Long-Term Extension Study of KRN23 in Adult Subjects With X-Linked Hypophosphatemia (XLH)

Inclusion Criteria:

1. Have participated in KHK's KRN23-INT-001 or KRN23-INT-002 studies (received at least 2
doses of KRN23)

2. Estimated glomerular filtration rate (eGFR) ≥60 mL/min at Screening.

3. Sexually active subjects must be willing to use an acceptable method of contraception
(e.g., double barrier method) while participating in the study and for 30 days after
receiving the last dose of KRN23.

Exclusion Criteria:

1. Subject experienced a safety-related event in the KRN23-INT-001 or KRN23-INT-002 study
that, in the opinion of the investigator and sponsor, precludes resuming KRN23
treatment.

2. Presence of nephrocalcinosis on renal ultrasound that, in the opinion of the
investigator and sponsor, precludes resuming KRN23 treatment.

3. Pregnant or breastfeeding at Screening or planning to become pregnant (self or
partner) at any time during the study.

4. Participation in an investigational drug or device trial within 30 days of enrollment
(other than KRN23-INT-001 or KRN23-INT-002).

5. Use of a pharmacologic vitamin D metabolite or analog (e.g., calcitriol,
doxercalciferol, and paricalcitol), phosphate, or aluminum hydroxide antacids (e.g.,
Maalox® and Mylanta®) within 21 days prior to Screening or during the study.

6. Use of medication to suppress PTH (e.g., Sensipar®, cinacalcet, calcimimetics) within
2 months prior to Screening.