Novel Preparation Methods of ⁵²Mn for ImmunoPET Imaging

52Mn (t1/2 =5.59 d, ß+ = 29.6%, Eßave = 0.24 MeV) shows promise in positron emission tomography (PET) and in dual-modality manganese-enhanced magnetic resonance imaging (MEMRI) applications including neural tractography, stem cell tracking, and biological toxicity studies. The extension to bioconjugate application requires high specific activity 52Mn in a state suitable for macromolecule labeling. To that end a 52Mn production, purification, and labeling system is presented, and its applicability in preclinical, macromolecule PET is shown using the conjugate 52Mn-DOTA-TRC105. 52Mn is produced by 60 µA, 16 MeV proton irradiation of natural chromium metal pressed into a silver disc support. Radiochemical separation proceeds by strong anion exchange chromatography of the dissolved Cr target, employing a semi-organic mobile phase, 97:3 (v:v) ethanol: HCl (11M, aqueous). The method is 62 ± 14% efficient (n=7) in 52Mn recovery, leading to a separation factor from Cr of (1.6 ± 1.0) x106 (n = 4), and an average effective specific activity of 0.8 GBq/µmol (n = 4) in titration against DOTA. 52Mn-DOTA-TRC105 conjugation and labeling demonstrate the potential for chelation applications. In vivo images acquired using PET/CT in mice bearing 4T1 xenograft tumors are presented. Peak tumor uptake is 18.7 ± 2.7 %ID/g at 24 hours post injection and ex vivo 52Mn biodistribution validates the in vivo PET data. Free 52Mn2+(as chloride or acetate) is used as a control in additional mice to evaluate the non-targeted biodistribution in the tumor model

⁴⁴Sc: An Attractive Isotope for Peptide-Based PET Imaging

The overexpression of integrin α_vβ₃ has been linked to tumor aggressiveness and metastasis in several cancer types. Because of its high affinity, peptides containing the arginine–glycine–aspartic acid (RGD) motif have been proven valuable vectors for noninvasive imaging of integrin α_vβ₃ expression and for targeted radionuclide therapy. In this study, we aim to develop a ⁴⁴Sc-labeled RGD-based peptide for <i>in vivo</i> positron emission tomography (PET) imaging of integrin α_vβ₃ expression in a preclinical cancer model. High quality ⁴⁴Sc (<i>t</i>_1/2, 3.97 h; β⁺ branching ratio, 94.3%) was produced inexpensively in a cyclotron, via proton irradiation of natural Ca metal targets, and separated by extraction chromatography. A dimeric cyclic-RGD peptide, (cRGD)₂, was conjugated to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and radiolabeled with ⁴⁴Sc in high yield (>90%) and specific activity (7.4 MBq/nmol). Serial PET imaging of mice bearing U87MG tumor xenografts showed elevated ⁴⁴Sc-DOTA-(cRGD)₂ uptake in the tumor tissue of 3.93 ± 1.19, 3.07 ± 1.17, and 3.00 ± 1.25 %ID/g at 0.5, 2, and 4 h postinjection, respectively (<i>n</i> = 3), which were validated by <i>ex vivo</i> biodistribution experiments. The integrin α_vβ₃ specificity of the tracer was corroborated, both <i>in vitro</i> and <i>in vivo</i>, by competitive cell binding and receptor blocking assays. These results parallel previously reported studies showing similar tumor targeting and pharmacokinetic profiles for dimeric cRGD peptides labeled with ⁶⁴Cu or ⁶⁸Ga. Our findings, together with the advantageous radionuclidic properties of ⁴⁴Sc, capitalize on the relevance of this isotope as an attractive alternative isotope to more established radiometals for small molecule-based PET imaging, and as imaging surrogate of ⁴⁷Sc in theranostic applications

<i>In Vivo</i> Integrity and Biological Fate of Chelator-Free Zirconium-89-Labeled Mesoporous Silica Nanoparticles

Traditional chelator-based radio-labeled nanoparticles and positron emission tomography (PET) imaging are playing vital roles in the field of nano-oncology. However, their long-term <i>in vivo</i> integrity and potential mismatch of the biodistribution patterns between nanoparticles and radio-isotopes are two major concerns for this approach. Here, we present a chelator-free zirconium-89 (⁸⁹Zr, <i>t</i>_1/2 = 78.4 h) labeling of mesoporous silica nanoparticle (MSN) with significantly enhanced <i>in vivo</i> long-term (>20 days) stability. Successful radio-labeling and <i>in vivo</i> stability are demonstrated to be highly dependent on both the concentration and location of deprotonated silanol groups (−Si–O^–) from two types of silica nanoparticles investigated. This work reports ⁸⁹Zr-labeled MSN with a detailed labeling mechanism investigation and long-term stability study. With its attractive radio-stability and the simplicity of chelator-free radio-labeling, ⁸⁹Zr-MSN offers a novel, simple, and accurate way for studying the <i>in vivo</i> long-term fate and PET image-guided drug delivery of MSN in the near future

Matching the Decay Half-Life with the Biological Half-Life: ImmunoPET Imaging with ⁴⁴Sc-Labeled Cetuximab Fab Fragment

Scandium-44 (<i>t</i>_1/2 = 3.9 h) is a relatively new radioisotope of potential interest for use in clinical positron emission tomography (PET). Herein, we report, for the first time, the room-temperature radiolabeling of proteins with ⁴⁴Sc for <i>in vivo</i> PET imaging. For this purpose, the Fab fragment of Cetuximab, a monoclonal antibody that binds with high affinity to epidermal growth factor receptor (EGFR), was generated and conjugated with <i>N</i>-[(R<i>)</i>-2-amino-3-(<i>para</i>-isothiocyanato-phenyl)­propyl]-<i>trans</i>-(<i>S</i>,<i>S</i>)-cyclohexane-1,2-diamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentaacetic acid (CHX-A″-DTPA). The high purity of Cetuximab-Fab was confirmed by SDS-PAGE and mass spectrometry. The potential of the bioconjugate for PET imaging of EGFR expression in human glioblastoma (U87MG) tumor-bearing mice was investigated after ⁴⁴Sc labeling. PET imaging revealed rapid tumor uptake (maximum uptake of ∼12% ID/g at 4 h postinjection) of ⁴⁴Sc–CHX-A″-DTPA–Cetuximab-Fab with excellent tumor-to-background ratio, which might allow for same day PET imaging in future clinical studies. Immunofluorescence staining was conducted to correlate tracer uptake in the tumor and normal tissues with EGFR expression. This successful strategy for immunoPET imaging of EGFR expression using ⁴⁴Sc–CHX-A″-DTPA–Cetuximab-Fab can make clinically translatable advances to select the right population of patients for EGFR-targeted therapy and also to monitor the therapeutic efficacy of anti-EGFR treatments

VEGF₁₂₁-Conjugated Mesoporous Silica Nanoparticle: A Tumor Targeted Drug Delivery System

The vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR) signaling cascade plays a critical role in tumor angiogenesis and metastasis and has been correlated with several poorly prognostic cancers such as malignant gliomas. Although a number of anti-VEGFR therapies have been conceived, inefficient drug administration still limits their therapeutic efficacy and raises concerns of potential side effects. In the present work, we propose the use of uniform mesoporous silica nanoparticles (MSNs) for VEGFR targeted positron emission tomography imaging and delivery of the anti-VEGFR drug (i.e., sunitinib) in human glioblastoma (U87MG) bearing murine models. MSNs were synthesized, characterized and modified with polyethylene glycol, anti-VEGFR ligand VEGF₁₂₁ and radioisotope ⁶⁴Cu, followed by extensive in vitro, in vivo and ex vivo studies. Our results demonstrated that a significantly higher amount of sunitinib could be delivered to the U87MG tumor by targeting VEGFR when compared with the non-targeted counterparts. The as-developed VEGF₁₂₁-conjugated MSN could become another attractive nanoplatform for the design of future theranostic nanomedicine