Longitudinal head-to-head comparison of C-11-PiB and F-18-florbetapir PET in a Phase 2/3 clinical trial of anti-amyloid-beta monoclonal antibodies in dominantly inherited Alzheimer's disease

PurposePittsburgh Compound-B (C-11-PiB) and F-18-florbetapir are amyloid-beta (A beta) positron emission tomography (PET) radiotracers that have been used as endpoints in Alzheimer's disease (AD) clinical trials to evaluate the efficacy of anti-A beta monoclonal antibodies. However, comparing drug effects between and within trials may become complicated if different A beta radiotracers were used. To study the consequences of using different A beta radiotracers to measure A beta clearance, we performed a head-to-head comparison of C-11-PiB and F-18-florbetapir in a Phase 2/3 clinical trial of anti-A beta monoclonal antibodies.MethodsSixty-six mutation-positive participants enrolled in the gantenerumab and placebo arms of the first Dominantly Inherited Alzheimer Network Trials Unit clinical trial (DIAN-TU-001) underwent both C-11-PiB and F-18-florbetapir PET imaging at baseline and during at least one follow-up visit. For each PET scan, regional standardized uptake value ratios (SUVRs), regional Centiloids, a global cortical SUVR, and a global cortical Centiloid value were calculated. Longitudinal changes in SUVRs and Centiloids were estimated using linear mixed models. Differences in longitudinal change between PET radiotracers and between drug arms were estimated using paired and Welch two sample t-tests, respectively. Simulated clinical trials were conducted to evaluate the consequences of some research sites using C-11-PiB while other sites use F-18-florbetapir for A beta PET imaging.ResultsIn the placebo arm, the absolute rate of longitudinal change measured by global cortical C-11-PiB SUVRs did not differ from that of global cortical F-18-florbetapir SUVRs. In the gantenerumab arm, global cortical C-11-PiB SUVRs decreased more rapidly than global cortical F-18-florbetapir SUVRs. Drug effects were statistically significant across both A beta radiotracers. In contrast, the rates of longitudinal change measured in global cortical Centiloids did not differ between A beta radiotracers in either the placebo or gantenerumab arms, and drug effects remained statistically significant. Regional analyses largely recapitulated these global cortical analyses. Across simulated clinical trials, type I error was higher in trials where both A beta radiotracers were used versus trials where only one A beta radiotracer was used. Power was lower in trials where F-18-florbetapir was primarily used versus trials where C-11-PiB was primarily used.ConclusionGantenerumab treatment induces longitudinal changes in A beta PET, and the absolute rates of these longitudinal changes differ significantly between A beta radiotracers. These differences were not seen in the placebo arm, suggesting that A beta-clearing treatments may pose unique challenges when attempting to compare longitudinal results across different A beta radiotracers. Our results suggest converting A beta PET SUVR measurements to Centiloids (both globally and regionally) can harmonize these differences without losing sensitivity to drug effects. Nonetheless, until consensus is achieved on how to harmonize drug effects across radiotracers, and since using multiple radiotracers in the same trial may increase type I error, multisite studies should consider potential variability due to different radiotracers when interpreting A beta PET biomarker data and, if feasible, use a single radiotracer for the best results.