Characterizing gadofosveset for use in quantitative tracer kinetic MRI studies

定量MRI已成为体内组织生理学和病理生理学评估的有力工具。特别是，动态对比增强（DCE） MRI与示踪动力学分析相结合，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）已被很好地表征并获准用于人体研究。研究人员使用啮齿动物模型来评估各种较大的示踪剂分子，这些示踪剂分子可以提供有关血容量和微血管通透性的更具体信息，但迄今为止，这些示踪剂尚未进入临床市场。最近被批准用于人体研究的一种新的示踪剂引起了人们的兴趣。Gadofosveset是一种gd - dtpa样造影剂，可与人血清白蛋白（HSA）可逆结合。因此，它的行为很像临床前研究中使用的较大的示踪剂，在血液池中停留较长时间，为MR血管造影提供有用的对比。这种行为有望用于表征微血管通透性，但由于与HSA的可逆结合，这种测量变得复杂；即在任何给定时间，gadofosveset的一部分将结合并表现为血管内示踪剂，而其余部分将自由并表现为Gd-DTPA并且分布更自由。本研究的目的是在定量DCE-MRI研究中应用示踪剂之前表征gadofosveset及其体内动力学。这将通过一系列具有以下具体目标的实验来实现：实验1 -表征gadofosveset的弛豫性。实验将在体外进行，以建立gadofosveset在人类和小鼠血清白蛋白中在场强范围（1.5 T, 3 T， 4.7 T和9.4 T）下的弛豫性。通过增加钆的浓度，可以确定束缚和自由示踪剂的弛豫度以及这些状态之间的交换率。pH值的影响也将被评估，因为它可能在病理上发生变化。实验2 -测量小鼠静脉注射后gadofosveset血管浓度与时间的关系（测定动脉输入功能（AIF））。这将根据Port等人（放射学调查40:565-573,2005）使用电感耦合等离子体质谱法建立gadofosveset的绝对浓度，并以实验1的工作为基础。这些结果将使我们能够通过体内T1测量来评估血管浓度。实验3 -评估体内gadofosveset动力学。成像研究将在老鼠身上进行，以评估肌肉和肿瘤等组织的摄取情况。已知血管密度和微血管通透性存在差异的肿瘤模型将用于测试gadofosveset增强MRI区分它们的能力。将在同一动物中与Gd-DTPA增强MRI进行比较。结果将与血管密度的组织学测量和使用Hoeschst 33542染色获得的灌注估计相关。实验4 -建立示踪动力学模型并应用于人体研究。这些研究的结果将有助于发展扩展的示踪动力学模型，以解释gadofosveset的可逆结合和随场强的效应变化。这些模型将使用从人体临床/志愿者研究中获得的数据进行测试。这些实验以及与同事的互动将为学生提供广泛的跨学科训练。除了数学建模和数据分析外，学生还将获得使用一系列系统和场强（幻影制备和动物监测），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.