Approximately 63 participants will be randomized to one of three doses to receive Recifercept either * Low Dose * Medium Dose * High Dose Participants will will attend the clinic at baseline and at Day 1, 4, 8, 15, 29 \& then Month 2, 3 6, 9 \& 12. Assessments include safety, blood sampling, physical examination, vital signs, anthropometric body measurements \& patient/caregiver quality of life questionnaires Participants will received treatment with Recifercept for 12 months. All participants who complete the study and in the opinion of the investigator, continue to have a positive risk:benefit profile, will be offered to enroll into an open-label extension (OLE) study. A PK cohort will include 12 participants who will randomly receive a single dose of 3 mg/kg of Phase 2 study (process 1c) formulation and a single dose of 3 mg/kg of the proposed Phase 3 (process 2) study formulation in a cross over study. Dose of the cohort could be changed due to emerging safety and efficacy data in the study.