Creatine/Creatinine Ratio and Myostatin as Biomarkers to Monitor Muscle Function in Duchenne Muscular Dystrophy...

Duchenne Muscular Dystrophy (DMD) is characterized by progressive muscle wasting leading to early loss of motor function. Functional tests monitor disease progression and serve as clinical trial endpoints but are influenced by maturation in younger patients, high intra-patient variability and patient motivation. Clinical milestones, instead, have been used to identify modifiers of disease progression but are rarely used as primary endpoints since clinical trials are usually too short to capture them. Blood biomarkers, that can reflect disease progression and objectively evaluate treatment responses, offer a valuable alternative. In this study, we investigated whether longitudinal observations of biomarkers myostatin and creatine/creatinine ratio (Cr/Crn) are associated with functional tests, such as 6-min walk test, North Star Ambulatory Assessment (NSAA), 10-m walk-run test velocity (10MWT), Performance of Upper Limb (PUL2.0) and disease milestones like loss of ambulation, overhead reach and hand-to-mouth function.

We used longitudinal data from an observational study on 74 DMD patients followed between 2009 and 2022 with annual visits to the LUMC outpatient clinic, linked to 408 serum samples. Associations between log2-biomarkers, functional tests and clinical milestones were assessed using linear mixed models and time-dependent Cox models. A post-hoc sample size calculation was performed to evaluate whether the biomarkers' lower intra-patient variability (NSAA SD: 0.79, 10MWT SD: 0.88, log2-myostatin and log2-Cr/Crn SDs: 0.57) may improve future clinical trials design.

Lower Cr/Crn and higher myostatin levels were associated with better functional performance and a less rapid decline in ambulation, given fixed treatment and BMI. Children with one-unit higher log2-myostatin levels had, on average, 4.73 points higher NSAA and 3.40 points higher PUL2.0 (p-values < 0.001) and were 42% less likely to lose ambulation over the following year. Conversely, children with one-unit lower log2-ratio levels had, on average, 11.40 points higher NSAA and 7.18 points higher PUL2.0 (p-values < 0.001) and were 3.67 times more likely to remain ambulant. We proved that incorporating log2-myostatin and log2-Cr/Crn as endpoints could reduce the required sample size for clinical trials by more than half without compromising statistical power. For instance, to detect a yearly drop of 3 points in the NSAA with 80% power, recruitment requires almost 80 participants in a 1:1 randomized trial, compared to a little more than 50 patients for the respective value of log2-myostatin or log2-Cr/Crn.

These findings support the potential of myostatin and Cr/Crn as monitoring biomarkers to enhance trial design and endpoints in clinical and interventional trials for DMD.