Study of the Safety, Pharmacodynamics (PD) and Efficacy of KRN23 in Children From 1 to 4 Years Old With X-linked Hypophosphatemia (XLH)

Inclusion Criteria:

1. Male or female, aged ≥1 year and <5 years

2. Diagnosis of XLH supported by ONE or more of the following

- Confirmed phosphate regulating gene with homology to endopeptidases located on
the X chromosome (PHEX) mutation in the patient or a directly related family
member with appropriate X-linked inheritance

- Serum fibroblast growth factor 23 (FGF23) level > 30 pg/mL by Kainos assay

3. Biochemical findings associated with XLH including:

- Serum phosphorus < 3.0 mg/dL (0.97 mmol/L)

- Serum creatinine within age-adjusted normal range

4. Radiographic evidence of rickets

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

6. Provide written informed consent by a legally authorized representative after the
nature of the study has been explained, and prior to any research-related procedures

7. Must, in the opinion of the investigator, be willing and able to complete all aspects
of the study, adhere to the study visit schedule, and comply with the assessments

Exclusion Criteria:

1. Unwilling to stop treatment with oral phosphate and/or pharmacologic vitamin D
metabolite or analog (e.g. calcitriol, alfacalcidol) during the screening period and
for the duration of the study

2. Presence of nephrocalcinosis on renal ultrasound grade 4 based on the following scale:
0 = Normal, 1 = Faint hyperechogenic rim around the medullary pyramids, 2 = More
intense echogenic rim with echoes faintly filling the entire pyramid, 3 = Uniformly
intense echoes throughout the pyramid, 4 = Stone formation: solitary focus of echoes
at the tip of the pyramid

3. Planned or recommended orthopedic surgery including staples, 8-plates or osteotomy,
within the clinical trial period

4. Hypocalcemia or hypercalcemia, defined as serum calcium levels outside the
age-adjusted normal limits

5. Presence or history of any condition that, in the view of the investigator, places the
subject at high risk of poor treatment compliance or of not completing the study

6. Presence of a concurrent disease or condition that would interfere with study
participation or affect safety

7. History of recurrent infection or predisposition to infection, or of known
immunodeficiency

8. Use of any investigational product or investigational medical device within 30 days
prior to screening, or requirement for any investigational agent prior to completion
of all scheduled study assessments