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

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，在正电子发射断层扫描（PET）和单光子发射计算机断层扫描（SPECT）上对烟碱乙酰胆碱受体（nAChRs）成像的特点在亚型选择性和毒性方面进行了进一步的研究。详细的动力学和饱和度分析证实，与5-[I-125]碘- a85380一样，2-氟- a -85380具有非常高的亲和力，具有均匀的结合位点群体，可能是啮齿动物和人类大脑中nAChRs的alpha4-beta2亚型。对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alpha4beta2受体的亲和力比它们对哺乳动物alpha7、肌肉和alpha3beta4亚型的亲和力高出三到五个数量级。此外，在缺乏nachr β 2亚基的“敲除”小鼠中，5-[I-125]碘- a85380在任何脑区都没有结合。这些发现和非人灵长类动物的体内研究表明，[F-18]2-fluoro-A85380优于其他可用于PET选择性成像alpha4beta2 nachr的放射配体。毒理学研究表明，在相对高剂量下，这些配体具有惊厥特性并影响心血管系统。小鼠惊厥的剂量大约是nachr成像所需剂量的1000到10000倍，这表明a -85380相关化合物比毒素依比替丁衍生物的新成像配体具有更大的安全范围。在对未麻醉的大鼠进行的初步心血管研究中，5-碘-A-85380、2-氟-A-85380和尼古丁产生了相当的、适度的短暂性血压升高，但只有尼古丁产生了明显的心率下降，这表明A-85380衍生物的心脏毒性比尼古丁小。[F-18]-2-Fluoro-A-85380穿过血脑屏障，并按已知的alpha4beta2 nachr密度成比例分布在小鼠和恒河猴的大脑中。利用含有少量alpha4beta2 nachr的猴子小脑估计特异性与非特异性结合的比例，显示在放射性示踪剂给药后4小时，丘脑的特异性结合水平很高。结合的特异性被胞氨酸阻断和置换研究证实，表明2-[F-18]- f - a -85380是人类受试者中央nachr PET成像的优秀候选者。另一个目标是改进PET-FDG方法，使用静脉而不是动脉血液取样来计算CMRglc。麻醉非人灵长类动物（NHPs）动、静脉血FDG和葡萄糖浓度时间曲线与动、静脉血CMRglc值吻合较好。用静脉取样代替动脉血取样将简化适用于人类和NHPs的实验程序，而不会显著降低准确性，并扩大了对同一受试者进行多次PET-FDG研究的能力。

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.