Long-Term Safety and Tolerability Study of NKTR-181 in Subjects With Chronic Low Back Pain or Chronic Non-Cancer Pain: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Long-Term Safety and Tolerability Study of NKTR-181 in Subjects With Chronic Low Back Pain or Chronic Non-Cancer Pain (NCT02367820), a na phase 3 study. The primary outcome is Number of Participants Reporting Adverse Events.

2. Clinical Question

The study addresses a safety question in people with chronic pain who received NKTR-181. Its open-label, single-group design is suited to describing adverse events observed during follow-up, but it does not establish how those event counts would compare with outcomes under a concurrent randomized control condition. The listed cohorts reflect different prior-study or opioid-experience labels, not randomized allocation. The registry does not report a prespecified causal comparison among these cohorts.

3. Trial Design

This was a Phase 3, multicenter, open-label study of NKTR-181 in participants with moderate to severe chronic low back pain or chronic non-cancer pain. The registry records the model as single group, allocation as NA, and masking as none. Its stated purpose was to evaluate long-term safety and tolerability. The official study title describes a 52-week study; the primary endpoint timeframe is screening baseline through end of study, an average of 57 weeks.

The registry reports enrollment of 638 participants and lists four cohorts in the safety information: Rollover NKTR-181, Rollover PBO, De Novo Opioid Experienced, and De Novo Naive. These are descriptive cohort labels. In particular, the presence of a cohort labeled Rollover PBO does not make this a randomized comparison: allocation is recorded as NA, and the registry explicitly cautions that the groups are not a like-for-like control comparison.

The intervention was NKTR-181 BID tablets. There was no masking. The listed secondary outcomes were change from baseline in the Brief Pain Inventory (BPI) Pain Intensity Item to Week 52 and change from baseline in the BPI Pain Interference Item to Week 52. The registry reports three outcome measures in total, but the details of the additional measure are not stated in the information available for this analysis.

The study status is completed. The registry lists the start date as 2015-04-14 and primary completion as 2017-12. It does not report the allocation procedure beyond NA, a protocol-based sample-size justification, or a comparative efficacy analysis. Those omissions matter because follow-up duration, cohort composition, and analysis choices affect how descriptive safety counts should be understood. The available record supports a description of the study structure and posted event counts, not an estimate of the effect of NKTR-181 relative to a randomized control.

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: The titration phase of the study was designed to titrate patients to a dose of NKTR-181 that provided adequate analgesia and acceptable side effects.

Overall Study

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: Safety Analysis Set is defined as all enrolled subjects who received at least 1 dose of study drug.

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants Reporting Adverse Events, measured over Screening baseline through end of study, an average of 57 weeks.

The primary endpoint is the number of participants reporting adverse events. The registry describes it as a count of subjects reporting treatment-emergent adverse events, with the timeframe running from screening baseline through end of study, an average of 57 weeks. This is a participant-level count: it concerns how many participants experienced an event, rather than how many separate event episodes occurred. The registry does not provide an overall value for this primary endpoint in the record analyzed.

Counting participants with events is a direct way to describe the occurrence of safety events during follow-up. It can communicate how many people in a cohort were affected, while avoiding treating multiple events in the same participant as multiple affected participants. The count alone does not describe event severity, duration, recurrence, or timing. The separate serious-adverse-event figures provide additional safety detail, but they do not replace a complete report of the primary endpoint.

Interpretation also depends on who was followed and for how long. The endpoint timeframe is an average of 57 weeks, whereas the study title describes a 52-week study. The registry does not provide participant-level follow-up durations or a method for adjusting counts for time under observation. The endpoint is therefore best read as a descriptive count over the stated study timeframe, not as a rate or a causal comparison.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is a count of participants reporting treatment-emergent adverse events, with a stated timeframe from screening baseline through end of study, an average of 57 weeks. The record also supplies cohort-specific counts for serious and other adverse events. These are descriptive statistics: they summarize how many participants in each listed cohort had an event, without estimating a causal difference between treatments.

What does a participant count represent?

A count of participants with at least one event differs from a count of all event episodes. If a participant experiences more than one event, a participant-level count still represents that person once for the outcome being counted. The registry labels the primary measure as a count of participants, but does not report the event-counting rules, such as how repeat events or event categories were handled. Those details are not reported in the registry record.

The cohort figures pair affected-participant counts with numbers at risk. For serious adverse events, the listed pairs are 10 and 214 for Rollover NKTR-181, 8 and 217 for Rollover PBO, 8 and 134 for De Novo Opioid Experienced, and 4 and 73 for De Novo Naive. For other adverse events, they are 136 and 214, 151 and 217, 110 and 134, and 62 and 73, respectively. The registry also lists zero deaths in each cohort. These figures are straightforward descriptions of the posted data. They are not adjusted for differences among cohorts or for time under observation.

Why is the design important?

In a randomized comparison, allocation is intended to make treatment groups comparable on average, supporting an estimate of the difference attributable to treatment. This study is recorded as single group, with allocation NA and masking none. Its cohorts are not identified as randomized comparison arms, and the record cautions that they are not a like-for-like control comparison. Thus, comparing their counts does not provide the same interpretation as comparing randomized groups. Differences could reflect cohort composition, prior-study history, exposure, or other factors; the record does not provide enough detail to evaluate those possibilities.

What would a confidence interval add?

General statistical explanation: A confidence interval describes the uncertainty around an estimated quantity under a statistical model. For an event proportion or a difference between groups, it can show which values are reasonably compatible with the observed data and the assumptions used. The registry information does not report confidence intervals or a comparative estimate for these outcomes. It is therefore not possible to use a confidence interval to judge the precision of a treatment contrast from the posted results.

Why do rates and follow-up matter?

General statistical explanation: A simple count does not account for how long each participant was observed. When follow-up varies, event rates based on time at risk can sometimes make exposure time more comparable, provided the necessary individual or aggregate follow-up information and an appropriate method are available. The record states an average timeframe of 57 weeks for the primary endpoint, but does not report participant-level follow-up or a time-adjusted analysis. A rate should not be inferred from the counts alone.

Over-dispersion is a statistical concern when event counts vary more across participants or groups than a simple model assumes. General statistical explanation: If a count model assumes less variability than the data contain, its uncertainty estimates can be too narrow. This study’s primary measure is a count of participants with events, not a count of recurrent events, and the registry does not report a count-model analysis or an assessment of over-dispersion. The concept therefore explains a general modeling issue, not a reported feature of this analysis.

What analysis is documented?

The registry lists three outcome measures and zero analysis entries. It does not report a hypothesis test, a model specification, adjustment variables, a confidence interval, or a comparative effect estimate. The stated primary endpoint is a count, and the available event figures are descriptive. No statistical significance claim can be made from the registry information. A sample-size justification for the trial is not reported in the registry record; any sample-size planning illustration is not the trial’s observed analysis or design justification.

The secondary outcomes concern change from baseline in BPI Pain Intensity and BPI Pain Interference to Week 52. Their analysis methods, estimates, uncertainty, and handling of missing measurements are not reported in the registry record. The safety counts should therefore be interpreted on their own terms and not as evidence about those secondary outcomes.

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("Rollover NKTR-181", "Rollover PBO", "De Novo Opioid Experienced", "De Novo Naive")
n      <- c(214, 217, 134, 73)
events <- c(138, 151, 110, 62)
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  Rollover NKTR-181  214  138  64.5  57.7  70.9  57.9  70.6  57.9  70.6
ARM  Rollover PBO  217  151  69.6  63.0  75.6  63.2  75.3  63.2  75.3
ARM  De Novo Opioid Experienced  134  110  82.1  74.5  88.2  74.7  87.7  74.7  87.6
ARM  De Novo Naive  73  62  84.9  74.6  92.2  75.0  91.4  75.0  91.3

11. Multiplicity, Missing Data and Analysis Populations

The registry lists the primary count of participants reporting adverse events and two secondary outcomes: change from baseline in the BPI Pain Intensity Item and BPI Pain Interference Item to Week 52. It does not report a multiplicity strategy for interpreting several outcomes or tests. No adjustment procedure, testing sequence, or allocation of an overall error rate is reported in the registry record. Because no analysis entries are listed, the record also does not provide a set of outcome-specific inferential results to interpret jointly.

Missing-data handling is not reported in the registry record. The record does not describe how participants without an end-of-study assessment were treated, whether partial follow-up contributed to the safety counts, or how missing BPI measurements were handled. It also does not report an imputation method or sensitivity analysis. These omissions limit assessment of whether the posted counts and any unreported outcome summaries represent all enrolled participants or a subset with available observations.

The analysis population is not defined in the primary outcome description. No separate intention-to-treat, safety, or per-protocol population is reported in the registry record. The listed cohort-specific numbers at risk clarify the denominators attached to the posted adverse-event counts, but they do not by themselves define the analysis population for every outcome.

Interim analyses and a protocol-based sample-size justification are not reported in the registry record. This means the record does not establish whether the planned follow-up, analysis populations, missing-data methods, or multiplicity approach were prespecified. These are reporting gaps, not evidence that a particular method was or was not used outside the registry record.

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: duodenitis (1.4%), pancreatitis (1.4%), generalized oedema (1.4%), osteoarthritis (1.4%), chronic obstructive pulmonary disease (1.4%).

The serious adverse-event figures are counts of affected participants paired with the number at risk in each cohort: Rollover NKTR-181, 10 of 214; Rollover PBO, 8 of 217; De Novo Opioid Experienced, 8 of 134; and De Novo Naive, 4 of 73. The registry reports zero deaths in each cohort. The separate other-adverse-event counts are 136 of 214, 151 of 217, 110 of 134, and 62 of 73, in the same cohort order.

These figures describe what was recorded in the named groups over the study timeframe. They are not randomized comparative estimates, and cohort differences should not be interpreted as treatment effects. The study is open label and single group, and the registry explicitly says the groups are not a like-for-like control comparison. Counts also do not convey event timing, duration, recurrence, or participant-level follow-up.

The registry lists serious-event terms and percentages: Duodenitis, Pancreatitis, Generalized Oedema, Osteoarthritis, and Chronic Obstructive Pulmonary Disease at 1.4% each, and Goitre at 0.7%. The cohort attribution for these percentages is not reported in the registry record. They should be read as listed terms and percentages, without assigning them to a particular group or treating them as comparative risks.

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

The posted results provide descriptive information about adverse events among participants in a completed, open-label Phase 3 study of NKTR-181. They include counts of participants with serious and other adverse events in four named cohorts and report zero deaths in each cohort. The primary outcome was defined as the count of participants reporting treatment-emergent adverse events during follow-up, but an overall primary endpoint result is not reported in the registry record.

These data do not establish that NKTR-181 caused or prevented any event. The study is recorded as single group, allocation NA, and masking none; the cohort labels are not randomized treatment assignments. In particular, Rollover PBO is a cohort label, not evidence of a concurrent randomized control comparison. Differences among the posted counts cannot be interpreted as causal treatment effects because the record does not establish that the groups are comparable.

The counts also do not describe every aspect of safety. They do not by themselves show when events occurred, how long they lasted, whether they recurred, or how event severity varied. The registry does not report participant-level exposure time, an adjusted event rate, confidence intervals, or comparative statistical tests. A count paired with a number at risk is useful descriptive information, but it is not a substitute for these missing details.

The record lists BPI Pain Intensity and Pain Interference change outcomes, but their results and analysis methods are not reported in the information available for this analysis. The safety findings therefore should not be used to draw conclusions about pain outcomes. The study’s stated aim was long-term safety and tolerability, and the available figures offer a limited descriptive view of adverse events during follow-up. They do not establish comparative safety or efficacy.

14. Why This Trial Matters Statistically

Statistically, this study illustrates the distinction between describing events and estimating a treatment effect. A single-group, open-label study can document adverse events observed during follow-up, and the registry provides cohort-specific participant counts for serious and other adverse events. Those counts are useful as a record of what was reported in the listed cohorts, but they do not answer what would have happened to comparable participants under a randomized alternative.

The cohort labels also show why study structure matters when reading safety summaries. Rollover NKTR-181, Rollover PBO, De Novo Opioid Experienced, and De Novo Naive are not identified as randomized arms. The registry records allocation as NA and cautions that the groups are not a like-for-like control comparison. Without that design support, a contrast among the groups can be confounded by differences in who entered each cohort or in their prior study history. The registry does not report enough information to assess such differences.

The endpoint framing is important as well. A count of participants with events answers a different question from a count of event episodes or a time-adjusted event rate. The primary outcome is a participant count over a timeframe stated as an average of 57 weeks. Without detailed exposure time, the counts cannot account for how long each person was observed. The record also does not report an overall primary endpoint result or inferential analysis.

Finally, the record demonstrates how missing analysis details constrain interpretation. The registry does not report the analysis population, missing-data methods, multiplicity strategy, confidence intervals, or sample-size justification. These gaps do not erase the descriptive information, but they limit the precision and scope of conclusions that can be drawn from it. The central statistical lesson is to match claims to design and reporting: these results describe observed safety counts, not a controlled estimate of treatment effect.

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 is the absence of a randomized comparison. The study is recorded as single group, allocation NA, and open label, and the registry cautions that the named cohorts are not a like-for-like control comparison. The Rollover PBO cohort should not be treated as a randomized control arm for this study. Cohort differences may therefore reflect differences in participants or their prior study history rather than a treatment effect; the registry does not provide information to separate those explanations.

The primary endpoint is defined, but its overall result is not reported in the registry record. The available safety information consists of cohort-specific counts for serious and other adverse events, plus listed serious-event terms and percentages without cohort attribution. The record does not report confidence intervals, comparative estimates, or hypothesis tests. The precision of the counts as estimates of broader event experience cannot be assessed from the reported information.

Follow-up is described as an average of 57 weeks for the primary endpoint, while the study title describes a 52-week study. Participant-level observation time and time-adjusted analyses are not reported. The record also does not describe event ascertainment details, missing-data handling, analysis populations, or whether the counts include all enrolled participants. These omissions limit interpretation of the denominators and completeness of follow-up.

Finally, the record lists BPI secondary outcomes but does not provide their results or analysis methods in the information available for this analysis. Multiplicity procedures, interim analyses, and a protocol-based sample-size justification are also not reported in the registry record. These reporting limitations constrain both safety interpretation and assessment of the broader study analysis.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Long-Term Safety and Tolerability Study of NKTR-181 in Subjects With Chronic Low Back Pain or Chronic Non-Cancer Pain — NCT02367820.
• Registry record last updated 2021-06-22; status last verified 2021-06.
• Results first posted 2020-10-08.
• 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.

19. Record Summary

NCT02367820 is a completed Phase 3, multicenter, open-label study of NKTR-181 BID tablets in participants with moderate to severe chronic low back pain or chronic non-cancer pain. The registry records a single-group model, allocation NA, no masking, and enrollment of 638 participants. The study title describes a 52-week study; the primary safety endpoint timeframe is screening baseline through end of study, an average of 57 weeks.

The primary outcome is the number of participants reporting treatment-emergent adverse events. The record lists serious and other adverse-event counts for Rollover NKTR-181, Rollover PBO, De Novo Opioid Experienced, and De Novo Naive, and reports zero deaths in each cohort. It does not report an overall primary endpoint result or a randomized comparison. The registry also lists BPI Pain Intensity and Pain Interference change outcomes, but their results are not reported in the information available for this analysis.

The record does not report inferential analyses, a sample-size justification, missing-data methods, or a multiplicity strategy. The posted safety figures should therefore be read as descriptive cohort counts, not causal estimates of a treatment effect.