This page provides an independent statistical analysis and educational interpretation of publicly reported results. ClinicalTrials.gov provides the official trial registry record.
1. Trial at a Glance
TRANSIT-1 was a completed phase 3, single-group study evaluating the tolerability and safety of transitioning patients with pulmonary arterial hypertension from inhaled treprostinil to oral selexipag. The registry reports an enrollment of 34 subjects and one intervention arm.
| Feature | TRANSIT-1 |
|---|---|
| Phase | Phase 3 |
| Condition | Pulmonary Arterial Hypertension |
| Design model | Single-group |
| Allocation | NA |
| Masking | None |
| Primary purpose | Treatment |
| Enrollment | 34 |
| Number of arms | 1 |
| Intervention | Selexipag |
| Status | Completed |
| Lead sponsor | Actelion |
| Sponsor type | Industry |
| Start date | 2015-10-12 |
| Primary completion date | 2016-12-05 |
| ClinicalTrials.gov | NCT02471183 |
2. Clinical Question
The central clinical question was whether patients with pulmonary arterial hypertension could transition from inhaled treprostinil to oral selexipag while maintaining a sustained treatment transition and an acceptable tolerability and safety profile.
Population
Patients with pulmonary arterial hypertension enrolled in the TRANSIT-1 study.
Intervention
Oral selexipag.
Transition
The study evaluated transition from inhaled treprostinil to oral selexipag.
Primary question
Can the treatment transition be sustained while its tolerability and safety are characterized through the specified follow-up periods?
3. Trial Design
Selexipag
- Oral selexipag
- Transition evaluated from inhaled treprostinil
- Sustained treatment transition assessed at Week 16
- Safety followed through the registry-specified periods
Single-group comparison framework
- No randomized comparator arm
- No masking
- One treatment arm
- Analyses are therefore primarily descriptive rather than between-group treatment comparisons
The absence of a randomized comparator is central to the statistical interpretation. A single-group study can estimate quantities such as the proportion of subjects achieving a predefined treatment-transition criterion and the frequency of adverse events, but it cannot by itself estimate a randomized treatment effect relative to another treatment.
4. Endpoints
The registry lists seven primary endpoints. They cover sustained treatment transition, adverse events, premature discontinuation, blood pressure, heart rate, maximal tolerated dose, and time to discontinuation of inhaled treprostinil.
| Endpoint | Time frame | Registry definition / assessment |
|---|---|---|
| Percentage of Subjects With Sustained Treatment Transition | At Week 16 | A sustained treatment transition requires all three criteria: being on study treatment (selexipag) at Week 16; not having study treatment interruption(s) totaling 8 days or more before Week 16; and absence of inhaled treprostinil or any prostanoid treatment after Week 8 up to Week 16. The percentage is calculated with a 95% confidence interval using the Clopper-Pearson method. |
| Percentage of Subjects With Treatment-emergent Adverse Events (AEs) | 26 weeks on average, from the first dose of selexipag up to 30 days after the last dose of selexipag | Percentage of subjects with treatment-emergent AEs, serious and non-serious, regardless of relationship to selexipag. |
| Number of Subjects With Adverse Events Leading to Premature Discontinuation of Selexipag | Up to 22 weeks on average | Number of subjects with adverse events leading to premature discontinuation of selexipag from the first dose through the last dose. |
| Absolute Change From Baseline Over Time in Blood Pressure | Baseline, Week 4, Week 12, Week 16 | Both systolic and diastolic arterial blood pressure were measured in a sitting position after at least 5 minutes of rest. Median change from baseline to prespecified post-baseline visits is calculated. |
| Absolute Change From Baseline Over Time in Heart Rate (HR) | Baseline, Week 4, Week 12, Week 16 | Pulse rate is measured after at least 5 minutes of rest in a sitting position. Median change from baseline to prespecified post-baseline visits is calculated. |
| Maximal Tolerated Dose | At Week 12, in subjects still on selexipag at Week 16 | Individual maximal tolerated dose observed at Week 12 in subjects still on selexipag at Week 16. MTD is defined as the dose of selexipag reached with the last dose change up to Week 12. |
| Time to Discontinuation of Inhaled Treprostinil | Baseline to Week 16 | Median time from baseline (Day 1) to the end of down-titration of inhaled treprostinil is calculated. |
The primary endpoint has a composite operational definition
The sustained treatment-transition endpoint is not simply whether a subject was receiving selexipag at Week 16. It requires three conditions simultaneously. This distinction matters statistically because the endpoint combines treatment status, treatment interruption, and continued absence of inhaled treprostinil or another prostanoid during a specified period.
All three criteria must be satisfied for a subject to count as having a sustained treatment transition.
5. Statistical Methodology
Exact binomial confidence interval
For the sustained treatment-transition endpoint, the registry explicitly specifies the Clopper-Pearson method for the 95% confidence interval. This is an exact binomial interval for a proportion and is particularly useful when the sample size is modest or when the observed proportion is near 0% or 100%.
where x is the number of subjects satisfying the sustained-transition definition and n is the relevant analysis population.
The Clopper-Pearson interval is constructed by inverting exact binomial tests rather than relying on a normal approximation. The resulting interval is therefore an interval estimate for the underlying proportion under the binomial model.
Descriptive analysis of adverse events
The treatment-emergent adverse-event endpoint is defined as a percentage of subjects experiencing an AE during the specified exposure and follow-up period. In a single-group study, the principal statistical quantity is the observed proportion, accompanied where appropriate by a confidence interval to describe uncertainty.
The registry does not identify a comparative hypothesis test for this endpoint. That distinction is important: an AE percentage in a single treatment group describes the observed safety experience but does not establish whether the rate is higher or lower than it would have been under another treatment.
Counts of premature discontinuation
The premature-discontinuation endpoint is defined as the number of subjects with adverse events leading to premature discontinuation of selexipag. A count is naturally summarized descriptively. A corresponding proportion can also be informative when the denominator is clearly defined, but the registry endpoint itself is specified as a number of subjects.
Repeated measurements of blood pressure and heart rate
Blood pressure and heart rate were measured at baseline, Week 4, Week 12, and Week 16, with median changes from baseline specified. Because these are repeated measurements on the same subjects, the observations are correlated within subject.
For an educational analysis, the prespecified median change is therefore best treated as a longitudinal descriptive summary unless a formal repeated-measures model is explicitly specified. A paired analysis can account for within-subject change, while a longitudinal mixed-effects model can account for multiple scheduled measurements simultaneously. Neither approach should be attributed to the registry unless it is explicitly reported there.
Maximal tolerated dose
MTD is an individual dose-level endpoint. The registry defines it as the dose reached with the last dose change up to Week 12 among subjects still receiving selexipag at Week 16. Its natural statistical summary is therefore a distribution of individual dose levels rather than a treatment-effect estimate.
Time to discontinuation of inhaled treprostinil
The registry specifies the median time from baseline (Day 1) to the end of down-titration of inhaled treprostinil. Time-to-event methods such as Kaplan-Meier estimation are commonly used when subjects can be censored before an event occurs. If every subject's event time is observed, the ordinary sample median can instead be used. The registry specifically states that the median time is calculated but does not identify a survival-analysis model.
6. Results and Posted Statistical Analysis Status
The ClinicalTrials.gov record indicates that results have been posted for the listed endpoints. However, the registry does not contain posted formal statistical analyses for these endpoints. The available record therefore supports descriptive reporting of the numerical safety information and the endpoint definitions, but not a formal treatment-effect analysis.
Serious adverse events
Selexipag: 2 affected subjects out of 34 at risk
| Safety measure | Selexipag |
|---|---|
| Serious adverse events, affected / at risk | 2/34 |
The reported 2/34 figure corresponds to approximately 5.9% if expressed as a simple arithmetic proportion, but the registry reports the safety result as 2/34, and that is the primary form used here. No additional adverse-event counts or percentages are identified in the record summarized here.
7. Planned Analysis
The registry-defined statistical structure is primarily descriptive and proportion-based. The main endpoint, sustained treatment transition at Week 16, is a binary subject-level outcome for which the registry specifies a percentage and a 95% Clopper-Pearson confidence interval.
| Endpoint type | What is measured | Typical statistical treatment |
|---|---|---|
| Binary transition endpoint | Whether each subject satisfies all three sustained-transition criteria at Week 16 | Observed proportion with an exact binomial confidence interval; the registry specifies Clopper-Pearson for the 95% CI |
| Adverse-event percentage | Whether each subject experiences a treatment-emergent AE during the specified period | Descriptive proportion, with an appropriate confidence interval when precision is to be quantified |
| Premature discontinuation | Number of subjects whose selexipag treatment is prematurely discontinued because of an AE | Descriptive count and, when useful, proportion using a clearly defined denominator |
| Blood pressure change | Median absolute change from baseline at prespecified visits | Descriptive longitudinal summaries; paired or repeated-measures methods can be used when formal inference is specified |
| Heart rate change | Median absolute change from baseline at prespecified visits | Descriptive longitudinal summaries; paired or repeated-measures methods can be used when formal inference is specified |
| Maximal tolerated dose | Individual dose reached with the last dose change through Week 12 among subjects still on selexipag at Week 16 | Descriptive distribution of individual dose levels |
| Time to discontinuation | Median time from baseline (Day 1) to the end of down-titration of inhaled treprostinil | Median time; Kaplan-Meier methods are typically appropriate when right censoring is present |
The absence of a randomized comparator changes the inferential target. The principal statistical question is how frequently the observed transition and safety outcomes occur within this study population, rather than whether selexipag causes a difference relative to a concurrent control treatment.
8. Statistical Methods Explained
Why is the sustained treatment transition a binary endpoint?
Each subject either satisfies all three criteria for a sustained treatment transition at Week 16 or does not. This produces a binary outcome that can be summarized as the number and percentage of subjects meeting the definition.
Why use the Clopper-Pearson method?
The study enrolled 34 subjects, so a normal-approximation confidence interval can be less reliable for a proportion, particularly near the boundaries of the possible range. The Clopper-Pearson method provides an exact binomial confidence interval and is the method explicitly specified by the registry for the sustained-transition endpoint.
What does a confidence interval for a proportion mean?
A 95% confidence interval quantifies statistical uncertainty around the estimated underlying proportion under the specified binomial framework. It does not mean that 95% of individual patients will fall inside the interval, nor does it describe the range of individual treatment responses.
Why are adverse events primarily descriptive here?
TRANSIT-1 is a single-group study. Without a concurrent randomized comparator, an observed AE frequency cannot by itself establish a relative increase or decrease attributable to selexipag. Descriptive safety summaries are therefore central to the design.
Why does the time frame matter for adverse events?
The treatment-emergent AE endpoint is defined from the first dose of selexipag through 30 days after the last dose, with the registry describing the period as 26 weeks on average. Changing the observation window can change the number of events observed, so the endpoint's time frame is part of its statistical definition.
Why is MTD a conditional endpoint?
The registry defines MTD at Week 12 specifically among subjects still on selexipag at Week 16. The analysis population is therefore conditional on later treatment status. This should be distinguished from an MTD summary calculated among all 34 enrolled subjects.
Why is time to discontinuation different from a simple percentage?
A percentage describes whether an event occurred during a defined period. A time-to-event endpoint additionally considers when the event occurred. If some subjects have not experienced the event by the end of observation, censoring can make Kaplan-Meier estimation more appropriate than calculating a simple arithmetic median from observed event times alone.
9. Analysis Population and Conditioning
The registry identifies an overall enrollment of 34 subjects and specifies several endpoints with endpoint-specific populations. In particular, maximal tolerated dose is assessed at Week 12 in subjects who are still on selexipag at Week 16.
Overall enrollment
34 subjects were enrolled in the single-group study.
Week 16 transition assessment
Sustained treatment transition requires subjects to be on selexipag at Week 16 and to satisfy the two additional transition criteria.
MTD population
MTD is evaluated among subjects still on selexipag at Week 16.
Safety follow-up
Treatment-emergent AEs are evaluated from first selexipag dose through 30 days after the last dose.
This distinction is statistically important because different endpoints can have different effective denominators. A denominator should always be read from the endpoint definition rather than assumed to be the full enrollment of 34 subjects.
10. Longitudinal Measurement Structure
Blood pressure and heart rate were measured at four scheduled time points: baseline, Week 4, Week 12, and Week 16. Both endpoints use absolute change from baseline and specify the median change at the prespecified post-baseline visits.
| Measurement | Baseline | Week 4 | Week 12 | Week 16 |
|---|---|---|---|---|
| Systolic arterial blood pressure | Measured | Measured | Measured | Measured |
| Diastolic arterial blood pressure | Measured | Measured | Measured | Measured |
| Heart rate | Measured | Measured | Measured | Measured |
The protocolized measurement conditions also matter. Blood pressure was measured in a sitting position after at least 5 minutes of rest, and pulse rate was measured after at least 5 minutes of rest in a sitting position. These standardized conditions reduce avoidable measurement variation and make changes across scheduled visits more interpretable.
11. Time-to-Discontinuation Endpoint
The registry defines the endpoint as the median time from baseline (Day 1) to the end of down-titration of inhaled treprostinil, with a baseline-to-Week-16 time frame.
The event is the end of down-titration rather than simply the initiation of oral selexipag.
The distinction between an event definition and its statistical summary is useful here. The endpoint specifies a median time, but a formal survival-analysis framework would require additional information about event occurrence and censoring. When censoring is present, Kaplan-Meier estimation is a standard approach because it uses each subject's observed follow-up rather than treating all incomplete observations as if they had the same event time.
12. Safety Interpretation
The registry reports 2 serious adverse events among 34 subjects in the selexipag arm. Serious adverse events are an important safety measure, but the statistic has to be interpreted within the single-group design.
The reported 2/34 indicates that two subjects were affected by serious adverse events among the 34 subjects at risk in the selexipag arm.
The figure does not establish a causal comparison between selexipag and inhaled treprostinil, because the study has no randomized comparator arm. It also does not describe the severity, timing, or individual clinical course of those events beyond the reported serious-adverse-event count.
Safety proportions are meaningful only when their denominator and observation period are understood. Here the registry reports the serious-adverse-event result explicitly as affected subjects over subjects at risk: 2/34.
13. No Formal Statistical Analyses Posted
The ClinicalTrials.gov record indicates that results are posted, but no formal statistical analyses are listed. Consequently, the registry information supports descriptive interpretation of the posted numerical safety result and the prespecified endpoint definitions rather than a formal hypothesis-test framework with treatment-effect estimates.
This is particularly relevant for TRANSIT-1 because the study is single-group. A p-value comparing selexipag with a concurrent control would require a comparator that is not part of the registered design. The most natural inferential quantities are therefore proportions, confidence intervals, medians, counts, and longitudinal descriptive summaries.
14. Important Limitations and Interpretation Issues
- Single-group design: without a randomized comparator, observed outcomes cannot be interpreted as a randomized treatment effect.
- Small enrollment: 34 subjects were enrolled, so estimates of proportions can have substantial statistical uncertainty even when the observed percentage appears high or low.
- Endpoint-specific populations: the MTD endpoint is restricted to subjects still receiving selexipag at Week 16, so its denominator can differ from overall enrollment.
- Composite transition criterion: sustained treatment transition requires three separate conditions to be met simultaneously.
- Safety observation window: treatment-emergent AEs are defined over a specific exposure and post-treatment period, so safety estimates depend on that observation framework.
- Descriptive versus comparative inference: percentages, counts, and median changes characterize the study population but do not establish superiority or inferiority to another treatment.
- Time-to-event methodology: a median time can be summarized directly when event times are observed, while censoring generally requires survival-analysis methods such as Kaplan-Meier estimation.
- Repeated measurements: blood pressure and heart rate are measured repeatedly within subjects, so observations across visits are correlated rather than independent.
- Posted analyses: the registry contains posted results but no formal posted statistical analyses for the endpoints.
15. Why This Trial Matters Statistically
TRANSIT-1 is a useful statistical teaching case because it illustrates a different analytical structure from the randomized, two-arm trials that dominate clinical-trial methodology. Its central problem is not estimating a between-group hazard ratio or treatment difference. Instead, it is defining and estimating clinically meaningful outcomes within a single treatment-transition cohort.
| Concept | How it appears in TRANSIT-1 |
|---|---|
| Single-group design | One selexipag arm with 34 enrolled subjects. |
| Binary endpoint | Sustained treatment transition at Week 16 is determined from three yes/no criteria. |
| Exact confidence interval | The registry specifies the Clopper-Pearson method for the 95% CI around the sustained-transition percentage. |
| Safety analysis | Treatment-emergent AEs are summarized as a percentage, with serious adverse events reported as 2/34. |
| Conditional analysis population | MTD is evaluated in subjects still on selexipag at Week 16. |
| Repeated measurements | Blood pressure and heart rate are assessed at baseline, Week 4, Week 12, and Week 16. |
| Robust descriptive statistics | Median change is specified for blood pressure and heart rate, rather than a mean change. |
| Time-to-event analysis | Time to the end of down-titration of inhaled treprostinil is summarized by its median. |
| Interpretation without a control | Observed frequencies describe the study population but do not estimate a randomized comparative effect. |
16. Clinical Interpretation vs Statistical Interpretation
Statistical interpretation
The study is structured around estimation of proportions, counts, median changes, dose levels, and a median time-to-discontinuation measure within a single treatment group. The sustained-transition endpoint has an explicitly specified exact 95% confidence interval method.
Clinical interpretation
The study addresses whether a transition from inhaled treprostinil to oral selexipag can be characterized in terms of sustained treatment status, tolerability, safety, dose experience, physiologic measurements, and discontinuation timing.
17. How to Read the Sustained Treatment Transition Endpoint
The sustained-transition endpoint is particularly instructive because its definition contains both an endpoint event and a treatment-history requirement. A subject must be receiving selexipag at Week 16, must not have accumulated a study-treatment interruption of 8 days or more before Week 16, and must have no inhaled treprostinil or other prostanoid treatment after Week 8 through Week 16.
Criterion 1
Subject is on study treatment, selexipag, at Week 16.
Criterion 2
No study treatment interruption(s) totaling 8 days or more before Week 16.
Criterion 3
No inhaled treprostinil or any prostanoid treatment after Week 8 through Week 16.
Endpoint classification
All three criteria must be met for the subject to count as having a sustained treatment transition.
This construction illustrates why endpoint definitions matter as much as the statistical formula. Changing any of the three conditions would change which subjects are classified as successes and therefore change the estimated proportion.
18. Statistical Concepts in This Trial
TRANSIT-1 illustrates several core principles in clinical-trial statistics: exact confidence intervals for proportions, descriptive safety analysis, conditional analysis populations, repeated-measures data, median-based summaries, and time-to-event endpoints.
19. Sources
- ClinicalTrials.gov: TRANSIT-1, NCT02471183.
- PubMed: Publication indexed under PMID 30391194.
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20. Record Summary
TRANSIT-1 is a phase 3, single-group study of 34 subjects evaluating the tolerability and safety of transitioning from inhaled treprostinil to oral selexipag in pulmonary arterial hypertension. Its statistical structure centers on a carefully defined Week 16 sustained-treatment-transition endpoint, for which the registry specifies a percentage and 95% Clopper-Pearson confidence interval, together with treatment-emergent adverse events, adverse-event-related premature discontinuation, longitudinal blood pressure and heart-rate changes, maximal tolerated dose, and time to discontinuation of inhaled treprostinil.
The principal posted safety result is 2/34 serious adverse events in the selexipag arm. Because the study has a single treatment group, the most appropriate interpretation is descriptive rather than comparative. The study is therefore a useful example of how clinical-trial statistics can focus on precise endpoint definitions, exact confidence intervals, conditional analysis populations, longitudinal measurements, and time-to-event summaries even when a randomized treatment comparison is not part of the design.