Substance P Challenge in Healthy Participants: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Substance P Challenge in Healthy Participants (NCT04676763), a non-randomized na study. The primary outcome is Part 1: Wheal Response-time Area Under the Curve (AUC) Over the 2 Hours Post-challenge Period Following Skin Challenge With Substance P- at Challenge Visit 1 (Day 1).

2. Clinical Question

The registry describes an enabling study intended to characterize wheal and flare responses over time after skin challenges with ascending Substance P concentrations. Part 1 was intended to help characterize those responses. Part 2 was intended to investigate their variability, with two challenge visits. Participants could enroll in Part 1 or Part 2, not both.

The primary endpoint applies to Part 1 and concerns the wheal response, not the flare response. Secondary outcomes include maximum wheal area, time to maximum wheal area, time to complete wheal-area disappearance and flare response-time AUC. The listed Part 2 outcomes include measurements across its two challenge visits. These objectives describe what was to be measured; the registry record does not report the observed endpoint values or the results of statistical analyses.

3. Trial Design

This was an open-label, single-centre, non-randomized, sequential study of Substance P skin challenges in healthy participants. The registry record lists enrollment of 32 and describes two parts. Participants could take part in Part 1 or Part 2, but not both. The study status is completed; the listed start date is 2021-03-02 and the primary completion date is 2021-07-21.

In both parts, eligible participants received saline and histamine control challenges, followed by challenges with ascending concentrations of Substance P. Part 1 included a single challenge visit. Part 2 included two challenge visits, at Challenge Visit 1 on Day 1 and Challenge Visit 2 at Week 2. The registry record does not report the number enrolled in each part or the numbers contributing data to particular outcomes.

The primary endpoint is a Part 1 measure at Challenge Visit 1 on Day 1. Its timeframe specifies measurements at 0, 5, 10, 15, 20, 40, 60, 90 and 120 minutes post-challenge. Part 2 is relevant to secondary outcomes, including measurements intended to describe responses across visits.

Because allocation was non-randomized and the registry identifies the arms as not being a like-for-like control comparison, observed values across study parts should not be interpreted as results from a randomized treatment comparison. The registry record does not report a protocol-level statistical plan, a sample-size justification, or details of interim analyses. A rationale for the chosen enrollment is 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: A total of 32 participants were enrolled (Enrolled population: All participants in the screened population who entered the study) in Part 1 (12 participants) and Part 2 (20 participants) of the study.

Part 1 (Up to Day 1)

Reasons for not completing this period:

Part 2 (Up to 3 Weeks)

Analysis population for the primary outcome, as defined in the registry: Safety Population consisted of all participants in the enrolled analysis set who received at least one challenge-related study procedure on their (first) challenge day. 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: Baseline characteristics were presented for Enrolled population which consisted of all participants in the screened population who entered the study.

6. Primary Endpoint

The registry lists the primary outcome as Part 1: Wheal Response-time Area Under the Curve (AUC) Over the 2 Hours Post-challenge Period Following Skin Challenge With Substance P- at Challenge Visit 1 (Day 1), measured over 0, 5, 10, 15, 20, 40, 60, 90 and 120 minutes post-challenge on Day 1 (Visit 1).

The primary endpoint is the Part 1 wheal response-time area under the curve (AUC) over the 2 hours after the Substance P skin challenge at Challenge Visit 1 on Day 1. The unit is millimeter square*minutes. The record describes calculating the AUC using the trapezoidal rule from wheal-response measurements taken at the specified post-challenge time points.

AUC combines measurements over time into a summary of the response’s magnitude and duration. As a general statistical interpretation, a larger AUC can reflect a larger response, a response that persists longer, or both; the summary alone does not separate those features. The listed measurement times provide the observations used to approximate the area between measurements. This is useful when the scientific question concerns the response across a period rather than a single measurement at one time point.

The endpoint is described as a mean with standard deviation. The analysis population is defined as participants in the enrolled analysis set who received at least one challenge-related study procedure on their first challenge day. Only participants with data available at the specified data points were analyzed. The registry record does not report the observed mean, standard deviation, or number analyzed for this endpoint, so its magnitude and precision cannot be assessed from the recorded results.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does the AUC represent?

The primary endpoint summarizes the wheal response across time after challenge. It is an area-under-the-curve measure rather than a single-time-point measurement. The registry record states that the trapezoidal rule was used. In general, this rule approximates the area under a curve by treating the measurements at neighboring time points as endpoints of straight-line segments and combining the areas between them. It is a practical way to summarize repeated measurements when observations are available at discrete times.

The AUC has units that combine response area and time: the registry lists millimeter square*minutes. It can reflect both the response’s size and how long it is present. As a general limitation of this summary, different response patterns can yield similar AUC values. AUC alone does not show the full time course, identify the peak, or indicate when the response disappeared. The registry therefore also lists secondary outcomes for maximum wheal area, time to maximum, time to complete disappearance and flare response-time AUC.

How should a mean and standard deviation be read?

The primary parameter is the mean, with standard deviation as the measure of dispersion. In general, a mean is the arithmetic average of the observations included in an analysis. A standard deviation describes the spread of those observations around their mean. It is not the same as a confidence interval: the standard deviation describes variation among observations, whereas a confidence interval expresses uncertainty about an estimated quantity, such as a population mean. No confidence interval is reported for the primary endpoint in the registry record.

The population definition matters when interpreting either statistic. The record defines the Safety Population as participants in the enrolled analysis set who received at least one challenge-related study procedure on their first challenge day. It further specifies that only those with data available at the stated data points were analyzed. Thus, the endpoint’s analyzed observations need not include every enrolled participant. The number with usable data for this endpoint is not reported in the registry record.

What does non-randomized, sequential allocation imply?

In general, random allocation is used to make treatment groups comparable in expectation with respect to both measured and unmeasured characteristics. This study is listed as non-randomized and sequential. Its parts have different visit structures, and participants enrolled in one part or the other. The registry specifically cautions that the arms are not a like-for-like control comparison. Accordingly, a contrast between parts cannot automatically be interpreted as the effect of a treatment or procedure. Differences could reflect other differences between groups or study procedures; the record does not report analyses addressing such possibilities.

Open-label means the study is listed with no masking. The record does not report a blinded assessment procedure. In general, knowledge of the challenge can affect how subjective measurements are assessed or recorded, although the registry information does not establish whether or how that occurred in this study. This is a design consideration, not a claim about the direction or size of any measurement bias.

Why can over-dispersion matter?

For count outcomes, some models assume that variability follows a specified relationship to the mean. In general, over-dispersion means the observed variability is greater than that assumption allows. If it is not accounted for, uncertainty estimates can be too small and tests can be poorly calibrated. The primary endpoint is continuous and summarized as a mean and standard deviation, not a count outcome. The registry record does not report an over-dispersion analysis or a model for the primary endpoint, so no such method can be attributed to this study.

What can planning assumptions establish?

The planning assumptions listed in the record describe a two-sided significance level of 0.05, equal allocation between two arms and a normal approximation. The effect to detect is expressed as a stated fraction of the observed effect. In general, sample-size calculations use assumptions about the effect and variability to illustrate what study size might be needed under a specified design. Such planning outputs are not observed results and do not show that a completed study had a particular probability of detecting an effect. They are never observed power.

The record notes that time-to-event planning quantities refer to total events, while other planning quantities refer to participants per arm. These are planning conventions, not observed event counts or enrollment results. A non-randomized study with arms that are not a like-for-like control comparison does not provide a sound basis for interpreting a two-arm planning calculation as if it were a randomized comparison. The registry does not report the observed effect estimate or the study’s own sample-size justification, so those planning assumptions cannot establish the precision or adequacy of this study’s results.

10. Multiplicity, Missing Data and Analysis Populations

The registry lists 46 outcome measures and zero analyses. The primary endpoint is one of several listed measurements; secondary outcomes cover maximum wheal area, timing of response features, disappearance of wheal area, flare AUC and assessments across visits. With multiple outcomes, there can be multiple opportunities to identify an apparently unusual result. As a general statistical principle, multiplicity procedures can help control error rates when a family of hypotheses is tested. Whether such procedures were used is not reported in the registry record.

No results, p-values, confidence intervals or analysis estimates for the primary or secondary outcomes are reported in the registry record. A statistical analysis plan specifying which outcomes were confirmatory, how comparisons were selected, or how multiplicity was handled is not reported in the registry record. The outcome list alone does not establish which measurements were analyzed or how findings were interpreted.

The endpoint population definition includes participants with data available at the specified data points. This makes data availability relevant to the analyzed population. The record does not report how much data were missing, why data were unavailable, whether missing observations were imputed, or whether sensitivity analyses were conducted. Those details are not reported in the registry record.

The registry also does not provide the endpoint-specific analysis counts needed to judge how many participants contributed to each estimate. Enrollment of 32 should therefore not be read as the denominator for every outcome. Nor does the record establish that every enrolled participant completed all measurements. Without outcome-specific counts and missing-data methods, the effect of incomplete observations on any estimated response cannot be assessed.

11. 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 non-serious adverse event terms

The registry record lists serious and other adverse-event counts separately for Part 1 and Part 2 skin challenges. For Part 1, 0 serious adverse events were reported among 9 participants at risk, and 1 other adverse event was reported among 9 at risk. For Part 2, 0 serious adverse events were reported among 20 participants at risk, and 2 other adverse events were reported among 20 at risk. Deaths are listed as 0 in each part.

These figures are counts within the reported study parts, not evidence from a randomized safety comparison. The study was non-randomized, and the registry cautions that the arms are not like-for-like control groups. The counts do not establish that one part had greater or lesser risk than the other, and they do not estimate risk relative to an untreated population.

The record does not list serious adverse-event terms. It also does not provide further information in these figures about event timing, severity details beyond the serious/other classification, or causality assessment. A zero count means no event in that category is listed for that group in the registry record; it does not establish the absence of all possible harms. These safety counts should be read as a descriptive report for the recorded study groups.

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

The record describes an enabling study of intradermal Substance P challenge procedures in healthy participants. Its objectives were to characterize wheal and flare responses over time and to investigate response variability. The primary endpoint was defined as Part 1 wheal response-time AUC during the 2 hours after challenge. This information explains the scientific question and the measurements chosen, but it does not report the observed response values.

Without reported endpoint estimates and their uncertainty, the record cannot establish the average observed AUC, how much responses varied, or how precisely an average was estimated. It also does not report comparative results for Substance P, saline or histamine. The presence of control challenges is not equivalent to a randomized comparison, particularly because allocation was non-randomized and the registry warns that the arms are not like-for-like controls.

The record’s outcome definitions should not be read as findings. Listed secondary outcomes describe measurements that were of interest, including peak wheal area, timing, disappearance, flare response and repeated-visit measures. Their inclusion does not demonstrate a particular response pattern, dose relationship or degree of repeatability. Numerical results for these outcomes are not reported in the registry record.

The safety counts are descriptive: no serious adverse events or deaths are listed in either part, while other adverse events are listed in both. These counts do not prove that challenges are free from risk, and the design does not support a randomized safety comparison.

In short, the registry record provides a study framework and endpoint definitions, not the statistical results needed to determine what response was observed or how uncertain its estimate was. It should not be used to claim that a treatment works or does not work. The conclusions that can be drawn are limited to the design, planned measurements and recorded adverse-event counts.

13. Why This Trial Matters Statistically

This study is statistically relevant because it focuses on how to measure a response over time, rather than only at one point. Wheal response-time AUC combines measurements from a defined post-challenge period into a single summary, while secondary outcomes address features that an AUC cannot isolate, such as maximum area and time to disappearance. Together, these endpoints illustrate how a repeated-measure response can be described through complementary summaries.

The two-part structure also distinguishes characterization from variability assessment. Part 1 included a single challenge visit, while Part 2 included two visits. Repeated visits can be useful for examining whether measurements vary across occasions, but the registry’s objective is not itself evidence that repeatability was demonstrated. Numerical results and analysis details would be needed to assess that question.

The design highlights why comparability matters. Saline and histamine were included as control challenges, but the study was non-randomized and the registry cautions against treating its arms as like-for-like controls. Statistical interpretation depends not only on the endpoint but also on how participants were allocated and what comparisons were actually analyzed.

Finally, the distinction between a clearly defined endpoint and a reported result is important. The registry gives a detailed endpoint definition, population description and measurement schedule, yet lists zero analyses. Without outcome estimates and uncertainty measures, the study’s statistical findings cannot be evaluated. This makes the record useful for understanding the measurement question while placing firm limits on claims about what the study observed.

14. 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 main limitation for interpretation is that the registry record does not report the primary endpoint estimate, its standard deviation, or an endpoint-specific analysis count. It also lists zero analyses. The response magnitude and precision therefore cannot be assessed from the recorded results.

The non-randomized, sequential design is another constraint. The registry explicitly states that the arms are not a like-for-like control comparison. Saline and histamine control challenges were part of the procedures, but the record does not report a randomized contrast or comparative estimates. Part 1 and Part 2 also differ in visit structure, so comparisons between them cannot be treated as if they arose from the same randomized design.

The population for each outcome may be smaller than the overall enrollment because the endpoint definition includes only participants with data at the specified time points. The number contributing to each outcome, the amount and reasons for missing data, and the handling of missing observations are not reported in the registry record. It is therefore not possible to assess how missingness might affect an estimate.

The study is open-label, with no masking listed. The record does not describe whether endpoint assessment involved any safeguards against knowledge of the challenge. It also does not report the statistical analysis plan, multiplicity handling, sample-size justification, or interim analyses. These omissions limit assessment of how outcome selection, uncertainty and multiple measurements were handled. The observed endpoint results and the analyses needed to interpret them are not reported in the registry record.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Substance P Challenge in Healthy Participants — NCT04676763.
• Registry record last updated 2025-01-20; status last verified 2025-01.
• Results first posted 2024-02-08.
• 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.

18. Record Summary

This completed, open-label, single-centre, non-randomized, sequential study enrolled 32 healthy participants. Participants could enter Part 1 or Part 2, not both. Each part included saline and histamine control challenges followed by ascending Substance P concentrations. Part 1 had a single challenge visit; Part 2 had two.

The primary endpoint was Part 1 wheal response-time AUC over the 2 hours after the challenge at Challenge Visit 1 on Day 1. It was calculated using the trapezoidal rule and summarized as a mean with standard deviation. The registry lists 46 outcome measures and zero analyses. Numerical primary and secondary outcome results are not reported in the registry record.

No serious adverse events or deaths are listed for either part; other adverse events are listed in both. The non-randomized design and absence of reported outcome analyses limit conclusions about response magnitude, precision and comparisons.