Characterization of the binding of tau imaging ligands to melanin-containing cells: putative off-target-binding site

Objective: Amyloid-β plaques and neurofibrillary tangles composed of tau protein are the neuropathological hallmarks of Alzheimer’s disease. In recent years, marked progress has been made in Alzheimer’s disease research using tau ligands for positron emission tomography (PET). However, the issue of off-target binding, that is, the binding of ligands to regions without tau pathology, remains unresolved. Tissues with melanin-containing cells (MCCs) have been suggested as binding targets for tau ligands. In the present study, we characterized the MCC-binding properties of representative tau PET ligands. Methods: Autoradiographic studies of [¹⁸F]AV-1451 and [¹⁸F]THK5351 were conducted using postmortem human midbrain sections. Saturation-binding assays of [¹⁸F]AV-1451 and [¹⁸F]THK5351 were performed with B16F10 melanoma cells. The blocking effects of 25 compounds against [¹⁸F]THK5351 binding to B16F10 cells were used to investigate the relationship between chemical structure and MCC binding. Results: Autoradiography demonstrated specific binding of the radioligands in the substantia nigra. [¹⁸F]AV-1451 and [¹⁸F]THK5351 exhibited saturable binding to melanoma cells ([¹⁸F]AV-1451: K_d = 669 ± 196 nM, B_max = 622 ± 269 pmol/mg protein; [¹⁸F]THK5351: K_d = 441 ± 126 nM, B_max = 559 ± 75.5 pmol/mg protein). In blocking studies with melanoma cells, compounds bearing multiple aromatic rings and an aminopyridine group, including tau ligands such as AV-1451, PBB3, and a lead compound of MK-6240, exhibited the inhibition of [¹⁸F]THK5351 binding comparable to self-blocking by THK5351 (> 70% at 10 µM). Conclusions: These studies suggest that the binding properties of [¹⁸F]AV-1451 and [¹⁸F]THK5351 are sufficient to expect highlighting of tissues with a high density of MCCs. The findings of the present study should aid the development of neuroimaging ligands that do not bind to MCC.