Prevalence and Impact of Bypassing or Overriding Phase 2 Trials in Neurologic Drug Development

Pivotal trials for neurologic drugs in clinical development are often initiated without a phase 2 trial ("bypass") or despite a negative phase 2 efficacy result ("override"). Such practices may degrade the risk/benefit ratio of phase 3 trials. The aim of this study is to estimate the proportion of phase 3 trials for 10 neurologic diseases started without a positive phase 2 trial, to identify factors associated with this practice, and to investigate any association with unfavorable phase 3 trial outcomes.

We searched ClinicalTrials.gov for phase 3 trials completed during 2011-2021, with at least 1 research site in the United States, Canada, the European Union, the United Kingdom, or Australia, and investigating drugs or biologics for treatment of 10 neurologic conditions. Our primary objective was to assess the prevalence of phase 2 bypass/override by searching for preceding phase 2 trials. We used Fisher exact tests to determine whether phase 3 trial characteristics and trial results were associated with phase 2 bypass/override.

Of the 1,188 phase 3 trials captured in our search, 113 met eligibility for inclusion. Of these, 46% were not preceded by a phase 2 trial that was positive on an efficacy endpoint (31% bypassed and 15% overrode phase 2 trial). Phase 2 bypass/override was not associated with industry funding (77% vs 89%, 95% CI 0.75-7.55, p = 0.13) or testing already approved interventions (23% vs 15%, 95% CI 0.60-5.14, p = 0.33). Overall, phase 3 trials based on phase 2 bypassed/override were statistically significantly less likely to be positive on their primary outcome (31% vs 57%, respectively, 95% CI 1.21-6.92, p = 0.01). This effect disappeared when indications characterized by nearly universal positive or negative results were excluded. Trials that bypassed/overrode phase 2 trials were not statistically significantly more likely to be terminated early because of safety or futility (29% vs 15%, respectively, 95% CI 0.15-1.18, p = 0.11) and did not show increased risk of adverse events in experimental arms (RR = 1.46, 95% CI 1.19-1.79, vs RR = 1.36, 95% CI 1.10-1.69, respectively, p = 0.65).

Almost half of the neurologic disease phase 3 trials were initiated without the support of a positive phase 2 trial. Although our analysis does not establish harm with bypass/override, its prevalence and the scientific rationale for phase 2 trial testing favor development of criteria defining when phase 2 bypass/override is justified.