Interventional Study of Infigratinib in Children < 3 Years Old With Achondroplasia (ACH)

This is a Phase 2, multicenter, randomized, placebo-controlled study to evaluate the safety and efficacy of infigratinib in participants \< 3 years old with ACH. The purposes of the SAD and Phase 2 portions are to identify and confirm the dose of infigratinib to be used in the Phase 2b portion, based on safety and PK. The purpose of the Phase 2b, placebo-controlled portion is to evaluate the safety and efficacy of infigratinib in children \< 3 years old with ACH at the selected dose.

PROPEL Infant \& Toddler (I\&T) is a Phase 2, multicenter, randomized, placebo-controlled study that comprises 4 portions: the single ascending dose (SAD) portion (open-label), the Phase 2 portion (open-label), the Phase 2b portion (placebo-controlled), and an Extension Portion (open-label). The study will evaluate children with ACH \< 3 years old being administered oral infigratinib.

Inclusion Criteria: * Diagnosis of ACH confirmed by genetic testing. If prospective participants had prior genetic testing, the diagnosis must be confirmed by a report from a certified laboratory, documenting the specific mutation. * Age 0 to 32 months (2 years and 8 months) at screening. * Signed informed consent, which must be obtained from each participant's parent(s) or legal guardian. * Parent(s)/Guardian(s) willing and able to attend all study visits and comply with all study requirements. * Parent(s)/Guardian(s) willing and able to comply with the routine care of the study participants according to local guidance for the management of infants and young children with ACH. * Able to swallow age-appropriate oral medication. * In participants \<1 year old, be compliant with recommended vitamin D supplementation of 5 10 μg/day or higher (or as recommended by country specific guidelines). Exclusion Criteria: * Participants who have hypochondroplasia or diagnosis of genetic condition other than ACH, or any clinical condition that can affect growth. * Gestational age at birth \<37 weeks and/or birth weight \<2500 grams. * Gastroesophageal reflux disease requiring prolonged treatment (\>1 week) with prohibited medications. * Evidence of cervicomedullary compression, as defined by an Achondroplasia Foramen Magnum Score (AFMS) 4, symptomatic or asymptomatic, diagnosed during MRI done at screening or a previous MRI done at any time if the participant had not undergone decompression surgery. * History of fracture of a long bone or spine within 6 months prior to screening. * Any other significant concurrent disease or condition that, in the view of the investigator and/or sponsor, would confound assessment of efficacy or safety of infigratinib and/or would require treatment with a prohibited medication (per protocol), and/or would place the participant at high risk for poor treatment compliance or for failure to complete the study. * Having received or planning to receive treatment with any other investigational or approved product for the treatment of ACH or short stature, including (but not limited to) r-hGH, IGF-1, CNP analog, FGF ligand trap, or treatment targeting FGFR inhibition at any time. * Regular long-term (\>3 weeks; more than twice/year) treatment with supraphysiologic doses of glucocorticoid therapy (ie, \>15 mg/m2/day of hydrocortisone or equivalent) or treatment with glucocorticoids at anti-inflammatory doses (for over 3 weeks within 6 months of the screening visit. NOTE: Low-dose topical, inhaled, or intranasal corticosteroids are acceptable. * Significant abnormality in screening laboratory results, * Allergy or hypersensitivity to any components of the study drug.