DEVELOPMENT OF TC-99M RENAL TUBULAR AGENTS

NIH赞助的这项研究的前六年导致了临床 引入Tc-99m MAG3（巯基乙酰基三甘氨酸），建立了其 剂量测定，并提高了我们对MAG3和OIH在 肾动脉狭窄 Tc-99m MAG3已取代OIH成为第二大 常用的肾脏放射性药物，占肾脏放射性药物的19%。 在美国进行扫描;但是，MAG 3相当昂贵，而且它的 血浆清除率仅为OIH的50 - 60%，使其成为次优药物 以估计有效肾血浆流量。 这些限制促使我们 在分子力学的指导下， 计算，以确定具体结构变化的影响， 电荷分布对有效肾转运的影响，从而设计和 合成改进的配体。 这项研究导致了新的代理，Tc-99m 清除率最高的N，N-1，2-亚乙基-二-d-半胱氨酸（d，d-EC） 在我们曾经测试过的任何管状Tc-99m试剂（包括MAG 3）的大鼠中; 此外，初步结果表明，它具有较高的清除率， 但仍低于OIH。 我们建议 合成新的Tc-99m、Re和Tc配合物， 成功二酰氨基-硫醚-硫醇盐、二酰氨基-氨基-硫醇盐和 二氨基-二硫醇盐配体类以及与二氨基-二硫醇盐配体相关的新衍生物 MAG 3。 将在大鼠中测量复合物的清除率， 普通志愿者 由于许多肾小管的高蛋白结合 包括MAG3在内的药物可能会限制肾小管的取出，并解释了 与OIH相比，此类药物的清除率不理想，我们建议（a） 测定所选蛋白的表观蛋白结合平衡常数， Tc-99 m试剂（B）测量它们在分离的灌注大鼠中的清除率 和（c）将这两个参数相关联 确定蛋白质结合特性的参数， 管状运输 为了增强配体设计，选择Tc-99和Re 化合物将通过X射线晶体学进行结构表征 用于分子力学建模。 我们将改进建模计算 包括溶剂和更广泛的构象空间搜索 为了更好地表征结构和电荷分布， 具有高效的管状运输和最佳的蛋白质结合。 的 改进的分子力学模型加上上级 对蛋白质结合的理解具有广泛的意义， 结果也可以应用于非肾性 放射性药物 第二代Tc-99m的更高清除率 管状剂将改善图像质量，增加肾脏 背景比，从而提高了准确性和可靠性， 基于相机的定量测量。 此外，改进的复合物 将减少肾功能受损患者的辐射剂量， 可以提供肾血浆流量的直接测量，并且将 有助于开发一种新的测试来评估输尿管功能。 重要的是，体内蛋白结合研究应减少 哺乳动物实验 最后，另一种Tc-99m的可用性 与MAG3竞争的肾小管药物应减少 放射性药物成本 按照目前MAG3的使用水平， 降低成本每年可节省180万美元。

The first six years of this NIH sponsored research led to the clinical introduction of Tc-99m MAG3 (mercaptoacetyltriglycine), established its dosimetry and enhanced our understanding of the use of MAG3 and OIH in renal artery stenosis. Tc-99m MAG3 has replaced OIH as the second most commonly used renal radiopharmaceutical and accounts for 19% of renal scans in the United States; however, MAG3 is quite expensive and its plasma clearance is only 50-60% that of OIH making it a suboptimal agent to estimate effective renal plasma flow. These limitations prompted us to pursue a systematic effort, guided by molecular mechanics calculations, to determine the effect of specific structural changes and charge distribution on efficient renal transport and thereby design and synthesize improved ligands. This research led to the new agent, Tc-99m N,N-1,2-ethylene-di-d-cysteine (d,d-EC) which has the highest clearance in rats of any tubular Tc-99m agent we have ever tested including MAG3; moreover, preliminary results suggest that it has a higher clearance than MAG3 in humans but it is still less than OIH. We propose to synthesize new Tc-99m, Re and Tc complexes with ligands in the successful diamido-thioether-thiolate, diamido-amino-thiolate, and diamino-dithiolate ligand classes as well as new derivatives related to MAG3. The clearance of the complexes will be measured in rats and normal volunteers. Since the high protein binding of many tubular agents, including MAG3, may limit tubular extraction and explain the suboptimal clearance of such agents compared to OIH, we propose to (a) determine the apparent protein-binding equilibrium constant for selected Tc-99m agents (b) measure their clearances in the isolated perfused rat kidney using a protein free perfusate and (c) correlate these two parameters to determine the protein binding properties which limit tubular transport. To enhance ligand design, selected Tc-99 and Re compounds will be structurally characterized by X-ray crystallography for molecular mechanics modeling. We will improve modeling calculations with inclusion of solvent and broader searches of conformational space to better characterize the structure and charge distribution associated with efficient tubular transport and optimal protein binding. The improved molecular mechanics modeling coupled with a superior understanding of protein binding has broad significance since the results can also be applied to development of non-renal radiopharmaceuticals. The higher clearance of second generation Tc-99m tubular agents will improve image quality, increase the kidney to background ratio and thereby increase the accuracy and reliability of quantitative camera based measurements. Furthermore, improved complexes will reduce the radiation dose to patients with impaired renal function, may provide a direct measurement of renal plasma flow, and will facilitate development of a new test to evaluate ureteral function. Importantly, in vivo protein binding studies should reduce the number of mammalian experiments. Finally, the availability of another Tc-99m renal tubular agent to compete with MAG3 should reduce radiopharmaceutical costs. At current levels of MAG3 use, a 30% reduction in cost would save 1.8 million dollars annually.