquired hypophosphatemic osteomalacia associated with Multiple Myeloma (MM). Case Report: A 45-year-old man with MM and systemic AL amyloidosis was referred to our clinic for evaluation of low bone mass on DXA scan. He presented with progressive back pain, proximal muscle weakness, weight loss, and worsening proteinuria, eventually leading to the diagnosis of AL amyloidosis on fat pad biopsy and MM on bone marrow biopsy. Biochemical work up was remarkable for elevated serum alkaline phosphatase at 463 U/L (reference range 46-116) determined to be of skeletal origin. Additional studies included serum phosphorus 1.9 mg/dl (2.4-4.5), creatinine 1.0 mg/dl (0.7-1.2), calcium 9.0 mg/dl (8.4-10.2), albumin 4.1 g/dl (3.5-5.2), PTH 17.1 pg/ml (15-77), and 25-OH-D 52 ng/ml (30-80). Skeletal survey and PET/CT identified diffuse osteopenia but no fracture or lytic lesion. DXA scan showed Z-score of -5.0 at the L-spine and -4.2 at the femoral neck. Further work-up of hypophosphatemia was notable for fractional excretion (FE) phosphate of 37% (<20), serum 1,25-OH2-D 74 pg/ml (18-64), FGF-23 <14 pg/ml (<59), hypouricemia with high FE uric acid, and normoglycemia with glucosuria. Given his marked bone pain, hypophosphatemia, hypouricemia, glucosuria, and elevated alkaline phosphatase, he was diagnosed with hypophosphatemic osteomalacia presumably secondary to light chain accumulation in kidneys leading to proximal tubule dysfunction (acquired Fanconi syndrome). In addition to MM treatment (5 cycles of Dara-CyBorD then stem cell transplantation), he was started on phosphate supplementation, with marked improvement in bone pain, muscle strength, and serum alkaline phosphatase over 6 months. No anti-resorptive agents were used in treating his MM. Conclusion: This case highlights an unusual presentation of hypophosphatemic osteomalacia. It is important to recognize that bone pain in patients with MM is not only caused by osteolytic lesion but can also result from osteomalacia due to Fanconi syndrome due to light chain kappa deposition. In this patient, treatment with phosphate supplementation resulted in clinical improvement of bone pain and biochemical markers. To date, there are only few reported cases of hypophosphatemic osteomalacia due to Fanconi syndrome in the setting of MM. Presentation: Thursday, June 15, 2023