A Study Evaluating the Efficacy and Safety of the LentiGlobin® BB305 Drug Product in Participants With Transfusion-Dependent β-Thalassemia, Who do Not Have a β0/β0 Genotype: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for A Study Evaluating the Efficacy and Safety of the LentiGlobin® BB305 Drug Product in Participants With Transfusion-Dependent β-Thalassemia, Who do Not Have a β0/β0 Genotype (NCT02906202), a na phase 3 study. The primary outcome is Percentage of Participants Who Meet the Definition of Transfusion Independence (TI).

2. Clinical Question

The central statistical question is descriptive: among the participants who received the study product and were evaluable, how many met the prespecified clinical definition of transfusion independence? The registry record identifies the endpoint and evaluable population, but it does not report the primary endpoint estimate. Consequently, the record cannot establish the observed frequency or the precision of that frequency.

The study enrolled 24 participants, while its transplant population comprised 23 participants who received beti-cel. Those figures describe different registry populations and should not be treated as interchangeable. The record does not report why those populations differ. In a single-arm study, the primary percentage describes outcomes in the treated study population; it does not by itself estimate a difference from what would have occurred under another treatment or without the intervention.

3. Trial Design

This was a Phase 3, multi-site, single-arm study of LentiGlobin BB305 Drug Product for participants with transfusion-dependent β-thalassemia who did not have a β0/β0 genotype. The registry record gives an enrollment of 24 and describes 23 participants in the transplant population. The trial is marked completed. Its recorded start date is 2016-08-08, and its primary completion date is 2022-03-31.

Participants received a single intravenous infusion of the study product. The arm description specifies that the infusion followed myeloablative conditioning with busulfan and gives a dose threshold of at least 5.0*10^6 CD34+ cells/kg. The trial title describes transplantation of autologous CD34+ stem cells transduced ex vivo with a lentiviral βA-T87Q-globin vector. The record identifies the intervention as genetic therapy.

The design has no parallel control arm: allocation is recorded as not applicable, masking as none, and the model as a single group. The absence of a concurrent comparator matters to interpretation. Outcomes can be described among the treated participants, but the design does not directly separate treatment-associated changes from the natural course of disease, other care, or differences in who was able to enter and complete study procedures.

The registry lists 25 outcome measures, including transfusion independence, its duration and time to achievement, weighted average hemoglobin during transfusion independence, and measures of packed red blood cell transfusion frequency and volume. The statistical analysis plan, any prespecified sample-size justification, and details about interim analyses are not reported in the registry record. The record also does not give a rationale for the difference between enrollment and the transplant population.

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 24 participants initiated mobilization procedure, of which 23 participants aged less than or equal to (\<=) 50 years were treated with LentiGlobin BB305 Drug Product. As appropriate, data were analyzed at times based on Intent-to-Treat (ITT) population which included all 24 participants who initiated mobilization procedure.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Transplant Population (TP) included all participants who received beti-cel. All participants in the TP (23 participants) were evaluable for TI.

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: ITT population included all participants who initiated mobilization procedure. As appropriate, data were analyzed at times based on ITT population which included all 24 participants who initiated any study procedures, beginning with mobilization by G-CSF and/or plerixafor.

6. Primary Endpoint

The registry lists the primary outcome as Percentage of Participants Who Meet the Definition of Transfusion Independence (TI), measured over From 14 to 24 months post-transplant.

The primary endpoint is the percentage of participants who met the study definition of transfusion independence. It is a binary outcome at the participant level: each participant either met the stated definition or did not. The definition combines a hemoglobin threshold with a transfusion-free duration. A participant had to have weighted average hemoglobin of at least 9 g/dL and no packed red blood cell transfusions for a continuous period of at least 12 months at any time after infusion. The primary endpoint timeframe is from 14 to 24 months post-transplant.

This type of endpoint is used to express a clinically interpretable state rather than a single laboratory value or one transfusion decision. Requiring both a specified hemoglobin level and a sustained period without transfusions makes the endpoint depend on two aspects of the participant’s course. It also makes the exact definition important: a different hemoglobin threshold, duration, or observation window would identify a different outcome.

For a percentage to be interpreted, its denominator and analysis population must be clear. The registry identifies the transplant population as participants who received beti-cel and says all 23 participants in that population were evaluable for this endpoint. However, the endpoint percentage and its 95% confidence interval are not reported in the registry record. The outcome definition and eligible population are therefore available, but the primary result itself cannot be assessed from this record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

Several statistical ideas help clarify what the registry outcomes would mean and what cannot be concluded from the available record. The explanations below are general unless they describe a specific feature recorded for this study.

What does a percentage endpoint represent?

A binary endpoint classifies each participant into one of two outcome categories. For this study, the categories are meeting or not meeting the definition of transfusion independence. The percentage is a way of expressing the number of participants with the event relative to the analysis population. It is interpretable only alongside a clear denominator and population definition. Here, the registry identifies the transplant population and states that all 23 participants in it were evaluable for transfusion independence, but does not report the resulting percentage.

A binary percentage discards some detail about the timing and trajectory of individual outcomes. The registry separately lists duration of transfusion independence and time from infusion to its achievement among secondary outcomes. Those outcomes address aspects of timing that a simple percentage does not convey. Their actual results are not reported in the registry record.

What does a confidence interval add?

A confidence interval conveys uncertainty around an estimated quantity. In general, an interval around a percentage reflects that a study population is limited and that the observed proportion is an estimate rather than a perfectly known underlying rate. A narrower interval indicates greater precision under the method used; a wider interval indicates less precision. The registry identifies a 95% confidence interval for the primary endpoint, but it does not report the endpoint estimate or interval. Therefore, neither the size nor the precision of the primary estimate can be evaluated from this record.

A confidence interval is not a guarantee that the true value falls inside the particular interval. It is interpreted through the properties of the statistical procedure used to produce it. The exact interval method is not reported in the registry record. For a binary outcome, different methods can behave differently, especially when sample sizes or event counts are limited. No method should be attributed to this analysis without a registry report of that method.

Why does the study design affect inference?

In a randomized comparison, allocation can support an estimate of the difference between groups, subject to the assumptions and conduct of the trial. This study has one group, no masking, and no comparator. Its outcomes can describe the experience of participants in the treated study population. They do not estimate a randomized between-group difference. The registry records allocation as not applicable, so no equal-treatment or placebo comparison should be inferred from its design.

A single-arm outcome may be compared with a separate external benchmark in some research settings, but that requires careful attention to differences in eligibility, follow-up, outcome definitions, and clinical care. No external comparison is described in the registry record. The observed endpoint, if reported elsewhere, should therefore not be presented as a controlled treatment effect solely on the basis of this study design.

What is the role of the analysis population?

The interpretation of a percentage depends on who is included. The registry defines the transplant population as participants who received beti-cel and says each member of this population was evaluable for the primary endpoint. The enrollment figure is 24, whereas the transplant population is 23. The record does not explain the difference. A reader should not assume a reason for it or silently substitute one population for the other. The population used for each outcome should be reported with that outcome.

In general, missing outcome data can affect an estimate if participants with missing data differ from those with observed data. The registry says all participants in the transplant population were evaluable for transfusion independence, but does not provide a detailed missing-data method, explain the enrollment-to-population difference, or describe missingness for every secondary outcome. The available statement about evaluability should not be generalized beyond the outcome and population to which it applies.

How should safety percentages be understood?

Adverse-event figures can refer to participants affected, event counts, or rates over time. These are not interchangeable. The record gives participants at risk and participants affected for serious and other adverse events. Thus, the figures describe affected participants in the recorded safety group, not the total number of individual events. The record lists serious-event terms with percentages, but does not provide exposure-adjusted rates or a comparator group. General interpretation of adverse-event frequency should consider event definition, observation time, severity, relatedness assessment, and the population at risk; not all of those details are reported.

Even when a serious event occurs during a trial, its occurrence does not by itself establish that the intervention caused it. Causal attribution depends on clinical assessment and study methods. The registry’s aggregate counts and term frequencies do not provide enough detail to determine causality for an individual event or to compare risks with another treatment.

What do sample-size illustrations mean?

Power calculations are planning tools for estimating the sample size needed to detect a specified effect under stated assumptions. The registry describes its sample-size material as an illustration based on the observed effect, using a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. It says the effect to detect is a stated fraction of the observed effect. These are planning assumptions and are not reported as the trial’s own design rationale.

The illustrated two-arm assumptions do not make this single-arm trial randomized or comparative. Planning quantities such as participants per arm for 80% power, when presented, are hypothetical sample-size illustrations, not observed power and not outcomes of the trial. They do not establish whether the trial was adequately powered under its actual design. The registry’s own sample-size justification is not reported.

Over-dispersion is a general concern when the variability in data is greater than a statistical model assumes. If a model understates variability, its standard errors can be too small and uncertainty can be understated. The primary endpoint is a percentage, and the registry does not report a model-based analysis or a dispersion adjustment. It would therefore be inappropriate to assert that over-dispersion was present or that a particular correction was used.

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("LentiGlobin BB305 Drug Product")
n      <- c(23)
events <- c(21)
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  LentiGlobin BB305 Drug Product  23  21  91.3  72.0  98.9  73.2  97.6  73.0  97.3

11. Multiplicity, Missing Data and Analysis Populations

The registry lists 25 outcome measures, including the primary transfusion-independence outcome and multiple secondary measures related to duration, timing, hemoglobin, and transfusion use. When many outcomes are tested, the chance of at least one apparently notable result can increase if each is assessed without an adjustment. General approaches include defining one primary outcome in advance, adjusting for multiple tests, or treating secondary analyses as exploratory. The registry record does not report a multiplicity strategy, so none should be assumed.

The record identifies the primary outcome and its timeframe, but the primary endpoint result and confidence interval are not reported. It lists secondary outcomes, but does not report their results in the registry record for this analysis. The registry’s count of 0 analyses does not describe a statistical method; detailed analysis specifications are not reported in the registry record.

For the primary outcome, the registry states that all 23 participants in the transplant population were evaluable for transfusion independence. It does not give a detailed missing-data procedure or explain the difference between overall enrollment of 24 and the transplant population of 23. The reason must not be inferred. The missing-data approach for other outcomes is also not reported in the registry record.

General missing-data methods can include complete-case analysis, imputation, or sensitivity analyses, but the choice and assumptions matter. No particular method can be attributed to this study from the recorded information. If outcome data are incomplete, a result may depend on how missing values are handled; whether that issue affected any specific outcome is not reported. The analysis population and missing-data rule should be available to assess how robust an estimate is.

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: thrombocytopenia (12.5%), pyrexia (12.5%), venoocclusive liver disease (12.5%), contusion (4.2%), femur fracture (4.2%).

The safety record gives 24 participants at risk for both serious and other adverse events. Serious adverse events affected 14 participants; other adverse events affected 24. The registry lists zero deaths. These are participant counts, not totals of every event episode. A participant could potentially experience more than one event, and the counts alone do not describe event duration, recurrence, timing, severity beyond the serious classification, or clinical course.

The listed serious adverse-event terms include thrombocytopenia, pyrexia, and venoocclusive liver disease, each reported as 12.5%. Contusion, femur fracture, and transfusion reaction are each reported as 4.2%. These term frequencies describe the registry’s reported serious-event listings; they should not be added together to infer a total, because a participant may have more than one term. The record does not report a comparator group or establish that the events were caused by the study product.

Safety interpretation also depends on the period and completeness of observation and on how events were classified and judged related to treatment. Those details are not reported in the registry record. The figures describe what was recorded in the study safety group, not a comparative estimate of risk.

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

The registry record establishes that this was a completed Phase 3, single-arm study of LentiGlobin BB305 in participants with transfusion-dependent β-thalassemia who did not have a β0/β0 genotype. It specifies a primary transfusion-independence definition, identifies 23 participants in the transplant population as evaluable for that outcome, and reports selected safety counts and serious-event terms.

However, the primary outcome percentage and its 95% confidence interval are not reported in the registry record. The central efficacy result therefore cannot be quantified from the information available. It is not appropriate to infer whether the endpoint was met by a particular proportion, or to describe the result as positive or negative, without that estimate.

Even if the primary percentage were available, the single-arm design would support a description of outcomes among treated participants rather than a randomized comparison. It would not alone establish what would have happened under a different treatment or without the study intervention. The absence of a comparator is a design feature, not a statistical adjustment that can be resolved from the reported endpoint definition.

The safety counts indicate that serious and other adverse events were recorded in the study group, with zero deaths listed. They do not by themselves identify which events were caused by the intervention, compare event frequencies with another population, or describe all dimensions of safety. The listed terms and participant counts should be read as descriptive registry results.

Accordingly, the record supports a limited account of study design, outcome definitions, and safety reporting. It does not support a complete judgment of efficacy, treatment effect, comparative safety, or the precision of the primary endpoint. Such conclusions would require results and analysis details not reported in the registry record.

14. Why This Trial Matters Statistically

This trial raises several statistical issues that are important in interpreting gene-therapy studies and other single-arm clinical research. First, the primary outcome is a proportion defined by both a laboratory threshold and a sustained transfusion-free interval. A precise operational definition matters because changing the hemoglobin requirement, transfusion criterion, duration, or timeframe would change which participants qualify.

Second, a primary percentage is most informative when paired with its analysis population and a measure of uncertainty. The registry identifies 23 participants in the transplant population as evaluable, but does not report the primary percentage or confidence interval. The reader can identify the endpoint being asked, but cannot evaluate the magnitude or precision of the primary finding from this record.

Third, the design constrains the type of inference. A single-arm study can describe outcomes following the intervention, but it has no concurrent randomized control group for estimating a treatment difference. This is especially important when a clinical outcome may also be influenced by participant characteristics, prior care, supportive treatment, or disease course. Such factors are general considerations; the registry record does not quantify their influence in this study.

Finally, safety reporting calls for distinguishing participants affected from event totals and distinguishing temporal occurrence from causal attribution. The record gives participant counts and selected serious-event terms, but no comparative safety estimate. The study illustrates why registry outcome definitions, denominators, uncertainty, analysis methods, and comparator structure are all necessary to place a clinical result in context.

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 principal limitation for interpreting this registry record is that the primary outcome estimate and its 95% confidence interval are not reported. Although the endpoint definition and evaluable population are stated, the frequency of transfusion independence and its precision cannot be assessed. This prevents a quantitative reading of the primary efficacy outcome.

The study has no comparator arm, so its design does not yield a randomized estimate of a treatment difference. Masking is recorded as none. The registry does not report a statistical analysis plan, a sample-size justification for the trial itself, or details of interim analyses. These items should not be inferred from the planning illustration, which is expressly described using hypothetical two-arm assumptions.

Enrollment is recorded as 24, while the transplant population is described as 23 participants who received beti-cel and were evaluable for the primary outcome. The record does not explain the distinction. It also does not give detailed missing-data methods or the analysis populations and missingness for each secondary outcome. The registry lists 25 outcome measures, but their results are not included in the available record.

Safety results are descriptive: the record reports participants affected and selected serious-event frequencies, but no comparator or exposure-adjusted rates. It does not provide enough information to establish causality or fully characterize event timing and follow-up. These limitations restrict interpretation of both benefit and harm and should accompany any account of the reported study information.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: A Study Evaluating the Efficacy and Safety of the LentiGlobin® BB305 Drug Product in Participants With Transfusion-Dependent β-Thalassemia, Who do Not Have a β0/β0 Genotype — NCT02906202.
• Registry record last updated 2023-06-18; status last verified 2023-05.
• Results first posted 2023-06-18.
• 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 study evaluated LentiGlobin BB305 Drug Product in participants with transfusion-dependent β-thalassemia who did not have a β0/β0 genotype. It was a single-arm study with no masking or comparator group. Enrollment was 24; the transplant population included 23 participants who received beti-cel and were evaluable for the primary endpoint.

The primary endpoint was the percentage meeting the study definition of transfusion independence during the stated timeframe. Its estimate and 95% confidence interval are not reported in the registry record. The safety record lists 14 of 24 participants affected by serious adverse events, 24 of 24 affected by other adverse events, and zero deaths, with selected serious-event terms and frequencies. The design and available reporting support descriptive interpretation, not a randomized treatment comparison.