M.T. Hooijmans

T2 relaxation times are increased in Skeletal muscle of DMD but not BMD patients

Exon‐skipping drugs in Duchenne muscular dystrophy (DMD) aim to restore truncated dystrophin expression, which is present in the milder Becker muscular dystrophy (BMD). MRI skeletal muscle T2 relaxation times as a representation of edema/inflammation could be quantitative outcome parameters for such trials. Methods: We studied T2 relaxation times, adjusted for muscle fat fraction using Dixon MRI, in lower leg muscles of DMD and BMD patients and healthy controls. Results: T2 relaxation times correlated significantly with fat fractions in patients only (P < 0.001). After adjusting for muscle fat, T2 relaxation times were significantly increased in 6 muscles of DMD patients (P < 0.01), except for the extensor digitorum longus. In BMD, T2 relaxation times were unchanged. Conclusions: T2 relaxation times could be a useful outcome parameter in exon‐skipping trials in DMD but are influenced by fat despite fat suppression. This should be accounted for when using quantitative T2 mapping to investigate edema/inflammation. Muscle Nerve 53: 38–43, 2016

Elevated phosphodiester and T2 levels can be measured in the absence of fat infiltration in Duchenne muscular dystrophy patients

Quantitative MRI and MRS are increasingly important as non‐invasive outcome measures in therapy development for Duchenne muscular dystrophy (DMD). Many studies have focussed on individual measures such as fat fraction and metabolite levels in relation to age and functionality, but much less attention has been given to how these indices relate to each other. Here, we assessed spatially resolved metabolic changes in leg muscles of DMD patients, and classified muscles according to the degree of fat replacement compared with healthy controls. Quantitative MRI (three‐point Dixon and multi‐spin echo without fat suppression and a tri‐exponential fit) and 2D–CSI 31P MRS scans were obtained from 18 DMD patients and 12 healthy controls using a 3 T and a 7 T MR scanner. Metabolite levels, T2 values and fat fraction were individually assessed for five lower leg muscles. In muscles with extensive fat replacement, phosphodiester over adenosine triphosphate (PDE/ATP), inorganic phosphate over phosphocreatine, intracellular tissue pH and T2 were significantly increased compared with healthy controls. In contrast, in muscles without extensive fat replacement, only PDE/ATP and T2 values were significantly elevated. Overall, our results show that PDE levels and T2 values increase prior to the occurrence of fat replacement and remain elevated in later stages of the disease. This suggests that these individual measures could not only function as early markers for muscle damage but also reflect potentially reversible pathology in the more advanced stages.

Non-uniform muscle fat replacement along the proximodistal axis in Duchenne muscular dystrophy

The progressive replacement of muscle tissue by fat in Duchenne muscular dystrophy (DMD) has been studied using quantitative MRI between, but not within, individual muscles. We studied fat replacement along the proximodistal muscle axis using the Dixon technique on a 3T MR scanner in 22 DMD patients and 12 healthy controls. Mean fat fractions per muscle per slice for seven lower and upper leg muscles were compared between and within groups assuming a parabolic distribution. Average fat fraction for a small central slice stack and a large coverage slice stack were compared to the value when the stack was shifted one slice (15 mm) up or down. Higher fat fractions were observed in distal and proximal muscle segments compared to the muscle belly in all muscles of the DMD subjects (p <0.001). A shift of 15 mm resulted in a difference in mean fat fraction which was on average 1-2% ranging up to 12% (p <0.01). The muscle end regions are exposed to higher mechanical strain, which points towards mechanical disruption of the sarcolemma as one of the key factors in the pathophysiology. Overall, this non-uniformity in fat replacement needs to be taken into account to prevent sample bias when applying quantitative MRI as biomarker in clinical trials for DMD.

Improved olefinic fat suppression in skeletal muscle DTI using a magnitude-based dixon method: DTI Dixon Olefinic Fat Suppression

To develop a method of suppressing the multi‐resonance fat signal in diffusion‐weighted imaging of skeletal muscle. This is particularly important when imaging patients with muscular dystrophies, a group of diseases which cause gradual replacement of muscle tissue by fat.

Spatially localized phosphorous metabolism of skeletal muscle in Duchenne muscular dystrophy patients: 24–month follow-up

Objectives To assess the changes in phosphodiester (PDE)-levels, detected by 31P magnetic resonance spectroscopy (MRS), over 24-months to determine the potential of PDE as marker for muscle tissue changes in Duchenne Muscular Dystrophy (DMD) patients. Methods Spatially resolved phosphorous datasets were acquired in the right lower leg of 18 DMD patients (range: 5–15.4 years) and 12 age-matched healthy controls (range: 5–14 years) at three time-points (baseline, 12-months, and 24-months) using a 7T MR-System (Philips Achieva). 3-point Dixon images were acquired at 3T (Philips Ingenia) to determine muscle fat fraction. Analyses were done for six muscles that represent different stages of muscle wasting. Differences between groups and time-points were assessed with non-parametric tests with correction for multiple comparisons. Coefficient of variance (CV) were determined for PDE in four healthy adult volunteers in high and low signal-to-noise ratio (SNR) datasets. Results PDE-levels were significantly higher (two-fold) in DMD patients compared to controls in all analyzed muscles at almost every time point and did not change over the study period. Fat fraction was significantly elevated in all muscles at all time points compared to healthy controls, and increased significantly over time, except in the tibialis posterior muscle. The mean within subject CV for PDE-levels was 4.3% in datasets with high SNR (>10:1) and 5.7% in datasets with low SNR. Discussion and conclusion The stable two-fold increase in PDE-levels found in DMD patients in muscles with different levels of muscle wasting over 2-year time, including DMD patients as young as 5.5 years-old, suggests that PDE-levels may increase very rapidly early in the disease process and remain elevated thereafter. The low CV values in high and low SNR datasets show that PDE-levels can be accurately and reproducibly quantified in all conditions. Our data confirms the great potential of PDE as a marker for muscle tissue changes in DMD patients.

Proton Magnetic Resonance Spectroscopy Indicates Preserved Cerebral Biochemical Composition in Duchenne Muscular Dystrophy Patients

BACKGROUND Duchenne muscular dystrophy (DMD) is caused by the absence of dystrophin. DMD is associated with specific learning and behavioural disabilities. In the brain, dystrophin is associated with GABAA receptors and aquaporin-4 in neurons and astrocytes, respectively, but little is known about its function. OBJECTIVE AND METHODS In this study we aimed to compare the biochemical composition between patients and healthy controls in brain regions that are naturally rich in dystrophin using magnetic resonance spectroscopy. Given previous conflicting results obtained at clinical field strengths, we obtained data using a 7 Tesla system with associated higher signal-to-noise ratio and spectral resolution. RESULTS Results indicated unchanged biochemical composition in all regions investigated, and increased variance in glutamate in the frontal cortex.

G.O.11

Exon skipping in Duchenne muscular dystrophy (DMD) aims to restore dystrophin expression and maintain muscle function. The largest RCT so far targeting exon 51 failed to reach its primary endpoint despite promising results from preceding studies. It underscores the need for biomarkers. Quantitative MRI (qMRI) is promising as it is a non-invasive method which can be repeated easily. qMRI was used in 5 boys from the 48week RCT (DMD114044) and open label extension (OLE) study (DMD114349) assessing exon 51 skipping with weekly subcutaneous dosing at 6mg/kg drisapersen. T1w, 3-point Dixon and quantitative fat suppressed T2 scans were acquired from the left lower leg at 3T at 3 occasions: n =5), around the start of the OLE phase ( n =5) and n =4). Two boys were on placebo during RCT. Outcome measures were % fatty infiltration, contractile cross-sectional area (cCSA), and mean and standard deviation (SD) of T2 relaxation times in five lower leg muscles. Results were compared to three DMD controls from our natural history study. Interval between scans on treatment varied between 322 and 825days. cCSA and fat% did not show a clear pattern. However, in treated boys, a reduction in mean value and SD of the T2 relaxation time was visible especially in the tibialis anterior muscle. In contrast, T2 values of controls increased. The oldest boy with the highest initial fat% showed a large reduction in fat% and increase in cCSA for all lower leg muscles during 10.5months on treatment. Nonetheless he lost ambulation during the extension phase. The observed shift in T2 values opposite to that of controls may point towards less damaged muscle tissue. Subclinical improvement in muscle structure can provide a base for developing more potent drugs or studying prolonged treatment in younger patients as disease progression may lead to a point of no return regarding ambulation. qMRI should be considered as a biomarker in DMD trials.