A Study to Assess Safety, Tolerability and Efficacy of Garetosmab Versus Placebo Administered Intravenously (IV) in Adult Participants With Fibrodysplasia Ossificans Progressiva (FOP)

Key Inclusion Criteria:

1. Clinical diagnosis of Fibrodysplasia Ossificans Progressiva (FOP) [(based onfindings of congenital malformation of the great toes, episodic soft tissueswelling, and/or progressive Heterotopic Ossification (HO)]

2. Confirmation of FOP diagnosis with documentation of Type I activin A receptor (ACVR1) FOP causing mutation

3. FOP disease activity within 1 year of screening visit. FOP disease activity isdefined as pain, swelling, stiffness, or other signs and symptoms associated withFOP flare-ups; or worsening of joint function, or radiographic progression of HOlesions (increase in size or number of HO lesions) with/without being associatedwith flare-up episodes

4. Willing and able to undergo CT imaging procedures and other procedures as defined inthe protocol

Key Exclusion Criteria:

1. Cumulative Analog Joint Involvement Scale (CAJIS) score at screening >19

2. Participant has significant concomitant illness or history of significant illnesssuch as but not limited to cardiac, renal, rheumatologic, neurologic, psychiatric,endocrine, metabolic, or lymphatic disease, that in the opinion of the studyinvestigator might confound the results of the study or pose additional risk to thepatient by their participation in the study

3. Previous history or diagnosis of cancer

4. Severely impaired renal function defined as estimated glomerular filtration rate <30milliliter per minute (mL/min) (/1.73 m^2 calculated by the Modification of Diet inRenal Disease equation

5. Uncontrolled diabetes defined as hemoglobin A1C (HbA1c) >9% at screening

6. History of poorly controlled hypertension, as defined by:

7. Systolic blood pressure ≥180 mm Hg or diastolic blood pressure ≥110 mm Hg atthe screening visit

8. Systolic blood pressure of 160 mm Hg to 179 mm Hg or diastolic blood pressureof 100 mm Hg to 10^9 mm Hg at the screening visit, AND a history of end-organdamage (including history of left-ventricular hypertrophy, heart failure,angina, myocardial infarction, stroke, transient ischemic attack, peripheralarterial disease, end-stage renal disease, and moderate-to-advanced retinopathy

9. Known history of cerebral vascular malformation

10. Cardiovascular conditions such as New York Heart Association class III or IV heartfailure, cardiomyopathy, intermittent claudication, myocardial infarction, or acutecoronary syndrome within 6 months prior to screening; symptomatic ventricularcardiac arrhythmia

11. History of severe respiratory compromise requiring oxygen, respiratory support (eg,bilevel positive airway pressure [biPAP] or continuous positive airway pressure [CPAP]), or a history of aspiration pneumonia requiring hospitalization

12. Prior use in the past year and concomitant use of bisphosphonates

13. Concurrent participation in another interventional clinical study or anon-interventional study with radiographic measures or invasive procedures (eg,collection of blood or tissue samples)

14. Treatment with another investigational drug, denosumab, imatinib or isotretinoin inthe last 30 days or within 5 half-lives of the investigational drug, whichever islonger

15. Pregnant or breastfeeding women

16. Women of childbearing potential (WOCBP) who are unwilling to practice highlyeffective contraception, as defined in the protocol

17. Male patients with WOCBP partners who are not willing to use condoms with WOCBPpartners to prevent potential fetal exposure, as defined in the protocol

Note: Other protocol defined Inclusion/Exclusion Criteria apply