PT - JOURNAL ARTICLE AU - Olafson, Emily AU - Tonietto, Matteo AU - Klein, Gregory AU - Teng, Edmond AU - Stephens, Andrew W. AU - Russell, David S. AU - Pickthorn, Karen AU - Sanabria Bohorquez, Sandra TI - In Vivo Head-to-Head Comparison of [<sup>18</sup>F]GTP1 with [<sup>18</sup>F]MK-6240 and [<sup>18</sup>F]PI-2620 in Alzheimer Disease AID - 10.2967/jnumed.124.268623 DP - 2025 Jan 02 TA - Journal of Nuclear Medicine PG - jnumed.124.268623 4099 - http://jnm.snmjournals.org/content/early/2025/01/02/jnumed.124.268623.short 4100 - http://jnm.snmjournals.org/content/early/2025/01/02/jnumed.124.268623.full AB - Alzheimer disease (AD) is characterized by the accumulation of tau neurofibrillary tangles that can be labeled with PET tracers. Multiple tau PET tracers have been used in clinical studies, including [18F]GTP1, [18F]PI-2620, and [18F]MK-6240. Standardized harmonization scales for comparing tau PET signals across tracers are currently under development and can be informed by comparisons of signals between tracers in both target and off-target regions of the brain. Methods: We conducted a head-to-head study comparing [18F]GTP1 with [18F]PI-2620 and [18F]MK-6240 in terms of dynamic range, magnitude of uptake, and correlation between tracers in participants with normal cognition and prodromal to mild AD. Results: [18F]GTP1 exhibited retention patterns that correlated with [18F]PI-2620 and [18F]MK-6240 for all Braak regions (except Braak II). Differences in tracer binding in AD target regions were relatively small, and off-target binding profiles were unique to each tracer. Conclusion: Our findings indicate that [18F]GTP1, [18F]PI-2620, and [18F]MK-6240 display similar uptake patterns in AD patients, suggesting that they detect the same tau pathology. However, the tracer-specific off-target signal distribution may impact their direct comparability, and for some use cases, tracer-specific considerations should be taken into account in the development of a standardized harmonization scale for tau PET.