Unraveling the in vivo fate of inhaled micro- and nanoplastics with PET imaging.

Microplastics and nanoplastics have become ubiquitous environmental pollutants. The threat these plastics pose to human health has fueled research focused on their pathophysiology and toxicology, yet many of their fundamental properties - for example, their in vivo pharmacokinetics - remain poorly understood. In this investigation, we have harnessed positron emission tomography (PET) to track the in vivo fate of micro- and nanoplastics administered to mice intratracheally and intravenously. To this end, 1 µm and 20 nm diameter amine-functionalized polystyrene particles were modified with an isothiocyanate-bearing variant of desferrioxamine (DFO) and radiolabeled with the positron-emitting radiometal [89Zr]Zr4+. Both radioplastics - [89Zr]Zr-DFO-PS1000 and [89Zr]Zr-DFO-PS20 - were produced in ∼95% radiochemical yield and found to be >85% stable to demetallation over one week at 37 °C in human serum and simulated lung fluid. The incubation of [89Zr]Zr-DFO-PS1000 and [89Zr]Zr-DFO-PS20 with MH-S cells revealed that the majority of the former were phagocytosed by alveolar macrophages within 4 h, while the latter largely evaded consumption. Finally, the in vivo behavior of the radioplastics was interrogated in mice upon intravenous and intratracheal administration. PET imaging and biodistribution experiments revealed that the intravenously injected plastics accumulated primarily in the liver and spleen, yielding hepatic radioactivity concentrations of 101 ± 48 %ID/g and 92 ± 22 %ID/g at 168 h post-injection for [89Zr]Zr-DFO-PS1000 and [89Zr]Zr-DFO-PS20, respectively. In contrast, the mice that received the radioplastics via intratracheal installation displayed the highest uptake in the lungs at the end of one week: 4 ± 2 %ID/g for [89Zr]Zr-DFO-PS1000 and 32 ± 6 %ID/g for [89Zr]Zr-DFO-PS20. Ultimately, this work illustrates the critical role that the route of exposure plays in the bioaccumulation of plastic particles, reveals that size dramatically influences the pulmonary retention of inhaled particles, and underscores the value of PET imaging as a tool for studying the pharmacokinetics of environmental pollutants.

Visualizing Galectin-3 Binding Protein Expression with ImmunoPET.

Galectin-3 binding protein (Gal-3BP) is a glycoprotein that is overexpressed and secreted by several cancers and has been implicated as a marker of both tumor progression and poor prognosis in melanoma, non-small cell lung cancer, head and neck squamous cell carcinoma, and breast cancer. The expression of Gal-3BP by a variety of neoplasms makes it an enticing target for both diagnostics and therapeutics, including immuno-positron emission tomography (immunoPET) probes and antibody-drug conjugates (ADCs). Herein, we report the development, in vitro characterization, and in vivo evaluation of a pair of Gal-3BP-targeting radioimmunoconjugates for 89Zr-immunoPET. A humanized anti-Gal-3BP antibody, 1959, and its corresponding ADC, 1959-sss/DM4 (DM4 = ravtansine), were modified with desferrioxamine (DFO) to yield DFO-1959 and DFO-1959-sss/DM4 immunoconjugates bearing 1-2 DFO/monoclonal antibody. Both DFO-modified immunoconjugates retained their affinity for Gal-3BP in enzyme-linked immunosorbent assay experiments. The chelator-bearing antibodies were radiolabeled with zirconium-89 (t1/2 ≈ 3.3 d) to produce radioimmunoconjugates ─ [89Zr]Zr-DFO-1959 and [89Zr]Zr-DFO-1959-sss/DM4 ─ with high specific activity (>444 MBq/mg, >12 mCi/mg) and stability (>80% intact after 168 h in human serum at 37 °C). In mice bearing subcutaneous Gal-3BP-secreting A375-MA1 xenografts, [89Zr]Zr-DFO-1959 clearly delineated tumor tissue, reaching a maximum tumoral activity concentration (54.8 ± 15.8%ID/g) and tumor-to-background contrast (tumor-to-blood = 8.0 ± 4.6) at 120 h post-injection. The administration of [89Zr]Zr-DFO-1959 to mice bearing subcutaneous Gal-3BP-expressing melanoma patient-derived xenografts produced similarly promising results. [89Zr]Zr-DFO-1959 and [89Zr]Zr-DFO-1959-sss/DM4 exhibited nearly identical pharmacokinetic profiles in the mice bearing A375-MA1 tumors, though the latter produced higher uptake in the spleen and kidneys. Both [89Zr]Zr-DFO-1959 and [89Zr]Zr-DFO-1959-sss/DM4 effectively visualized Gal-3BP-secreting tumors in murine models of melanoma. These results suggest that both probes could play a role in the clinical imaging of Gal-3BP-expressing malignancies, particularly as companion theranostics for the identification of patients likely to respond to Gal-3BP-targeted therapeutics such as 1959-sss/DM4.

Inverse electron demand Diels-Alder click chemistry for pretargeted PET imaging and radioimmunotherapy.

Radiolabeled antibodies have shown promise as tools for both the nuclear imaging and endoradiotherapy of cancer, but the protracted circulation time of radioimmunoconjugates can lead to high radiation doses to healthy tissues. To circumvent this issue, we have developed an approach to positron emission tomography (PET) imaging and radioimmunotherapy (RIT) predicated on radiolabeling the antibody after it has reached its target within the body. This in vivo pretargeting strategy is based on the rapid and bio-orthogonal inverse electron demand Diels-Alder reaction between tetrazine (Tz) and trans-cyclooctene (TCO). Pretargeted PET imaging and RIT using TCO-modified antibodies in conjunction with Tz-bearing radioligands produce high activity concentrations in target tissues as well as reduced radiation doses to healthy organs compared to directly labeled radioimmunoconjugates. Herein, we describe how to prepare a TCO-modified antibody (humanized A33-TCO) as well as how to synthesize two Tz-bearing radioligands: one labeled with the positron-emitting radiometal copper-64 ([64Cu]Cu-SarAr-Tz) and one labeled with the ß-emitting radiolanthanide lutetium-177 ([177Lu]Lu-DOTA-PEG7-Tz). We also provide a detailed description of pretargeted PET and pretargeted RIT experiments in a murine model of human colorectal carcinoma. Proper training in both radiation safety and the handling of laboratory mice is required for the successful execution of this protocol.

Dual Radionuclide Theranostic Pretargeting.

Recent years have played witness to the advent of nuclear theranostics: the synergistic use of "matched pair" radiopharmaceuticals for diagnostic imaging and targeted radiotherapy. In this investigation, we report the extension of this concept to in vivo pretargeting based on the rapid and bioorthogonal inverse electron demand Diels-Alder reaction between tetrazine (Tz) and trans-cyclooctene (TCO). We demonstrate that a single injection of a TCO-modified immunoconjugate can be used as a platform for pretargeted PET imaging and radiotherapy via the sequential administration of a pair of Tz-bearing radioligands labeled with the positron-emitting radiometal copper-64 (t1/2 ≈ 12.7 h) and the beta-emitting radiometal lutetium-177 (t1/2 ≈ 6.7 days). More specifically, a mouse model of human colorectal carcinoma received a dose of the A33 antigen-targeting immunoconjugate huA33-TCO, followed 24 and 48 h later by injections of [64Cu]Cu-SarAr-Tz and [177Lu]Lu-DOTA-PEG7-Tz, respectively. This approach produces high activity concentrations of both radioligands in tumor tissue (16.4 ± 2.7 %ID/g for [64Cu]Cu-SarAr-Tz at 48 h post-injection and 18.1 ± 2.1 %ID/g for [177Lu]Lu-DOTA-PEG7-Tz at 120 h post-injection) as well as promising tumor-to-healthy organ activity concentration ratios. Ultimately, we believe that this work could not only have important implications in nuclear theranostics-most excitingly with isotopologue-based radioligand pairs such as [64Cu]Cu-SarAr-Tz and [67Cu]Cu-SarAr-Tz-but also in the delivery of fractionated doses during pretargeted radioimmunotherapy.