Folate Dendrimers as Tumor Specific Contrast Agents

卵巢癌是女性妇科肿瘤的头号杀手，而颅内室管膜瘤是儿童中第三常见的原发脑肿瘤。该项目的长期目标是开发一种针对卵巢肿瘤、儿童室管膜瘤和脉络丛肿瘤的磁共振成像(MRI)造影剂，从而提高这些肿瘤诊断和治疗监测的特异性。本项目的具体目的是开发一种高弛豫度的树枝状大分子-Gd(III)-螯合物为基础的磁共振造影剂，利用低分子靶向剂(叶酸)对表达高亲和力叶酸受体的细胞具有很强的亲和力，并在体内主动靶向肿瘤细胞。我们正在检验两个假设。其一是具有更快质子交换率的双功能Gd(III)螯合物在与树枝状大分子偶联后将具有比已有的临床批准试剂的双官能螯合物更高的驰豫(更好的效率)，以及另一种是这些试剂具有足够高的分子效率(驰豫)以在1.5T增强表达高亲和力叶酸受体的肿瘤的T1加权图像。实验方法包括将一种新的双功能Gd(III)大环螯合物连接到一类新的树枝状大分子上。这类树枝状大分子允许我们控制靶向和报告分子的确切数量，并提供纯化合物。在合成这种试剂后，我们将表征其磁性，确定最佳靶向分子的数量，确定改变肿瘤对比度所需的Gd(III)-螯合物的数量，证明体内靶向特异性，并确定药代动力学和生物分布。我们有两个体内肿瘤模型，由人卵巢癌裸鼠移植瘤组成，它们要么表达高亲和力叶酸受体，要么缺乏高亲和力叶酸受体。开发相对低分子量(相对于抗体靶向系统)的肿瘤特异性MRI造影剂的意义在于，它将使肿瘤更好地可视化，为癌症诊断提供更好的解释，最重要的是，它将是一种监测肿瘤治疗的非侵入性方法。

Ovarian cancer is the leading killer of women with tumors of gynecological origin, and intracranial ependymomas are the third most common primary brain tumors found in children. The long term goal of this project is to develop a magnetic resonance imaging (MRI) contrast agent specific for ovarian tumors, childhood ependymomas, and choroid plexus tumors thus improving the specificity of both the diagnosis of these tumors and the monitoring of the treatment of these tumors. The specific aim of this project is to develop a high relaxivity dendrimer-Gd(III)-chelate based MRI contrast agent with strong avidity to cells expressing the high affinity folate receptor using low molecular weight targeting agents (folic acid) and actively target it to tumor cells in vivo. We are testing two hypotheses. One is that bifunctional Gd(III) chelates with faster proton exchange rates will have higher relaxivities (better efficiency) than those already achieved with bifunctional chelates of clinically approved agents following conjugation to dendrimers, and the other is that these agents have a high enough molecular efficiency (relaxivity) to enhance T1 weighted images, at 1.5 T, of tumors that express the high affinity folate receptor. The experimental approach consists of attaching a new bifunctional Gd(III) macrocyclic chelate to a new class of dendrimers. This class of dendrimers allows us to control the exact number of targeting and reporter molecules and provides a pure compound. Following the synthesis of this agent we will characterize the magnetic properties, determine the optimum number of targeting molecules, determine the number of Gd(III)- chelate complexes needed to alter the tumor contrast, prove targeting specificity in vivo, and determine the pharmacokinetics and biodistribution. We have two in vivo tumor models consisting of human ovarian tumor xenografts in nude mice that either express the high affinity folate receptor or lack it. The significance of developing a relatively low molecular weight (relative to antibody targeted systems) tumor specific MRI contrast agent is that it will allow better tumor visualization, interpretation for cancer diagnosis, and most significantly a noninvasive method for monitoring tumor therapy.