P1-244: Comparing [3 H]MK-6240 and [3 H]AV-1451(T-807) in in VITRO Binding Studies in Brain Tissues

Alzheimer’s disease (AD) is the most common cause of dementia. Neurofibrillary tangles (NFTs), which consist of tau protein aggregates, are a pathological hallmark of AD brain. NFT levels are associated with the severity of dementia and disease progression. Thus, therapies to reduce brain NFTs may be beneficial to disease modification. A tau PET tracer for imaging brain tauopathy could aid in the development of such therapies. Here, we report in vitro study of a novel tau PET tracer, [3H]MK-6240, in comparison to [3H]AV-1451. Frozen AD and non-AD brains were used in the study. [3H]MK-6240 and [3H]AV-1451 were synthesized in house. Autoradiography (ARG) was performed using frozen brain slices. Brain homogenates were used for the filtration binding assays. Self-block, T-808, Pittsburgh Compound-B (PIB) and MAO-A inhibitor (clorgyline) were used as blockers for defining binding specificity of tracers. Immunohistochemistry (IHC) was performed using antibodies made against amyloid plaque (6E10) and NFTs (PHF6 & AT8) with brain slices adjacent to those of ARG study. In AD brain slices, [3H]MK-6240 binds AD tauopathy brain regions showing abundant NFTs. [3H]MK-6240 binding is inhibited by self-block and T-808, but not by PIB. [3H]MK-6240 binding patterns in NFT-rich AD brain slices are similar to IHC stain patterns of NFTs from the adjacent tissue slices. In non-AD brain slices, [3H]MK-6240 exhibits minimal binding while [3H]AV-1451 displays dense displaceable binding in the gray matter of cortex and hippocampus by self-block, but not by T-808. In tissue homogenate binding assays, [3H]MK-6240 shows great binding potential (Bmax/Kd = 226) in NFT-rich AD brain cortex by self-block. [3H]MK-6240 binds to one site with high affinity (Kd = 0.28 ± 0.09 nM). In non-AD brain cortex, [3H]MK-6240 displays minimal non-saturable binding. In contrast, [3H]-AV-1451 binds to more than one site in AD brain and shows displaceable binding in non-AD brain. Clorgyline inhibits [3H]AV-1451 but not MK-6240 binding in non-AD brain cortex (Ki = 0.43 nM). The study demonstrates [3H]MK-6240 is a selective tau PET tracer with great binding potential and minimal non-specific binding in human AD brains. The data supports further development of MK-6240 as a tau-selective PET tracer.