Study of KRN23 in Adults With X-linked Hypophosphatemia (XLH)

Inclusion criteria

• . Male or female, aged 18 - 65 years, inclusive
• . Diagnosis of XLH supported by classic clinical features of adult XLH (such as short stature or bowed legs) and at least ONE of the following at Screening:
• . Biochemical findings consistent with XLH at Screening Visit 2 following overnight fasting (min. 8 hours):
• . Presence of skeletal pain attributed to XLH/osteomalacia, as defined by a score of ≥ 4 on Brief Pain Inventory (BPI) Questions 3 (Worst Pain) at Screening Visit 1. (Skeletal pain that, in the opinion of the investigator, is attributed solely to causes other than XLH/osteomalacia-for example, back or joint pain in the presence of severe osteoarthritis by radiograph in that anatomical location-in the absence of any skeletal pain likely attributed to XLH/osteomalacia should not be considered for eligibility)
• . Estimated glomerular filtration rate (eGFR) ≥ 60 mL/min (using the Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] equation) or eGFR of 45 60 mL/min at Screening Visit 2 with confirmation that the renal insufficiency is not due to nephrocalcinosis
• . If taking chronic pain medications (including narcotic pain medications/opioids), must be on a stable regimen for at least 21 days prior to Screening Visit 1, and be willing to maintain medications at the same stable dose(s) and schedule throughout the Placebo-controlled Treatment Period of the study. The dose must not exceed 60 mg oral morphine equivalents/day
• . Provide written informed consent or if a minor, provide written consent and have a legally authorized representative willing and able to provide written informed consent, after the nature of the study has been explained, and prior to any research-related procedures