Efficacy and Safety of Losmapimod in Subjects With Facioscapulohumeral Muscular Dystrophy (FSHD): Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Efficacy and Safety of Losmapimod in Subjects With Facioscapulohumeral Muscular Dystrophy (FSHD) (NCT04003974), a randomized phase 2 study. The primary outcome is Change From Baseline in Double Homeobox 4 (DUX4) Activity in Affected Skeletal Muscle.

2. Clinical Question

3. Trial Design

NCT04003974 is a completed phase 2, randomized, double-blind, placebo-controlled, parallel-group trial sponsored by Fulcrum Therapeutics. The registry lists an enrollment of 80 participants. Its stated purpose was to evaluate the safety and efficacy of losmapimod in people with facioscapulohumeral muscular dystrophy over 48 weeks.

Participants were assigned to losmapimod or placebo. The treatment arm received losmapimod 15 mg twice daily by mouth, and the control arm received placebo twice daily. Both arm descriptions specify treatment for 48 weeks. The registry identifies masking as quadruple, but does not report which specific roles were masked.

The primary outcome was change from baseline in DUX4 activity in affected skeletal muscle. Muscle biopsies were collected at baseline and after baseline; the outcome timeframe is listed as baseline and Week 16 to Week 36. The primary analysis included the Full Analysis Set, restricted to participants with data available at the specified data points. The numbers analyzed for the primary comparison were 39 in the losmapimod arm and 38 in the placebo arm.

The registry lists 19 outcome measures and 11 secondary outcomes with posted results. Secondary outcomes include adverse-event summaries and changes in muscle fat fraction, lean muscle volume, and muscle fat infiltration. The registry record does not report the original sample-size justification or the trial’s original statistical assumptions. The sample-size illustrations associated with the posted comparison are planning exercises based on its observed effect, not evidence about the original design.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Before assignment: This study was a randomized, double-blind placebo-controlled treatment period for 48 weeks which evaluated the efficacy and safety of losmapimod.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Full Analysis Set. Only those participants with data available at the specified data points were analyzed.

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: Full Analysis Set: included all participants who were randomly assigned to receive double-blind study drug in the placebo-controlled treatment period.

6. Primary Endpoint

The registry lists the primary outcome as Change From Baseline in Double Homeobox 4 (DUX4) Activity in Affected Skeletal Muscle, measured over Baseline and Week 16 to Week 36.

The primary endpoint measures change in DUX4 activity in affected skeletal muscle. The registry describes muscle biopsies collected at baseline and post-baseline. A validated quantitative RT-PCR assay measured expression of six genes known to be regulated by DUX4. Each gene’s raw cycle-threshold result was normalized to reference genes; the six normalized values were then averaged to form the activity measure.

Change was defined as the post-baseline average delta Ct across the six genes minus the baseline average delta Ct. The reported unit is delta threshold cycle, or Ct. This is a laboratory-based measure of gene expression in muscle tissue, rather than a direct measurement of muscle strength, function, or symptoms.

General statistical context: a biological measure can be selected as an endpoint because it captures a process relevant to the study question and can be measured consistently. The registry record describes the assay and the genes used, but does not report a clinically meaningful threshold for interpreting the size of a change. It also does not specify a direction of change that should be interpreted as beneficial. The estimated group difference therefore needs to be read as a contrast on the assay’s scale, not as a direct statement about clinical benefit.

7. Results Posted in the Registry

• Losmapimod 15 Milligrams (mg) Twice Daily (BID): mean 0.83 versus 0.40 on control; difference 0.43 (95% CI -1.32 to 2.18), standardized effect d = 0.11; z-test p = 0.632.

8. Statistical Interpretation

9. Statistical Methods Explained

What does a least-squares mean difference represent?

A mean summarizes the average value of a continuous measure in a group. The primary result is reported as a least-squares mean difference, which is an estimate of the difference between groups from the sponsor’s analysis. The comparison is losmapimod relative to placebo. A value above zero indicates that the estimated change on the stated scale was higher in the losmapimod group; it does not, by itself, indicate whether that direction is favorable. The registry does not give an interpretation threshold for the scale.

The sponsor’s analysis used ANCOVA, or analysis of covariance. General statistical context: ANCOVA compares groups while accounting for specified covariates in a model. This can produce an adjusted comparison rather than a simple subtraction of the two observed group averages. The registry identifies ANCOVA as the method but does not report the covariates, model specification, or other details needed to reproduce the adjustment.

How should the recalculated comparison be understood?

A separate recalculation uses the posted group summaries to form a simple, unadjusted comparison. The group summaries are least-squares means with standard errors, while the recalculation uses an approximate pooled standard deviation derived from those summaries. It is therefore an approximation, not a replacement for the sponsor’s ANCOVA. The registry marks no discrepancy between the recalculated result and the sponsor’s result. The sponsor’s analysis remains the reported analysis of record.

General statistical context: an unadjusted comparison and a covariate-adjusted analysis can differ because they use different information and model assumptions. A recalculation from summary statistics cannot recover individual participant data or reproduce the sponsor’s model when the model details and covariates are not reported. It is useful as a check on the broad interpretation, not as an alternative claim about the trial.

What does the standard error describe?

The arm-level dispersion is reported as a standard error. A standard error describes the uncertainty in an estimated mean, rather than the spread of individual participant measurements. General statistical context: standard errors tend to be smaller when an estimate is more precise, while the participant-level variability can be larger. Because the registry reports standard errors rather than standard deviations for the group means, translating them into a pooled standard deviation for a simple recalculation requires an approximation.

Over-dispersion is a general concern when the variability in data exceeds what a chosen statistical model expects. In continuous-outcome settings, an incorrect variability model can make uncertainty estimates too small or too large. The registry does not report an over-dispersion assessment for this endpoint, and the available record does not justify a claim that over-dispersion occurred.

What role does the p-value play?

The sponsor reports a p-value of 0.562 for the group comparison. General statistical context: a p-value describes how compatible the observed data are with a specified null model, under the assumptions of the analysis. It is not the probability that the null hypothesis is true, and it does not measure the probability that a treatment is beneficial. Interpretation should consider the estimated difference and confidence interval as well as the p-value. In this case, the reported confidence interval crosses zero, so the data are compatible with differences in either direction.

What does the endpoint analysis population mean?

The primary analysis population is identified as the Full Analysis Set, with only participants who had data available at the specified data points included. General statistical context: analysis populations define which randomized participants contribute to a particular estimate. An available-data approach can yield different analysis counts across outcomes or assessment times. The registry does not describe an imputation method or a detailed missing-data sensitivity analysis for this endpoint.

The method name alone does not answer every question about analysis conduct. Covariates, assumptions about missing observations, any interim analyses, and the original analysis plan are not reported in the registry record. As a result, the posted estimate can be interpreted as reported, but a full independent reconstruction of its statistical model is not possible from the record.

10. Confidence Intervals and Precision

The sponsor’s 95% confidence interval for the primary group difference extends from -1.038 to 1.895. It includes zero, so the data do not rule out no difference under the model used. It also includes values on either side of zero, leaving uncertainty about the direction of the group contrast. The estimated difference is 0.428, but the interval shows why the point estimate should not be treated as a precise or definitive value.

General statistical context: a confidence interval is a range generated by a statistical procedure that, under its assumptions, reflects uncertainty in an estimated quantity. A wider interval indicates less precision than a narrower interval on the same scale. It is not a range guaranteed to contain the true value, nor does it assign probabilities to individual values within its bounds.

The interval is tied to the sponsor’s ANCOVA analysis, while an unadjusted recalculation from summary data has a different interval. The adjusted analysis should guide interpretation because it is the sponsor’s reported method. The registry does not report a clinical threshold for the DUX4 activity measure, so the interval cannot be classified as excluding or including a clinically important difference. Its breadth, together with the crossing of zero, supports a cautious reading of the estimated contrast.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The sample-size illustrations are based on the observed effect and are planning illustrations, not observed power. They assume a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is set to a stated fraction of the observed effect. The figures are participants per arm, not total participants or total events.

For planning for 100% of the observed effect, the illustration gives 1,313 participants per arm for 80% power and 1,757 participants per arm for 90% power. For planning for 75% of the observed effect, it gives 2,334 participants per arm for 80% power and 3,124 participants per arm for 90% power. For planning for 50% of the observed effect, it gives 5,250 participants per arm for 80% power and 7,028 participants per arm for 90% power.

In plain language, the 80% power column describes a planning scenario intended to have an 80% chance of detecting the specified effect if that effect is the true one and the assumptions hold. The 90% column describes the corresponding scenario with a higher target chance of detection. Smaller effects require more participants under these assumptions, and a higher target power also requires more participants. These calculations use the observed effect as their starting point; they do not reveal the original trial’s design assumptions or justify its enrollment. The original sample-size justification is not reported in the registry record.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = 0.428; pooled standard deviation σ = 3.917.
2. n = 2 × (1.96 + 0.842)² × 3.917² / 0.428² = 1,312.4, rounded up to 1,313 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
delta <- 0.4284                      # observed difference in means
sigma <- 3.91703164984235                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(4003974)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 1,313 (80% power), 1,757 (90% power)
 75% of observed effect: n per arm = 2,334 (80% power), 3,124 (90% power)
 50% of observed effect: n per arm = 5,250 (80% power), 7,028 (90% power)
power.t.test: 1313.3 
Simulated power at n = 1,313 per arm: 0.800 (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Analyses Reported by the Sponsor

The sponsor’s posted primary analysis is an ANCOVA comparison, with a least-squares mean difference of 0.428, a 95% confidence interval from -1.038 to 1.895, and a p-value of 0.562. The recalculation is a simple, unadjusted comparison based on posted summary statistics and an approximate pooled standard deviation. It is not the same model as ANCOVA and cannot reproduce covariate adjustment from group summaries alone.

The registry indicates no discrepancy between the recalculation and the sponsor’s result. Both support the same broad reading: the estimated group difference is uncertain, and the interval includes zero. The sponsor’s analysis should be used as the main reported result because it is the trial analysis described in the registry. The recalculation is a check of the summary data, not a competing adjusted result. Details of the ANCOVA covariates and fuller model specification are not reported in the registry record.

14. Multiplicity, Missing Data and Analysis Populations

The registry lists 19 outcome measures and 11 secondary outcomes with posted results. The primary endpoint is a change in DUX4 activity; the secondary outcomes include adverse events and changes in muscle imaging measures at specified follow-up visits. When a study assesses multiple outcomes or time points, the chance of at least one apparently unusual result can increase if each is assessed separately. General statistical context: multiplicity procedures can control this issue by adjusting how results are evaluated, but the appropriate approach depends on the planned testing strategy.

Whether multiplicity was controlled, how any testing hierarchy was defined, and whether secondary analyses were intended to be confirmatory are not reported in the registry record. The record lists one primary analysis, but does not provide enough information to infer a complete prespecified testing plan. The reported p-value should therefore be interpreted as the result for the named primary comparison, without assuming that other outcomes were adjusted or that they were part of a formal hierarchy.

The primary analysis population is the Full Analysis Set, and the registry states that only participants with data available at the specified data points were analyzed. A detailed account of missing observations, reasons for missingness, imputation, or sensitivity analyses is not reported in the registry record. Available-data analyses can be affected if the participants with measured outcomes differ systematically from those without measurements; the record does not establish whether that occurred.

The original statistical analysis plan, interim analyses, and original sample-size justification are not reported in the registry record. These omissions limit assessment of prespecification and missing-data handling, but do not change the figures that are posted.

15. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: postoperative wound infection (2.5%), alcohol poisoning (2.5%), suicide attempt (2.5%).

The serious adverse-event summary lists 2 affected participants among 40 at risk in the losmapimod group and 0 among 40 at risk in the placebo group. Deaths are listed as 0 in each group. The serious-event terms shown are postoperative wound infection, alcohol poisoning, and suicide attempt, each reported as 2.5%. These term-level figures should be read as registry entries; the record does not describe their timing or provide enough detail to determine whether terms occurred in distinct participants.

Other adverse events are listed for 29 of 40 participants in the losmapimod group and 23 of 40 in the placebo group. These are descriptive counts, not adjusted comparisons. The registry also lists adverse-event outcomes by type, severity, relationship, special interest, and premature discontinuation, but those details are not fully presented in the summary figures cited. No conclusion about comparative safety should be based on serious-event counts alone, particularly without further information about event definitions, exposure, and follow-up.

16. What the Result Does, and Does Not, Mean

The primary result is an estimate of the difference between losmapimod and placebo in change in a muscle-biopsy measure of DUX4 activity. The sponsor’s ANCOVA estimate is 0.428, and its 95% confidence interval ranges from -1.038 to 1.895. Because the interval includes zero and the reported p-value is 0.562, the posted primary analysis does not establish a clear difference between groups on this measure.

That finding is not proof that the groups were identical. The confidence interval leaves room for a difference in either direction, and the registry does not define a clinically meaningful threshold for the assay. The estimate should therefore be understood as uncertain rather than as evidence of equivalence or as a precise estimate of effect.

The endpoint is a molecular assay result from affected skeletal muscle. It is not a direct measure of function, symptoms, or quality of life. The registry record’s primary result alone cannot establish whether participants experienced a clinical change, and it does not establish effects on outcomes that are not part of this comparison.

The safety summaries report serious adverse events in the losmapimod group and none in the placebo group, alongside no deaths in either group. These counts are descriptive and do not, by themselves, establish a cause or a comparative risk. The record gives additional adverse-event outcome categories, but the posted summary information does not support a comprehensive benefit-risk judgment.

In sum, the registry reports an uncertain primary comparison on DUX4 activity, with a confidence interval spanning zero. That statistical result should not be extended into claims about clinical benefit, lack of benefit, or outcomes beyond those measured and reported.

17. Why This Trial Matters Statistically

This trial illustrates why a point estimate, a confidence interval, and a p-value answer related but different questions. The estimated difference gives the direction and scale of the comparison; the interval shows substantial uncertainty around it; and the p-value summarizes compatibility with a null model under the sponsor’s analysis. Reading all three together avoids treating a single threshold as a complete interpretation.

It also highlights the distinction between a molecular endpoint and a clinical outcome. DUX4 activity was measured through gene expression in muscle biopsies. Statistical evidence about that assay is not automatically evidence about function or symptoms. Interpreting the endpoint requires both the assay description and a clinically meaningful threshold, which is not reported in the registry record.

The comparison also shows why the analysis method matters. The sponsor reports an ANCOVA least-squares mean difference, whereas a recalculation from summary statistics is unadjusted and approximate. The two are related checks, but they are not interchangeable. A registry result can be read more responsibly when the analysis population, outcome definition, and model are kept distinct.

Finally, the planning illustrations demonstrate that sample-size calculations depend on assumptions about the effect to detect and target power. They are not a retrospective measure of whether the completed trial was adequately powered and do not disclose the original design rationale. The original rationale is not reported in the registry record.

18. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record leaves several trial-specific details unavailable. It does not report the original sample-size justification, the covariates included in the ANCOVA, the complete model specification, interim analyses, or the original statistical analysis plan. These gaps limit independent assessment of prespecification and reproducibility.

The primary analysis included participants with data available at the stated data points. The record does not detail missingness by group or visit, explain reasons for missing biopsies, or describe imputation and sensitivity analyses. The analysis counts are 39 for losmapimod and 38 for placebo, while enrollment is listed as 80; the record does not explain all differences between these counts.

The endpoint is an assay-based measure, and no clinically meaningful threshold or interpretive direction is reported. This restricts translation of the numerical group contrast into clinical terms. The follow-up period for the primary endpoint is recorded as baseline and Week 16 to Week 36, while the overall study duration is 48 weeks; additional detail about the timing of the primary post-baseline measurement is not reported in the registry record.

The registry lists 19 outcome measures but posts 11 secondary outcomes. It does not report whether multiplicity was controlled or how secondary outcomes were prioritized. Safety figures are descriptive and do not include enough detail in the cited summary to evaluate event timing, exposure-adjusted rates, or the relationship of every event to treatment. These limitations call for restraint in interpreting the primary result and the safety counts.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Efficacy and Safety of Losmapimod in Subjects With Facioscapulohumeral Muscular Dystrophy (FSHD) — NCT04003974.
• Registry record last updated 2024-07-10; status last verified 2024-06.
• Results first posted 2024-07-10.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

22. Record Summary

NCT04003974 is a completed randomized, double-blind, placebo-controlled phase 2 parallel-group study of losmapimod for FSHD. The registry lists 80 enrolled participants and a 48-week treatment period. The primary endpoint was change in DUX4 activity measured from skeletal-muscle biopsy gene expression.

The sponsor’s ANCOVA analysis reports a least-squares mean difference of 0.428, a 95% confidence interval from -1.038 to 1.895, and a p-value of 0.562. The interval includes zero, leaving uncertainty about the direction and size of the group difference. The registry also reports descriptive adverse-event counts. The original sample-size rationale, detailed model specification, and missing-data methods are not reported in the registry record.