Characterization and Optimization of Benzimidazopyrimidine and Pyridoimidazopyridine Derivatives as Tau-SPECT Probes

Abstract

The accumulation of hyperphosphorylated tau protein in the brain is regarded as one of the hallmarks of Alzheimer's disease (AD). In vivo imaging of tau aggregates is helpful for diagnosis and monitoring of the progression of AD. In this study, we designed and synthesized novel radioiodinated benzimidazopyrimidine (BIPM) and pyridoimidazopyridine (PIP) derivatives with a monomethylamino, monoethylamino, monopropylamino, or diethylamino group as tau imaging probes for single-photon-emission computed tomography (SPECT). On in vitro autoradiography with AD brain sections, [¹²⁵I]PIP-NHMe showed the highest selective binding affinity for tau aggregates among the radioiodinated BIPM and PIP derivatives. In a biodistribution study using normal mice, [¹²⁵I]PIP-NHMe and [¹²⁵I]PIP-NHEt displayed high initial uptake (6.62 and 6.86% ID/g, respectively, at 2 min postinjection) into and rapid clearance from the brain, with brain_{2 min}/brain_{30 min} ratios of 38.9 and 28.6, respectively. These results suggest that [¹²³I]PIP-NHMe may be a novel SPECT probe that is useful for detecting tau aggregates in the AD brain.

Figure 1

Chemical structures of radioiodinated BIPM and PIP derivatives.

Scheme 1. Synthetic Route to BIPM and PIP Derivatives

Reagents: (a) di-tert-butyl dicarbonate, N,N-dimethylpyridine-4-amine, Et₃N, THF; (b) CH₃I, NaH, DMF; (c) CH₃CH₂I, NaH, DMF; (d) CH₃CH₂CH₂I, NaH, DMF; (e) 2,5-dibromoaniline, CuI, Cs₂CO₃, 1,10-phenanthroline, xylene; (f) 2,5-dibromopyridine-3-amine, CuI, Cs₂CO₃, 1,10-phenanthroline, xylene; (g) bis(tributyltin), Pd(PPh₃)₄, dioxane, Et₃N; (h) I₂, CHCl₃, (i) TFA, CH₂Cl₂.

Scheme 2. Radioiodination Reactions of BIPM and PIP Derivatives

Reagents: (a) (1) [¹²⁵I]NaI, H₂O₂, 1 N HCl, EtOH; (2) TFA, CHCl₃; (b) (1) [¹²⁵I]NaI, acetic acid, N-chlorosuccinimide, MeOH; (2) TFA, CHCl₃; (c) [¹²⁵I]NaI, H₂O₂, 1 N HCl, EtOH.

Figure 2

Comparison of in vitro autoradiography of (A, B) [¹²⁵I]27 ([¹²⁵I]BIPM-NHMe), (C, D) [¹²⁵I]28 ([¹²⁵I]BIPM-NHEt), (E, F) [¹²⁵I]29 ([¹²⁵I]BIPM-NHPr), and (G, H) [¹²⁵I]39 ([¹²⁵I]BIPM-NEt₂) in brain sections from an AD patient. A, C, E, and G show results in Aβ(+)/tau(−) brain sections. B, D, F, and H show results in Aβ(+)/tau(+) brain sections.

Figure 3

Comparison of in vitro autoradiography of (A, B) [¹²⁵I]30 ([¹²⁵I]PIP-NHMe), (C, D) [¹²⁵I]31 ([¹²⁵I]PIP-NHEt), (E, F) [¹²⁵I]32 ([¹²⁵I]PIP-NHPr), and (G, H) [¹²⁵I]40 ([¹²⁵I]PIP-NEt₂) in brain sections from an AD patient. A, C, E, and G show results in Aβ(+)/tau(−) brain sections. B, D, F, and H show results in Aβ(+)/tau(+) brain sections.

Figure 4

Quantitative analysis of [¹²⁵I]27 ([¹²⁵I]BIPM-NHMe) (A), [¹²⁵I]28 ([¹²⁵I]BIPM-NHEt) (B), [¹²⁵I]29 ([¹²⁵I]BIPM-NHPr) (C), [¹²⁵I]39 ([¹²⁵I]BIPM-NEt₂) (D), [¹²⁵I]30 ([¹²⁵I]PIP-NHMe) (E), [¹²⁵I]31 ([¹²⁵I]PIP-NHEt) (F), [¹²⁵I]32 ([¹²⁵I]PIP-NHPr) (G), and [¹²⁵I]40 ([¹²⁵I]PIP-NEt₂) (H) on in vitro autoradiography of AD brain sections. a: gray matter of Aβ(+)/tau(−) brain section, b: white matter of Aβ(+)/tau(−) brain section, c: gray matter of Aβ(+)/tau(+)brain section, d: white matter of Aβ(+)/tau(+) brain section.