Characterizing gadofosveset for use in quantitative tracer kinetic MRI studies

定量磁共振成像已成为一个非常强大的工具，在体内的组织生理和病理生理的评估。特别是，动态对比增强（DCE）MRI结合示踪剂动力学分析，使研究人员能够以非侵入性和可重复的方式探测微血管生理学。然而，很少有示踪剂可用于此类研究，只有一种小分子（Gd-DTPA）已得到充分表征，并获准用于人体研究。已经使用啮齿动物模型进行了研究，以评估各种较大的示踪剂分子，这些分子提供了关于血容量和微血管渗透性的更具体的信息，但迄今为止，这些示踪剂都没有进入临床市场。最近批准用于人类研究的一种新示踪剂令人感兴趣。Gadofosveset是一种与人血清白蛋白（HSA）可逆结合的Gd-DTPA样造影剂。因此，其表现非常类似于临床前研究中使用的较大示踪剂，其在血池中保留较长时间，并为MR血管造影提供有用的造影剂。这种行为对于表征微血管渗透性是有希望的，但是这种测量由于与HSA的可逆结合而变得复杂;即，在任何给定时间，一部分钆磷维司将被结合并且表现得像血管内示踪剂，而剩余部分将是游离的并且表现得像Gd-DTPA并且更自由地分布。本研究的目的是在定量DCE-MRI研究中应用示踪剂之前表征钆磷维司及其体内动力学。这将通过一系列具有以下特定目标的实验来实现：1 -表征钆磷维司的弛豫性。将在体外进行实验，以确定钆磷维司在一定范围的场强（1.5 T、3 T、4.7 T和9.4 T）下在人和小鼠血清白蛋白中的弛豫率。通过逐步增加gadfosveset浓度，可以确定结合和游离示踪剂的弛豫率以及这些状态之间的交换率。还将评估pH值的影响，因为这可能在病理学中发生变化。Exp. 2 -测量小鼠静脉注射后随时间变化的钆磷维司的血管浓度（测定动脉输入函数（AIF））。这将根据Port等人（Investigative Radiology 40：565-573，2005）使用电感耦合等离子体质谱法进行，以确定钆磷维司的绝对浓度，并建立在exp. 1.这些结果将使我们能够从体内进行的T1测量中评估血管浓度。Exp. 3 -评估钆磷维司体内动力学。将在小鼠中进行成像研究，以评估肌肉和肿瘤等组织中的摄取。将采用已知血管密度和微血管渗透性差异的肿瘤模型来测试钆磷维塞增强MRI区分它们的能力。将在相同动物中使用Gd-DTPA增强MRI进行比较。结果将与血管密度的组织学测量值和使用Hoeschst 33542染色获得的灌注估计值相关。Exp. 4 -模拟示踪剂动力学并应用于人体研究。这些研究的结果将有助于扩展示踪动力学模型的发展，以解释钆磷维司的可逆结合和随场强的变化效果。将使用从人体临床/志愿者研究中获得的数据对这些模型进行检测。这些实验以及与帮助运行它们的同事的互动将为学生提供广泛的跨学科培训。除了数学建模和数据分析，学生还将获得使用一系列系统和场强（体模制备和动物监测），ICP-MS（样品制备和处理）和人体成像研究的体内MRI经验。

Quantitative MRI has become a very powerful tool for the assessment of tissue physiology and patho-physiology in vivo. In particular, dynamic contrast-enhanced (DCE) MRI combined with the analysis of tracer kinetics enables the researcher to probe microvascular physiology in a non-invasive and repeatable manner. However, there are few tracers available for such studies and only one small molecule (Gd-DTPA) has been well characterized and is licensed for studies in humans. Studies have been performed using rodent models to assess a variety of larger tracer molecules that provide more specific information about blood volume and microvascular permeability but to date none of these tracers has reached the clinical market. One new tracer that has recently been approved for human studies is of interest. Gadofosveset is a Gd-DTPA-like contrast agent that binds reversibly with human serum albumin (HSA). As such, it behaves much like the larger tracers used in pre-clinical studies remaining in the blood pool for extended periods of time and providing useful contrast for MR angiography. Such behaviour is promising for characterizing microvascular permeability but such measures are complicated by the reversible binding to HSA; i.e. at any given time a fraction of the gadofosveset will be bound and behave like an intravascular tracer while the remaining fraction will be free and behave like Gd-DTPA and distribute more freely. The objective of this study is to characterize gadofosveset and its in vivo kinetics prior to application of the tracer in quantitative DCE-MRI studies. This will be achieved using a series of experiments with the following specific aims: Exp. 1 - to characterize the relaxivity of gadofosveset. Experiments will be performed in vitro to establish the relaxivity of gadofosveset at a range of field strengths (1.5 T, 3 T, 4.7 T & 9.4 T) in human and mouse serum albumin. By escalating the gadfosveset concentration the relaxivity of bound and free tracer and exchange rate between these states may be determined. The effect of pH will also be assessed since this may change in pathology. Exp. 2 - to measure the vascular concentration of gadofosveset as a function of time following intravenous injection in mice (determination of the arterial input function (AIF)). This will be performed according to Port et al. (Investigative Radiology 40:565-573, 2005) using inductively coupled plasma mass spectroscopy to establish the absolute concentration of gadofosveset and building upon the work in exp. 1. These results will enable us to assess vascular concentration from T1 measurements performed in vivo. Exp. 3 - to assess gadofosveset kinetics in vivo. Imaging studies will be performed in mice to assess uptake in tissues such as muscle and tumour. Tumour models with known differences in vessel density and microvascular permeability will be employed to test the ability of gadofosveset enhanced MRI to differentiate between them. Comparisons will be made, in the same animals, with Gd-DTPA enhanced MRI. The results will be correlated with histological measures of vessel density and estimates of perfusion obtained using Hoeschst 33542 staining. Exp. 4 - to model the tracer kinetics and apply to human studies. The results of these studies will aid the development of extended tracer kinetics models to account for the reversible binding of gadofosveset and the variation of effect with field strength. These models will be tested using data obtained from clinical/volunteer studies performed in humans. These experiments and the interactions with colleagues helping to run them will provide the student with a broad, interdisciplinary training. In addition to mathematical modelling and data analysis the student will gain experience with in vivo MRI using a range of systems and field strengths (phantom preparation and animal monitoring), ICP-MS (sample preparation and processing) and human imaging studies.