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

Exclusion criteria

• . 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
• . 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
• . Planned or recommended orthopedic surgery including staples, 8-plates or osteotomy, within the clinical trial period
• . Hypocalcemia or hypercalcemia, defined as serum calcium levels outside the age-adjusted normal limits
• . 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
• . Presence of a concurrent disease or condition that would interfere with study participation or affect safety
• . History of recurrent infection or predisposition to infection, or of known immunodeficiency
• . 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