There is a pressing need for new and more efficient boron delivery agents to tumor cells for use in boron neutron capture therapy (BNCT). A class of boronated unnatural cyclic amino acids has demonstrated a remarkable selectivity toward tumors in animal and cell culture models, far superior to currently used agents in clinical BNCT. One of these amino acids, 1-amino-3-boronocyclopentanecarboxylic acid (ABCPC), has shown a tumor to blood ratio of 8 and a tumor to normal brain ratio of nearly 21 in a melanoma bearing mouse model. This work represents further biological characterization of this compound for tumor targeting in an EMT6 murine mammary carcinoma mouse model and a T98G human glioblastoma cell line. Female BALB/c mice bearing EMT6 tumors were injected with the fructose complex form of racemic mixtures of cis and trans isomers of ABCPC in identical concentrations. Boron concentrations were measured in the tumor, blood, brain, skin, and liver tissues at 1, 3, and 5 h post-injection. These observations revealed a remarkable difference in racemic mixtures of cis and trans isomers in tumor targeting by boron. This implies that further separation of the L and D forms of this compound may enhance tumor targeting to an even higher degree than that provided by the racemic mixtures. Since the uptake measurements were made in homogenized tumor and normal tissues, little is known about the subcellular location of the boron arising from the various isomeric forms of the amino acid. To study subcellular delivery of boron from ABCPC in T98G human glioblastoma cells, we employed secondary ion mass spectrometry (SIMS) based technique of ion microscopy, which is capable of quantitatively imaging isotopic (elemental) gradients in cells and tissues at 500 nm spatial resolution. The T98G cells were exposed to the nutrient medium containing 100 ppm boron equivalent of a mixture of both L and D isomers of ABCPC in the form of a fructose complex for 1 h. Following this treatment, the cells were fast frozen, freeze-fractured, and freeze-dried for SIMS analysis. Within an hour of exposure, ABCPC provided partitioning of intracellular to extracellular boron of 3/1. SIMS imaging revealed that boron from ABCPC was distributed throughout the cell, including the nucleus. This level of boron delivery within an hour of exposure is superior to p-boronophenylalanine (BPA) and sodium borocaptate (BSH), which have been previously studied by SIMS in the same cell line. These encouraging observations provide compelling support for further isomeric separations of ABCPC into the D and L forms for enhanced tumor targeting and continued testing of these compounds as new boron carriers in BNCT.

中文翻译：

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硼化非天然氨基酸作为新型硼载体的生物学评价。

对于用于硼中子俘获疗法 (BNCT) 的肿瘤细胞，迫切需要新的和更有效的硼递送剂。一类硼化非天然环状氨基酸在动物和细胞培养模型中表现出对肿瘤的显着选择性，远远优于目前临床 BNCT 中使用的药物。其中一种氨基酸，1-氨基-3-硼环戊烷羧酸 (ABCPC)，在携带黑色素瘤的小鼠模型中显示出肿瘤与血液的比率为 8，肿瘤与正常脑的比率接近 21。这项工作代表了该化合物在 EMT6 鼠乳腺癌小鼠模型和 T98G 人胶质母细胞瘤细胞系中用于肿瘤靶向的进一步生物学表征。携带 EMT6 肿瘤的雌性 BALB/c 小鼠被注射以相同浓度的 ABCPC 顺式和反式异构体的外消旋混合物的果糖复合物形式。在注射后 1、3 和 5 小时测量肿瘤、血液、脑、皮肤和肝脏组织中的硼浓度。这些观察结果揭示了硼靶向肿瘤中顺式和反式异构体的外消旋混合物的显着差异。这意味着与外消旋混合物相比，进一步分离该化合物的 L 和 D 形式可以将肿瘤靶向性提高到更高的程度。由于摄取测量是在均质化的肿瘤和正常组织中进行的，因此对由氨基酸的各种异构形式产生的硼的亚细胞定位知之甚少。为了研究 T98G 人胶质母细胞瘤细胞中 ABCPC 硼的亚细胞传递，我们采用了基于二次离子质谱 (SIMS) 的离子显微镜技术，该技术能够以 500 nm 的空间分辨率对细胞和组织中的同位素（元素）梯度进行定量成像。将 T98G 细胞暴露于含有 100 ppm 硼当量的果糖复合物形式的 ABCPC L 和 D 异构体混合物的营养培养基中 1 小时。在此处理后，将细胞快速冷冻、冷冻破碎并冷冻干燥用于 SIMS 分析。在暴露一小时内，ABCPC 提供了 3/1 的细胞内硼与细胞外硼的分配。SIMS 成像显示来自 ABCPC 的硼分布在整个细胞中，包括细胞核。暴露后一小时内的这种硼传递水平优于对硼苯丙氨酸 (BPA) 和硼烷酸钠 (BSH)，SIMS 之前曾在同一细胞系中研究过它们。这些令人鼓舞的观察结果为进一步将 ABCPC 异构体​​分离成 D 和 L 形式提供了有力的支持，以增强肿瘤靶向性和继续测试这些化合物作为 BNCT 中新的硼载体。