Abstract
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Objectives: Recently, we have demonstrated that radioiodinated APCs are excellent targeting probes for molecular imaging and targeted radiotherapy of several primary and metastatic malignancies. Our goal is to perform proof-of-principle studies in a series of novel APC chelates radiolabeled with 64Cu for in vivo positron emission tomography of several murine models of cancer.
Methods: Chelators DOTA, NOTA, and DTPA were conjugated to 18-(p-aminophenyl)octadecyl phosphocholine via amide or thiourea bond formation. Compounds were purified by HPLC and their structure confirmed through mass spectrometry. Radiolabeling with 64Cu proceeded in NaAc buffer pH 5.0 at 95°C for 1h. Purification of the radiolabeled APCs was performed using solid-phase HLB cartridges (Waters, MA) and radiochemical purity assessed by instant-TLC. The tumor targeting properties of 64Cu-DOTA-APC were evaluated in vivo in mice bearing glioblastoma (U87MG), lung (A549), colorectal (HT-29), prostate (PC-3), or pancreatic cancer (MiaPaca) xenografts (n = 3-4). Sequential PET scans were acquired using an Inveon microPET/CT (Siemens, TN) in tumor-bearing mice at 4, 24, and 48 h after the intravenous injection of 7.4-11.1 MBq of 64Cu-DOTA-APC. Region-of-interest analysis of the PET images was carried out to determine the temporal distribution of the compound, expressed as percent injected dose per gram of tissue (%ID/g). Following the terminal imaging time point at 48 h postinjection of the tracer, mice were sacrificed, and tissues were collected for ex vivo biodistribution.
Results: Mass spectrometry confirmed the structural identity of DOTA-APC, NOTA-APC, and DTPA-APC. Quantitative radiolabeling (>95% yield) with 64Cu was achieved in all three compounds, which showed excellent specific activity (>3.4 GBq/µmol) and radiochemical purities surpassing 95%. Upon intravenous administration, 64Cu-DOTA-APC presented a prominent hepatic and gut uptake that significantly declined over time (29.83 ± 2.02 %ID/g vs. 6.57 ± 0.56 %ID/g at 4 h and 48 h postinjection) indicating the hepatobiliary clearance of the tracer. Tumor uptake peaked at the initial imaging time point 4 h after administration, but declined at a slower rate compared to normal tissues, indicating enhanced tumor retention. U87MG xenografts exhibited the highest tracer accretion (4.90 ± 0.70 %ID/g) followed by HT-29 (3.27 ± 0.32 %ID/g %ID/g), A549 (2.90 ± 0.17 %ID/g), PC-3 (2.80 ± 1.58 %ID/g), and MiaPaca (2.73 ± 0.85 %ID/g). 64Cu-DOTA-APC uptake in normal tissues remained low throughout the study resulting in adequate tumor-to-background ratios. Ex vivo biodistribution corroborated the accuracy of PET quantification.
Conclusion: Our preliminary data demonstrated the targeting efficacy of radiolabeled APC chelates in a several malignancies. These results align well with our previous studies showing broad-spectrum tumor-targeting using radioiodinated APCs. The ability of APC chelates to coordinate a plethora of radiometals opens the door to the implementation of novel molecular imaging and targeted radiotherapy strategies with great potential to improve the clinical management of cancer. Research Support: Department of Radiology, University of Wisconsin Madison. $$graphic_F764C491-E7ED-4832-841F-BEE7008A23A3$$