• Part 1: Inclusion Criteria:

‣ Male or female patients aged 18 to 65 years inclusive at the time of signing the ICF.

⁃ Diagnosed with XLH (as documented by the investigator).

⁃ Have a value of fasting serum phosphorus \< 2.5 mg/dL (0.81 mmol/L) at Screening.

⁃ Have a value of renal TmP/GFR \< 2.5 mg/dL at Screening.

⁃ eGFR ≥ 60 mL/min (using the Chronic Kidney Disease Epidemiology Collaboration equation \[Inker, 2021\]) at Screening.

⁃ Have a corrected serum calcium level \< 10.8 mg/dL (2.7 mmol/L) at Screening.

⁃ Provide a signed ICF.

⁃ Agree not to change diet and exercise regimen from one week prior to dosing to end of study.

⁃ Have a negative pregnancy test at Screening and be willing to have additional pregnancy tests during the study (female participants only).

‣ If taking chronic pain medications (including narcotic pain medications/opioids), must be on a stable regimen for at least 21 days prior to the Screening visit, and be willing to maintain medications at the same stable dose(s) and schedule throughout the clinical trial. The dose must not exceed 60 mg oral morphine equivalents/day.

‣ Be willing to use an effective method of contraception while participating in the study and for 5 months after the last dose (all sexually active participants of childbearing potential).

∙ Postmenopausal status of female participants will be confirmed with a Screening serum follicle-stimulating hormone level \>40 mIU/mL.

‣ Must, in the opinion of the investigator, be willing and able to complete all aspects of the study, adhere to the study visit schedule and comply with the assessments.