Abstract
Tumor-associated macrophages (TAMs) are increasingly investigated in cancer immunology, and are considered a promising target for better and tailored treatment of malignant growth. Although TAMs also have high diagnostic and prognostic value, TAM imaging still remains largely unexplored. Here, we describe the development of reconstituted high-density lipoprotein (rHDL)-facilitated TAM positron emission tomography (PET) imaging in a breast cancer model. Methods: Radiolabeled rHDL nanoparticles incorporating the long-lived positron-emitting nuclide 89Zr were developed using two different approaches. The nanoparticles were composed of phospholipids and apolipoprotein A-I (ApoA-I) in a 2.5:1 weight ratio. 89Zr was complexed with DFO, conjugated to either a phospholipid or ApoA-I protein to generate 89Zr-PL-HDL and 89Zr-AI-HDL, respectively. In vivo evaluation was carried out in an orthotopic mouse model of breast cancer and included pharmacokinetic analysis, biodistribution studies, and PET imaging. Ex vivo histological analysis of tumor tissues to assess regional distribution of 89Zr radioactivity was also performed. Fluorescent analogs of the radiolabeled agents were used to determine cell-targeting specificity using flow cytometry. Results: The phospholipid- and apoA-I-labeled rHDL were produced at 79 ± 13 % (n = 6) and 94 ± 6 % (n = 6) radiochemical yield, respectively, with excellent radiochemical purity (> 99 %). Intravenous administration of both probes resulted in high tumor radioactivity accumulation (16.5 ± 2.8 and 8.6 ± 1.3 %ID/g for ApoA-I- and phospholipid-labeled rHDL, respectively) at 24 hours post injection. Histological analysis showed good co-localization of radioactivity with TAM-rich areas in tumor sections. Flow cytometry revealed high specificity of rHDL for TAMs, which had the highest uptake per cell (6.8-fold higher than tumor cells for both DiO@Zr-PL-HDL and DiO@Zr-AI-HDL) and accounted for 40.7 % and 39.5% of the total cellular DiO@Zr-PL-HDL and DiO@Zr-AI-HDL in tumors, respectively. Conclusion: We have developed 89Zr-labeled TAM imaging agents based on the natural nanoparticle rHDL. In an orthotopic mouse model of breast cancer, we have demonstrated their specificity for macrophages, a result that was corroborated by flow cytometry. Quantitative macrophage PET imaging with our 89Zr-rHDL imaging agents could be valuable for non-invasive monitoring of TAM immunology and targeted treatment.
- Copyright © 2015 by the Society of Nuclear Medicine and Molecular Imaging, Inc.