DEVELOPMENT OF NEW PET AND SPECT RADIOTRACERS AND NEW APPROACHES TO PET DATA

新型 PET 和 SPECT 放射示踪剂的开发以及 PET 数据的新方法

• 批准号: 6431947
• 负责人: ALANE S KIMES
• 金额: --
• 依托单位: NATIONAL INSTITUTE ON DRUG ABUSE
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/6431947
• 关键词: Primates; autoradiography; chemical synthesis; iodine; nicotinic receptors; positron emission tomography; radiotracer; single photon emission computed tomography; technology /technique development

The features of novel radiotracers, specifically the 5-iodo, 2- fluoro, 6-fluoro and 5-iodo-6-fluoro analogs of A-85380, developed to image the nicotinic acetylcholine receptors (nAChRs) with positron emission tomography (PET) and single photon emission computed tomography (SPECT) have been further investigated in terms subtype selectivity and toxicity. Detailed kinetic and saturation analyses confirmed that as with 5-[I-125] iodo-A85380, 2-fluoro-A-85380 labels with very high affinity a homogeneous population of binding sites, presumably the alpha4-beta2 subtype of nAChRs in rodent and human brain. Results of competition assays for four different types of receptors demonstrated that the affinities of 5-iodo- and 2-fluoro-A-85380 for alpha4beta2 receptors exceed their affinities for the mammalian alpha7, muscular, and alpha3beta4 subtypes by three to five orders of magnitude. Further, 5-[I-125] iodo-A85380 shows no binding in any brain region in "knockout" mice lacking the beta2 subunit of nAChRs. These findings and in vivo studies in non-human primates demonstrate that [F-18]2-fluoro-A85380 is superior to other radioligands available for selectively imaging alpha4beta2 nAChRs with PET. Toxicology studies demonstrated that at relatively high doses these ligands have convulsant properties and affect the cardiovascular system. Convulsions in mice were manifested at doses approximately 1000 to 10,000 times larger than doses needed to image nAChRs, demonstrating that the A-85380-related compounds have a wider margin of safety than new imaging ligands that are derivatives of the toxin epibatidine. In preliminary cardiovascular studies in unanesthetized rats, 5-iodo-A-85380, 2-fluoro-A-85380 and nicotine produced equivalent, modest transient increases in blood pressure, but only nicotine produced marked decreases in heart rate, suggesting that derivatives of A-85380 are less cardiotoxic than nicotine. [F-18]-2-Fluoro-A-85380 crosses the blood-brain barrier and distributes in the brains of mice and Rhesus monkeys in proportion to known densities of alpha4beta2 nAChRs. Estimating the ratio of specific to non-specific binding in monkey using the cerebellum, which contains few alpha4beta2 nAChRs, showed a high level of specific binding in the thalamus at 4 h after radiotracer administration. The specificity of binding was confirmed by blocking and displacement studies with cytisine indicating that 2-[F-18]-F-A-85380 is an excellent candidate for PET imaging of central nAChRs in human subjects. Another goal has been to improve PET-FDG methodology by using venous instead of arterial blood sampling to calculate CMRglc. There was a good agreement between the time curves of FDG and glucose concentration obtained from arterial and venous blood samples and values of CMRglc calculated using arterial and venous blood in anesthetized non human primates (NHPs). Substitution of arterial blood sampling by venous sampling will simplify the experimental procedure as applied to both humans and NHPs without a significant loss of accuracy and expands the ability to conduct multiple PET-FDG studies on the same subject.

用正电子发射断层扫描和单光子发射计算机断层扫描对新型放射性示踪剂，特别是A-85380的5-碘、2-氟、6-氟和5-碘-6-氟类似物进行了进一步的亚型选择性和毒性研究。详细的动力学和饱和度分析证实，与5-[I-125]碘-A85380、2-氟-A-85380具有很高亲和力的标记一样，在啮齿动物和人脑中有一组均一的结合位点，推测是α4-β2亚型的nAChRs。四种不同类型受体的竞争分析结果表明，5-碘和2-氟-A-85380对α4β2受体的亲和力比它们对哺乳动物α7、肌肉和α3β4亚型的亲和力高三到五个数量级。此外，5-[I-125]IODO-A85380在缺乏nAChRs Beta2亚基的“基因敲除”小鼠的任何大脑区域都没有结合。这些发现和在非人类灵长类动物体内的研究表明，[F-18]2-氟-A85380比其他可用于用PET选择性地成像α4β2 nAChRs的放射性配基更优越。毒理学研究表明，在相对较高的剂量下，这些配体具有惊厥特性，并影响心血管系统。在小鼠中表现出惊厥的剂量大约是成像nAChRs所需剂量的1,000到10,000倍，这表明与A-85380相关的化合物比作为毒素表巴替丁衍生物的新成像配体具有更广泛的安全边际。在对未麻醉的大鼠进行的心血管初步研究中，5-碘-A-85380、2-氟-A-85380和尼古丁产生了同等的、温和的一过性血压升高，但只有尼古丁能显著降低心率，这表明A-85380的衍生物对心脏的毒性低于尼古丁。[F-18]-2-Fluoro-A-85380可通过血脑屏障，以已知的α4β2 nAChRs密度分布于小鼠和猕猴的脑内。用含有少量α4β2 nAChRs的小脑估算猴的特异性结合与非特异性结合的比率，发现在注射放射性示踪剂后4小时，丘脑的特异性结合水平很高。2-[F-18]-F-A-85380与胞嘧啶的阻断和置换研究证实了结合的特异性，表明2-[F-18]-F-A-85380是一种很好的人体中枢nAChRs的正电子发射计算机断层成像的候选分子。另一个目标是改进PET-FDG方法，使用静脉血样本而不是动脉血样本来计算CMRglc。在麻醉的非人灵长类动物(NHP)中，由动、静脉血测得的FDG和葡萄糖浓度的时间曲线与由动、静脉血计算的CMRglc值符合良好。用静脉采血替代动脉采血将简化适用于人类和NHP的实验程序，而不会显著损失准确性，并扩大了对同一对象进行多项PET-FDG研究的能力。