Activatable MR Imaging Probes

描述(申请人提供)：可激活磁共振成像探头可激活磁共振(MR)成像探头提供前所未有的生物洞察力。已经报道了响应于钙离子通量、锌离子通量、报告基因、酶活性、pO2和pH的MR探针。这种探针的潜在应用包括神经元电流的直接成像、胰岛活性、基因激活和异质肿瘤微环境的关键因素。MR允许在高场系统上以细胞分辨率(~105m)、在临床扫描仪上以亚毫米分辨率对完整的、不透明的生物进行三维询问。深度的组织渗透和高分辨率使磁共振可以直接将研究结果从细胞转换到老鼠再到人类。抑制其翻译的可激活MR探头的根本局限性在于难以区分“主动”和“非主动”探头。在磁共振成像中，主要是对水进行成像，并通过其对水信号的影响间接检测探头。这种效应依赖于探头浓度和探头的弛豫度。弛豫度取决于许多分子因素，包括探针的水化状态及其旋转扩散速率。可以设计可激活或“智能”的探针，其中弛豫度随环境刺激而变化，例如，探针通过酶反应从低弛豫度状态转变为高弛豫度状态，或者探针的弛豫度在结合分析物(例如钙)时发生变化。尽管弛豫度对刺激非常敏感，但磁共振信号的变化取决于弛豫度和探头浓度：两个未知数。在体外，在浓度不变的情况下，这些探针充当优雅的传感器。然而，在体内，探针浓度是未知的，并且随时间变化。与正常组织相比，由于内皮通透性增加，病变组织的浓度可能更高，或者由于血流灌注不佳，浓度可能更低。信号变化可能是非活性探针分布的结果，也可能是探针激活的结果。目前还没有实用的方法来区分活动和非活动的探测器。我们提出了可激活的探测器，这些探测器在“关闭”状态下完全是MR静默的。这样，探头注入后MR信号的任何变化都必须是由于探头的激活。我们提出了一种基于锰的还原-氧化(氧化)化学的可激活磁共振探针的新范式。二价锰(Mn2+)是一种有效的MR弛豫剂，但Mn3+通常不是一种很差的弛豫剂。我们将制备稳定的锰络合物，它可以在环境刺激下从真正的MR-OFF状态(Mn3+)可逆地转换到MR-On状态(Mn2+)。随着可调氧化还原核心的发展，有可能设计出对pH、酶活性、离子通量或特定半抗原敏感的探针。在这项应用中，我们将重点开发一种用于低氧成像的磁共振氧传感器，其中磁共振信号仅在低氧区域产生。我们将通过与PO2电极直接测量、正电子发射断层扫描低氧探头和依赖于血氧水平的MR进行比较，在小鼠和兔肿瘤模型中验证这种低氧探头。

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.