A Macrocyclic Hybrid PET/MRI Probe for Quantitative Perfusion Imaging In Vivo.

Perfusion dynamics play a vital role in delivering essential nutrients and oxygen to tissues while removing metabolic waste products. Imaging techniques such as magnetic resonance imaging (MRI), computed tomography (CT), and positron emission tomography (PET) use contrast agents to visualize perfusion and clearance patterns; however, each technique has specific limitations. Hybrid PET/MRI combines the quantitative power and sensitivity of PET with the high functional and anatomical detail of MRI and holds great promise for precision in molecular imaging. However, the development of dual PET/MRI probes has been hampered by challenging synthesis and radiolabeling. Here, we present a novel PET/MRI probe, [18F][Gd(FL1)], which exhibits excellent stability comparable to macrocyclic MRI contrast agents used in clinical practice. The unique molecular design of [18F][Gd(FL1)] allows selective and expeditious radiolabeling of the gadolinium chelate in the final synthetic step. Leveraging the strengths of MRI and PET signals, the probe enables quantitative in vivo mapping of perfusion and excretion dynamics through an innovative voxel-based analysis. The diagnostic capabilities of [18F][Gd(FL1)] were demonstrated in a pilot study on healthy mice, successfully detecting early cases of unilateral renal dysfunction, a condition that is typically challenging to diagnose. This study introduces a new approach for PET/MRI and emphasizes a streamlined probe design for practical synthesis and improved diagnostic accuracy.

Molecular MRI of collagen to diagnose and stage liver fibrosis.

BACKGROUND & AIMS: The gold standard in assessing liver fibrosis is biopsy despite limitations like invasiveness and sampling error and complications including morbidity and mortality. Therefore, there is a major unmet medical need to quantify fibrosis non-invasively to facilitate early diagnosis of chronic liver disease and provide a means to monitor disease progression. The goal of this study was to evaluate the ability of several magnetic resonance imaging (MRI) techniques to stage liver fibrosis. METHODS: A gadolinium (Gd)-based MRI probe targeted to type I collagen (termed EP-3533) was utilized to non-invasively stage liver fibrosis in a carbon tetrachloride (CCl4) mouse model and the results were compared to other MRI techniques including relaxation times, diffusion, and magnetization transfer measurements. RESULTS: The most sensitive MR biomarker was the change in liver:muscle contrast to noise ratio (ΔCNR) after EP-3533 injection. We observed a strong positive linear correlation between ΔCNR and liver hydroxyproline (i.e. collagen) levels (r=0.89) as well as ΔCNR and conventional Ishak fibrosis scoring. In addition, the area under the receiver operating curve (AUR0C) for distinguishing early (Ishak ≤ 3) from late (Ishak ≥ 4) fibrosis was 0.942 ± 0.052 (p<0.001). By comparison, other MRI techniques were not as sensitive to changes in fibrosis in this model. CONCLUSIONS: We have developed an MRI technique using a collagen-specific probe for diagnosing and staging liver fibrosis, and validated it in the CCl4 mouse model. This approach should provide a better means to monitor disease progression in patients.

Is macrocycle a synonym for kinetic inertness in Gd(III) Complexes? Effect of coordinating and noncoordinating substituents on inertness and relaxivity of Gd(III) chelates with DO3A-like ligands.

Gadolinium chelates with octadentate ligands are widely used as contrast agents for magnetic resonance imaging (MRI), with macrocyclic ligands based on DO3A being preferred for the high kinetic inertness of their Gd chelates. A major challenge in the design of new bifunctional MRI probes is the need to control the rotational motion of the chelate, which greatly affects its relaxivity. In this work we explored facile alkylation of a secondary amine in macrocyclic DO3A-like ligands to create a short, achiral linkage to limit the undesired internal motion of chelates within larger molecular constructs. The acetate moiety on the trans nitrogen was also replaced with either a bidentate (ethoxyacetate, L1 or methyl picolinate, L2) or bulky monodentate (methyl phosphonate, L3) donor arm to give octa- or heptadentate ligands, respectively. The resultant Gd(III) complexes were all monohydrated (q = 1) and exhibited water residency times that spanned 2 orders of magnitude (τM = 2190 ± 170, 3500 ± 90, and 12.7 ± 3.8 ns at 37 °C for GdL1, GdL2, and GdL3, respectively). Alkylation of the secondary amine with a noncoordinating biphenyl moiety resulted in coordinatively saturated q = 0 complexes of octadentate ligands L1 and L2. Relaxivities were limited by slow water exchange and/or lack of water coligand. All complexes showed decreased inertness compared to [Gd(DO3A)] despite higher ligand denticity, and inertness was further decreased upon N-alkylation. These results demonstrate that high kinetic inertness and in vivo safety of Gd chelates with macrocyclic ligands should not be generalized.

Gd(DOTAla): a single amino acid Gd-complex as a modular tool for high relaxivity MR contrast agent development.

MR imaging at high magnetic fields benefits from an increased signal-to-noise ratio; however T(1)-based MR contrast agents show decreasing relaxivity (r(1)) at higher fields. High field, high relaxivity contrast agents can be designed by carefully controlling the rotational dynamics of the molecule. To this end, we investigated applications of the alanine analogue of Gd(DOTA), Gd(DOTAla). Fmoc-protected DOTAla suitable for solid phase peptide synthesis was synthesized and integrated into polypeptide structures. Gd(III) coordination results in very rigid attachment of the metal chelate to the peptide backbone through both the amino acid side chain and coordination of the amide carbonyl. Linear and cyclic monomers (GdL1, GdC1), dimers (Gd(2)L2, Gd(2)C2), and trimers (Gd(3)L3, Gd(3)C3) were prepared and relaxivities were determined at different field strengths ranging from 0.47 to 11.7 T. Amide carbonyl coordination was indirectly confirmed by determination of the hydration number q for the EuL1 integrated into a peptide backbone, q = 0.96 ± 0.09. The water residency time of GdL1 at 37 °C was optimal for relaxivity, τ(M) = 17 ± 2 ns. Increased molecular size leads to increased per Gd relaxivity (from r(1) = 7.5 for GdL1 to 12.9 mM(-1) s(-1) for Gd(3)L3 at 1.4 T, 37 °C). The cyclic, multimeric derivatives exhibited slightly higher relaxivities than the corresponding linearized multimers (Gd(2)C2: r(1) = 10.5 mM(-1) s(-1) versus Gd(2)C2-red r(1) = 9 mM(-1) s(-1) at 1.4 T, 37 °C). Overall, all six synthesized Gd complexes had higher relaxivities at low, intermediate, and high fields than the clinically used small molecule contrast agent [Gd(HP-DO3A)(H(2)O)].

Molecular MR imaging of liver fibrosis: a feasibility study using rat and mouse models.

BACKGROUND & AIMS: Liver biopsy, the current clinical gold standard for fibrosis assessment, is invasive and has sampling errors, and is not optimal for screening, monitoring, or clinical decision-making. Fibrosis is characterized by excessive accumulation of extracellular matrix proteins including type I collagen. We hypothesize that molecular magnetic resonance imaging (MRI) with a probe targeted to type I collagen could provide a direct and non-invasive method of fibrosis assessment. METHODS: Liver fibrosis was induced in rats with diethylnitrosamine and in mice with carbon tetrachloride. Animals were imaged prior to and immediately following i.v. administration of either collagen-targeted probe EP-3533 or non-targeted control Gd-DTPA. Magnetic resonance (MR) signal washout characteristics were evaluated from T1 maps and T1-weighted images. Liver tissue was subjected to pathologic scoring of fibrosis and analyzed for gadolinium and hydroxyproline. RESULTS: EP-3533-enhanced MR showed greater signal intensity on delayed imaging (normalized signal enhancement mice: control=0.39 ± 0.04, fibrotic=0.55 ± 0.03, p<0.01) and slower signal washout in the fibrotic liver compared to controls (liver t(1/2)=51.3 ± 3.6 vs. 42.0 ± 2.5 min, p<0.05 and 54.5 ± 1.9 vs. 44.1 ± 2.9 min, p<0.01 for fibrotic vs. controls in rat and mouse models, respectively). Gd-DTPA-enhanced MR could not distinguish fibrotic from control animals. EP-3533 gadolinium concentration in the liver showed strong positive correlations with hydroxyproline levels (r=0.74 (rats), r=0.77 (mice)) and with Ishak scoring (r=0.84 (rats), r=0.79 (mice)). CONCLUSIONS: Molecular MRI of liver fibrosis with a collagen-specific probe identifies fibrotic tissue in two rodent models of disease.

Paramagnetic encoding of molecules.

Contactless digital tags are increasingly penetrating into many areas of human activities. Digitalization of our environment requires an ever growing number of objects to be identified and tracked with machine-readable labels. Molecules offer immense potential to serve for this purpose, but our ability to write, read, and communicate molecular code with current technology remains limited. Here we show that magnetic patterns can be synthetically encoded into stable molecular scaffolds with paramagnetic lanthanide ions to write digital code into molecules and their mixtures. Owing to the directional character of magnetic susceptibility tensors, each sequence of lanthanides built into one molecule produces a unique magnetic outcome. Multiplexing of the encoded molecules provides a high number of codes that grows double-exponentially with the number of available paramagnetic ions. The codes are readable by nuclear magnetic resonance in the radiofrequency (RF) spectrum, analogously to the macroscopic technology of RF identification. A prototype molecular system capable of 16-bit (65,535 codes) encoding is presented. Future optimized systems can conceivably provide 64-bit (~10^19 codes) or higher encoding to cover the labelling needs in drug discovery, anti-counterfeiting and other areas.

PAMAM dendrimers conjugated with an uncharged gadolinium(III) chelate with a fast water exchange: the influence of chelate charge on rotational dynamics.

A bifunctional ligand, DO3A-py(NO-C) (DO3A-py(NO-C) = 10-[(4-carboxy-1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid), was attached to different generations (G0, G1, G2, and G4) of ethylenediamine-cored PAMAM dendrimers (PAMAM = polyamidoamine). The gadolinium(III) complex of this ligand possesses one molecule of water in its first coordination sphere and has a unique combination of a short water residence lifetime (tau(M) = 34 ns), a neutral overall charge, and a possibility for rigid attachment to molecules bearing primary amino groups. These favorable properties predestine the ligand for constructions of highly efficient nanosized contrast agents for magnetic resonance imaging (MRI). The coupling reaction between the carboxylic group on the pyridine-N-oxide moiety of the protected ligand molecule and primary amines in the dendrimers was achieved by an active ester method under nonaqueous conditions using the coupling agent TBTU. This reaction afforded conjugates with high loadings (80-100% of the theoretically available primary amines) and of high purity. The gadolinium(III) complexes of the conjugates were studied by variable-temperature 17O NMR and 1H NMRD measurements. The water residence lifetime (tau(M) = 55 ns) found in the largest conjugate G4-[Gd(H2O)(do3a-py(NO-C))]57, though somewhat longer compared to the "monomeric" complex, is still short enough not to limit the relaxivity. Surprisingly, compared with analogous conjugates with negatively charged chelates, the prepared uncharged compounds displayed much faster global rotational correlation times (tau(g)) and lower relaxivities. This phenomenon can be explained on the basis of Coulomb interactions. The motion of the charged chelates is restrained due to interactions with their counterions and the chelates themselves, while the uncharged chelates are not affected. Comparison of the PAMAM-based conjugates bearing uncharged and (1-)-charged chelates based on relaxometric data, 1H DOSY spectra, and SAXS measurements reveals that tau(g) reflects the rotational motion of large segments (dendrons) of the conjugates rather than that of the whole macromolecule.

Lanthanide(III) complexes of pyridine-N-oxide analogues of DOTA in solution and in the solid state. A new kind of isomerism in complexes of DOTA-like ligands.

The replacement of one of the acetate pendant arms with a 2-methylpyridine-N-oxide group in the molecule of H4dota significantly alters the coordination properties of the ligand in Ln(III) complexes. The structural properties of the complexes are investigated both in solution and in the solid state. The variable-temperature 1H NMR spectra of Nd(III), Eu(III), and Yb(III) complexes show that the twisted-square-antiprismatic (TSA) isomer is strongly destabilized and suppressed in solution and the complexes exist mostly as the square-antiprismatic (SA) isomers (98% for Eu(III) at -35 degrees C). The exchange between the TSA and SA isomers is fast at room temperature compared to that of the NMR time scale. The flexibility of the six-membered chelate ring formed by coordination of the 2-methylpyridine-N-oxide group to the central ion allows two orientations of this pendant arm relative to the acetate arms: syn-SA (pyridine in the direction of the acetates) and anti-SA (pyridine opposite to the acetates). The syn-SA form was found in the X-ray structure of the Nd(III) complex; the anti-SA forms were found in the structures of Dy(III), Tm(III), and Yb(III) complexes. The UV-vis and 1H NMR spectra of the Eu(III) complex suggest that both forms are in dynamic equilibrium in solution. A derivatization of the pyridine-N-oxide group with a carboxylic group in the 4 position has no significant effect on the properties of the Ln(III) complexes.

Pyridine-N-oxide analogues of DOTA and their gadolinium(III) complexes endowed with a fast water exchange on the square-antiprismatic isomer.

Two macrocyclic ligands derived from H4dota containing three acetate pendant arms and one 2-methylpyridine-N-oxide coordinating unit were synthesized. The ligand H3do3apy(NO) (H3L1, 10-[(1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) contains an unsubstituted pyridine-N-oxide ring; the ligand H4do3apy(NO-C) (H4L2, 10-[(4-carboxy-1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) is functionalized with a carboxylic group in the 4 position of the pyridine ring to allow attachment to other molecules. The ligands form octadentate (N4O4 environment) Ln(III) complexes, with one water molecule completing the coordination sphere in a capping position. The complexes are present in solution exclusively as square-antiprismatic isomers over the whole lanthanide series. The introduction of the carboxylic group to the pyridine-N-oxide unit in H4L2 has no significant effect on the hydration number (q = 1) and the water exchange rate of the [Gd(H2O)(L2)]- complex compared to the parent [Gd(H2O)(L1)] complex (water residence times: tauM = 39 ns for [Gd(H2O)(L1)] and tauM = 34 ns for [Gd(H2O)(L2)]- at 298 K).

Radiolabeling of the antibody IgG M75 for epitope of human carbonic anhydrase IX by 61Cu and 64Cu and its biological testing.

Human carbonic anhydrase IX is a membrane enzyme that is significantly expressed in some types of cancer cells, while copper radioisotopes offer wide range of diagnostic, therapeutic and theranostic properties. The work was focused on a new approach to the labelling of antibody IgG M75 for epitope human carbonic anhydrase IX with copper radioisotopes 61Cu and 64Cu and its in vivo testing in mice with inoculated colorectal cancer. Monoclonal antibody IgG M75 for epitope human carbonic anhydrase IX was successfully conjugated with copper-specific chelator "phosphinate" and labelled with 61Cu and 64Cu The obtained molecule has considerable potential as a radioimmuno pharmaceutical suitable for imaging of tumours expressing carbonic anhydrase IX by positron emission tomography (PET).

Molecular MR imaging of fibrosis in a mouse model of pancreatic cancer.

Fibrosis with excessive amounts of type I collagen is a hallmark of many solid tumours, and fibrosis is a promising target in cancer therapy, but tools for its non-invasive quantification are missing. Here we used magnetic resonance imaging with a gadolinium-based probe targeted to type I collagen (EP-3533) to image and quantify fibrosis in pancreatic ductal adenocarcinoma. An orthotopic syngeneic mouse model resulted in tumours with 2.3-fold higher collagen level compared to healthy pancreas. Animals were scanned at 4.7 T before, during and up to 60 min after i.v. injection of EP-3533, or of its non-binding isomer EP-3612. Ex-vivo quantification of gadolinium showed significantly higher uptake of EP-3533 compared to EP-3612 in tumours, but not in surrounding tissue (blood, muscle). Uptake of EP-3533 visualized in T1-weighted MRI correlated well with spatial distribution of collagen determined by second harmonic generation imaging. Differences in the tumour pharmacokinetic profiles of EP-3533 and EP-3612 were utilized to distinguish specific binding to tumour collagen from non-specific uptake. A model-free pharmacokinetic measurement based on area under the curve was identified as a robust imaging biomarker of fibrosis. Collagen-targeted molecular MRI with EP-3533 represents a new tool for non-invasive visualization and quantification of fibrosis in tumour tissue.

Lanthanide(III) complexes of a pyridine N-oxide analogue of DOTA: exclusive M isomer formation induced by a six-membered chelate ring.

The presence of a six-membered chelate ring involving a pyridine N-oxide moiety induces exclusive M isomer formation throughout the whole lanthanide series endowed with a fast water exchange in the case of the Gd(III) complex.

Microscopic magnetic stimulation of neural tissue.

Electrical stimulation is currently used to treat a wide range of cardiovascular, sensory and neurological diseases. Despite its success, there are significant limitations to its application, including incompatibility with magnetic resonance imaging, limited control of electric fields and decreased performance associated with tissue inflammation. Magnetic stimulation overcomes these limitations but existing devices (that is, transcranial magnetic stimulation) are large, reducing their translation to chronic applications. In addition, existing devices are not effective for deeper, sub-cortical targets. Here we demonstrate that sub-millimeter coils can activate neuronal tissue. Interestingly, the results of both modelling and physiological experiments suggest that different spatial orientations of the coils relative to the neuronal tissue can be used to generate specific neural responses. These results raise the possibility that micro-magnetic stimulation coils, small enough to be implanted within the brain parenchyma, may prove to be an effective alternative to existing stimulation devices.

Radiolabeling of PAMAM dendrimers conjugated to a pyridine-N-oxide DOTA analog with ¹¹¹In: Optimization of reaction conditions and biodistribution.

Polyamidoamine dendrimers (PAMAMs) of generations 1 (G1) and 4 (G4) were conjugated with a bifunctional pyridine-N-oxide DOTA analog, 10-[(4-carboxy-1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (H(4)do3a-py(NO-C)), through the pyridine-4-carboxylic acid group, and the conjugates were radiolabeled with indium-111. Reaction conditions for the radiolabelling were optimized. Both radiolabeled conjugates, G1-[(111)In(do3a-py(NO-C))] and G4-[(111)In(do3a-py(NO-C))], were kinetically stable for at least 48h after preparation; in the presence of competitive ligands, the radiochemical purity of the conjugates slightly decreased (4-7%) over the same time period. The preclinical pharmacokinetics of both agents were evaluated. Biodistribution and elimination in rats were more favorable for the G1-[(111)In(do3a-py(NO-C))] conjugate than G4-[(111)In(do3a-py(NO-C))] conjugate. However, the G1-[(111)In(do3a-py(NO-C))] conjugate was rapidly eliminated from the body, mainly through urine, while, significant and long-term radioactivity uptake in the liver and kidney was observed for the G4-[(111)In(do3a-py(NO-C))] conjugate.

Densely packed Gd(III)-chelates with fast water exchange on a calix[4]arene scaffold: a potential MRI contrast agent.

A pyridine-N-oxide functionalized DOTA analogue has been conjugated to a calix[4]arene and the corresponding Gd-complex was characterized with respect to its suitability as MRI contrast agent. The compound forms spherical micelles in water with a cmc of 35 microM and a radius of 8.2 nm. The relaxivity of these aggregates is 31.2 s(-1) mM(-1) at 25 degrees C and 20 MHz, which corresponds to a molecular relaxivity of 125 s(-1) mM(-1). The high relaxivity mainly originates from the short tau(M) (72.7 ns) and the size of the micelles. The interaction with bovine serum albumin (BSA) was studied and an observed relaxivity of up to 40.8 s(-1) mM(-1) (163.2 s(-1) mM(-1) per binding place) at 20 MHz and 37 degrees C was found in the presence of 2.0 mM protein.