Garetosmab
An antibody that blocks the signal turning soft tissue to bone in an ultra-rare disease, approved in 2026 after a phase 2 that missed its primary endpoint
In fibrodysplasia ossificans progressiva a mutated receptor is switched on by activin A, a protein that should switch it off, and muscle and ligament turn into bone. Garetosmab blocks activin A. Its route to approval was not smooth: the phase 2 missed its primary endpoint and five patients died in its open-label period.
What is it?
Fibrodysplasia ossificans progressiva is caused by a gain-of-function mutation in the activin receptor type 1A. The mutation inverts a signal: activin A, which normally quietens that receptor, instead activates it, and bone forms inside ligaments, tendons and muscle. Garetosmab is an antibody that takes activin A out of circulation, which is why blocking a protein the body makes on purpose is the treatment.
The phase 2 trial, LUMINA-1, is an unusual record. Forty-four adults took 10 mg/kg or placebo every four weeks for 28 weeks. The primary efficacy endpoint, total lesion activity on PET-CT, was not met. But no new bone lesions formed on the drug, so the second period's endpoint was prospectively changed to new lesion count, and no placebo patient who crossed over developed a new lesion, against 40.9% in the first period.
Its safety record is the part to read carefully. Every patient had at least one adverse event in period 1 — nosebleeds, loss of eyelashes and eyebrows, skin abscesses. Five of 44 patients died during the open-label period, 11.4%, and while the deaths were considered unlikely to be related, the trial report says causality cannot be ruled out. Drug levels gave no clue: patients who had nosebleeds or died had the same exposure as those who did not.
The phase 3 trial, OPTIMA, settled the question it was designed for. Sixty-three adults were randomised to placebo, 3 mg/kg or 10 mg/kg monthly for 56 weeks. New bone lesions on whole-body CT totalled 19 on placebo, against 1 on the lower dose and 2 on the higher, reductions of 94% and 90%. Clinician-assessed flare-ups fell sharply only on 10 mg/kg (9 against 66 on placebo), and nobody died.
Pasatru was approved in August 2026 to reduce new bone lesions and flare-ups in adults. The label starts at 10 mg/kg by infusion every four weeks, with 3 mg/kg if that is not tolerated, and lists abscesses, acne, increased hair growth, eyelash loss, mouth ulcers and nosebleeds as the common reactions. It must not be used in pregnancy: in monkeys it caused fetal harm. It did not arrive alone: FDA approved zilurgisertib (Atebrioz), a small-molecule drug for the same disease, on September 25, 2026, for patients aged 12 and over.
Mechanism of Action
Garetosmab is a human IgG4 antibody that binds activin A and stops it activating the mutant ACVR1 receptor that drives bone formation in FOP (label).
Activin A neutralised
Binds activin A specifically and blocks its signalling (label).
Mutant receptor kept quiet
In FOP, mutant ACVR1 (ALK2) is switched on by activin A; removing the ligand reduces new heterotopic bone.
Antibody kinetics
Steady state is reached by about 12 weeks; after the last 10 mg/kg dose it takes a median of about 24 weeks to become undetectable (label).
Published Research
These are the trials that describe garetosmab, read from their PubMed abstracts: first-in-human, the phase 2 and its flare-up analysis, and the phase 3.
LUMINA-1, the phase 2
44 adults took 10 mg/kg or placebo every four weeks for 28 weeks, then open-label. The primary endpoint, total lesion activity on PET-CT, was not met. No new bone lesions formed on the drug, and no crossover patient developed one, against 40.9% in period 1. Five of 44 died in the open-label period.
OPTIMA, the phase 3
63 adults at 18 sites in 16 countries were randomised to monthly placebo, 3 mg/kg or 10 mg/kg for 56 weeks. New bone lesions totalled 19 on placebo against 1 on 3 mg/kg and 2 on 10 mg/kg (rate ratios 0.06 and 0.10); adverse events of special interest occurred in 17% on 10 mg/kg, and no one died.
What flare-ups mean
In a post hoc analysis, 71% of placebo patients had flare-ups and 59% of those developed a new bone lesion somewhere; 24% of flare-ups matched the location of a new lesion. The analysis ties the painful episodes patients report to the bone that forms afterwards.
First in humans
40 healthy women of non-childbearing potential received a single dose of 0.3, 1, 3 or 10 mg/kg intravenously, 300 mg subcutaneously, or placebo. There were no dose-limiting toxicities, and the antibody showed non-linear pharmacokinetics with target-mediated elimination.
LUMINA-1 clinical pharmacology
Steady state was reached 12 to 16 weeks after the first dose (mean trough 105 mg/L); 1 of 43 patients formed low-titre antibodies, and drug levels showed no link to efficacy, nosebleeds or deaths.
Dosing Data
Pasatru is infused intravenously over 60 minutes every four weeks at 10 mg/kg, or 3 mg/kg if 10 mg/kg is not tolerated, after a negative pregnancy test.
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Side Effects & Contraindications
Reported Side Effects
From the trials above, where the adverse-event rate was total.
Contraindications & Cautions
Contraindication and warnings from the Pasatru label (DailyMed, effective August 14, 2026).
Legal Status
Garetosmab is approved in the US as Pasatru.
Frequently Asked Questions
?What is garetosmab?
A fully human monoclonal antibody that blocks activin A, approved as Pasatru for fibrodysplasia ossificans progressiva.
?Why block activin A?
The mutation that causes the disease inverts the receptor's response: activin A, which should quieten it, switches it on instead and bone forms in soft tissue.
?Did the phase 2 succeed?
Not on its own terms. The primary endpoint, lesion activity on PET-CT, was missed. New lesions stopped forming, and the second period's endpoint was changed to that.
?What about the deaths?
Five of 44 patients died during the open-label period, 11.4%. The investigators considered them unlikely to be related to garetosmab but could not rule causality out.
?Can it be used in pregnancy?
No. The label contraindicates it in pregnancy because it caused fetal harm and infant deaths in monkeys; women who could become pregnant need a pregnancy test before starting and contraception during treatment and for 6 months afterwards.
?Does it reduce flare-ups?
Clinician-assessed flare-ups did fall on the 10 mg/kg dose: 9 through week 56, against 66 on placebo. On 3 mg/kg there were 53, not clearly different from placebo, and the share of patients reporting flare-ups themselves did not differ between groups.
?What are the common side effects?
In the phase 3 trial, reactions more frequent than on placebo included abscesses (35% on 10 mg/kg), acne, increased hair growth, loss of eyelashes or eyebrows, mouth ulcers and nosebleeds. One nosebleed needed hospital treatment.
?How long does it stay in the body?
After the last 10 mg/kg dose, the label estimates a median of about 24 weeks before it can no longer be detected, and about 17 weeks after 3 mg/kg.
References
- [pubmed] Di Rocco M, et al. "Garetosmab in fibrodysplasia ossificans progressiva: a randomized, double-blind, placebo-controlled phase 2 trial." Nat Med, 2023;29(10):2615-2624. PMID: 37770652.
- [pubmed] Papa R, et al. "Efficacy and safety of garetosmab, an activin A-blocking antibody, in fibrodysplasia ossificans progressiva (OPTIMA): a randomised, double-blind, placebo-controlled, phase 3 trial." Lancet, 2026. PMID: 42805227.
- [pubmed] Keen R, et al. "Characterization of flare-ups and impact of garetosmab in adults with fibrodysplasia ossificans progressiva: a post hoc analysis of the randomized, double-blind, placebo-controlled LUMINA-1 trial." J Bone Miner Res, 2024;39(10):1486-1492. PMID: 39216107.
- [pubmed] Vanhoutte F, et al. "Pharmacokinetics and Pharmacodynamics of Garetosmab (Anti-Activin A): Results From a First-in-Human Phase 1 Study." J Clin Pharmacol, 2020;60(11):1424-1431. PMID: 32557665.
- [other] US FDA, Drugs@FDA (openFDA): PASATRU (garetosmab-grts) injection, Regeneron, BLA 761508, approved August 19, 2026, prescription. Read September 30, 2026.
- [other] US FDA, Novel Drug Approvals for 2026: Pasatru (garetosmab-grts), August 19, 2026; Atebrioz (zilurgisertib), September 25, 2026, for fibrodysplasia ossificans progressiva in patients 12 and older. Content current as of September 28, 2026; read September 30, 2026.
- [fda-pi] Pasatru (garetosmab-grts) injection Prescribing Information, sections 1 and 2 (DailyMed version 1, effective August 14, 2026; read September 30, 2026).
- [pubmed] Wang Y, et al. "Garetosmab in Fibrodysplasia Ossificans Progressiva: Clinical Pharmacology Results from the Phase 2 LUMINA-1 Trial." J Clin Pharmacol, 2024;64(2):264-274. PMID: 37694449.
Sources & Citations
Studies were read from their PubMed abstracts; the Pasatru label (DailyMed version 1, effective August 14, 2026) was read October 6, 2026.
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