Anti-FGF23 (Burosumab) in Adult Patients With XLH

Inclusion Criteria: * Male or female, aged ≥ 18 years, inclusive * Diagnosis of X-linked Hypophosphatemia supported by classic clinical features of adult XLH (e.g. short statue or bowed legs, clinical symptoms as judged by the investigator) and at least one of the following at Screening visit: * documented PHEX mutation in either the patient, or * in a directly related family member with appropriate X-linked inheritance * Increased serum levels of c-term FGF23 or iFGF23 * Biochemical findings consistent with XLH at Screening visit following overnight fasting: * Serum phosphorus level or * TmP/GFR below lab specific lower limit of normal (LLN) * Estimated glomerular filtration rate (eGFR) ≥ 60 mL/min (using the Chronic Kidney Disease Epidemiology Collaboration equitation) or eGFR of 30 up to 60 mL/min at Screening visit with confirmation that the renal insufficiency is not due to nephrocalcinosis * Subjects who provide written informed consent after the nature of the study has been explained, and prior to any research-related procedures. * Participants 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. * Females of child-bearing potential must have a negative urine pregnancy test at Screening and be willing to have additional pregnancy tests during the study. Females considered not to be of child-bearing potential include those who have been in menopause for at least 2 years prior to Screening, or have had tubal ligation at least one year prior to Screening, or have had a total hysterectomy or bilateral salpingo-oophorectomy. * Female Participants of child-bearing potential who are sexually active must consent to use an effective method of contraception as determined by the site investigator (i.e. oral hormonal contraceptives, patch hormonal contraceptives, vaginal ring, intrauterine devices, surgical hysterectomy, vasectomy, tubal ligation, or true abstinence) from the period following the signing of the informed consent through 12 weeks after the last dose of study drug. Exclusion Criteria: * Hypocalcemia or hypercalcemia, defined as serum calcium levels outside the age-adjusted normal limits and deemed as clinically significant in the opinion of the investigator. * Vitamin D deficiency (25OH D3 \< 20ng/ml); if Vitamin D is low at screening, substitution is allowed and recompensation has to be confirmed before treatment start by normalized levels of Vitamin D (25OH D3 ≥ 20ng/ml) * Serum intact parathyroid hormone (iPTH) \>2.5-fold the upper limit of normal (ULN) * Severe renal insufficiency with a Glomerular filtration rate (eGFR) \<30 at screening * Treatment with oral phosphate and / or active vitamin D analogues in addition to Burosumab treatment. (In order to ensure appropriate patient care and preclude any harm due to deficient supply, required supplementation with oral phosphate salts and/or active vitamin D analogues at screening can be continued during the run-in phase but has to be stopped before Baseline and Initiation of treatment with Burosumab.) * Treatment with bisphosphates or Denosumab within the last 6 months * Treatment with Teriparatide within the last 3 months * Intake of calcimimetics within 30 days before screening * Patients with known hypersensitivity to Burosumab and the active substances of any of the excipients of Burosumab * Presence of a concurrent disease or condition that would interfere with study participation or affect safety in the opinion of the investigator * Use of any investigational product other than Burosumab or investigational medical device within 30 days prior to screening, or requirement for any investigational agent prior to completion of all scheduled study assessments.