This is a phase 2 randomized, 3 arm (3 active doses of Recifercept), parallel group dose finding study of safety, tolerability, PK and efficacy~The total number of participants is 63 in 2 age straified cohorts of 0-2 years and 6-10 years old.~The study will enroll approximately 54 children with achondroplasia aged 2-10 years (inclusive) who will be enrolled and randomized to receive one of three doses of recifercept~Low Dose~Medium Dose~High Dose~A total of 18 participants will be enrolled per dose 18 per dosesuch that at least 15 participants per dose are evaluable. An interim analysis is planned when at least 15 participants per dose aged ≥2 to <11 years have received 6 months of treatment with recifercept. eDMC will review safety, PK and efficacy data to confirm ongoing positive benefit:risk in participants.~Additionally, an exploratory cohort of approximately 9 children with achondroplasia, ages 0-2 years, will be enrolled later in the study (n=3 per dose).~Enrollment will follow an age and dose-staggered approach (descending age and ascending dose) with review of safety and PK data by the study team before progression to the next enrollment block If certain pre-defined safety signals occur then a meeting of the eDMC will be convened to make a decision on progression of enrollment. The PK data collected in block A will be used in the PopPK model (developed using healthy adult data) to confirm the dosing for younger children (ie, ≥2 to <6 years and 0-<2 years).~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~All participants will receive 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.~PK Cohort:~Multiple changes have been made in the manufacturing process of the drug product (process 2) which will be used in Phase 3. Therefore, an additional PK cohort (at selected sites only) has been added, to evaluate the PK of Phase 2 formulation (process 1c) and Phase 3 formulation (process 2).~PK Cohort:~At selected sites only, an additional PK cohort has been added to evaluate the PK of two recifercept formulations. A total of 12 children with achondroplasia aged 2- <11 years will be enrolled in the PK cohort (6 in each treatment sequence). Each participant will receive 2 treatments (3 mg/kg Phase 2 formulation [process 1c] and 3 mg/kg Phase 3 formulation [process 2]) in a randomized manner. Dose of the cohort could be changed due to emerging safety and efficacy data in the study.~PK samples collected following each dose will be analyzed to evaluate the exposures of two formulations.
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.