[18F]GTP1 (Genentech Tau Probe 1), a radioligand for detecting neurofibrillary tangle tau pathology in Alzheimer’s disease

Objective Neurofibrillary tangles (NFTs), consisting of intracellular aggregates of the tau protein, are a pathological hallmark of Alzheimer’s disease (AD). Here we report the identification and initial characterization of Genentech Tau Probe 1 ([ 18 F]GTP1), a small-molecule PET probe for imaging tau pathology in AD patients. Methods Autoradiography using human brain tissues from AD donors and protein binding panels were used to determine [ 18 F]GTP1 binding characteristics. Stability was evaluated in vitro and in vivo in mice and rhesus monkey. In the clinic, whole-body imaging was performed to assess biodistribution and dosimetry. Dynamic [ 18 F]GTP1 brain imaging and input function measurement were performed on two separate days in 5 β-amyloid plaque positive (Aβ+) AD and 5 β-amyloid plaque negative (Aβ-) cognitive normal (CN) participants. Tracer kinetic modeling was applied and reproducibility was evaluated. SUVR was calculated and compared to [ 18 F]GTP1-specific binding parameters derived from the kinetic modeling. [ 18 F]GTP1 performance in a larger cross-sectional group of 60 Aβ+ AD participants and ten (Aβ- or Aβ+) CN was evaluated with images acquired 60 to 90 min post tracer administration. Results [ 18 F]GTP1 exhibited high affinity and selectivity for tau pathology with no measurable binding to β-amyloid plaques or MAO-B in AD tissues, or binding to other tested proteins at an affinity predicted to impede image data interpretation. In human, [ 18 F]GTP1 exhibited favorable dosimetry and brain kinetics, and no evidence of defluorination. [ 18 F]GTP1-specific binding was observed in cortical regions of the brain predicted to contain tau pathology in AD and exhibited low (< 4%) test-retest variability. SUVR measured in the 60 to 90-min interval post injection correlated with tracer-specific binding (slope = 1.36, r 2 = 0.98). Furthermore, in a cross-sectional population, the degree of [ 18 F]GTP1-specific binding increased with AD severity and could differentiate diagnostic cohorts. Conclusions [ 18 F]GTP1 is a promising PET probe for the study of tau pathology in AD.