Development of radioiodinated pyrimidinopyridone derivatives as targeted imaging probes of activated p38α for single photon emission computed tomography

p38α, a member of the mitogen-activated protein kinase superfamily, is ubiquitously expressed in a variety of mammalian cells. Activated p38α induces inflammatory responses to external stimuli, suggesting that non-invasive detection of activated p38α would be valuable for diagnosing inflammatory diseases. For this purpose, we designed radiolabeled compounds [¹²³I]2-IR and [¹²³I]4-IR based on a potent p38α selective inhibitor R1487 for use with single photon emission computed tomography (SPECT). In this study, we used ¹²⁵I instead of ¹²³I due to its more usable radiochemical properties, synthesized [¹²⁵I]2-IR and [¹²⁵I]4-IR, and evaluated their effectiveness as activated p38α imaging probes.

[¹²³I]2-IR and [¹²³I]4-IR were designed by introduction of a ¹²³I atom at the 2- or 4-ositions of the phenoxy ring, preserving the pyrimidinopyridone structure of R1487. We synthesized 2-IR and 4-IR via a 7-step process. The inhibitory potencies of 2-IR, 4-IR, and p38α inhibitors were measured using an ADP-Glo™ kinase assay system. Radioiodination of 2-IR and 4-IR was performed via an organotin-radioiodine exchange reaction using the corresponding tributyltin precursors. Biodistributions were evaluated by determining radioactivity in tissues of interest after intravenous administration of [¹²⁵I]2-IR and [¹²⁵I]4-IR in normal ddY mice and turpentine oil-induced inflammation model mice. In vivo inhibition study was also performed in inflammation model mice after intravenous administration of [¹²⁵I]4-IR with pretreatment of p38α inhibitors.

We synthesized 2-IR and 4-IR at total yields of 17.5% and 19.2%, respectively. 4-IR had higher p38α inhibitory potency than 2-IR; both compounds were significantly less potent than R1487. [¹²⁵I]2-IR and [¹²⁵I]4-IR were successfully obtained from tributyltin precursors with high radiochemical yield (> 65%), purity (> 97%), and molar activity (~ 81 GBq/µmol). [¹²⁵I]4-IR showed high radioactivity accumulation in the inflamed tissue (7.0 ± 1.2%D/g), rapid delivery throughout the body, and rapid blood clearance, resulting in a high inflammation-to-blood ratio (6.2 ± 0.4) and a high inflammation-to-muscle ratio (5.2 ± 1.3) at 30 min, while [¹²⁵I]2-IR showed low radioactivity accumulation in inflamed tissue over the experimental period. Further, radioactivity accumulation in inflamed tissue after [¹²⁵I]4-IR administration was significantly decreased by pretreatment with selective inhibitors.

[¹²³I]4-IR would be a promising imaging agent for detection of activated p38α.