【分享】小动物PET技术在药物研究中的应用
小动物PET 用于药物的开发和研究，引起了全球医药界的极大关注，目前世界上至少有200 个科研机构和动物显像实验室常规使用小动物PET 进行药物研究。在药物的研究过程中，我们必须解决以下几个问题：① 药物在血液和靶组织中的浓度能否达到所需要的生物浓度，即生物分布。②药物与靶点的结合情况即特异性。③药物是否能达到特定的生物效果，如抑制增生扩散或减少组织乏氧，即间接评价。④药物的代谢速率是否合适。只有这些问题能在动物模型上得到满意的解决，才有可能进入临床研究。小动物PET能够很好解决这些问题，提供了一个从发现前药到用于人体、从动物实验的结果外推到临床的合理程序，架起了从动物研究到人体研究的桥梁，大大提高了新药研究的准确性和有效性，缩短了新药研究的周期，减少了新药研究的投入资金，加速了创新药物进入临床应用的时间，将极大地推进药物开发的进程。该技术能够无创伤地、动态地、定量地显像正电子标记的放射性药物在活体内的分布；人类基因测序的完成又提出了大量的药物可能作用的生物靶点，为现代创新药物的研究创造了新的条件，结果导致体外实验证明有活性的先导化合物的数量大大增加，该技术可以验证这些药物在活体内是否通过识别特异的靶点起作用，用于从候选药物中筛选先导化合物；该技术可提供药物的定量动力学、体内药代动力学和药效学数据，帮助随后的人体实验研究确定初始剂量。1. 新药的筛选在新药研究的第一阶段，小动物PET 是一个最理想的药物筛选的平台技术。
高通量的合成系统只有配置小动物PET，才能真正地达到高通量地筛选先导化合物。在体外检测能结合到特定靶点的候选药物，使候选药物的数量减少到一个容易控制的范围内，找出与靶器官亲和力最好并具有高效性的先导化合物进行小动物PET活体实验。例如，在神经系统新药的研究中，利用小动物PET 的定量示踪技术和一种特定的核受体分子探针就可对单一受体进行大量的化合物筛选。张等通过PET显像接种CWR22前列腺癌细胞的小鼠对FDG的吸收，来研究抗前列腺癌药物治疗效果，发现在第四天肿瘤体积虽然没有明显缩小，但是对FDG的吸收已经下降了43%。
2. 药代动力学、药效学和临床前药理学
小动物PET可以获得放射性标记药物或者类似药物详细的药代动力学信息，即药物的ADME（吸收、分布、分泌、排泄）的信息。也就是说，该技术可以观测药物是否穿越血脑屏障，是否有器官特异性的聚集，是否识别靶受体以及血浆与组织中药物含量比值。总之，通过小动物PET技术可以监测药物作用全过程，提供活体动态信息，并对比治疗前后动物疾病模型的状况，还可以知道可能产生的组织损伤并进行人与动物的对比研究。例如：意大利的科研人员用11C标记化合物PK11195的类似物VC701，注射入大鼠体内后，观察由于对PBR受体的识别而导致的分布规律，经过拮抗剂PK11195的竞争性阻断后，11C -VC701与受体的结合明显降低。
该技术还可以通过标记一个示踪剂评价药物的治疗效果，这种类型的研究是通过用小动物PET成像葡萄糖、氨基酸以及脂质代谢的变化，来了解药物是如何起作用的。
3. 药物作用靶点研究
小动物PET可以显示药物是否与特定的分子靶点相结合。PET显像能确定药物在体内被转运并结合到靶组织以及在体内的药代动力学，如用18F标记的galacto-RGD来研究它对αvβ3的竞争结合。αvβ3是一个细胞支持受体，在有活性的内皮细胞层表达，可诱导转导，能识别并结合到一个特异的三肽序列R00GD 配体。用专门的小动物PET显像可显示18F标记的galacto-RGD 与老鼠模型皮下肿瘤αvβ3结合的数量，得出随着αvβ3拮抗剂环五肽cyclo（-Arg-Gly-Asp-Phe-Val-）数量的增加，18F标记的galacto-RGD与αvβ3的结合减少。这些研究可无创伤性的评价药物对其他生物学位点的抑制作用，而且可用于肿瘤病人优化治疗剂量，利用适量的αvβ3拮抗剂，保证与18F标记的galacto-RG 适当程度的竞争结合。
4. 肿瘤细胞多药耐药性和逆转成分的研究
临床上肿瘤化疗失败主要是由先天性和获得性肿瘤细胞多药耐药引起的。获得性多药耐药由多种病理机制形成，例如DNA拓朴异构酶、细胞谷胱甘肽升高、P-糖蛋白（ P-gp）表达异常等。小动物PET是研究体内功能性转运的有效技术。因此，在肿瘤细胞多药耐药的基础研究和多药耐药逆转成分的研究中，可发挥独到的作用。例如，11C-colchicine（ 秋水仙素）可用于肿瘤P-gp功能显像。进而为研究肿瘤细胞多药耐药性和逆转成分提供了实验手段。
5. 中药学的研究
在现代中医药的研究领域，小动物PET 提供了一个最新的研究方法和途径。特别是对天然药物、仿生药物以及中医药复方的研究将是一场革命。成为传统中医药走向现代化、全球化的快通道和核心平台
转自dxy Synthesis and Biologic Evaluation of
Monocationic Asymmetric 99mTc-Nitride
Heterocomplexes Showing High Heart Uptake
and Improved Imaging Properties
Alessandra Boschi, PhD1; Licia Uccelli, PhD1; Cristina Bolzati, PhD1; Adriano Duatti, PhD1; Nicola Sabba, PhD1;
Elena Moretti, BS1; Giovanni Di Domenico, PhD2; Guido Zavattini, PhD2; Fiorenzo Refosco, PhD3;
and Melchiore Giganti, MD1
1Laboratory of Nuclear Medicine, Department of Clinical and Experimental Medicine, University of Ferrara, Ferrara, Italy;
2Department of Physics, University of Ferrara, Ferrara, Italy; and 3Istituto di Chimica Inorganica e delle Superfici, Consiglio
Nazionale delle Ricerche, Padua, Italy
The preparation, characterization, and first biologic evaluation in
rats of a novel class of monocationic 99mTc heart imaging agents
are reported. The complexes are represented by the general
formula [99mTc(N)(PNP)(L)], where L is the monoanionic form of
a dithiocarbamate ligand of the type [R1(R2)-N-C(S)S], PNP is
a diphosphine ligand of the type [(R3)2P-(CH2)2]2-N(R4), and
R1–R4 are organic functional groups. Methods: The new complexes
were prepared by use of both liquid and freeze-dried
formulations through a 2-step procedure. The first step involved
the formation of the [Tc'N]2 group through the reaction of
99mTcO4
with succinic dihydrazide as a donor of nitride nitrogen
atoms (N3) in the presence of Sn2 ions. This step was followed
by the simultaneous addition to the reaction solution of the
ligand PNP and of the sodium salt of the dithiocarbamate ligand
(NaL) to afford the final products, [99mTc(N)(PNP)(L)]. The chemical
identities of the resulting 99mTc complexes were determined
by comparing their chromatographic properties with those of
the corresponding 99gTc analogs prepared by use of the longlived
isotope 99gTc and characterized by spectroscopic and
crystallographic techniques. Ex vivo biodistribution studies
were conducted in rats. In vivo tomographic images of the rat
heart were obtained by use of a small-animal SPECT scanner.
Results: The [99mTc(N)(PNP)(L)] complexes are monocationic
and possess a distorted square–pyramidal geometry in which
the Tc'N multiple bond occupies an apical position and the
diphosphine and dithiocarbamate ligands span the residual 4
coordination positions on the basal plane through the 2 phosphorus
atoms and the 2 sulfur atoms, respectively. Imaging and
biodistribution studies demonstrated that these radiopharmaceuticals
localize selectively in the myocardium of rats and are
retained in this region for a prolonged time. The kinetics of heart
uptake and clearance were found to be influenced by variations
in the lateral R1–R4 groups. Blood and lung washouts were
extremely fast. Elimination occurred mostly through the kidneys
and the liver. Surprisingly, at 1 h after injection, liver activity was
almost negligible. Analysis of heart-to-liver and heart-to-lung
uptake ratios showed that these values increased exponentially
over time and became much higher than those determined for
99mTc-sestamibi and 99mTc-tetrofosmin. These findings were
confirmed by analysis of high-quality SPECT images collected
in rats for the new complexes and compared with images obtained
with 99mTc-sestamibi and 99mTc-tetrofosmin. Conclusion:
The high myocardial uptake and the very high heart-to-lung and
heart-to-liver uptake ratios indicate that the [99mTc(N)(PNP)(L)]
complexes exhibit very favorable distribution properties and
could be used to obtain SPECT cardiac images with improved
quality.
Key Words: 99mTc heart perfusion imaging agents; asymmetric
99mTc-nitride heterocomplexes; mixed diphosphino-dithiocarbamate
99mTc-nitride complexes
J Nucl Med 2003; 44:806–814
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