Novel hexadentate and pentadentate chelators for ⁶⁴Cu-based targeted PET imaging

Positron Emission Tomography Imaging of Vessel Wall Matrix Metalloproteinase Activity in Abdominal Aortic Aneurysm

BACKGROUND

Matrix metalloproteinases (MMPs) play a key role in the pathogenesis of abdominal aortic aneurysm (AAA). Imaging aortic MMP activity, especially using positron emission tomography to access high sensitivity, quantitative data, could potentially improve AAA risk stratification. Here, we describe the design, synthesis, characterization, and evaluation in murine AAA and human aortic tissue of a first-in-class MMP-targeted positron emission tomography radioligand, 64Cu-RYM2.

METHODS

The broad spectrum MMP inhibitor, RYM2 was synthetized, and its potency as an MMP inhibitor was evaluated by a competitive inhibition assay. Toxicology studies were performed. Tracer biodistribution was evaluated in a murine model of AAA induced by angiotensin II infusion in Apolipoprotein E-deficient mice. 64Cu-RYM2 binding to normal and aneurysmal human aortic tissues was assessed by autoradiography.

RESULTS

RYM2 functioned as an MMP inhibitor with nanomolar affinities. Toxicology studies showed no adverse reaction in mice. Upon radiolabeling with Cu-64, the resulting tracer was stable in murine and human blood in vitro. Biodistribution and metabolite analysis in mice showed rapid renal clearance and acceptable in vivo stability. In vivo positron emission tomography/computed tomography in a murine model of AAA showed a specific aortic signal, which correlated with ex vivo measured MMP activity and Cd68 gene expression. 64Cu-RYM2 specifically bound to normal and aneurysmal human aortic tissues in correlation with MMP activity.

CONCLUSIONS

64Cu-RYM2 is a first-in-class MMP-targeted positron emission tomography tracer with favorable stability, biodistribution, performance in preclinical AAA, and importantly, specific binding to human tissues. These data set the stage for 64Cu-RYM2-based translational imaging studies of vessel wall MMP activity, and indirectly, inflammation, in AAA.

Metals in Imaging of Alzheimer's Disease

One of the hallmarks of Alzheimer's disease (AD) is the deposition of amyloid plaques in the brain parenchyma, which occurs 7-15 years before the onset of cognitive symptoms of the pathology. Timely diagnostics of amyloid formations allows identifying AD at an early stage and initiating inhibitor therapy, delaying the progression of the disease. However, clinically used radiopharmaceuticals based on 11C and 18F are synchrotron-dependent and short-lived. The design of new metal-containing radiopharmaceuticals for AD visualization is of interest. The development of coordination compounds capable of effectively crossing the blood-brain barrier (BBB) requires careful selection of a ligand moiety, a metal chelating scaffold, and a metal cation, defining the method of supposed Aβ visualization. In this review, we have summarized metal-containing drugs for positron emission tomography (PET), magnetic resonance imaging (MRI), and single-photon emission computed tomography (SPECT) imaging of Alzheimer's disease. The obtained data allow assessing the structure-ability to cross the BBB ratio.

Evaluation of 64Cu-Based Radiopharmaceuticals that Target Aβ Peptide Aggregates as Diagnostic Tools for Alzheimer's Disease

Positron emission tomography (PET) imaging agents that detect amyloid plaques containing amyloid beta (Aβ) peptide aggregates in the brain of Alzheimer's disease (AD) patients have been successfully developed and recently approved by the FDA for clinical use. However, the short half-lives of the currently used radionuclides 11C (20.4 min) and 18F (109.8 min) may limit the widespread use of these imaging agents. Therefore, we have begun to evaluate novel AD diagnostic agents that can be radiolabeled with 64Cu, a radionuclide with a half-life of 12.7 h, ideal for PET imaging. Described herein are a series of bifunctional chelators (BFCs), L1-L5, that were designed to tightly bind 64Cu and shown to interact with Aβ aggregates both in vitro and in transgenic AD mouse brain sections. Importantly, biodistribution studies show that these compounds exhibit promising brain uptake and rapid clearance in wild-type mice, and initial microPET imaging studies of transgenic AD mice suggest that these compounds could serve as lead compounds for the development of improved diagnostic agents for AD.

Evaluation of Cu-64 and Ga-68 Radiolabeled Glucagon-Like Peptide-1 Receptor Agonists as PET Tracers for Pancreatic β cell Imaging

PURPOSE

Copper-64 (Cu-64) and Galium-68 (Ga-68) radiolabeled DO3A and NODA conjugates of exendin-4 were used for preclinical imaging of pancreatic β cells via targeting of glucagon-like peptide-1 receptor (GLP-1R).

PROCEDURES

DO3A-VS- and NODA-VS-tagged Cys(40)exendin-4 (DO3A-VS-Cys(40)-exendin-4 and NODA-VS-Cys(40)-exendin-4, respectively) were labeled with Cu-64 and Ga-68 using standard techniques. Biodistribution and dynamic positron emission tomography (PET) were carried out in normal Sprague-Dawley (SD) rats. Ex vivo autoradiography imaging was conducted with freshly frozen pancreatic thin sections.

RESULTS

DO3A-VS- and NODA-VS-Cys(40)-exendin-4 analogues were labeled with Cu-64 and Ga-68 to a specific activity of 518.7 ± 3.7 Ci/mmol (19.19 ± 0.14 TBq/mmol) and radiochemical yield above 98 %. Biodistribution data demonstrated pancreatic uptake of 0.11 ± 0.02 %ID/g for [(64)Cu]DO3A-VS-, 0.14 ± 0.02 %ID/g for [(64)Cu]NODA-VS-, 0.11 ± 0.03 for [(68)Ga]DO3A-VS-, and 0.26 ± 0.03 for [(68)Ga]NODA-VS-Cys(40)-exendin-4. Excess exendin-4 and exendin-(9-39)-amide displaced all four Cu-64 and Ga-68 labeled exendin-4 derivatives in blocking studies.

CONCLUSIONS

[(64)Cu]/[(68)Ga]DO3A-VS-Cys(40)- and [(64)Cu]/[(68)Ga]NODA-VS-Cys(40)-exendin-4 can be used as PET imaging agents specific for GLP-1R expressed on β cells. Here, we report the first evidence of pancreatic uptake visualized with exendin-4 derivative in a rat animal model via in vivo dynamic PET imaging.