Evaluation, optimization and labeling of substituted 1,3,4-oxadiazole derivatives as tracers for imaging of human telomerase reverse transcriptase (hTERT) activity in tumors

人类的端粒可以被称为染色体的“端帽”。它们由特定的重复核苷酸序列和相关蛋白组成端粒酶主要由（人类）端粒酶逆转录酶（TERT）催化亚基和端粒RNA （TR或TERC）组成。其主要功能是维持并因此合成线性染色体等位基因末端的端粒重复单元[2,3]。一旦端粒达到临界长度，相应的细胞进入复制性衰老或诱导细胞凋亡。不断分裂的细胞，如生殖细胞、干细胞，最重要的是，绝大多数癌细胞（85-95%）依赖于高端粒酶活性来确保它们的生存。相反，端粒酶活性在大多数成年体细胞中几乎检测不到。[4-6]鉴于上述特点，提示端粒酶可作为肿瘤成像的靶点。尽管hTERT具有开发显像剂的理想特性，并进行了大量的研究工作，以及定向（放射）治疗[7,8]和间接成像[9-12]的例子，但迄今为止还没有发现用于诊断成像方式的小分子抑制剂。对这些配体进行放射性标记，通过正电子发射断层扫描（PET）进行有针对性的非侵入性成像，可能是一种特别有吸引力的方法，用于没有积累或不充分积累临床建立的代谢示踪剂，如[18F]氟脱氧葡萄糖（FDG）或[18F]氟基甲基乳酸（FET）的肿瘤，特别是前列腺-[13]、乳腺-[14]和肾-[15]癌在增殖示踪剂（如3'-脱氧-3'[18F]-氟胸嘧啶，[18F]FLT）有一定限制的情况下，潜在的hTERT示踪剂可能提供有价值的信息此外，当使用潜在的恶二唑类显像剂时，炎症组织中非肿瘤特异性fdg摄取不会构成问题。[16,18 - 20]一类1,3,4-恶二唑衍生物已被选择作为正电子发射断层扫描的示踪剂。在第一种方法中，将制备两种化合物，并用[18F]氟（一种发射正电子的核素）标记合适的前体。在首次分布研究和体外试验之后，对进一步研究的中心决定将决定在拟议研究的非常早期阶段进行进一步调查，使用1,3,4-恶二唑衍生物的荧光特性作为潜在示踪剂后期积累的指标。为了能够响应较长的示踪剂积累时间，考虑了不同的标记策略。潜在的最终示踪剂将在体外和体内进行充分评估，使用表达和不表达hTERT的细胞系产生的肿瘤模型。

Human telomeres can be referred to as "end-caps" of chromosomes. They are composed of specific repeating nucleotide sequences and associated proteins.[1] The telomerase is consisting mainly of the (human) telomerase reverse transcriptase ((h)TERT) catalytic subunit and a telomeric RNA (TR or TERC) component. The primary function is the maintenance and therefore synthesis of telomere repeating units at the ends of alleles of linear chromosomes.[2,3] Once the telomeres reach a critical length, the corresponding cells enter replicative senescence or apoptosis is induced. Continuously dividing cells such as germ cells, stem cells and, most importantly, a vast majority of cancer cells (85-95%) depend on a high telomerase activity to ensure their survival. In contrast, telomerase activity is barely detectable in most adult somatic cells.[4–6] Given the above features suggests telomerase as a target for imaging of tumors. Despite the ideal properties of hTERT for development of imaging agents and numerous research efforts as well as examples of directed (radio)therapy[7,8] and indirect imaging[9–12], no small molecule inhibitor labeled for the use in diagnostic imaging modalities is known to date. Radiolabeling of these ligands for targeted, non-invasive imaging by positron emission tomography (PET) might be an especially attractive approach for tumors not accumulating or insufficiently accumulating clinically established metabolic tracers such as [18F]fluorodeoxyglucose (FDG) or [18F]fluorethyltyrosin (FET), in particular prostate-[13], breast-[14] and renal[15] cancers.[16] The potential hTERT tracers may add valuable information in cases where proliferation tracers (like 3'-deoxy-3'[18F]-fluorothymidine, [18F]FLT) exhibit certain limits.[17] Also non-tumor-specific FDG-uptake in inflamed tissue would not pose a problem when using the potential oxadiazole based imaging agents.[16,18–20]The class of 1,3,4-oxadiazole derivatives has been chosen to be developed as tracers for positron emission tomography. In a first approach two compounds will be prepared and suitable precursors will be labeled with [18F]fluorine, a positron emitting nuclide. After first distribution studies and in vitro tests a central decision on further research will determine further investigations on a very early stage of the proposed research, using the fluorescent properties of 1,3,4-oxadiazole derivatives as an indicator for late accumulation of the potential tracers. To be able to respond to long tracer accumulation times, different labeling strategies are taken into account. The potential final tracers will be fully evaluated in vitro and in vivo, using tumor models arising from hTERT expressing and non-expressing cell lines.