Department of Biofunctional Analysis, Graduate School of Pharmaceutical Sciences, Osaka Medical and Pharmaceutical University, 4-20-1 Nasahara, Takatsuki, Osaka, 569-1094, Japan.
Biomedical Imaging Research Center, University of Fukui, 23-3 Matsuoka-shimoaizuki, Eiheiji-cho, Yoshida-gun, Fukui, 910-1193, Japan.
Ann Nucl Med. 2022 Oct;36(10):845-852. doi: 10.1007/s12149-022-01764-2. Epub 2022 Jun 27.
p38α, a member of the mitogen-activated protein kinase superfamily, is activated by external stimuli, followed by nuclear translocation for the regulation of inflammatory responses at the transcriptional and translational levels in inflammatory diseases. Thus, activated p38α would be an appropriate target molecule for in vivo noninvasive imaging and targeted radionuclide therapy. For this purpose, we designed a radiobrominated compound, 6-(4-[Br]bromo-2-fluorophenoxy)-8-methyl-2-(tetrahydro-2H-pyran-4-ylamino)-pyrido[2,3-d]pyrimidin-7(8H)-one ([Br]4-BR), based on a potent p38α selective inhibitor, R1487, for use with single-photon emission computed tomography. We synthesized [Br]4-BR and evaluated its effectiveness as an activated p38α imaging probe compared with our previous radioiodinated probe (6-(2-fluoro-4-[I]iodophenoxy)-8-methyl-2-(tetrahydro-2H-pyran-4-ylamino)-pyrido[2,3-d]pyrimidin-7(8H)-one ([I]4-IR)) in a mouse inflammatory model.
We designed [Br]4-BR by replacing the radioiodine of [I]4-IR or the fluorine of R1487 with radiobromine at the 4-position of the phenoxy ring. We synthesized 4-BR via a four-step process. The inhibitory potency of 4-BR was measured using an ADP-Glo™ kinase assay system. Radiosynthesis of [Br]4-BR was performed via an organotin-radiobromine exchange reaction using the corresponding tributyltin precursor. Radioactivity biodistribution was evaluated in normal ddY mice and turpentine oil-induced inflammation model mice for 120 min after intravenous administration of [Br]4-BR. The temporal changes in radioactivity in blood fractions were compared between [Br]4-BR and [I]4-IR.
4-BR was synthesized at a total yield of 9.1% and showed a p38α inhibitory potency similar to that of 4-IR. [Br]4-BR was successfully obtained from a tributyltin precursor with high radiochemical yield (89.9%), purity (95.9%), and molar activity (2.0 TBq/µmol). [Br]4-BR showed accumulation of high radioactivity in the inflamed tissue (3.4% ± 0.9% ID/g, peaking at 15 min), rapid delivery throughout the body, and rapid blood clearance with approximately half of the blood radioactivity existing as an intact form at 60 min. Although the maximum radioactivity accumulation in inflamed tissue after [Br]4-BR administration was approximately half that of [I]4-IR because of its faster blood clearance and lower free fraction in the input function, the inflamed tissue-to-blood ratio was comparable between [Br]4-BR and [I]4-IR.
[Br]4-BR would be a promising imaging agent for detecting activated p38α in inflammatory diseases.
丝裂原活化蛋白激酶超家族的成员 p38α 被外部刺激激活，随后在转录和翻译水平上发生核易位，以调节炎症反应。因此，激活的 p38α 将是体内非侵入性成像和靶向放射性核素治疗的合适靶标分子。为此，我们设计了一种放射性溴化化合物 6-（4-[Br]溴-2-氟苯氧基）-8-甲基-2-（四氢-2H-吡喃-4-基氨基）-吡啶并[2,3-d]嘧啶-7(8H)-one([Br]4-BR)，基于一种有效的 p38α 选择性抑制剂 R1487，用于单光子发射计算机断层扫描。我们合成了[Br]4-BR，并在小鼠炎症模型中评估了其作为激活的 p38α 成像探针的有效性，与我们之前的放射性碘标记探针（6-（2-氟-4-[I]碘苯氧基）-8-甲基-2-（四氢-2H-吡喃-4-基氨基）-吡啶并[2,3-d]嘧啶-7(8H)-one([I]4-IR)）进行了比较。
我们通过在苯氧基环的 4 位用放射性溴取代[I]4-IR 的放射性碘或 R1487 的氟，设计了[Br]4-BR。我们通过四步反应合成了 4-BR。使用 ADP-Glo™激酶测定系统测量 4-BR 的抑制效力。通过使用相应的三丁基锡前体进行有机锡-放射性溴交换反应来进行[Br]4-BR 的放射性合成。在静脉注射[Br]4-BR 后 120 分钟，在正常 ddY 小鼠和松节油诱导的炎症模型小鼠中评估[Br]4-BR 的放射性生物分布。比较了[Br]4-BR 和[I]4-IR 之间血液各部分放射性的时间变化。
4-BR 的总收率为 9.1%，显示出与 4-IR 相似的 p38α 抑制效力。[Br]4-BR 可从三丁基锡前体中以高放射化学产率（89.9%）、高纯度（95.9%）和高摩尔活性（2.0 TBq/µmol）获得。[Br]4-BR 在炎症组织中积累了高放射性（3.4%±0.9% ID/g，在 15 分钟时达到峰值），全身迅速分布，并在 60 分钟时迅速清除血液，约有一半的血液放射性以完整形式存在。尽管由于[Br]4-BR 的血液清除速度较快和输入函数中的游离分数较低，其在炎症组织中的最大放射性积累约为[I]4-IR 的一半，但[Br]4-BR 和[I]4-IR 之间的炎症组织与血液的放射性比值相当。
[Br]4-BR 将是一种有前途的成像剂，可用于检测炎症性疾病中的激活的 p38α。
