A Study of Fitusiran (ALN-AT3SC) in Severe Hemophilia A and B Patients Without Inhibitors: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Study of Fitusiran (ALN-AT3SC) in Severe Hemophilia A and B Patients Without Inhibitors (NCT03417245), a randomized phase 3 study. The primary outcome is Estimated Annualized Bleeding Rate (ABR) for Treated Bleeds During the Efficacy Period.

2. Clinical Question

3. Trial Design

NCT03417245 is a completed phase 3, randomized, parallel-group study of fitusiran in severe hemophilia A or B without inhibitors. The registry lists 120 enrolled participants. The primary outcome analysis includes 40 participants in the Factor On-demand arm and 79 in the Fitusiran 80 mg Prophylaxis arm; the registry describes the primary analysis population as the intent-to-treat population, with the analyzed number defined as participants evaluable for that outcome.

The trial was open-label, with no masking. In the fitusiran arm, participants received 80 mg subcutaneously once monthly as prophylaxis from Day 1 for up to a total of 9 months. They could also receive on-demand factor concentrates for breakthrough bleeding. In the comparator arm, participants received factor concentrates as needed for episodic bleeding, rather than on a regular regimen intended to prevent spontaneous bleeding. Investigator discretion applied to on-demand treatment.

The primary efficacy period began on Day 29 and continued to Day 246 or the last day of bleeding follow-up, whichever came first. The registry says this period was selected to begin when fitusiran's antithrombin-lowering capacity had reached the therapeutic target range. The outcome's maximum efficacy-period duration was from Day 29 to Day 246. The treatment-period outcomes instead began on Day 1 and ended by Day 246 or the last day of follow-up, whichever came first.

The study start date was 2018-03-01, primary completion was 2021-01-26, and status is completed. Original design details beyond the registered design features, including the trial's sample-size justification, are not reported in the registry record.

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: 120 participants were randomized in 2:1 ratio to fitusiran prophylaxis and on-demand arms by interactive response system; stratified by the number of bleeding episodes in the 6 months prior to Screening (less than or equal to \[\<=10\] versus greater than \[\>\]10) and by hemophilia type (hemophilia A or B).

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Analysis was performed on ITT population. Here, overall number of participants analyzed = participants evaluable for this outcome measure.

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: Analysis was performed on intent-to-treat (ITT) population which included all randomized participants analyzed according to the randomized arm.

6. Primary Endpoint

The registry lists the primary outcome as Estimated Annualized Bleeding Rate (ABR) for Treated Bleeds During the Efficacy Period, measured over From Day 29 up to Day 246 or up to the last day of bleeding follow up (any day up to Day 246), whichever was the earliest.

The primary endpoint is the estimated annualized bleeding rate for treated bleeding episodes during the efficacy period. It is expressed as episodes per participant per year. Annualizing the count puts participants with differing follow-up durations on a common time-rate scale; as a general statistical concept, a rate describes events in relation to time at risk or exposure, rather than simply counting events without regard to observation time.

A treated bleeding episode was defined as a hemorrhage requiring administration of a bypassing agent or factor. An episode began at the first sign of bleeding and ended no more than 72 hours after the last treatment for that bleed. Symptoms at the same location or injections no more than 72 hours apart were treated as part of the same episode. The definition therefore counts qualifying treated episodes, not every symptom or injection as a separate event.

The efficacy period ran from Day 29 through Day 246 or the last day of bleeding follow-up, whichever was earliest. The registry describes the endpoint as estimated by applying a negative binomial regression model to data collected during that period. Such a model estimates an event rate while accounting for the count nature of the endpoint. This type of endpoint is useful for comparing recurrent bleeding events, for which a participant can experience more than one event. It does not, by itself, convey how events were distributed among participants; arm-level rates and their confidence intervals help show the scale and uncertainty of the estimates.

7. Results Posted in the Registry

• Fitusiran 80 mg Prophylaxis: 3.13 versus 30.99 (episodes per participant per year) on control; rate ratio 0.10 (95% CI 0.06–0.17, derived from the posted confidence limits); p ≈ <0.001.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Estimated Annualized Bleeding Rate (ABR) for Treated Bleeds During the Treatment Period

• Endpoint type: Event rate: events per unit of follow-up, compared as a rate ratio.
• Test: Poisson rate ratio, with the standard error taken from the posted confidence limits.
• Result:
  • Fitusiran 80 mg Prophylaxis: 4.09 versus 31.44 (episodes per participant per year) on control; rate ratio 0.13 (95% CI 0.09–0.20, derived from the posted confidence limits); p ≈ <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference (a ratio of 1), so the event rate is statistically lower for Fitusiran 80 mg Prophylaxis than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

Observed Annualized Bleeding Rate (ABR) for Treated Bleeds During the Treatment Period

• Endpoint type: Event rate: events per unit of follow-up, compared as a rate ratio.
• Test: Poisson rate ratio, based on the estimated number of events and follow-up time.
• Result:
  • Fitusiran 80 mg Prophylaxis: 1.80 versus 25.20 (episodes per participant per year) on control; rate ratio 0.07 (95% CI 0.00–2.05, approximate, Poisson model); p ≈ 0.123.
• Interpretation:
  • The 95% confidence interval includes no difference (a ratio of 1), so these data do not show a statistically significant difference between Fitusiran 80 mg Prophylaxis and the control. The interval gives the range of effects that remain compatible with the data.

Estimated Annualized Spontaneous Bleeding Rate for Treated Bleeds During the Efficacy Period

• Endpoint type: Event rate: events per unit of follow-up, compared as a rate ratio.
• Test: Poisson rate ratio, with the standard error taken from the posted confidence limits.
• Result:
  • Fitusiran 80 mg Prophylaxis: 1.82 versus 22.04 (episodes per participant per year) on control; rate ratio 0.08 (95% CI 0.05–0.15, derived from the posted confidence limits); p ≈ <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference (a ratio of 1), so the event rate is statistically lower for Fitusiran 80 mg Prophylaxis than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

The secondary outcomes extend the primary question in several ways: some use the full treatment period rather than the efficacy period, and others focus on spontaneous or joint bleeding. The registry also lists health-related quality-of-life outcomes at Month 9. Among the secondary results with posted estimates, the estimated treated-bleeding rates and the estimated spontaneous-bleeding rate show the same broad direction of lower rates in the fitusiran arm as the primary result. They therefore provide related, but not independent, context for the primary comparison.

Observed treatment-period bleeding is summarized differently from the estimated rates: it is reported as a median and inter-quartile range. That descriptive outcome does not have a posted sponsor analysis in the record. A different summary measure answers a different statistical question, so it should not be read as interchangeable with the negative binomial estimated rate. The registry lists joint bleeding and quality-of-life outcomes, but their numerical results are not reported in the registry record.

Secondary outcomes generally carry less inferential weight than the designated primary endpoint when no multiplicity procedure is reported. Testing many outcomes increases the chance that at least one result may appear statistically notable by chance. This is general statistical context; whether a particular adjustment was planned or applied cannot be determined because multiplicity handling is not reported in the registry record. The reported secondary p-values are sponsor analyses for selected endpoints, but their status relative to any prespecified hierarchy is not reported.

The secondary outcomes should therefore be viewed as supporting context, not as a collection of equally definitive tests. Their differing periods, definitions, and summaries matter, and the registry does not post results for every outcome it lists. They do not change the interpretation of the primary endpoint, nor do they establish effects on unreported outcomes.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Estimated Annualized Bleeding Rate (ABR) for Treated Bleeds During the Treatment Period
• Observed Annualized Bleeding Rate (ABR) for Treated Bleeds During the Treatment Period
• Estimated Annualized Spontaneous Bleeding Rate for Treated Bleeds During the Efficacy Period
• Observed Annualized Spontaneous Bleeding Rate for Treated Bleeds During the Efficacy Period
• Estimated Annualized Joint Bleeding Rate for Treated Bleeds During the Efficacy Period
• Observed Annualized Joint Bleeding Rate for Treated Bleeds During the Efficacy Period
• Health-related Quality of Life (HRQOL): Change From Baseline in Haemophilia Quality of Life Questionnaire for Adults (Haem-A-QOL) Physical Health Score at Month 9
• Health-related Quality of Life (HRQOL): Change From Baseline in Haem-A-QOL Total Score at Month 9

9. Statistical Interpretation

10. Statistical Methods Explained

The primary endpoint counts treated bleeding episodes over a defined period and expresses the result as an annualized rate. Because this is a count outcome, the analysis must address both the number of events and the time over which events could occur. General statistical explanation: an event rate relates a count to exposure time, which allows rate comparisons when follow-up time is not identical for every participant. Annualization presents that rate on a common one-year scale; it does not mean every participant was followed for a year.

What does the negative binomial model do?

The registry says the estimated primary results were obtained using negative binomial regression. In general, negative binomial regression is a model for count outcomes that can accommodate over-dispersion: greater variation in counts than a simple Poisson model assumes. This matters for recurrent bleeding counts because participants may have different event experiences. The model can estimate rates and a relative rate comparison while accounting for exposure time. The registry does not report all model settings, such as any covariates, offset specification, or checks of model fit. Those details are therefore not reported in the registry record.

How should the rate ratio be read?

The registry reports a rate ratio of 0.101 for fitusiran relative to on-demand factor concentrates, with a 95% confidence interval from 0.064 to 0.159. A ratio of 1 would represent equal estimated rates; a ratio below 1 represents a lower estimated rate in the numerator arm than in the reference arm. This is a general interpretation of rate ratios. The ratio is a relative comparison, while the arm-specific annualized rates communicate the estimated event burden on the stated rate scale. The two types of result complement each other.

The registry also reports p < 0.001 for the sponsor's negative binomial analysis. A p-value evaluates how compatible the observed data are with a specified null model, assuming the analysis conditions hold. It is not the probability that the null hypothesis is true, nor a measure of the size or importance of an effect. The registry classifies this comparison as superiority. The detailed statistical testing plan and the rationale for its specific choices are not reported in the registry record.

What role do confidence intervals play?

A confidence interval gives a range of estimates compatible with the observed data under the statistical procedure and its assumptions. In repeated use of a method, a 95% confidence interval procedure is designed to cover the target parameter in 95% of repetitions. For an individual result, it should not be read as a probability statement that the true value lies within that particular interval. The interval around the rate ratio is especially relevant to the comparison, while the intervals around arm rates show uncertainty in each arm's estimate.

Interval width depends on data quantity and variability as well as the modeling approach. General statistical context: broader intervals indicate less precision, and narrower intervals indicate greater precision under the method used. These statements do not establish that model assumptions are correct. The registry supplies the interval estimates but does not report a detailed assessment of their robustness.

Why separate efficacy and treatment periods?

The primary efficacy period began on Day 29 and ended at Day 246 or the last day of bleeding follow-up, whichever was earliest. The treatment-period secondary outcome began on Day 1 and used the same stated end rule. The registry explains that the efficacy period began when antithrombin-lowering capacity was expected to have reached the therapeutic target range. Thus, the primary endpoint focuses on a predefined period after treatment initiation, while the treatment-period rate includes the time from Day 1. These definitions should remain attached to the estimates when interpreting or comparing them.

Different outcomes also use different summaries. Estimated annualized rates are modeled rate estimates with confidence intervals. The observed treatment-period bleeding outcome is reported as a median with an inter-quartile range. A median describes the middle of the observed distribution, while an inter-quartile range describes its central spread. General statistical explanation: a median can be less affected by unusually high values than a mean, but it does not estimate the same quantity as a model-based mean rate. The observed median and estimated rate should not be treated as interchangeable measures.

The registry names the ITT population for the primary outcome and says the analyzed count equals the participants evaluable for the outcome. The exact handling of missing observations, exclusions, or incomplete follow-up is not reported in the registry record. Nor is a detailed account of how outcome data were collected or adjudicated reported there. Those gaps limit independent evaluation of the analysis population and the assumptions underlying the estimates.

11. Confidence Intervals and Precision

The sponsor's primary rate ratio is 0.101, with a 95% confidence interval from 0.064 to 0.159. This interval quantifies uncertainty in the relative rate estimate under the sponsor's negative binomial regression analysis. It lies below 1 throughout, which is consistent with a lower estimated treated-bleeding rate in the fitusiran arm in that model. The registry also reports p < 0.001 for the comparison.

The individual arm estimates are 30.99 episodes per participant per year for Factor On-demand, with a 95% confidence interval from 21.11 to 45.49, and 3.133 for Fitusiran 80 mg Prophylaxis, with a 95% confidence interval from 2.269 to 4.326. These intervals describe uncertainty around each arm's estimated rate. They are not substitutes for the interval around the relative comparison, which directly addresses the rate ratio.

Confidence intervals describe statistical precision, not the range of individual participants' outcomes. A general statistical interpretation is that the procedure used to form a 95% confidence interval would capture the target parameter in 95% of repeated applications under its assumptions. It is not correct to interpret the particular interval as assigning a 95% probability to the true value. Precision also does not address possible bias, unmeasured outcomes, or whether a model is correctly specified.

The registry's reported interval for the primary ratio is from the sponsor's regression analysis. The separate simple recalculation uses a different approach, so any distinction between its interval and the sponsor interval should not be treated as a replacement for the sponsor's result. The sponsor analysis is the appropriate basis for describing the registry's formal conclusion.

12. 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 planning figures are hypothetical design illustrations based on the observed effect, not the trial's original design calculations. The original sample-size justification and assumptions are not reported in the registry record. These planning illustrations use 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 the stated fraction of the observed effect. The figures are participants per arm, not total participants or total events.

For each stated effect scenario, the 80% power column gives the planned number of participants per arm under those assumptions for an 80% chance of rejecting the null hypothesis if that scenario's effect is the true effect. The 90% power column gives the corresponding number per arm for a 90% chance. These are prospective planning concepts; they do not describe the chance that the completed trial detected an effect after results were known.

The scenarios include planning for 100%, 75%, and 50% of the observed effect. A smaller target effect generally requires more participants to distinguish it from no difference with the same statistical design assumptions. The stated counts vary by outcome and scenario, so each figure should be read alongside its associated endpoint and effect assumption. For the primary endpoint, the listed counts are 7 and 9 participants per arm when planning for 100% of the observed effect; 8 and 10 for 75%; and 11 and 14 for 50%, for the 80% and 90% power targets respectively.

This is not observed power. It is a set of planning illustrations, not evidence about the probability that the completed trial's result is correct or reproducible. The sample-size display should be read as a sensitivity exercise showing how a hypothetical design responds to different target effects under the stated assumptions, not as a retrospective judgment about the trial's success.

13. 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, λ₀ and λ₁ are event rates per patient-year, and T is follow-up per participant in years. This assumes Poisson counts and equal follow-up.

Worked calculation for this trial

1. Control rate λ₀ = 30.9910; treatment rate λ₁ = 3.1330 events per patient-year; follow-up T = 0.08 years.
2. n = (1.96 + 0.842)² × (1/30.9910 + 1/3.1330) / (0.08 × [ln(0.101)]²) = 6.6, rounded up to 7 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
lambda0 <- 30.991                      # control rate, events per patient-year
lambda1 <- 3.133                      # treatment rate
tt      <- 0.0793976728268309                       # follow-up per participant, years
n_per_arm <- function(power, frac = 1) {
  l1 <- lambda0 * (lambda1 / lambda0)^frac   # smaller effect = fraction of the observed log rate ratio
  ceiling(round((z_a + qnorm(power))^2 * (1 / lambda0 + 1 / l1) / (tt * log(l1 / lambda0)^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 = ",")))

# Simulation 1: Poisson counts. Simulation 2: over-dispersed counts (variance = 1.5 x mean), analysed with the matching variance.
set.seed(3417245)
n    <- n_per_arm(0.80)
reps <- 20000
m0 <- lambda0 * n * tt; m1 <- lambda1 * n * tt
x0 <- rpois(reps, m0); x1 <- rpois(reps, m1)
z  <- log(x1 / x0) / sqrt(1 / x0 + 1 / x1)
cat(sprintf("Simulated power, Poisson counts, n = %s per arm: %.3f\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE)))
phi <- 1.5
y0 <- rnbinom(reps, mu = m0, size = m0 / (phi - 1)); y1 <- rnbinom(reps, mu = m1, size = m1 / (phi - 1))
z2 <- log(y1 / y0) / sqrt(phi * (1 / y0 + 1 / y1))
cat(sprintf("Simulated power, over-dispersed counts (phi = 1.5), same n: %.3f\n", mean(abs(z2) > z_a, na.rm = TRUE)))

Output from running the code

100% of observed effect: n per arm = 7 (80% power), 9 (90% power)
 75% of observed effect: n per arm = 8 (80% power), 10 (90% power)
 50% of observed effect: n per arm = 11 (80% power), 14 (90% power)
Simulated power, Poisson counts, n = 7 per arm: 0.964
Simulated power, over-dispersed counts (phi = 1.5), same n: 0.760

The first simulation draws Poisson counts and should land close to the target power. The second allows extra variability (variance 1.5 times the mean, a purely illustrative value), which lowers the power at the same sample size, so rate trials usually need more participants than the Poisson formula suggests.

14. Analyses Reported by the Sponsor

The registry's own primary analysis uses negative binomial regression and reports an ABR ratio of 0.101, a 95% confidence interval from 0.064 to 0.159, and p < 0.001. This is the sponsor's model-based result and is the basis for the registry's superiority classification.

A separate simple recalculation is derived from posted summary statistics rather than from the full participant-level analysis. It gives a similar rounded rate-ratio magnitude, but its interval and unadjusted p-value are not identical to the sponsor's. The recalculation is not an alternative sponsor analysis and does not reproduce the full regression model. Differences can arise because a summary-statistic calculation does not use the same modeling procedure and information as the sponsor's analysis.

The sponsor's own analysis takes precedence for interpreting the formal result. The recalculation can serve as a check on the general direction and scale of the comparison, but it should not replace the reported model-based estimate, its confidence interval, or its p-value. The registry record does not provide enough detail to independently reconstruct all features of the sponsor's model.

15. Multiplicity, Missing Data and Analysis Populations

The registry identifies a primary endpoint and lists multiple secondary outcomes, including bleeding rates and health-related quality-of-life measures. It reports sponsor analyses for the primary estimated bleeding rate, the estimated treatment-period bleeding rate, and the estimated spontaneous bleeding rate. Multiplicity handling, such as whether tests were ordered in a hierarchy or adjusted to account for multiple comparisons, is not reported in the registry record.

This omission matters for interpretation. As a general statistical principle, examining multiple outcomes or comparisons can raise the probability of finding at least one apparently notable result even when there is no true effect. A prespecified testing strategy or adjustment can address that issue, but the record does not state whether one was used. Accordingly, the primary result is distinct from secondary findings, and the secondary findings should not automatically be treated as confirmatory tests with the same evidential status.

The primary population is described as ITT, with the number analyzed defined as participants evaluable for the outcome measure. The primary analysis counts are 40 for Factor On-demand and 79 for Fitusiran 80 mg Prophylaxis. How participants became evaluable, how missing outcome data were handled, and whether any imputation or sensitivity analyses were used are not reported in the registry record. The method for accounting for incomplete follow-up is also not described beyond the endpoint's specified follow-up window and annualized-rate approach.

The record does not report interim analyses, stopping rules, a detailed statistical analysis plan, or the model's full specification. It also does not provide enough detail to evaluate whether the analysis population and endpoint definitions were finalized before outcome data were examined. These are limits on what can be inferred from the registry entry; they do not establish that such procedures were absent, only that they are not reported there.

16. 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 5% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: cholelithiasis (2.5%), epidural haemorrhage (2.5%), humerus fracture (2.5%), subdural haemorrhage (2.5%), tibia fracture (2.5%).

For serious adverse events, the registry reports 5 affected participants among 40 at risk in the Factor On-demand arm and 5 among 79 at risk in the Fitusiran 80 mg Prophylaxis arm. The corresponding figures for participants with other adverse events are 8 among 40 and 45 among 79. The record reports 0 deaths in each arm. These counts describe participants with events, not the total number of event episodes.

The listed serious-event terms include Cholelithiasis, Epidural Haemorrhage, Humerus Fracture, Subdural Haemorrhage, Tibia Fracture, and Diplopia, each reported as 2.5%. The summary does not provide enough information to infer the timing, duration, severity beyond the serious-event classification, or causal relationship of each event to treatment. The listed terms should not be treated as a complete narrative of each participant's experience.

These figures are descriptive and do not establish a comparative safety effect. The groups differ in size, and interpretation of safety requires context about exposure duration, event definitions, and follow-up. The registry record does not report a statistical comparison of these adverse-event counts or detailed attribution information. Counts alone cannot determine whether an event was caused by a study intervention.

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

The primary result means that, in the registry's negative binomial regression analysis of treated bleeding episodes during the defined efficacy period, the estimated rate in the fitusiran arm was lower relative to the on-demand factor arm. The sponsor reported a rate ratio of 0.101 with a 95% confidence interval from 0.064 to 0.159 and p < 0.001. The statistical result is specific to the enrolled trial population, endpoint definition, follow-up window, and analysis model.

It does not mean that every participant experienced a lower rate, that bleeding was eliminated, or that the group estimate predicts an individual's future experience. A rate ratio is a group-level relative measure. Arm-level rates provide the absolute rate scale, while their confidence intervals express uncertainty around the estimates. Neither form of summary conveys the full distribution of participants' outcomes.

The result also does not answer every clinical or statistical question about fitusiran. The primary endpoint concerns treated bleeding episodes, not every possible bleeding event, quality-of-life outcome, or long-term outcome. The registry lists additional outcomes, but results for joint bleeding and health-related quality of life are not reported in the registry record. The posted efficacy period begins on Day 29, so the primary estimate does not cover the entire treatment period from Day 1.

Statistical significance is not the same as clinical importance. The reported p-value addresses compatibility with a statistical null under the model; it does not measure the size of practical benefit or establish an individual's expected outcome. Likewise, a confidence interval expresses statistical precision under its assumptions and does not rule out bias or limitations in measurement and analysis. The open-label design, incompletely reported analytic details, and missing multiplicity and missing-data information are relevant when judging the scope of the findings.

18. Why This Trial Matters Statistically

This trial illustrates several important statistical features of clinical research on recurrent events. The endpoint is not simply whether a participant had any bleeding; it is an annualized rate of treated episodes. That choice uses information about repeated events and observation time, and the rate ratio provides a direct relative comparison between randomized arms.

It also demonstrates why the endpoint's time window matters. The primary efficacy period begins on Day 29, while a secondary treatment-period outcome begins on Day 1. These definitions produce answers to related but different questions. Keeping the endpoint definition attached to its estimate avoids conflating the post-Day-29 efficacy period with the complete treatment period.

The reported use of negative binomial regression highlights a common modeling issue with count data: event counts can vary more than a Poisson model assumes. In general, allowing for over-dispersion can be important for recurrent events. At the same time, model details and diagnostics matter for evaluating the assumptions behind an estimate, and those details are not reported in the registry record.

Finally, the distinction between sponsor analysis and a simple recalculation is instructive. Summary statistics can support a transparent cross-check, but they cannot reproduce a full regression analysis. The sponsor's own analysis is the relevant formal result. Secondary outcomes, safety counts, and planning illustrations answer different questions and should not be collapsed into one conclusion. Together, these features show why statistical interpretation depends on outcome definition, analysis method, uncertainty, and the completeness of the record.

19. 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.
• Rate comparisons use a Poisson approximation unless the sponsor posted confidence limits; real event counts are often over-dispersed.

The study was open-label, so participants and investigators were not masked to the assigned group. The registry does not describe procedures to reduce or assess potential effects of that design on treatment decisions, reporting, or outcome assessment. The comparator regimen also involved on-demand factor concentrates per investigator discretion, while the fitusiran arm included prophylaxis and on-demand factor treatment for breakthrough bleeding. These treatment approaches are described, but the record does not provide further detail about variation in use across participants.

The primary analysis used an ITT population, but the registry defines the analyzed number as participants evaluable for the outcome. It does not explain how evaluability was determined, how missing observations were handled, or what sensitivity analyses were performed. The sponsor's regression method is named, but covariates, model diagnostics, and other implementation details are not reported.

The efficacy period starts on Day 29, so the primary endpoint does not summarize the full time from treatment initiation. The endpoint also counts treated bleeds under the registry's specific definition; it is not a count of all bleeding symptoms or events regardless of treatment. Results from the primary endpoint should be understood within those boundaries.

The record lists secondary outcomes for joint bleeding and health-related quality of life, but their numerical results are not reported in the registry record. Multiplicity procedures, interim analyses, and the original sample-size rationale are also not reported. The registry's safety summary gives participant counts but limited detail on timing, attribution, and event narratives. These reporting gaps restrict assessment of robustness and completeness without demonstrating that unreported methods or analyses were not conducted.

20. Related Tutorials

21. Related Statistical Calculators

22. Sources

• ClinicalTrials.gov: A Study of Fitusiran (ALN-AT3SC) in Severe Hemophilia A and B Patients Without Inhibitors — NCT03417245.
• Registry record last updated 2022-03-28; status last verified 2022-03.
• Results first posted 2022-02-04.
• 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.

23. Record Summary

NCT03417245 is a completed randomized, open-label, parallel-group phase 3 study in people with severe hemophilia A or B without inhibitors. It compared once-monthly subcutaneous fitusiran 80 mg prophylaxis, with on-demand factor treatment permitted for breakthrough bleeding, against on-demand factor concentrates.

The primary outcome was the estimated annualized rate of treated bleeding episodes during the efficacy period, which began on Day 29. The sponsor's negative binomial regression analysis reported a rate ratio of 0.101, a 95% confidence interval from 0.064 to 0.159, and p < 0.001. Secondary results offer related context, while several listed outcomes have no numerical results reported in the registry record.

The study began on 2018-03-01, reached primary completion on 2021-01-26, and is listed as completed. Important details about multiplicity, missing-data handling, model specification, and the original sample-size rationale are not reported in the registry record.