Presentation on theme: "1 Implementing Adaptive Designs in Clinical Trials: Risks and Benefits Christopher Khedouri, Ph.D.*, Thamban Valappil, Ph.D.*, Mohammed Huque, Ph.D.* *"— Presentation transcript:
1 Implementing Adaptive Designs in Clinical Trials: Risks and Benefits Christopher Khedouri, Ph.D.*, Thamban Valappil, Ph.D.*, Mohammed Huque, Ph.D.* * U.S. Food and Drug Administration, Center for Drug Evaulation and Research (FDA/CDER) ASA Joint Statistical Meetings in Seattle, Washington on August 7, 2006 Disclaimer: Views expressed are those of the authors and not necessarily those of the FDA
2 Outline Overview of risks & benefits of 3 types of sample size designs in a non-sequential (NS) two-stage setting Fixed sample design (FS): No sample size re-estimation (SSR) Variance Re-estimation (VR): SSR based on σ 2 only. Adaptive design for SSR (AD): SSR based on δ, σ 2 and other factors. Specific risks of ADs in non-inferiority (NI) trials Sample Size (SS) Increases (Due to Chance Variation) and Inefficiencies Reduction of Treatment Effect, Standards Bio-creep Influence from Confounders Operational Biases Misclassification Biases (Non-differential or Differential) Conclusions: ADs may not be appropriate in a NI trial setting, especially in Phase III confirmatory trials
3 Risks & Benefits: Fixed Sample (FS) vs. Variance Re-estimation (VR) vs. Adaptive Designs (AD) (1) FS Risks: Lack of flexibility (pre-specification of fixed δ T, σ 2 ) Loss of efficiency if δ T and σ 2 not pre-specified correctly. FS Benefits: Limited operational biases Efficiently and reliably implemented. Good statistical properties Easily interpreted/compared with historical studies Widely used/accepted VR Risks: Pre-specification of fixed δ T Careful implementation of SSR needed to limit operational biases
4 Risks & Benefits: Fixed Sample vs. Variance Re- estimation vs. Adaptive Designs (2) VR Benefits Flexible to unexpected increases in variance Good statistical properties More easily implemented than ADs Less potential for biases with blinded interim looks AD Risks (NI and Superiority) Limited use and acceptance in Phase III trials Limited advantages due to regulatory restrictions Sample size (SS) increase due to chance variation Inefficiencies due to large SS increases. Logistical constraints (e.g. new enrollment) Inconsistencies in Stage I & II Statistical vs. clinical significance Unequal patient weighting
5 Risks & Benefits: Fixed Sample vs. Variance Re- estimation vs. Adaptive Designs (3) AD Risks (Additional Risks in NI Studies) Unclear comparisons with historical controls. “Sub-optimal” new treatments Unclear comparisons with other drugs. Potential bio-creep issues Unclear assay sensitivity Operational and misclassification biases Inflation of type I error given underlying misclassification biases ‘Confirmatory’ vs. ‘Exploratory’ AD Benefits Flexibility to faulty design assumptions affecting 1 st stage test statistic (e.g. assumed δ and σ 2 ) Flexibility towards other considerations (e.g. logistical constraints) Increased power over FS and VR designs.
6 Specific Risks of Adaptive Designs in NI trials: Sample Size Increases and Inefficiencies (1) In NI trials for anti-infective drugs (AIs), “typical conditions” for SS estimation include: Assumption of equal cure rates for treatment and comparator Justified overall cure rate and NI margin (e.g. 10%) Pre-specified α (two-sided)=.05, =.10 to.20 Pre-specified sample size rule (no decreases to initial N) Simulations under “typical conditions” compared SS increases and inefficiencies among FS, VR & ADs based on Conditional (Proschan et al. ‘94 ) & Unconditional (Cui et al. ‘99) power: First stage data based on 50% of initial N N max = 2N & 4N used for both ADs 80% Power, δ NI = 10% and δ T = 0%. Assumed cure rate of 80% Treatment effects of +2%, 0%, -2%, -4%, -6%
7 Specific Risks of Adaptive Designs in NI trials: Sample Size Increases and Inefficiencies (2) Change in Sample Size & Power: FS & ADs for NI
8 Specific Risks of Adaptive Designs in NI trials: Sample Size Increases and Inefficiencies (3) Simulation results indicated that: Adaptive designs for SSR (ADs) often required substantial increases in sample size even when study power was adequate. ADs resulted in especially large SS increases if based on unconditional power or a large N max. ADs became less efficient as treatment effects became less favorable and required larger SS increases. ADs (compared to FS design) remained substantially under- powered given an unfavorable δ T with a large SS increase. Overall, ADs did not prove to be an effective strategy for salvaging a trial with poor treatment performance! Note: ADs are not a fix for poorly planned studies
9 Specific Risks of Adaptive Designs in NI trials: Reduction of Treatment Effects, Standards (1) In NI trials for AIs, placebo controlled trials (PCTs) are rare. NI trials must rely upon assay sensitivity, the ability to distinguish an effective treatment from an ineffective treatment. NI trials should adhere to the following steps: NI trials must have historical evidence (about the comparator) from PCTs to show “sensitivity of drug effect “ NI trials should be carefully planned, and should adhere closely to the PCTs from which “sensitivity of drug effect” was determined NI trials must justify an acceptable NI margin taking into account historical data and all relevant clinical & statistical considerations Conduct of NI trials should adhere to conduct of historical PCTs ADs may compromise any of the latter 3 steps of a NI trial including justification of an appropriate NI margin.
10 Specific Risks of Adaptive Designs in NI trials: Reduction of Treatment Effects, Standards (2) ADs can reduce the proportion of the 95% CI (of the treatment difference) due to σ 2 and increase the proportion due to T which: Allows wider (less favorable) margins for T Changes initial assumptions in choosing NI Reduce standards and complicate cross-study comparisons Assuming 50% power in our previous simulations, a treatment 3% worse (than comparator) under a FS design could be up to 5.6% worse under an AD with same lower bound of 95% CI near -10%. In NI trials for AIs, bio-creep may also be more likely:
11 Specific Risks of Adaptive Designs in NI trials: Bio-Creep (Non-inferiority margin =10%) 80% 70% 60% 50% 40% Placebo Drug A Drug B Drug C Drug D Placebo Drug E
12 Specific Risks of Adaptive Designs in NI trials: Confounders (1) Assessing a Clinically Meaningful Treatment Benefit Over Placebo Reductions of T (standards) and potential bio-creep and assay sensitivity issues can be especially problematic in AI studies due to the presence of other confounders. These confounders include: Lack of placebo-controlled trials Old or poorly controlled historical trials Lack of Constancy Assumption Different Doses /Treatment durations Heterogeneous historical population Different inclusion/exclusion criteria Different endpoints used Regional differences /change in clinical practice Different concomitant or adjunct therapies
13 Specific Risks of Adaptive Designs in NI trials: Confounders (2) Assessing Treatment vs. Comparator Additional confounders exist in assessing the treatment effect relative to active comparator: Heterogeneity in disease characteristics at baseline Heterogeneity in patient characteristics. Severity of the disease Combinational therapies (for MRSA, gram+) Adjunct Therapies Concomitant Medications IV to Oral Switch Misclassification of outcomes
14 Specific Risks of Adaptive Designs in NI trials: Operational Bias (1) Unblinded interim looks in ADs (or VR) can lead to unwarranted “data driven adjustments” that compromise the overall study results. Such “adjustments” can be more influential (and problematic) in NI trials where biases tend to make treatments appear more similar. ADs can increase opportunities for such “adjustments” due to the increased flexibility and reduced transparency in procedures. Such “adjustments” in NI trials may include but not be limited to: Enrolling subjects from favorable sites/investigators Enrolling subjects with favorable baseline/disease characteristics Enrolling subjects with high expected cure rates Influencing trial conduct/outcome assessment of investigators
15 Specific Risks of Adaptive Designs in NI trials: Operational Bias (2) Assigning an “independent” statistician to conduct the interim analysis is no guarantee against operational bias. Statistician “independence” Statistician and Sponsor interests Statistician and Sponsor contact Sponsor oversight and authority There are many other potential sources for leaks in the data flow! Currently, ADs do not have clearly defined or proven procedures for implementation that can reliably safeguard against operational biases.
16 Specific Risks of Adaptive Designs in NI trials: Misclassification Bias Differential outcome misclassification bias can seriously inflate the type-I error rate in both NI and superiority trials. In NI trials, non-differential bias can also seriously inflate the type I error rate Kim, Goldberg et. al, 2001).This inflation of the type I error rate becomes larger as N increases (Kim, Goldberg et. al, 2001). ADs, using a greatly increased N under inferiority would inflate the overall type I error rate more severely.
17 Conclusions ADs can offer increased power and flexibility but may involve unwanted SS increases due to chance variation. ADs can also become inefficient for larger increases in SS. ADs can substantially reduce the allowable T under a FS design. ADs can reduce assay sensitivity in NI studies. ADs can introduce serious operational or misclassification biases in NI studies where statistical testing is very challenging. ADs may not be appropriate in a NI trial setting, especially in Phase III confirmatory trials