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

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 (3)H-PI2620 in AD, CBD and PSP brains with different binding affinities (K(d) ranging from 0.2 to 0.7 nM) and binding site densities (following the order: B(max)AD > B(max)CBD > B(max)PSP). Competitive binding studies complemented these findings, demonstrating the presence of two binding sites [super-high affinity (SHA): IC(50(1)) = 8.1 pM; and high affinity (HA): IC(50(2)) = 4.9 nM] in AD brains. Regional binding distribution studies showed that (3)H-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). (3)H-PI2620, (3)H-MK6240 and (3)H-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 (3)H-PI2620 but not for (3)H-MK6240 or (3)H-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.