Leung Kam
National Center for Biotechnology Information, NLM, NIH, Bethesda, MD
Optical fluorescence imaging is increasingly being used to monitor biological functions of specific targets (1-3). However, the intrinsic fluorescence of biomolecules poses a problem when fluorophores that absorb visible light (350–700 nm) are used. Near-infrared (NIR) fluorescence (700–1,000 nm) detection avoids the natural background fluorescence interference of biomolecules, which provides a high contrast between target and background tissues in small animals. NIR fluorophores have a wider dynamic range and minimal background fluorescence as a result of reduced scattering compared with visible fluorescence detection. NIR fluorophores also have high sensitivity, attributable to low background fluorescence, and high extinction coefficients, which provide high quantum yields. The NIR region is also compatible with solid-state optical components such as diode lasers and silicon detectors. NIR fluorescence imaging is a non-invasive complement to radionuclide imaging in small animals. Among the various NIR agents, only indocyanine green (ICG), with absorption at 780 nm and emission at 820 nm, is approved by the United States Food and Drug Administration for clinical applications in angiography, blood flow evaluation, and liver function assessment (4). It is also under evaluation in several clinical trials for other applications. However, ICG has a plasma half-life of 2–4 min because it forms aggregates in the blood and is rapidly cleared from blood circulation (5). ICG is prone to photobleaching and non-specific quenching its extensive binding to proteins. On the other hand, the fluorescence efficiency of some cyanine dyes can increase by ~1,000-fold upon binding to proteins and nucleic acids (6). Organic anion transporters (OATPs) are responsible for the transport of a diverse group of substances (hormones, xenobiotics, and bile acids) (7). Some OATPs have been shown to be overexpressed in various human cancer tissues and cancer cell lines (8, 9). Lysosomes are membranous vesicles found in most mammalian cells (10). Lysosomes contain various types of proteases, which have been implicated in migration, invasion, and metastasis of malignant cancer cells by degradation of extracellular matrix (11). Lysosomes are found to be larger in malignant cancer cells than in benign cancer cells (12). 2-[2-[2-Chloro-3-[2-[1,3-dihydro-3,3-dimethyl-1-(4-sulfobutyl)-2-indol-2-ylidene]-ethylidene]-1-cyclohexen-1-yl]-ethenyl]-3,3-dimethyl-1-(4-sulfobutyl)-3-indolium (IR-783) has been shown to accumulate in the mitochondria and lysosomes of cancer cells (13). Uptake of IR-783 may be dependent upon OATPs. Yang et al. (13) have evaluated the usefulness of IR-783 as a probe for NIR imaging of tumors.
光学荧光成像越来越多地用于监测特定靶点的生物学功能(1-3)。然而，当使用吸收可见光(350-700nm)的荧光团时，生物分子的固有荧光会带来问题。近红外(NIR)荧光(700-1000nm)检测避免了生物分子的天然背景荧光干扰，这在小动物的靶组织和背景组织之间提供了高对比度。与可见荧光检测相比，由于散射减少，近红外荧光团具有更宽的动态范围和最小的背景荧光。近红外荧光团还具有高灵敏度，这归因于低背景荧光，以及高消光系数，可提供高量子产率。近红外区域也与固态光学组件兼容，如二极管激光器和硅探测器。近红外荧光成像是小动物放射性核素成像的一种非侵入性补充。在各种近红外试剂中，只有吲哚菁绿(ICG)，其吸收峰在780nm，发射峰在820nm，被美国食品药品监督管理局批准用于血管造影、血流评估和肝功能评估的临床应用(4)。它也在其他应用的几项临床试验中接受评估。然而，ICG的血浆半衰期为2-4分钟，因为它在血液中形成聚集体并迅速从血液循环中清除(5)。ICG容易发生光漂白和非特异性猝灭，因为它与蛋白质广泛结合。另一方面，一些花青染料与蛋白质和核酸结合后，荧光效率可提高约1000倍(6)。有机阴离子转运体(OATPs)负责多种物质(激素、外源性物质和胆汁酸)的转运(7)。一些OATPs已被证明在各种人类癌症组织和癌细胞系中过表达(8,9)。溶酶体是大多数哺乳动物细胞中发现的膜性囊泡(10)。溶酶体含有各种类型的蛋白酶，这些蛋白酶通过降解细胞外基质参与恶性癌细胞的迁移、侵袭和转移(11)。发现恶性癌细胞中的溶酶体比良性癌细胞中的溶酶体更大(12)。2-[2-[2-氯-3-[2-[1,3-二氢-3,3-二甲基-1-(磺丁基)-2-吲哚亚基]-亚乙基]-1-环己烯-1-基]-乙烯基]-3,3-二甲基-1-(磺丁基)-3-吲哚鎓(IR-783)已被证明积聚在癌细胞的线粒体和溶酶体中(13)。IR-783的摄取可能依赖于OATPs。Yang等人(13)评估了IR-783作为肿瘤近红外成像探针的实用性。
