A Multi-center, Single-arm Trial Exploring the Safety and Clinical Effectiveness of RBX2660 Administered by Colonoscopy to Adults With Recurrent Clostridioides Difficile Infection: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Multi-center, Single-arm Trial Exploring the Safety and Clinical Effectiveness of RBX2660 Administered by Colonoscopy to Adults With Recurrent Clostridioides Difficile Infection (NCT05831189), a na phase 3 study. The primary outcome is Number of Participants With RBX2660-related Treatment-emergent Adverse Events (TEAEs) After RBX2660 Treatment Delivered by Colonoscopy Through 8 Weeks, or Treatment Failure.

2. Clinical Question

The clinical question combines a safety focus with broader exploratory aims. Secondary outcomes include recurrence of CDI within 8 weeks, time to recurrence, treatment-emergent adverse events, serious adverse events, and adverse events of special interest. Other secondary outcomes concern physician perceptions and experience, clinician global impression of improvement, and patient experience interviews. These outcomes address different aspects of treatment and should not be treated as interchangeable measures of benefit or harm.

Because the trial has one group, its central question is descriptive: what outcomes were recorded among adults receiving RBX2660 by colonoscopy? A one-group study can characterize observations in participants who received the intervention, but it cannot by itself establish how those outcomes compare with what would have happened under another treatment or no treatment. The registry record does not report the primary endpoint result or comparative findings.

3. Trial Design

This is a Phase 3, multicenter, single-group trial of RBX2660 delivered by colonoscopy to adults with recurrent Clostridioides difficile infection. The registry record lists an enrollment of 41 and a completed status. The recorded start date is 2023-04-21, and the primary completion date is 2025-01-17. Allocation is listed as not applicable, and masking is listed as none.

A single-group design means that participants receive the intervention under study without a concurrent control arm. The trial can therefore describe outcomes observed after colonoscopic administration, but the design does not separate the intervention’s effect from other influences on outcomes. For example, the observed course could also reflect participant characteristics, the underlying condition, or other care. A randomized comparison would be needed to assess differences between treatment strategies while reducing some sources of bias.

The stated purpose includes exploring safety and clinical effectiveness, physician usability experience, and patient experience. The primary outcome is the number of participants with RBX2660-related treatment-emergent adverse events through 8 weeks after treatment or treatment failure. The secondary outcomes include CDI recurrence and time to recurrence, other adverse-event measures, clinician global impression of improvement, physician questionnaires, and patient interviews. These measures span event counts, time-to-event outcomes, and experience assessments.

The registry record does not report the detailed eligibility criteria, the treatment administration protocol, the definition of treatment failure, or the methods used to collect and analyze questionnaire and interview responses. It also does not report a statistical analysis plan or the rationale for the enrollment target. Those details should not be inferred from the study title or its stated objectives. The registered design supports a descriptive reading of the available event counts, not a direct estimate of comparative treatment effects.

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.

Overall Study

Reasons for not completing this period:

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.

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants With RBX2660-related Treatment-emergent Adverse Events (TEAEs) After RBX2660 Treatment Delivered by Colonoscopy Through 8 Weeks, or Treatment Failure, measured over 8 weeks after RBX2660 treatment delivered by colonoscopy.

The primary endpoint is the number of participants with RBX2660-related treatment-emergent adverse events after RBX2660 treatment delivered by colonoscopy, assessed through 8 weeks or treatment failure. It is expressed as a count of participants, rather than as a continuous measurement or a time-to-event summary. The event is qualified both by being treatment-emergent and by being considered related to RBX2660.

Counting participants with at least one event is a common way to describe how many people experienced a specified safety outcome during a defined observation period. It is clinically interpretable as a participant-level summary, but the count alone does not convey the number of events per participant, the event’s duration or severity, or when it occurred. Those details require additional outcome information.

The endpoint is used because the stated trial purpose includes exploring safety after delivery by colonoscopy. The time window anchors the safety assessment to a period after treatment, while the treatment-failure provision allows the endpoint window to include that stated stopping condition. The registry record does not provide an endpoint result, event definition details, or the method used to judge whether an event was related to treatment. The available serious and other adverse-event counts are not identified as the result for this specific primary endpoint and should not be substituted for it.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a single-group design estimate?

In a single-group study, participants receive the intervention under study, and outcomes are described among those participants. For an event-count endpoint, the basic result would be the number of participants meeting the event definition during the stated period. The registry record describes this trial as single-group, with no masking and no randomized allocation. It does not list a control group.

That design answers a descriptive question about observed outcomes in the enrolled group. It does not directly estimate a contrast between RBX2660 and another treatment. In a comparative trial, the contrast between groups helps distinguish differences associated with treatment assignment from changes that might occur over time or reflect other factors. Without such a group, a count cannot be interpreted as a treatment-attributable increase or decrease. This is a general feature of single-group designs, not a claim about the outcomes of this trial.

How should an event count be read?

The primary endpoint is a count of participants with RBX2660-related treatment-emergent adverse events through 8 weeks or treatment failure. A participant-level count differs from an event-level count: one participant may experience more than one event, while a participant count generally indicates how many people met the endpoint definition. The registry record does not provide the primary count or clarify the operational rules used to determine treatment-emergence and relatedness.

The posted serious and other adverse-event figures are separate descriptive information. Such counts should be interpreted with their stated at-risk denominators and event categories. They do not automatically answer a different endpoint’s question, even when both concern safety. The record’s serious-event terms and their listed percentages provide further description, but do not establish timing, severity details, or causality beyond what the registry labels report.

What can a confidence interval add?

General statistical explanation: a confidence interval presents a range of values compatible with an estimate under a specified statistical method. It helps readers assess precision, not just the direction or size of an estimate. An interval is wider when the data contain less information about the quantity being estimated, and narrower when they contain more information, all else being equal. A confidence interval does not remove bias from an uncontrolled design, and it does not turn an association into a causal effect.

No confidence interval for the primary endpoint is reported in the registry record. No primary estimate is reported either, so statistical precision for that endpoint cannot be evaluated. The registry record also lists no analyses. It is therefore not possible to describe a trial-specific inferential method, adjustment, or uncertainty estimate.

Why can variability matter?

General statistical explanation: for count outcomes, variability can exceed what a simple probability model expects. This is called over-dispersion. When present, a model that assumes less variability than the data actually contain can make uncertainty appear too small. Analysts may use models that account for extra variability, but the appropriate approach depends on the outcome and study design. The registry record does not describe a model for the registered outcomes, and the primary result is not reported; no claim about over-dispersion in this trial is possible.

Different outcome types also call for different analytical approaches. A binary recurrence outcome records whether recurrence occurred during a specified period; a time-to-recurrence outcome uses information about when recurrence occurred and whether observation ended before an event. Questionnaire and interview outcomes require their own definitions and summaries. These are general distinctions. The statistical methods used for the trial’s secondary outcomes are not reported in the registry record.

How should planning illustrations be understood?

General statistical explanation: sample-size calculations are planning tools. Their results depend on assumptions about the effect to detect, variability, significance level, allocation, and analysis method. A planning calculation does not show that a study had a particular probability of reaching a significant result after the data were collected. That retrospective interpretation is often called observed power and is not a substitute for an estimate with an uncertainty interval.

The stated planning illustration uses a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation, with the effect to detect expressed as a fraction of the observed effect. Time-to-event illustrations are described as total event counts, while other illustrations are described as participants per arm. Those assumptions describe hypothetical planning comparisons; the registered trial itself is single-group. The registry record does not report the trial’s own sample-size justification. A planning illustration based on an observed effect should not be mistaken for the design rationale or for an observed treatment effect.

10. Exact Estimates for Each Arm

No two-arm comparison is shown for this trial, but each arm's result can still be summarized with exact interval estimates computed from the posted counts. The Clopper-Pearson interval is exact and conservative; the Wilson interval is a common closer-to-nominal alternative; the Bayesian credible interval uses a uniform Beta(1, 1) prior. For small groups the three can differ noticeably.

R code and output

arm    <- c("RBX2660")
n      <- c(41)
events <- c(18)
for (i in seq_along(n)) {
  ci <- binom.test(events[i], n[i])$conf.int          # Clopper-Pearson exact interval
  cr <- qbeta(c(0.025, 0.975), 1 + events[i], 1 + n[i] - events[i])   # Bayesian, uniform Beta(1,1) prior
  wi <- (function(x, n) { p <- x / n; z <- qnorm(0.975); d <- 1 + z^2 / n
    c((p + z^2 / (2 * n) - z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d, (p + z^2 / (2 * n) + z * sqrt(p * (1 - p) / n + z^2 / (4 * n^2))) / d) })(events[i], n[i])
  cat(sprintf("ARM|%s|%d|%d|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f|%.1f\n", arm[i], n[i], events[i], 100 * events[i] / n[i],
              100 * ci[1], 100 * ci[2], 100 * wi[1], 100 * wi[2], 100 * cr[1], 100 * cr[2]))
}

ARM  RBX2660  41  18  43.9  28.5  60.3  29.9  59.0  29.8  59.0

11. Multiplicity, Missing Data and Analysis Populations

The registry record lists 10 outcome measures and no analyses. Multiple outcomes can raise a multiplicity issue: if many statistical tests are performed, the chance of at least one apparently notable result can increase. General approaches include prespecifying a primary outcome, controlling the error rate across a set of tests, or treating secondary results as exploratory. The registry record does not report a multiplicity strategy, a hierarchy among secondary outcomes, or an analysis plan. It is therefore not possible to determine how multiple outcomes were handled in the trial’s analyses.

The primary outcome is clearly described as a participant count for RBX2660-related treatment-emergent adverse events through 8 weeks or treatment failure. However, its result is not reported in the registry record. Results for the registered secondary outcomes are also not reported in the information described: this includes recurrence, time to recurrence, physician and patient experience, clinician global impression of improvement, and several safety outcomes. Their planned inclusion should not be confused with evidence that results were analyzed or posted.

Missing-data handling is not reported in the registry record. The record does not describe whether follow-up was incomplete, how participants with incomplete observation were handled, or whether any imputation or sensitivity analysis was used. Analysis populations are also not reported in the registry record; there is no description of which participants would be included in each outcome analysis.

These omissions limit assessment of both bias and precision. In general, incomplete outcome information can affect estimates if participants with missing follow-up differ in their outcomes from those observed. The direction and extent of any such effect depend on the pattern and reasons for missingness, which are not reported in the registry record. No missingness pattern, imputed result, or analysis population should be inferred from the enrollment figure alone.

12. 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: colitis ulcerative (2.4%), brain neoplasm (2.4%).

The registry record reports 2 participants with serious adverse events among 41 at risk, and 6 participants under other adverse events among 41 at risk. It reports 0 deaths. These are safety counts in their respective categories. The record also lists Colitis Ulcerative and Brain Neoplasm as serious-event terms, each at 2.4%.

These figures are descriptive. The presence of an adverse event after treatment does not, by itself, show that treatment caused it. The listed serious-event terms should be read as recorded event descriptions, not as conclusions about relatedness. The registry record does not state in this information whether the serious events were considered related to RBX2660. It also does not provide event-level timing or detail about severity beyond the serious-event classification.

The serious and other adverse-event counts are not the stated primary endpoint result. The primary outcome specifically concerns RBX2660-related treatment-emergent adverse events through 8 weeks or treatment failure, and its result is not reported in the registry record. The categories may overlap or differ in their definitions; overlap is not described. No conclusions about comparative safety are possible because the trial has no control group.

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

The registry record establishes that a completed, multicenter, single-group Phase 3 trial enrolled 41 adults with recurrent Clostridioides difficile infection to explore RBX2660 delivered by colonoscopy. It identifies a primary safety outcome and a range of secondary outcomes covering recurrence, adverse events, and treatment experience. It also reports limited adverse-event counts, including 2 participants with serious adverse events, 6 under other adverse events, and 0 deaths.

These registry entries do not establish the result for the primary endpoint. The registry record does not report the number of participants with RBX2660-related treatment-emergent adverse events through 8 weeks or treatment failure. Nor does it report results for the registered recurrence, time-to-recurrence, physician-experience, patient-experience, or clinician-impression outcomes. A study objective is not itself a finding, and the listed outcomes cannot be treated as evidence of effectiveness or safety in the absence of their results.

The posted safety counts do not establish causation. They describe events recorded in the study group, while the trial has no comparator for estimating how event frequency might differ under another treatment or no treatment. A single-group count is not a randomized treatment effect. Even a complete outcome report would need to be interpreted alongside the event definitions, follow-up, analysis population, and uncertainty estimates.

The design and available registry information therefore support a limited conclusion: the trial was intended to examine colonoscopic administration and registered relevant safety and clinical outcomes, but the primary and secondary outcome results needed to evaluate those questions are not reported in the registry record. The record does not justify claims that RBX2660 did or did not work, or that it did or did not cause the recorded events. Those conclusions would require results and analysis details that are not available in the registry information described.

14. Why This Trial Matters Statistically

This trial raises a statistical question that is important whenever an intervention is studied without a concurrent control group: what can event counts tell readers, and what remains uncertain? The study is registered as multicenter and single-group, with 41 enrolled participants. That design can provide descriptive information about participants’ experiences and recorded events after colonoscopic administration. It cannot, on its own, provide a randomized comparison with another treatment.

The distinction matters because an outcome occurring after treatment is not necessarily an outcome caused by treatment. This is relevant to safety events, recurrence, and patient experience. A comparator can help distinguish treatment-group differences from background events and changes over time. In its absence, interpretation depends heavily on careful event definitions, complete follow-up, transparent analysis methods, and cautious language about causation.

The registry record registers a primary endpoint that is specific to treatment-related, treatment-emergent adverse events, as well as secondary outcomes that cover recurrence, experience, and other safety categories. Keeping these measures distinct is statistically important. A general adverse-event count is not interchangeable with a count meeting the primary endpoint’s relatedness and timing criteria. Likewise, an outcome’s registration does not show that it was observed or reported.

The record also illustrates why reporting the estimate and its uncertainty matters. Counts without the primary result, confidence intervals, or analysis description do not show how precisely the safety outcome was estimated. The available information includes a few descriptive safety figures, but does not report the trial’s statistical analyses. Clear results and methods would allow readers to assess both the observed outcomes and the limits of inference.

For researchers, the main statistical lesson is to align claims with design and reporting. A single-group study can contribute descriptive evidence and inform further study, but comparative claims require a basis for comparison. The registry record’s absent endpoint results leave the central registered question unresolved in the information available.

15. 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 interpreting this trial is the absence of the primary endpoint result in the registry record. Although the outcome is defined as a count of participants with RBX2660-related treatment-emergent adverse events through 8 weeks or treatment failure, the corresponding count is not reported. The record also does not report results for the registered secondary outcomes, including recurrence and time to recurrence.

The single-group design is another important limitation. With no control group, outcomes among treated participants cannot be directly compared with outcomes under another treatment or no treatment. This restricts causal interpretation, even if descriptive event information is available. The lack of masking is also recorded; the registry record does not describe measures used to reduce possible assessment or reporting effects.

Key details needed to assess the reported outcomes are not reported in the registry record. These include the detailed eligibility criteria, treatment protocol, operational definition of treatment failure, event-relationship assessment method, analysis plan, analysis populations, handling of missing data, and rationale for the enrollment target. Follow-up completeness and participant-level outcome data are also not described in the information available.

The reported safety information is limited to counts and listed serious-event terms. It does not provide event timing, participant overlap between categories, or a full account of severity and relatedness. The serious-event terms and percentages should therefore be read as descriptive entries, not as a complete safety assessment or evidence of causation.

The record lists no analyses, so there are no reported inferential estimates or confidence intervals for judging statistical precision. Together, the missing outcome results and limited analysis detail make it impossible to evaluate the registered primary question from the registry information alone. These limitations concern what can be concluded from this record and do not establish what the full study results may show.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: A Multi-center, Single-arm Trial Exploring the Safety and Clinical Effectiveness of RBX2660 Administered by Colonoscopy to Adults With Recurrent Clostridioides Difficile Infection — NCT05831189.
• Registry record last updated 2026-04-24; status last verified 2026-03.
• Results first posted 2026-04-24.
• 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.

19. Record Summary

This completed Phase 3 trial studied RBX2660 administered by colonoscopy to adults with recurrent Clostridioides difficile infection. It was multicenter, single-group, unmasked, and enrolled 41 participants. The registry record lists no randomized allocation and no analyses.

The primary outcome was the number of participants with RBX2660-related treatment-emergent adverse events through 8 weeks or treatment failure. Secondary outcomes included recurrence, time to recurrence, physician and patient experience, clinician global impression of improvement, and additional safety measures. The primary and secondary outcome results are not reported in the registry record.

Reported safety information includes 2 participants with serious adverse events, 6 under other adverse events, and 0 deaths. Colitis Ulcerative and Brain Neoplasm are listed as serious-event terms, each at 2.4%. These figures are descriptive and do not establish treatment causality or a comparative effect.