Developing New Multicolor Polypeptide MR Contrast Agents for Stem Cell Tracking

描述（由申请人提供）： 开发新的对比材料对于扩大MR在细胞和分子水平上的诊断能力非常重要。特别是，提供关于细胞迁移、运输和归巢的信息将有助于开发使用干细胞和祖细胞的细胞疗法。先前的细胞追踪研究已经采用了含有氧化铁、钆或其他金属的试剂，通过影响水质子的弛豫时间来产生对比度。使用这些试剂进行细胞追踪的缺点包括细胞被检测为阳性或阴性斑点的特征，其可能难以与成像伪影区分开，以及关于体内长期金属毒性的不确定性。化学交换饱和转移（CEST）试剂是一类新的MR试剂，迄今为止尚未用于细胞跟踪。通过在试剂的质子上施加饱和脉冲来用这些试剂产生对比度，所述试剂的质子与水进行快速化学交换，转移信号损失，交换的速度从CEST试剂上的单个可交换位点产生多个水质子的饱和。我们建议开发一种新的分子探针用于细胞跟踪，将其插入细胞并监测其体内有效性。将测试两种不同类型的生物聚合物CEST试剂，由D-氨基酸（而不是L）组成的多肽和由L-氨基酸组成但具有适当的耐降解N-末端基团的多肽。这些试剂的优点是它们的对比度是饱和频率依赖性的，允许试剂的多种“颜色”，这将有助于用MRI独立地跟踪多个细胞群（与基于金属的试剂的简单开/关对比度相比）。我们有兴趣为MR成像开发四种不同的“颜色”，类似于光学/荧光成像的能力。我们还将优化这些药物的敏感性和递送。我们假设这些试剂的检测是可能的，在体内，为了实现这一目标，我们将合成一系列的这些试剂，并检测转染的神经干细胞在活的动物。由于造影剂的化学交换放大作用，这些造影剂应被证明是高度敏感的。此外，因为这些药剂仅由多肽组成，所以可以采用新的递送/靶向策略。

DESCRIPTION (provided by applicant): Development of new contrast materials is important to expand the diagnostic capabilities of MR at the cellular and molecular level. In particular, providing information on cell migration, trafficking and homing would be useful for the development of cell therapies using stem cells and progenitors. Previous cell tracking studies have employed agents containing iron oxide, gadolinium or other metals to generate contrast by influencing the relaxation time of water protons. The drawbacks of using these agents for cell tracking include the feature that cells are detected as positive or negative spots which can be difficult to distinguish from imaging artifacts and also the uncertainty about long-term metal toxicity in-vivo. Chemical Exchange Saturation Transfer (CEST) agents are a new class of MR agents which to date haven't been used for cell tracking. Contrast is generated with these agents by applying a saturation pulse on the agent's protons which rapid chemical exchange with water transferring the signal loss, with the speed of the exchange producing a saturation of multiple water protons from a single exchangeable site on the CEST agent. We are proposing developing a new class of molecular probes for use in cell tracking, inserting these into cells and monitoring their effectiveness in-vivo. Two different types of biopolymer CEST agents will be tested, poly-peptides consisting of D-amino acids (instead of L) and poly-peptides consisting of l-amino acids but with appropriate N-terminal groups resistant to degradation. The advantage of these agents is that their contrast is saturation frequency dependent, allowing multiple "colors" of agent, which would be useful for tracking multiple cell populations independently with MRI (as compared with the simple on/off contrast of metal based agents). We are interested in developing four different "colors" for MR imaging, analogous to the capabilities of optical/fluorescent imaging. We will also optimize the sensitivity and delivery of these agents. We hypothesize that the detection of these agents is possible in-vivo and to achieve this goal we will synthesize a series of these agents, and detect transfected neural stem cells individually in live animals. These contrast agents should prove to be highly sensitive due to the chemical exchange amplification of the contrast. In addition, because these agents are composed exclusively of polypeptides, new delivery/targeting strategies may be employed.