Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare disorder, characterized by progressive heterotopic ossification (HO) and painful soft-tissue inflammatory flare-ups. This was a post hoc analysis from a phase 2 (NCT03188666) trial in which adults with FOP received intravenous anti-activin A antibody garetosmab 10 mg/kg or placebo every 4 wk over 28 wk (Period 1), followed by a 28-wk open-label treatment and extension (Periods 2 and 3). Here we describe flare-ups, their relationship to new HO lesions, and the impact of garetosmab on flare-ups. Volume of new HO lesions was measured by CT. Patient-reported flare-ups were defined by any 2 of the following: new onset of pain, swelling, joint stiffness, decrease in movement, or perceived presence of HO. Flare-ups were experienced by 71% (17/24) of placebo-treated patients, 59% (10/17) of whom developed a new HO lesion irrespective of flare-up location; 24% of flare-ups location-matched new HO lesions. Twenty-nine new HO lesions occurred in the placebo cohort by week 28, of which 12 (41%) occurred in the same location as new or ongoing flare-ups. A higher volume of newly formed heterotopic bone (week 28) occurred in placebo-treated patients who had experienced a prior flare-up vs those without (median [Q1:Q3] of 16.6 [12.0:31.1] vs 3.2 cm3). Garetosmab was previously shown to decrease patient-reported flare-up frequency in Period 1; here, garetosmab reduced the median (Q1:Q3) duration of patient-reported flares (15.0 [6.0:82.0] vs 48.0 [15.0:1.00] d) and the severity of flare-ups vs placebo. Frequency of corticosteroid use was numerically reduced in those treated with garetosmab (40.0%) vs placebo (58.3%). In this analysis, 71% of placebo-treated adults with FOP experienced flare-ups over 28 wk, which were associated with an increased volume of newly formed heterotopic bone. Garetosmab reduced the severity and duration of flare-ups, with effects sustained during the entire trial.
Keywords: activin A; fibrodysplasia ossificans progressiva; flare-ups; heterotopic ossification; natural history.
Fibrodysplasia ossificans progressiva (FOP) is a very rare genetic disorder caused by mutations in the ACVR1 gene. People with FOP experience growth of new bone in places where bone does not usually develop. Soft tissues (like skeletal muscles) and connective tissues (like tendons and ligaments) are gradually replaced by bone beyond the normal skeleton—a process called heterotopic ossification (HO). People with FOP experience flare-ups, which are painful swellings of the soft tissues. In this clinical study in people with FOP, we looked at the number of flareups, whether flareups were linked to new HO lesions, and the impact of garetosmab (a monoclonal antibody) on flareups. At random, about half the patients received placebo, or inactive drug, with the other half receiving garetosmab, the study drug. Of the patients who received placebo, 71% had flare-ups and 59% percent of those who had flare-ups also had a new HO lesion, which was not always related to the location of the flare-up. We have previously shown that garetosmab reduces the number of flareups patients report. In this study, we show that garetosmab reduces the length and pain severity of flare-ups too. The treatment effects were maintained for the whole study.
© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society for Bone and Mineral Research.