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

Inclusion Criteria:

1. Male or female Chinese patients, aged 18 to 65 years (inclusive) at the time of signing the ICF

2. Diagnosis of XLH supported by classic clinical features of adult XLH (such as short stature or bowed legs) and at least either of the following at screening:

   • Confirmed PHEX mutation (prior to the study with historic record) in the patient or a directly related family member with appropriate X linked inheritance
   • Serum iFGF23 level ≥30 pg/mL by the Kainos assay at Screening

3. Biochemical findings consistent with XLH following overnight fasting (≥8 hours) at Screening:

   • Serum phosphorus <2.5 mg/dL (0.81 mmol/L). Serum phosphorus level may be re tested (once only) at least 7 days after discontinuation of therapy, if applicable.
   • TmP/GFR of <2.5 mg/dL

4. Presence of skeletal pain attributed to XLH/osteomalacia, as defined by a score of ≥4 on the BPI Worst Pain question at Screening (Skeletal pain that, in the opinion of the investigator or subinvestigator, is attributed solely to causes other than XLH/osteomalacia [e.g., back pain 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)

5. Patients who are taking chronic pain medications (including narcotic pain medications/opioids) must be on a stable regimen for at least 21 days before signing the ICF and be willing to maintain the medications at the same stable dose(s) and schedule throughout the study. The dose must not exceed 60 mg oral morphine equivalents/day

6. Able to receive conventional therapy (oral phosphate and pharmacologic vitamin D [or metabolites/analogs])

7. Written informed consent provided after the nature of the study has been explained and prior to any research related procedures

8. Willing to provide access to prior medical records for the collection of biochemical and radiographic data and disease history

9. Have a negative pregnancy test at Screening and be willing to have additional pregnancy tests during the study (female patients of child-bearing potential only)

10. Be willing to use an effective method of contraception while participating in the study (sexually active patients of child bearing potential) and for 12 weeks after last dose of study drug. Women of non child bearing potential are defined as permanently sterile (i.e. due to hysterectomy or bilateral oophorectomy) or postmenopausal (defined as at least 12 months postcessation of menses without an alternative medical cause). Postmenopausal status of female patients will be confirmed with a Screening serum follicle stimulating hormone (FSH) level >40 mIU/mL

11. 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

12. Have completed entries for ≥4 of 7 consecutive days of the patient diaries before Week -14

Exclusion Criteria:

1. Use of a pharmacologic vitamin D, its metabolites, or analogs, and oral phosphate for treatment of XLH within 14 days prior to Screening.
2. Use of aluminum hydroxide antacids, acetazolamide, thiazide diuretics and/or systemic corticosteroids within 14 days prior to Week -14
3. Corrected serum calcium level ≥10.8 mg/dL (2.69 mmol/L) at Screening
4. Plasma iPTH ≥2.5 times the upper limit of normal at Screening
5. Uncontrolled diabetes mellitus, defined as HbA1c >7.5% at Screening
6. Use of medication to suppress PTH (e.g., Sensipar®, cinacalcet, calcimimetics) within 60 days before signing the ICF
7. Use of oral bisphosphonates in the 2 years before signing the ICF
8. Planned or recommended orthopedic surgery within the clinical trial period
9. History of traumatic fracture or orthopedic surgery within 6 months before signing the ICF
10. Use of KRN23, or any other therapeutic mAb within 90 days before signing the ICF
11. Use of any investigational product or investigational medical device within 30 days before signing the ICF, or requirement for any investigational agent prior to completion of all scheduled study assessments
12. Pregnant or breastfeeding at Screening or Week -14, or intention to become pregnant (the patient or partner) at any time during the study
13. Unable or unwilling to withhold prohibited medications throughout the study
14. Presence or history of any hypersensitivity, or allergic or anaphylactic reactions to any mAb or KRN23 excipients that, in the judgment of the investigator or subinvestigator, places the patient at increased risk for adverse effects
15. Positive for human immunodeficiency virus (HIV) antibody, hepatitis B surface antigen (HBsAg), and/or hepatitis C virus (HCV) antibody at Screening, or prior history of positive test
16. History of recurrent infection or predisposition to infection, or of known immunodeficiency
17. Presence of malignant neoplasm (except basal cell carcinoma)
18. Presence of a concurrent disease or condition that would interfere with study participation or affect safety
19. Presence or history of any condition that, in the view of the investigator or subinvestigator, places the patient at high risk of poor treatment compliance or of not completing the study