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

Objective p38 alpha, a member of the mitogen-activated protein kinase superfamily, is ubiquitously expressed in a variety of mammalian cells. Activated p38 alpha induces inflammatory responses to external stimuli, suggesting that non-invasive detection of activated p38 alpha would be valuable for diagnosing inflammatory diseases. For this purpose, we designed radiolabeled compounds [I-123]2-IR and [I-123]4-IR based on a potent p38 alpha selective inhibitor R1487 for use with single photon emission computed tomography (SPECT). In this study, we used I-125 instead of I-123 due to its more usable radiochemical properties, synthesized [I-125]2-IR and [I-125]4-IR, and evaluated their effectiveness as activated p38 alpha imaging probes. Methods [I-123]2-IR and [I-123]4-IR were designed by introduction of a I-123 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 p38 alpha inhibitors were measured using an ADP-Glo (TM) 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 [I-125]2-IR and [I-125]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 [I-125]4-IR with pretreatment of p38 alpha inhibitors. Results We synthesized 2-IR and 4-IR at total yields of 17.5% and 19.2%, respectively. 4-IR had higher p38 alpha inhibitory potency than 2-IR; both compounds were significantly less potent than R1487. [I-125]2-IR and [I-125]4-IR were successfully obtained from tributyltin precursors with high radiochemical yield (> 65%), purity (> 97%), and molar activity (similar to 81 GBq/mu mol). [I-125]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 [I-125]2-IR showed low radioactivity accumulation in inflamed tissue over the experimental period. Further, radioactivity accumulation in inflamed tissue after [I-125]4-IR administration was significantly decreased by pretreatment with selective inhibitors. Conclusions [I-123]4-IR would be a promising imaging agent for detection of activated p38 alpha.