Assessment of rAAVrh.74.MHCK7.micro-dystrophin Gene Therapy Using Magnetic Resonance Imaging in Children With Duchenne Muscular Dystrophy

This case-control study uses magnetic resonance imaging and spectroscopy to evaluate the association between treatment with recombinant adeno-associated virus serotype rh74 (rAAVrh74) and muscle quality in children with Duchenne muscular dystrophy.


Introduction
Microdystrophin gene transfer using recombinant adeno-associated virus serotype rh74 (rAAVrh74) driven by a skeletal and cardiac muscle-specific promoter with enhanced cardiac expression (MHCK7) (SRP-9001) showed promise in an open-label gene transfer study in Duchenne muscular dystrophy (DMD), with robust transgene expression on muscle biopsy (74%-96% of fibers). 1 However, biopsy data reflect a small sample of muscle, whereas muscle quality in large muscle groups can be objectively and noninvasively measured using quantitative magnetic resonance  imaging (qMRI) and spectroscopy (qMRS). qMRI and qMRS are powerful tools for monitoring disease progression and therapeutic response in DMD, and they correlate with and identify future loss of ambulatory function. [2][3][4][5] We hypothesized that qMRI and qMRS data (muscle fat fraction and bulk MRI transverse relaxation time [qT 2 ]) would be reduced in 3 boys treated with SRP-9001 compared with an age-matched natural history cohort treated with standard of care and a control group of individuals without DMD.

Methods
This study was approved by the University of Florida institutional review board, and parents provided written informed consent. This study followed the Strengthening the Reporting of Observational (ImagingDMD) study (NCT01484678). A natural history comparison cohort was drawn retrospectively from the ImagingDMD study. We identified 54 participants who had a total of 110 study visits (between 2011 and 2018) aged between 4.9 and 7.9 years, matching the age of the participants who received SRP-9001. All participants in the natural history cohort were treated with corticosteroids, and none of them were treated with dystrophin restoration therapy. 3 We also retrospectively identified 17 age-matched individuals without DMD for a comparison control cohort. In all participants, qMRI and qMRS was used to measure leg muscle fat fraction and qT 2 , as previously published. 3 Briefly, we acquired multiecho axial gradient echo images and multiecho spin echo images in the calves and thighs and  The fat fraction (A) and qT 2 (B) were lower in participants in the SRP-9001 cohort across multiple leg muscles compared with those in the natural history cohort. Due to motion artifact, no upper leg qT 2 data were available for 1 participant in the SRP-9001 cohort.
Although statistical analysis was not performed, group mean data reflected lower quantitative MR biomarker values in the SRP-9001 cohort. Dots indicate individual data points; center lines, means; whiskers, SDs. shown in Figure 1. Minimal fat infiltration was observed on MR images from the participants who received SRP-9001 compared with the participants from the natural history cohort, whose muscles have dark patterning throughout, particularly in the biceps femoris long head and adductor magnus.

Raw
Similarly, spectra from the VL showed a visible fat peak, reflecting accumulation of intramuscular fat upper and lower leg muscles was greater in the natural history cohort than in the SRP-9001 or control cohorts, as shown in Figure 2B (

Discussion
These MR data indicate marked sparing and minimal fat infiltration in boys with DMD who received SRP-9001 compared with an age-matched natural history cohort. qMRI and qMRS biomarkers are valuable adjuncts to clinical assessments because of their sensitivity to subclinical disease progression and lack of dependence on participant growth, maturation, and motivation. 3 In 3 boys with DMD who received SRP-9001, VL fat fraction, which is associated with loss of ambulation, was within unaffected control ranges up to 24 months after gene therapy. qT 2 , which is influenced by inflammation and fat infiltration, was also lower in the boys who received SRP-9001 than those in the natural history cohort. These data support previously published histological and functional evidence for milder disease involvement following gene replacement in DMD. 1 However, study limitations, including small sample size and lack of MR data before gene therapy, warrant further investigation.
The MR results in this report also demonstrate the value of noninvasive qMRI and qMRS measures in examining the efficacy of novel therapeutics in clinical trials in young boys with DMD.