Imaging plays a major role in basic research, diagnostics and monitoring of disease. Live, whole-animal imaging using antibodies bound to near-IR dyes has enabled the non-invasive study of biological processes, disease progression and response to potential therapeutics. Unfortunately, antibodies can often trigger a biological response that affects the tissue or process being studied. Despite the many advances in imaging, some cancers still require earlier detection through more sensitive imaging to accelerate treatment and improve outcomes (2). In an effort to increase the accuracy and sensitivity of in vivo imaging, several research groups are currently working with aptamers for targeted imaging. Aptamers are small, single-stranded oligonucleotides with selectivity and affinity properties similar to antibodies. Due to their small size, aptamers are generally non-immunogenic and penetrate tumor tissue more quickly and easily than an antibody. Binding of more aptamer molecules to target tissue (vs. larger antibodies) may also increase signal generation and improve imaging sensitivity. Aptamers are chemically synthesized and easily modified for detection and enhanced in vivo stability (Read “Enhancing Aptamer Stability”). They can be selected to differentiate between highly similar compounds and conjugated to a wide range of molecules without affecting target binding. Selectivity, biocompatibility and flexibility make aptamers ideal targeting agents for in vivo imaging (1,4). Learn More About Aptamers and Aptamer Selection
Aptamer-Mediated Near-Infrared (NIR) Imaging
Researchers at Fudan University in China recently selected an aptamer to GPC3, a cell surface protein that is highly expressed in hepatocellular carcinoma (HCC) tissue, the most common type of liver cancer. Aptamer AP613-1 was shown to selectively target Huh-7 (GPC3+) HCC cells vs. A549 lung cancer cells in vitro. The aptamer was labeled with Alexa Fluor™ 750, a near-infrared dye, and used to image subcutaneous Huh-7 HCC tumors in nude mice both in vivo and ex vivo. Experimental and control mice showed similar levels of fluorescence in major organs, but the experimental group showed elevated levels of fluorescence at the tumor site, showing successful aptamer targeting (2). Average fluorescence intensities were approximately twenty to thirty percent higher when organs were imaged ex vivo compared with in vivo imaging, but the results showed strong correlation (correlation coefficient of 0.968). The in vivo imaging data accurately showed the bio-distribution of the imaging agent in major organs. Data also showed that selective aptamers can be used to detect subcutaneous tumors via NIR fluorescence imaging, making it possible to monitor tumor response in an animal model in vivo, without sacrificing the animal. Aptamer complexing with magnetic nanoparticles or 18F would enable enhanced imaging via MRI or PET-CT for early detection and monitoring of deep-tissue tumors (2).
Aptamer Targeting in Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging (MRI) is a non-invasive imaging technology known for superior spatial resolution and contrast in soft tissue (1,3). Traditional small molecule imaging agents used in MRI offer limited contrast enhancement, as binding is non-specific and agents are rapidly cleared. Specific antibodies have been used as targeting agents, but have difficulty diffusing into tumor tissue and are rapidly cleared by the body’s reticuloendothelial system (1). A growing number of researchers are overcoming these limitations with aptamers. Researchers at JiangSu University in China selected an aptamer against hypoxia-inducible factor-1α (HIF-1α) for enhanced imaging of hypoxia-induced cancer stem cells, believed to play an important role in cancer metastasis, therapy resistance, and tumor recurrence. PEG- and Mn(II)-modified magnetic nanoparticles were bound to the HIF-1α aptamer and evaluated in MRI both in vitro and in vivo. In vitro imaging of Panc-1 and Bxpc-3 pancreatic carcinoma cell lines showed a 7-fold increase in signal with aptamer targeting. In vivo imaging of a xenograft of Panc-1 in a mouse model showed a 3.5-fold increase in signal with aptamer targeting. Histopathological examination of major organs showed no toxic effects from the nanoparticles. The biocompatible aptamer-nanoparticles successfully targeted cancer cells in hypoxic regions and significantly enhanced signal and contrast in MRI (3). Aptamer-targeting can be applied to a wide range of imaging applications, from preclinical research to drug development and distribution studies to early detection and treatment monitoring for many forms of cancer.
Aptamer Selection for In Vivo Imaging
Aptamers can be selected for targeting of specific cell surface markers or specific cell types (without a known surface marker) for non-invasive detection and monitoring. Aptamers can be selected for targeting of metabolites and synthetic small molecules to facilitate the study of bio-distribution, drug metabolism, and drug clearance. Base Pair has successfully developed aptamers to a number of small molecules and metabolites using proprietary techniques for selection and characterization. Base Pair’s patented multiplex SELEX technology enables simultaneous selection of aptamers to several targets, decreasing the time and cost of aptamer development. Aptamer development can include modification for enhanced stability and conjugation to a wide range of linkers and imaging agents.
References: 1. Zhang, Y. et al. Aptamer-targeted magnetic resonance imaging contrast agents and their applications. Journal of Neuroscience and Nanotechnology. 2018. 18:3759-3774. 2. Zhao, M., et al. In vivo fluorescence imaging of hepatocellular carcinoma using a novel GPC3-specific aptamer probe. Quantitative Imaging in Medicine and Surgery. 2018. 8(2) :151-160. 3. Zhu, H., et al. Aptamer-PEG-modified Fe3O4@Mn as a novel T1- and T2- dual-model MRI contrast agent targeting hypoxia-induced cancer stem cells. Nature: Scientific Reports. 2016. 6:39245. 4. Shi, H. et al. Activatable aptamer probe for contrast-enhanced in vivo cancer imaging based on cell membrane protein-triggered conformation alteration. PNAS. 108(10):3900-3905.