Activatable MR Imaging Probes

描述（由申请人提供）：可激活磁共振成像探头可激活磁共振（MR）成像探头具有提供前所未有的生物学见解的潜力。已经报道了响应于Ca 2+通量、Zn 2+通量、报告基因、酶活性、pO 2和pH的MR探针。这种探针的潜在应用包括神经元电流、胰岛活力、基因激活和异质性肿瘤微环境的关键要素的直接成像。MR允许在高场系统上以细胞分辨率（~10 5 m）和临床扫描仪上以亚毫米分辨率在三维空间中询问完整的、不透明的生物体。深层组织穿透和高分辨率使MR能够直接将发现从细胞到小鼠再到人类。 抑制其平移的可激活MR探针的基本限制是难以区分“活性”探针和“非活性”探针。在MRI中，主要对水成像，并且通过其对水信号的影响间接检测探针。这种效应取决于探针浓度和探针的弛豫率。弛豫率取决于许多分子因素，包括探针的水合状态及其旋转扩散速率。可以设计可活化或“智能”探针，其中弛豫率响应于环境刺激而改变，例如探针经由酶促反应从低弛豫率状态转变为高弛豫率状态，或者探针的弛豫率在结合分析物（例如钙）时改变。虽然弛豫率对刺激非常敏感，但MR信号的变化取决于弛豫率和探针浓度：两个未知数。在体外，浓度不变，这些探针作为优雅的传感器。然而，在体内，探针浓度是未知的，并随时间变化。相对于正常组织，由于内皮渗透性增加，患病组织中的浓度可能较高，或由于灌注不良而较低。信号变化可能是无活性探针分布的结果，也可能是由于探针激活。目前还没有一种实用的方法来区分活性探针和非活性探针。 我们提出了在“关闭”状态下完全MR沉默的可激活探针。以这种方式，在探针注入之后MR信号的任何变化必须是由于探针的激活。我们提出了一个新的范例，可激活的MR探针的基础上的还原-氧化（氧化还原）化学锰。二价锰（Mn 2+）是有效的MR弛豫剂，但Mn 3+通常是差的弛豫剂。我们将制备稳定的锰络合物，其可以在环境刺激的存在下从真正的MR关闭状态（Mn 3+）可逆地转换为MR开启状态（Mn 2+）。随着可调氧化还原核心的开发，可以设计对pH、酶活性、离子通量或特定半抗原敏感的探针。在这个应用中，我们将专注于开发一种用于缺氧成像的MR氧传感器，其中MR信号仅在缺氧区域产生。我们将通过与直接pO 2电极测量、正电子发射断层扫描缺氧探针和血氧水平依赖性MR进行比较，在小鼠和兔肿瘤模型中验证该缺氧探针。 公共卫生相关性：该项目的目标是开发一种磁共振成像探头，可用于非侵入性地识别和量化肿瘤中的低氧水平（缺氧）区域。表征缺氧肿瘤可能在指导患者接受的治疗选择方面是有价值的。

DESCRIPTION (provided by applicant): Activatable MR Imaging Probes Activatable magnetic resonance (MR) imaging probes offer the potential to provide unprecedented biological insights. MR probes responsive to Ca2+ flux, Zn2+ flux, reporter genes, enzymatic activity, pO2, and pH have been reported. Potential applications of such probes include direct imaging of neuronal currents, pancreatic islet viability, gene activation, and key elements of the heterogeneous tumor microenvironment. MR allows the interrogation of intact, opaque organisms in three dimensions at cellular resolution (~10 5m) on high field systems and sub millimeter resolution on clinical scanners. The deep tissue penetration and high resolution make MR make it possible to directly translate findings from cells to mice to humans. The fundamental limitation of activatable MR probes that stifles their translation is the difficulty in distinguishing "active" from "inactive" probe. In MRI primarily water is imaged and the probe is detected indirectly by its effect on the water signal. This effect depends on probe concentration and the probe's relaxivity. Relaxivity is dependent on a number of molecular factors including the hydration state of the probe and its rotational diffusion rate. It is possible to design activatable or "smart" probes where the relaxivity changes in response to an environmental stimulus, e.g. the probe is transformed via enzymatic reaction from a low relaxivity state to a high relaxivity state, or the probe's relaxivity changes upon binding an analyte, e.g. calcium. Although relaxivity can be exquisitely sensitive to a stimulus, the MR signal change depends on both relaxivity and probe concentration: two unknowns. In vitro, where concentration does not change, these probes act as elegant sensors. However in vivo, the probe concentration is unknown and changes with time. Relative to normal tissue, concentrations may be higher in diseased tissue due to increased endothelial permeability, or lower because of poor perfusion. Signal change could be a result of distribution of inactive probe or could be due to probe activation. Currently there is no practical way to distinguish active from inactive probe. We propose activatable probes that are completely MR silent in the "off" state. In this way, any change in MR signal after probe injection must be due to activation of the probe. We propose a new paradigm for activatable MR probes based on the reduction-oxidation (redox) chemistry of manganese. Divalent manganese (Mn2+) is a potent MR relaxation agent but Mn3+ is generally a poor relaxor. We will prepare stable manganese complexes that can reversibly convert from a truly MR-off state (Mn3+) to a MR-on state (Mn2+) in the presence of an environmental stimulus. With development of a tunable redox core, it is possible to design probes sensitive to pH, enzymatic activity, ion flux, or specific haptens. In this application we will focus on developing an MR oxygen sensor for hypoxia imaging, where MR signal is only generated in hypoxic regions. We will validate this hypoxia probe in mouse and rabbit tumor models by comparing to direct pO2 electrode measurements, a positron emission tomography hypoxia probe, and blood oxygen level dependent MR. PUBLIC HEALTH RELEVANCE: Project Narrative This goal of this project is to develop a magnetic resonance imaging probe that can be used to noninvasively identify and quantify regions of low oxygen levels in tumors (hypoxia). Characterizing hypoxic tumors may be valuable in guiding the choice of therapy that a patient receives.