Abstract
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Objectives: The use of PET imaging surrogates for the prediction of biodistribution and dosimetry of targeted radionuclide therapy (TRT) agents is a common practice in the field of nuclear medicine oncology. Although isotopes such as 177Lu, 213Bi, and 225Ac have shown much promise in TRT of neuroendocrine and prostate cancers, they lack a definitive PET imaging surrogate. The goal of our study was to investigate the influence of different PET radiometals on biodistribution and tumor targeting of an alkylphosphocholine analog (NM600) and determine which of the investigated radiometals is most relevant as a PET surrogate for 177Lu-NM600.
Methods: NM600, an alkylphosphocholine analog containing a DO3A chelating moiety, was radiolabeled with 64Cu, 86Y, and 89Zr for PET imaging in a mouse model of breast cancer. 64CuCl2, 86YCl3, and 177LuCl3 in 0.1 M HCl were buffered with NaAc 0.1 M (pH=5.5), mixed with 0.3 nmol/MBq of NM600 and incubated at 95°C for 30 min. For 89Zr radiolabeling, 89Zr-oxalate was first converted to 89ZrCL4 by trapping in a Sep-Pak Accell Plus QMA cartridge and elution in 1M HCl. 89ZrCl4 was then buffered in HEPES (pH≍7.5) and reacted with 0.3 nmol/MBq of NM600 at 95°C for 1h. Radiolabeled NM600 was purified via solid-phase extraction using HLB cartridges. Radiochemical yield and purity were assessed by iTLC using EDTA (50 mM; pH=5.0) as mobile phase. Ten-week-old female Balb/c mice bearing mammary adenocarcinoma (4T1) xenografts were intravenously injected with 7.4-9.25 MBq (200-250 µCi) of 64Cu-NM600, 86Y-NM600, or 89Zr-NM600, and longitudinal PET scans were acquired at 3, 14, 24, and 48 h post-injection. Region-of-interest analysis of the PET images was performed to determine and compare radiotracer uptake (%ID/g) between 4T1 tumors and normal tissues over time. Ex vivo biodistribution at 48h p.i. was performed to validate the PET data, and biodistribution of the imaging probes was compared to that of 177Lu-NM600.
Results: Quantitative (>95%) radiolabeling yields were achieved with 64Cu, 86Y, and 177Lu while 89Zr efficiency was 50-60%, plausibly due to competing hydrolysis processes. PET imaging unveiled significant differences (P < 0.001) in terms of peak tumor uptake between tracers: 8.97 ± 1.27 %ID/g, 4.30 ± 0.35 %ID/g, and 6.67 ± 0.12 %ID/g for 86Y-NM600, 64Cu-NM600, and 89Zr-NM600, respectively. Blood circulation showed a marked disparity among radiotracers with terminal half-lives of 20.9 h, 14.1 h, and 0.8 h for the 86Y, 89Zr, and 64Cu-labeled NM600. Liver uptake, which followed the trend 89Zr-NM600 > 64Cu-NM600 > 86Y-NM600, was elevated in all cases, demonstrating the hepatobiliary excretion of the compounds. Overall uptake in normal tissue including muscle was low (<1.5 %ID/g). Ex vivo biodistribution at 48h p.i. corroborated the accuracy of the PET findings and showed significant differences in tumor, blood, liver, and spleen uptake among the imaging tracers and 177Lu-NM600. 86Y-NM600 uptake most closely represented 177Lu-NM600 biodistribution. Conclusions: Herein, we demonstrated that the choice of radiometal can dramatically impact the in vivo tumor targeting and pharmacokinetic profile of small molecule radiotracers. Our findings have direct implications to the field of theranostics where the use of surrogate imaging isotopes is common practice. At a minimum, thorough in vivo studies must be carried out to ensure the validity of imaging surrogates to describe the behavior of therapeutic analogs in vivo, or, when possible, the use of isotopic pairs such as 86Y/90Y should be favored.
