Imaging of O6-Alkylguanine-DNA Alkyltransferase

描述(申请人提供)：化疗，特别是烷基化化疗，是癌症治疗的主要模式之一；然而，应答率往往很低。许多患者预后不良的一个主要原因是耐药性的产生，导致肿瘤再生长和最终患者死亡。这种耐药性是由于DNA损伤被修复蛋白O6烷基DNA烷基转移酶(AGT)修复所致，DNA损伤是由烷化剂将鸟嘌呤和胸腺嘧啶残基烷基化而产生的。AGT通过将烷基从修饰的DNA转移到其活性部位的半胱氨酸来修复损伤。这是一个自杀的过程，因为没有再生蛋白质的机制，因此耐药性或治疗效果取决于肿瘤中AGT的含量。一种非侵入性定量肿瘤AGT的方法将在烷化剂化疗的计划、监测药物在治疗过程中对AGT水平的影响以及新型烷化剂化疗药物的开发方面大有裨益。苄基鸟嘌呤(BG)已被证明是AGT的一种很好的灭活剂，有几项临床试验正在进行中，评估在化疗前使用BG来耗尽AGT。这项建议的长期目标是开发放射性碘或18F标记的BG类似物，用于通过闪烁成像进行AGT的非侵入性标测。4-位含氟的BG(4-FBG)已经被证明是BG的一个很好的类似物。将开发用于4-FBG和放射性碘化BG衍生物的18F标记的方法。虽然可能使用这些化合物来证明原理，但也将努力开发更好的类似物，这些类似物更容易制备，更有效，代谢稳定性更高，肿瘤保留时间更长。所考虑的化合物包括4-benzyloxy-2，6-diamino-5-nitro/nitroso-pyrimidine，的衍生物和在其C-8和N-9位具有取代基的各种BG衍生物。新的未标记化合物将首先用转AGT的中国仓鼠卵巢(CHO)细胞评估其AGT失活能力。将评估标记化合物与纯AGT的结合以及它们在含有AGT的细胞中的摄取和保留。他们的体外代谢也将被研究。肿瘤AGT含量和标记BG类似物的肿瘤摄取之间的相关性将被建立。首先，TE-671人髓母细胞瘤异种移植将用于这些研究。肿瘤摄取作为药物浓度和时间的函数将被研究。潜在的代谢物，特别是肿瘤中的代谢物，将被确定。同样，在异种移植模型中，肿瘤摄取和肿瘤AGT含量之间的相关性也将被研究。总之，这项建议旨在确定开发一种适合肿瘤AGT成像的试剂是否有助于化疗计划，从而改善烷化化疗的结果。随后，这些示踪剂将在接受异种移植的无菌小鼠身上进行评估。首先，TE-671人髓母细胞瘤异种移植将用于这些研究。肿瘤摄取作为药物浓度和时间的函数将被研究。潜在的代谢物，特别是肿瘤中的代谢物，将被确定。同样，在异种移植模型中，肿瘤摄取和肿瘤AGT含量之间的相关性也将被研究。总之，这项建议旨在确定开发一种适合肿瘤AGT成像的试剂是否有助于化疗计划，从而改善烷化化疗的结果。

DESCRIPTION (provided by applicant): Chemotherapy, especially alkylator chemotherapy, is one of the primary modes for the treatment of cancer; however, the response rate is often low. A major reason for poor prognosis in many patients is the development of drug resistance, resulting in tumor regrowth and eventual patient death. The drug resistance is due to the repair of DNA lesions, generated by alkylation of guanine and thymine residues by the alkylating agents, by the repair protein O6 alkyl-DNA alkyltrasferase (AGT). AGT repairs the lesions by transferring the alkyl groups from the modified DNA to a cysteine in its active site. This is a suicidal process as there is no mechanism to regenerate the protein and hence the resistance, or alternatively the therapeutic efficacy, depends on the amount of AGT content in the tumor. A means to noninvasively quantitate tumor AGT will go a long way in the planning of alkylator chemotherapy, monitoring the effect of drugs on AGT levels during the therapeutic course, and in the development of newer alkylator chemotherapeutic agents. Benzylguanine (BG) has been shown to be an excellent inactivator of AGT and there are several clinical trials in progress evaluating the use of BG to deplete AGT prior to chemotherapy. The long-term objective of this proposal is to develop radioiodine- or 18F-labeled BG analogues useful in the noninvasive mapping of AGT by scintigraphic imaging. BG containing fluorine at the 4-position of its benzyl group (4-FBG) has already been shown to be an excellent analogue of BG. Methods will be developed for the 18F-labeling of 4-FBG as well as radioiodinated BG derivatives. While it may be possible to demonstrate the proof of principle using these compounds, efforts will also be directed in the development of better analogues that are easier to prepare, more potent, and will have higher metabolic stability, and longer tumor retention. Among the compounds considered are the derivatives of 4-benzyloxy-2,6-diamino-5-nitro/nitroso-pyrimidine, and various BG derivatives with substituents at its C-8 and N-9 positions. Novel unlabeled compounds will be first evaluated for their AGT inactivating ability using Chinese hamster ovary (CHO) cells transfected with AGT. Labeled compounds will be evaluated for their binding with pure AGT and for their uptake and retention in AGT-containing cells. Their in vitro metabolism also will be studied. Correlativity of tumor AGT content and tumor uptake of labeled BG analogues will be Established. Primarily, TE-67 1 human medulloblastoma xenografts will be utilized for these studies. Tumor uptake as a function of drug concentration and time will be studied. Potential metabolites, especially in the tumor, will be determined. Again, the correlativity between tumor uptake and tumor AGT content will be investigated in xenograft models as well. In summary, this proposal seeks to determine whether the development of a suitable agent for the imaging of tumor AGT will facilitate chemotherapy planning, thereby improving the outcome of alkylator chemotherapy. Subsequently these tracers will be evaluated in athymic mice hosting xenografts. Primarily, TE-671 human medulloblastoma xenografts will be utilized for these studies. Tumor uptake as a function of drug concentration and time will be studied. Potential metabolites, especially in the tumor, will be determined. Again, the correlativity between tumor uptake and tumor AGT content will be investigated in xenograft models as well. In summary, this proposal seeks to determine whether the development of a suitable agent for the imaging of tumor AGT will facilitate chemotherapy planning, thereby improving the outcome of alkylator chemotherapy.