A Study of Andecaliximab in Participants With Fibrodysplasia Ossificans Progressiva (FOP)

This study is researching an experimental drug called andecaliximab. The study will include pediatric and adult patients with fibrodysplasia ossificans progressiva (FOP). The study will evaluate how safe and effective andecaliximab is in patients with FOP. The study is looking at several research questions, including: * Safety of andecaliximab in participants with FOP * Whether andecaliximab reduces the number of new heterotopic bone lesions (Heterotopic Ossification; HO) * Whether andecaliximab reduces the number or severity of flare-ups * Pharmacokinetics/pharmacodynamics (PK/PD): How much study drug is in your blood at different times and its impact on blood biomarker(s) * Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)

Inclusion Criteria (Part 1a, 1b, and 2): 1. Participant and/or guardian able and willing to give informed consent and/or assent as applicable, and willing to adhere to the visits schedule and study procedures. 2. Clinical diagnosis of FOP including congenital malformation(s) consistent with FOP (e.g., of the great toes), and either episodic soft tissue swelling consistent with an FOP flare-up and/or progressive HO. 4\. CAJIS score of ≤19. 5. Disease activity within 1 year of screening visit. Disease activity is defined as physician confirmed flare-up like symptoms or clinical progression including newly identified HO or worsening joint function. 6\. Able to understand, undergo, and perform all protocol related procedures, including low-dose WBCT-LH scan without sedation. Assistance from a caregiver is allowed. 7\. Agree to provide access to all relevant current and historical medical records (including radiographs or radiographic records) and growth records. Inclusion Criteria (Part 1a only): 1. Male or female ≥ 15 years of age. 2. Serum creatinine ≤ upper limit of normal for age. 3. No open growth plates on bilateral PA hand/wrist or AP knee films at baseline 4. Able to receive IV radiotracer \[both IV access and no history of a reaction to radiotracer\]. 5. No use of bisphosphonates or bone active agent within the past year. 6. At least 1 active HO lesion at baseline per Na18F PET/CT Additional inclusion criteria apply including those listed above for all parts of the study. Inclusion Criteria (Part 1b only): 1. Male or female ≥12 years of age. 2. History of multiple flare-up episodes within the past 6 months (to be reviewed and confirmed as qualifying by the PI together with the Sponsor). Qualifying flare-up episodes include any of the following: * At least 3 qualifying flare-ups in the past 6 months each with continuous symptoms for at least 1 week * Migratory flare-up swellings across the back * Multiple flare-up episodes the sum of which lasted at least 21 days The timing of the individual flare-ups can be overlapping; they may be ongoing at the time of enrollment or resolved. 3. The qualifying flare-ups must involve at least 2 of the following flare-up symptoms: * Pain * Soft tissue swelling * Warmth * Redness * Joint stiffness * Decreased range of motion Additional inclusion criteria apply including those listed above for all parts of the study. Inclusion Criteria (Part 2 only): 1. Initial enrollment age requirement is ≥12 years 2. Enrollment may be extended to participants ≥6 years of age after safety in adult and participants age 12-17 years has been established. 3. Enrollment of participants ≥2 to \10mg/day prednisone equivalent of corticosteroids, eligibility will be discussed with the Sponsor. * NSAIDs of higher than doses recommended by the May 2022 ICCFOP guidelines within 1 week of Study Day 1. If the participant requires chronic use of NSAIDs at doses higher than those recommended by the May 2022 ICCFOP guidelines doses, eligibility will be discussed with the Sponsor. * Current or chronic use of tetracycline drugs (e.g., tetracycline, demeclocycline, doxycycline, or minocycline). 11. Chronic use of any of unproven therapies for FOP. 12. Palovarotene * Within 1 month of Study Day 1 for all participants * Within 2 years of Study Day 1 for female participants \