219P Enhancing clinical trial eligibility criteria in FSHD: validating whole-body MRI as a key outcome measure

The heterogeneous disease progression in FSHD makes assessing treatment response in clinical trials challenging. Quantitative whole-body MRI has been recognized as a promising technique to address these challenges, but there are limited data from its use in natural history studies. The overall aim of the CTRN FSHD France study (NCT03458832) is to validate new outcome measures, define minimal clinically important change, and establish FSHD characteristics useful for determining clinical trial eligibility criteria. Up to 70 ambulatory FSHD1 patients with symptomatic limb weakness aged 18-75 will be included and followed for 24 months. In addition to whole-body MRI, the study will also assess muscle strength and function, as well as several patient-reported outcome measures. 68 patients successfully completed the baseline analysis, including MRI. The median (min, max) age was 50 (21, 75) years, with CSS 6 (1, 9) and 6 (2, 10) D4Z4-repeats. The muscle fat fraction (MFF) was 7% (1, 91), 21% (2, 96), 6% (1, 87), and 31% (2, 100) in the arms, legs, rotator cuffs, and torso, respectively. We have successfully implemented quantitative whole-body MRI in a natural history study of a FSHD cohort with baseline characteristics resembling what can be expected in clinical trials. Initial analysis of disease progression after 12 months is expected to be available this fall.