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

Inclusion Criteria:

1. Have a clinical diagnosis of TIO based on evidence of excessive FGF23 that is not amenable to cure by surgical excision of the offending tumor (documented by Investigator)
2. Male or female Chinese patients aged ≥18 years at the time of signing the informed consent form
3. Have a fasting serum phosphorus level < 2.5 mg/dL (0.81 mmol/L) at Screening
4. Have a serum iFGF23 level ≥ 100 pg/mL by Kainos assay at Screening
5. Have a TmP/GFR < 2.5 mg/dL at Screening
6. Have an estimated glomerular filtration rate (eGFR) ≥ 60 mL/min/1.73m2(using CKD-EPI formula) at Screening. Subjects with an eGFR ≥ 30 but < 60 mL/min at screening will be considered eligible so long as in the opinion of the Investigator the decline in renal function is not related to nephrocalcinosis
7. Have a corrected serum calcium level < 10.8 mg/dL (2.69 mmol/L) at Screening (Corrected serum calcium = serum calcium in mg/dL + 0.8 × [4 - serum albumin in g/dL])
8. 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)
9. 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
10. Be willing to provide access to prior medical records to determine eligibility including imaging, biochemical, and diagnostic, medical, and surgical history data
11. Provide written informed consent after the nature of the study has been explained, and prior to any research-related procedures
12. Be willing and able to complete all aspects of the study, adhere to the study visit schedule and comply with the assessments (in the opinion of the Investigator)

Exclusion Criteria:

1. Use of the following drugs within 14 days prior to screening: pharmacologic vitamin D metabolites or analogs, or drugs for treating TIO including oral phosphate, aluminum hydroxide antacids, acetazolamide, or thiazide diuretics
2. Medication to suppress parathyroid hormone (PTH) (e.g., cinacalcet hydrochloride) within 60 days prior to screening
3. Blood or blood product transfusion within 60 days prior to screening
4. History of malignancy within 5 years of study entry with the exception of PMT-MCT (phosphaturic mesenchymal tumors of the mixed connective tissue type)
5. 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
6. Predisposition to infection, or history of recurrent infection or known immunodeficiency
7. Pregnant or breastfeeding at screening or intention to become pregnant during the study; for male subjects, the partner's intention to become pregnant during the study
8. Use of an investigational product (IP) or device within 4 months prior to screening, or planning to receive other IP before completing all assessments in this study.
9. Use of KRN23, or any other therapeutic mAb within 90 days before signing the informed consent form.
10. History of allergic or anaphylactic reactions to KRN23, any of the KRN23 ingredients, or any other monoclonal antibodies
11. Anyone otherwise considered unsuitable participation in the study by the investigator or subinvestigator