Can a drug for one bone disease help another? a Rare-Disorder trial puts it to the test

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: In order to be eligible to participate in this study, an individual must meet all of the following criteria: 1. Patient has confirmed CSHS by physician diagnosis 2. Patient has confirmed FGF23 elevations in the context of a low fasting serum phosphorous \< 2.5 mg/dL 3. Patient able to tolerate burosumab treatment 4. Have a corrected serum calcium level \< 10.8 mg/dL 5. Have an eGFR \>25 mL/min/1.73m2 (using CKD-EPI equation) 6. Must be willing in the opinion of the investigators, to comply with study procedures and schedule 7. Provide written informed consent by the subject or a Legal Authorized Representative (LAR) after the study has been explained and prior to any research related procedures begin 8. Must have a negative pregnancy test at Screening and be willing to have additional pregnancy tests during the study. 9. Must be willing to use a highly effective method of contraception for the duration of the study and for at least 12 weeks after the last dose of the study drug. Highly effective methods of contraception include: combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (e.g., oral, intravaginal, transdermal), progestogen-only hormonal contraception associated with inhibition of ovulation (e.g., oral, injectable, implantable), intrauterine device (IUD) or intrauterine hormone-releasing system (IUS), bilateral tubal occlusion, or sexual abstinence (i.e., refraining from heterosexual intercourse during the entire period of risk associated with the study treatments, when this is in line with the preferred and usual lifestyle of the subject) Exclusion Criteria: An individual who meets any of the following criteria will be excluded from participation in this study: 1. Concomitant use of active vitamin D (i.e. calcitriol) and/or exogenous phosphate supplementation during burosumab therapy. Subjects will be allowed over the counter Vitamin D should levels drop below \<20 ng/ml 2. Blood phosphorus level within or above the normal range while not taking phosphate or active Vitamin D. 3. Severe renal impairment or end-stage renal disease, defined as an eGFR of less than 25 ml/min/1.73m2 4. The use or enrollment in studies using other investigational therapies including other monoclonal antibodies 5. Subject or Legally Authorized Representative not willing or not able to give written informed consent 6. In the investigator's opinion, the subject may not be able to meet all the requirements for study participation 7. History of hypersensitivity to burosumab excipients that in the opinion of the investigator, places the subject at an increased risk of adverse effects 8. Subject has a condition that in the opinion of the investigator could present a concern for subject safety or data interpretation.