THU466 XLH During Pregnancy: A Closer Look Based On Real-World Experience

Disclosure: D.S. Ali: None. A.A. Khan: Advisory Board Member; Self; Amgen Inc, Takeda. Grant Recipient; Self; Alexion Pharmaceuticals, Inc., Amgen Inc, Radius Health, Inc, Takeda, Ultragenyx. Background: X-linked hypophosphatemia (XLH) is a genetic disorder caused by mutation in the PHEX gene leading to excess serum levels of the phosphate-regulating hormone fibroblast growth factor 23 (FGF23). High FGF23 causes renal phosphate wasting, decreased production of serum 1,25-dihydroxy vitamin D, and increased metabolism of 1,25-dihydroxy vitamin D, leading to hypophosphatemia. Maintaining normal serum phosphorus levels in pregnant women with XLH is critically important to ensure mineralization of the fetal skeleton. Multiple factors may influence phosphorus homeostasis in pregnancy, including an increase in PTHrP production and a tripling of calcitriol beginning in the first trimester. Case study: 30 years old Gravida 1 Para 0 with XLH, diagnosed at the age of 18 months. She has a pathogenic heterozygous mutation of the PHEX gene (c.1645+1G>A). Treated with phosphate and calcitriol since diagnosis. Had no history of neonatal seizure or respiratory distress, and no dental or renal manifestations. No prior fractures or corrective surgeries and has a normal BMD. No history of enthesopathy, however, has chronic joint pain > 1 year. The pregnancy progressed well with no issues; the patient chose not to undergo amniocentesis for fetal diagnosis. She was monitored closely every 3 weeks, aiming to maintain the serum phosphorus within the normal range to ensure adequate mineralization of the fetal skeleton. The requirements for phosphate increased by 50% in the 2(nd) trimester and by 125% in the 3(rd) trimester compared to baseline. While the calcitriol dose increased by 25% in the 3 trimesters compared to baseline. Because further increases in the calcitriol dose were associated with hypercalciuria, the dose was carefully titrated. The mean serum phosphorus level was maintained in the low normal reference range at 0.79 - 0.8 mmol/L (NR: 0.8-1.45), mean TmP/GFR 0.611 (NR: 0.96-1.44). She was induced at 37 weeks due to cholestasis, and delivered with a vaginal delivery. The baby boy weighed 2.690 kg and had slightly low serum phosphorus and calcium levels at birth, which corrected spontaneously. DNA analysis confirmed that the baby also has a pathogenic mutation of the PHEX gene. Conclusion: There was a noticeable variation in the serum phosphorus levels during the 1(st) and 2(nd) trimesters and less so in the 3(rd) trimester and during lactation. The dose of phosphate and active vitamin D required careful upward titration to maintain a normal serum phosphorus level. Therefore, close monitoring is essential. Although the rate of c-sections is higher (76%) amongst the XLH patient population compared to the normal population 27.1%, our patient delivered vaginally with no adverse skeletal outcomes in the mother or neonate. Presentation: Thursday, June 15, 2023