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Abstract：
The sphingosine-1-phosphate receptor 1 (S1PR1) radiotracer [11C]CS1P1 has shown promise in proof-of-concept PET imaging of neuroinflammation in multiple sclerosis (MS). Our HPLC radiometabolite analysis of human plasma samples collected during PET scans with [11C]CS1P1 detected a radiometabolite peak that is more lipophilic than [11C]CS1P1. Radiolabeled metabolites that cross the blood-brain barrier complicate quantitative modeling of neuroimaging tracers; thus, characterizing such radiometabolites is important. Here, we report our detailed investigation of the metabolite profile of [11C]CS1P1 in rats, nonhuman primates, and humans. CS1P1 is a fluorine-containing ligand that we labeled with C-11 or F-18 for preclinical studies; the brain uptake was similar for both radiotracers. The same lipophilic radiometabolite found in human studies also was observed in plasma samples of rats and NHPs for CS1P1 labeled with either C-11 or F-18. We characterized the metabolite in detail using rats after injection of the nonradioactive CS1P1. To authenticate the molecular structure of this radiometabolite, we injected rats with 8 mg/kg of CS1P1 to collect plasma for solvent extraction and HPLC injection, followed by LC/MS analysis of the same metabolite. The LC/MS data indicated in vivo mono-oxidation of CS1P1 produces the metabolite. Subsequently, we synthesized three different mono-oxidized derivatives of CS1P1 for further investigation. Comparing the retention times of the mono-oxidized derivatives with the metabolite observed in rats injected with CS1P1 identified the metabolite as N-oxide 1, also named TZ82121. The MS fragmentation pattern of N-oxide 1 also matched that of the major metabolite in rat plasma. To confirm that metabolite TZ82121 does not enter the brain, we radiosynthesized [18F]TZ82121 by the oxidation of [18F]FS1P1. Radio-HPLC analysis confirmed that [18F]TZ82121 matched the radiometabolite observed in rat plasma post injection of [18F]FS1P1. Furthermore, the acute biodistribution study in SD rats and PET brain imaging in a nonhuman primate showed that [18F]TZ82121 does not enter the rat or nonhuman primate brain. Consequently, we concluded that the major lipophilic radiometabolite N-oxide [11C]TZ82121, detected in human plasma post injection of [11C]CS1P1, does not enter the brain to confound quantitative PET data analysis. [11C]CS1P1 is a promising S1PR1 radiotracer for detecting S1PR1 expression in the CNS.
摘要：
鞘氨醇-1-磷酸受体1（S1PR1）放射性示踪剂[11C]CS1P1在多发性硬化症（MS）的神经炎症的概念验证PET成像中显示出希望。我们用[11C]CS1P1在PET扫描期间收集的人血浆样品的HPLC放射性代谢物分析检测到比[11C]CS1P1更亲脂性的放射性代谢物峰。穿过血脑屏障的放射性标记代谢物使神经影像示踪剂的定量建模复杂化；因此，表征这种放射性代谢物是重要的。这里,我们报告了我们对大鼠[11C]CS1P1代谢物谱的详细调查，非人灵长类动物,和人类。CS1P1是含氟配体，我们用C-11或F-18标记用于临床前研究；两种放射性示踪剂的脑摄取相似。在用C-11或F-18标记的CS1P1的大鼠和NHP的血浆样品中也观察到在人类研究中发现的相同亲脂性放射性代谢物。我们在注射非放射性CS1P1后使用大鼠详细表征了代谢物。为了验证这种放射性代谢物的分子结构，我们给大鼠注射8mg/kg的CS1P1以收集血浆进行溶剂萃取和HPLC注射，随后进行相同代谢物的LC/MS分析。LC/MS数据表明CS1P1的体内单氧化产生代谢物。随后，我们合成了CS1P1的三种不同的单氧化衍生物用于进一步研究。比较单氧化衍生物与在注射CS1P1的大鼠中观察到的代谢物的保留时间，将代谢物鉴定为N-氧化物1，也称为TZ82121。N-氧化物1的MS碎片模式也与大鼠血浆中主要代谢物的MS碎片模式相匹配。为了确认代谢物TZ82121不会进入大脑，我们通过[18F]FS1P1的氧化放射性合成了[18F]TZ82121。放射性HPLC分析证实[18F]TZ82121与注射[18F]FS1P1后在大鼠血浆中观察到的放射性代谢物相匹配。此外，SD大鼠的急性生物分布研究和非人灵长类动物的PET脑成像显示[18F]TZ82121不进入大鼠或非人灵长类动物的大脑.因此,我们得出的结论是，注射[11C]CS1P1后在人血浆中检测到的主要亲脂性放射性代谢物N-氧化物[11C]TZ82121不会进入大脑，从而无法进行定量PET数据分析。[11C]CS1P1是用于检测中枢神经系统中S1PR1表达的有前途的S1PR1放射性示踪剂。
