Patient Population

Patients were eligible for enrollment in the trial if they were between 55 and 85 years of age and if they met standard research and clinical criteria for dementia that was probably due to Alzheimer’s disease.12,13 All the patients underwent appropriate medical and neurologic evaluations, including magnetic resonance imaging (MRI) (or computed tomography if MRI was contraindicated), to exclude patients who had alternative causes of dementia. Entry criteria included a score of 15 to 26 on the Mini–Mental State Examination (MMSE), which represented mild or moderate dementia (scores range from 0 to 30, with lower scores indicating poorer cognitive performance).14 Patients could have been receiving an acetylcholinesterase inhibitor, memantine, or both, provided that they were receiving a stable dose for 3 months before screening. The diagnosis of Alzheimer’s disease was confirmed by an independent expert who reviewed the investigator’s written narrative of the patient’s history, including pertinent laboratory data and baseline clinical measures.

Trial Design

The trial was conducted at 238 centers in 21 countries from November 2012 through April 2017. A list of investigators is provided in the Supplementary Appendix, available with the full text of this article at NEJM.org. The trial consisted of a randomized, double-blind, placebo-controlled, parallel-group, 78-week trial period (part 1), followed by an optional extension period with a total planned duration of up to 5 years (part 2). Part 1 was designed to include a phase 2 lead-in safety cohort component that was intended to transition to a phase 3 trial if satisfactory safety results were observed. In the phase 2 lead-in safety period, patients were randomly assigned to receive, once daily, one of three oral dose levels of verubecestat (12 mg, 40 mg, or 60 mg) or placebo. These dose levels were selected on the basis of data from phase 1 studies in humans that suggested that doses of 12 mg and 40 mg reduced levels of Aβ-40 and Aβ-42, the major metabolites of BACE-1 cleavage of APP, in cerebrospinal fluid by 60% (12 mg) or 75% (40 mg).11 The 60-mg dose level was included in the phase 2 lead-in safety cohort to explore the safety of this high dose and was prespecified to be dropped for the phase 3 component of the trial. All the assigned trial regimens were administered as identical-appearing tablets.

The first planned interim analysis, which was conducted 3 months after the randomization of 200 patients, informed the decision to progress to phase 3. The data from these first 200 patients were excluded from the primary efficacy and safety analyses. Randomization continued during the 3 months after the 200th patient was enrolled, during which time approximately 200 additional patients were randomly assigned to a trial group, including 53 patients who were assigned to receive a 60-mg dose of verubecestat and whose data were excluded from the primary analyses. The patients who had been assigned to receive the 60-mg dose were switched to the 40-mg dose for the remainder of the trial. Patients who completed the 78-week trial period could enter the extension period, in which patients in the placebo group were switched to the 40-mg dose while patients who had been receiving the 12-mg or the 40-mg dose continued to receive the same dose to which they had been assigned, with preserved masking of doses. The trial design is described in detail in the protocol, available at NEJM.org.

An interactive voice-response system randomly assigned patients according to a computer-generated assignment schedule. Randomization was stratified according to geographic region, baseline severity of disease (mild [MMSE score of 21 to 26] or moderate [MMSE score of 15 to 20]), and use of memantine or anticholinesterase-inhibiting medications.

We performed biomarker substudies to evaluate certain biomarkers in cerebrospinal fluid and to assess amyloid burden with the use of positron-emission tomography (PET). All the patients enrolled in the trial were eligible for participation in the substudy of biomarkers in cerebrospinal fluid (provided that the investigative site where the patient was enrolled was willing to participate in the substudy), but the assessment of amyloid burden (PET amyloid substudy) was conducted only at sites that were near a PET ligand production facility at the start of enrollment and were willing to participate in the substudy.

Assessments

Evaluation of clinical efficacy included assessment of cognition according to the 11-item cognitive subscale of the Alzheimer’s Disease Assessment Scale (ADAS-cog; scores range from 0 to 70, with higher scores indicating worse dementia)15 and according to the MMSE,14 assessment of dementia according to the Clinical Dementia Rating Scale–Sum of Boxes (CDR-SB; scores range from 0 to 18, with higher scores indicating worse dementia),16 and assessment of daily function according to the 23-item version of the Alzheimer’s Disease Cooperative Study Activities of Daily Living Inventory scale (ADCS-ADL; scores range from 0 to 78, with lower scores indicating worse function).17 Sessions during which trial personnel performed the outcome assessments in patients were recorded, and a subset of these sessions underwent quality review by independent central experts, who provided feedback to the trial personnel for more than 15,000 interviews. Assessment of neuropsychiatric symptoms was performed with the use of the Neuropsychiatric Inventory (NPI; scores range from 1 to 144, with higher scores indicating more severe symptoms).18

Safety assessments included evaluation of adverse events, routine laboratory testing, electrocardiography, and physical examinations. Initially, routine MRI was performed to assess possible instances of amyloid-related imaging abnormalities, but the use of MRI was subsequently discontinued during the trial on the basis of regulatory feedback and feedback from the members of the data and safety monitoring committee, who indicated that it was no longer required. Comprehensive ophthalmologic and dermatologic examinations were also performed at baseline and at selected clinic visits as described in the protocol. Suicidality was assessed at every clinic visit with the use of the Columbia Suicide Severity Rating Scale, a six-question instrument that qualitatively rates the degree of suicidality.19

MRI structural measures of hippocampal volume were assessed by means of an automated segmentation method. The change in hippocampal volume was determined with the use of a tensor-based morphometry algorithm developed by Bioclinica. For the PET amyloid substudy, brain amyloid load was assessed by PET with the use of 18F-flutemetamol. A composite cortical index of amyloid burden was computed as the average of the regional standardized uptake value ratio in the following cortical areas, with a subcortical white-matter region used as the reference: frontal, temporal, and parietal lobes; the anterior and posterior cingulate cortex; and the precuneus.20 No partial volume correction was applied. For the substudy of biomarkers in cerebrospinal fluid, concentrations of total tau, phosphorylated tau, Aβ-40, Aβ-42, and sAPPβ were measured in a subgroup of patients.21

Oversight

The trial was conducted in accordance with International Conference on Harmonisation Good Clinical Practice guidelines and was approved by the relevant institutional review boards. Written informed consent was provided by the patients or their legal representatives. The sponsor (Merck) designed the trial in consultation with the academic authors. Data were collected by the investigators, analyzed by the sponsor, and interpreted by all the authors. The first draft of the manuscript was prepared by a professional medical writer (employed by the sponsor) and the first author. All the authors approved subsequent drafts and agreed to submit the manuscript for publication. The authors had full access to the trial data and vouch for the accuracy and completeness of the data and for the fidelity of the trial to the protocol. The trial was governed by three committees as described in the protocol.

Outcomes

The coprimary efficacy outcomes were the change from baseline in the score on the ADAS-cog and the ADCS-ADL at week 78. The secondary outcomes were the change from baseline at week 78 in the CDR-SB score, the total hippocampal volume as assessed by MRI, the concentration of total tau in cerebrospinal fluid, the brain amyloid load as assessed by PET, the MMSE score, and the NPI score. Exploratory outcomes included the change from baseline in other cerebrospinal fluid measures.

Statistical Analysis

The efficacy analyses were performed in the primary population, which included all patients who underwent randomization except the first 200 patients enrolled in the study and the patients who were randomly assigned to receive the 60-mg dose of verubecestat. We performed the efficacy analyses in a subgroup of the primary population — the full-analysis set — using a modified intention-to-treat approach. The full-analysis set included patients who received at least one dose of the trial regimen and who had both a baseline outcome measurement and at least one postrandomization outcome measurement that was obtained within a window of 6 weeks before to 6 weeks after a scheduled assessment visit. When individual subscores were missing, they may have been imputed (subject to restrictions) with the use of the last-observation-carried-forward method to enable the computation of a total score. No imputation was used for total scores. Sensitivity analyses that were planned to explore the results of the efficacy analyses under the assumption that data were not missing at random were not performed given the negative results of the trial.

We used a longitudinal analysis of covariance model to analyze changes in scores, with time considered to be a categorical variable. The model included adjustment for geographic region, trial-group assignment, sex, APOE4 genotype (carrier vs. noncarrier), baseline use of vitamin E (0 to 400 IU per day vs. >400 IU per day), baseline use of medication for Alzheimer’s disease (use vs. no use), trial cohort (safety cohort [patients enrolled before the decision to progress to phase 3] vs. main cohort [patients enrolled after the decision to progress to phase 3]), and the interaction between time and trial-group assignment, with the baseline values of MMSE score and age included as continuous covariates. The baseline value of the dependent variable and the interaction between the baseline value and time were also included. The mean differences between the trial groups (each verubecestat dose group vs. placebo) in the changes from baseline to week 78, as well as the confidence intervals and two-sided P values, were estimated from this model. An unstructured covariance matrix was used to model the correlation among repeated measurements. A Bonferroni correction (which was applied to both dose levels of verubecestat) in conjunction with a closed sequential testing approach was used to control for the type 1 error rate, with testing of primary outcomes and then secondary outcomes, in the order described in the statistical plan in the protocol.

All patients in the primary population who received at least one dose of verubecestat or placebo were included in the safety analyses. Prespecified adverse events of interest included amyloid-related imaging abnormalities of microhemorrhage, superficial siderosis, or macrohemorrhage and amyloid-related imaging abnormalities of incident vasogenic edema on MRI of the head; delirium; and clinically significant rash. All statistical analyses were performed with the use of SAS software, versions 9.3 and 9.4 (SAS Institute).

We calculated that 570 patients per trial group would be needed to provide the trial with 90% overall power to show a significant difference between at least one of the dose levels of verubecestat and placebo in both coprimary efficacy outcomes. This calculation was based on an anticipated dropout rate of 5.8% every 13 weeks (which would represent a 30% cumulative dropout rate at 78 weeks) and on an assumed drug effect of 35% for both dose levels (which would correspond to a 2-point difference in the ADAS-cog score and a 3.4-point difference in the ADCS-ADL score between patients receiving verubecestat and those receiving placebo, with 18-month rates of disease progression in the placebo group estimated primarily from data published by Schneider and Sano22). Although subgroup analyses that were based on patient characteristics were prespecified, the trial was not adequately powered for such analyses. Interim analyses performed during the trial are described in the protocol and in the Supplementary Appendix.