Discriminative binding of tau PET tracers PI2620, MK6240 and RO948 in Alzheimer’s disease, corticobasal degeneration and progressive supranuclear palsy brains

Abstract

Recent mechanistic and structural studies have challenged the classical tauopathy classification approach and revealed the complexity and heterogeneity of tau pathology in Alzheimer’s disease (AD) and primary tauopathies such as corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP), progressing beyond distinct tau isoforms. In this multi-tau tracer study, we focused on the new second-generation tau PET tracers PI2620, MK6240 and RO948 to investigate this tau complexity in AD, CBD, and PSP brains using post-mortem radioligand binding studies and autoradiography of large and small frozen brain sections. Saturation binding studies indicated multiple binding sites for 3H-PI2620 in AD, CBD and PSP brains with different binding affinities (Kd ranging from 0.2 to 0.7 nM) and binding site densities (following the order: BmaxAD > BmaxCBD > BmaxPSP). Competitive binding studies complemented these findings, demonstrating the presence of two binding sites [super-high affinity (SHA): IC50(1) = 8.1 pM; and high affinity (HA): IC50(2) = 4.9 nM] in AD brains. Regional binding distribution studies showed that 3H-PI2620 could discriminate between AD (n = 6) and control cases (n = 9), especially in frontal cortex and temporal cortex tissue (p < 0.001) as well as in the hippocampal region (p = 0.02). 3H-PI2620, 3H-MK6240 and 3H-RO948 displayed similar binding behaviour in AD brains (in both homogenate competitive studies and one large frozen hemispherical brain section autoradiography studies) in terms of binding affinities, number of sites and regional patterns. Our small section autoradiography studies in the frontal cortex of CBD (n = 3) and PSP brains (n = 2) showed high specificity for 3H-PI2620 but not for 3H-MK6240 or 3H-RO948. Our findings clearly demonstrate different binding properties among the second-generation tau PET tracers, which may assist in further understanding of tau heterogeneity in AD versus non-AD tauopathies and suggests potential for development of pure selective 4R tau PET tracers.

In Silico Discovery and Subsequent Characterization of Potent 4R-Tauopathy Positron Emission Tomography Radiotracers

“…[ 18 F]­florzolotau and [ 18 F]­APN-1607) and [ 18 F]­PI-2620 are the most advanced (Figure ). Preclinical investigation of these ligands with autoradiography demonstrated that these ligands bind both AD (3R/4R) and non-AD 4R tauopathies. , Furthermore, human imaging studies with [ 18 F]­PI-2620 and [ 18 F]­PM-PBB3 show that they may be capable of detecting 4R-tau deposits in non-AD tauopathies. – While encouraging, the PET outcome measures (SUVR or DVR) of these radioligands in non-AD tauopathies were modest, and the tracers require further characterization before entering routine clinical use. , …”

Efficient characterization of multiple binding sites of small molecule imaging ligands on amyloid-beta, 4-repeat/full-length tau and alpha-synuclein

“…For fluorescent probes, radiometric analysis has been used to estimate the binding sites with less accuracy. Using radioligand binding assays, multiple ligand binding sites on Aβ fibrils (LeVine, 2005; Ni et al, 2017; Ni et al, 2013a; Ni et al, 2021a) and tau fibrils (Malarte et al, 2020; Ni et al, 2018; Yap et al, 2021), e.g., using THK-5351 (Lemoine et al, 2017; Lemoine et al, 2020; Stepanov et al, 2017), MK-6240 (Lemoine et al, 2021; Malarte et al, 2022; Malarte et al, 2020), and PBB3 (Ono et al, 2017), have been reported. Different tau ligands show distinct binding towards tau in AD and different primary tauopathy (Shi et al, 2021a; Yap et al, 2021).…”

Insight into the Binding of First- and Second-Generation PET Tracers to 4R and 3R/4R Tau Protofibrils

“…The binding mode of MK6240 on PSP is, therefore, different in all of the sites on the AD protofibril (Figure 6D), which is also observed in a previous small section of autoradiography. 31 The time length of simulations may impact the distribution of the tracer most contacted sites. Additionally, we performed 70 (starting from sites S1-S7 for each of the 10 tracers) 500-ns MD simulations for the tracers on PSP protofibril with new seeds generating initial velocities.…”

“…37 As discussed above, in our settings of CVs, the dynamics of protein are not greatly perturbated by the bias potential. CBD2115, PI2620, Flortaucipir, and PMPBB3 are the know tracers that can bind both 3R/4R and 4R tau fibrils, [25][26][27]31,46 while MK6240 shows few bindings to the 4R tau fibrils. At an overall level, the well depth and numbers of local minima on these FESs also indicate the better PSP binding preference for PMPBB3, CBD2115, Flortaucipir, and PI2620 than MK6240 (Figure 7).…”