The purpose of this study is to assess the safety, pharmacokinetics and efficacy of KRN23 in pediatric Chinese patients with XLH

Inclusion Criteria:

1. Male or female Chinese patients, aged 1 to ≤12 years at ICF signature with radiographic evidence of rickets

2. Diagnosis of XLH supported by either of the following:

   • Confirmed PHEX mutation (prior to the study with historic record) in the patient or a directly related family member with approximate X linked inheritance
   • Serum intact FGF23 level ≥30 pg/mL by Kainos assay at Screening

3. Able to receive conventional therapy (oral phosphate and pharmacologic vitamin D)

4. Biochemical findings associated with XLH: Serum phosphorus <3.0 mg/dL (0.97 mmol/L). Serum phosphorus may be re tested (once only) at least 7 days after discontinuation of therapy, if applicable ([see Section 3.1])

5. Serum creatinine within age-adjusted normal range (based on overnight fasting [minimum 4 hours] values collected at Screening)

6. Serum 25(OH)D above or equal to the lower limit of normal (≥16 ng/mL) at Screening. If 25(OH)D levels are below the normal range, 25(OH)D supplementation will be prescribed. Assuming a patient meets all other eligibility requirements, the patient may be re tested for serum 25(OH)D after a minimum of 7 days of treatment

7. Willing to provide access to prior medical records for the collection of historical growth and radiographic data and disease history

8. Written or verbal assent (as appropriate for the patient and region) by the patient and written informed consent by legally authorized representatives provided after the nature of the study has been explained, and prior to any research-related procedures

9. Be willing and able to complete all aspects of the study, adhere to the study visit schedule and comply with the assessments, as judged by the investigator or subinvestigator

10. Female patients who have reached menarche must have a negative pregnancy test at Screening and be willing to have additional pregnancy testing during the study. If sexually active, male and female patients must be willing to use an effective method of contraception for the duration of the study and for 12 weeks after the last dose of IP

Exclusion Criteria:

1. Positive for human immunodeficiency virus (HIV) antibody, hepatitis B surface antigen (HBsAg), and/or hepatitis C virus (HCV) antibody at Screening
2. Tanner stage 4 or higher through physical examination
3. Height percentile >50% based on country specific norms
4. Use of a pharmacologic vitamin D, its metabolites, or analogs, oral phosphate for treatment of XLH, aluminum hydroxide antacids, acetazolamide, thiazide diuretics, and/or systemic corticosteroids within 7 days prior to Week -14
5. Current or prior use of leuprorelin, triptorelin, goserelin, or other drugs known to delay puberty
6. Use of growth hormone therapy within 12 months before ICF signature
7. Have uncontrolled diabetes mellitus, defined as glycated hemoglobin (HbA1c) >8.5% at Screening
8. Presence of nephrocalcinosis on renal ultrasound Grade 4 based on the following scale:
9. Planned or recommended orthopedic surgery (implantation or removal), including staples, 8 plates, or osteotomy, within the first 40 weeks of the study
10. Hypocalcemia or hypercalcemia, defined as serum calcium levels outside the age adjusted normal limits (based on overnight fasting [minimum 4 hours] values collected at the Screening)
11. Evidence of hyperparathyroidism (parathyroid hormone [PTH] levels 2.5 × ULN)
12. Use of medication to suppress PTH (e.g., Sensipar®, cinacalcet, calcimimetics) within 2 months prior to ICF signature
13. Presence or history of any condition that, in the view of the investigator or subinvestigator, places the subject at high risk of poor treatment compliance or of not completing the study
14. Presence of a concurrent disease or condition that would interfere with study participation or affect safety
15. History of recurrent infection or predisposition to infection, or of known immunodeficiency
16. Use of therapeutic mAb within 90 days prior to ICF signature or history of allergic or anaphylactic reactions to any mAb
17. Presence or history of any hypersensitivity to KRN23 excipients that, in the judgment of the investigator or subinvestigator, places the patient at increased risk for adverse effects
18. Use of any investigational product or investigational medical device within 30 days prior to ICF signature, or need for the use of any investigational agent prior to completion of all scheduled study assessments
19. Other patients who are considered to be ineligible for the study by the investigator or subinvestigator for reasons other than the above