Real-Time insight into in vivo redox status utilizing hyperpolarized [1-13C] N-acetyl cysteine.

Drastic sensitivity enhancement of dynamic nuclear polarization is becoming an increasingly critical methodology to monitor real-time metabolic and physiological information in chemistry, biochemistry, and biomedicine. However, the limited number of available hyperpolarized 13C probes, which can effectively interrogate crucial metabolic activities, remains one of the major bottlenecks in this growing field. Here, we demonstrate [1-13C] N-acetyl cysteine (NAC) as a novel probe for hyperpolarized 13C MRI to monitor glutathione redox chemistry, which plays a central part of metabolic chemistry and strongly influences various therapies. NAC forms a disulfide bond in the presence of reduced glutathione, which generates a spectroscopically detectable product that is separated from the main peak by a 1.5 ppm shift. In vivo hyperpolarized MRI in mice revealed that NAC was broadly distributed throughout the body including the brain. Its biochemical transformation in two human pancreatic tumor cells in vitro and as xenografts differed depending on the individual cellular biochemical profile and microenvironment in vivo. Hyperpolarized NAC can be a promising non-invasive biomarker to monitor in vivo redox status and can be potentially translatable to clinical diagnosis.

Monitoring PSMA Responses to ADT in Prostate Cancer Patient-Derived Xenograft Mouse Models Using [18F]DCFPyL PET Imaging.

PURPOSE: PSMA overexpression has been associated with aggressive prostate cancer (PCa). However, PSMA PET imaging has revealed highly variable changes in PSMA expression in response to ADT treatment ranging from increases to moderate decreases. To better understand these PSMA responses and potential relationship to progressive PCa, the PET imaging agent, [18F]DCFPyL, was used to assess changes in PSMA expression in response to ADT using genomically characterized LuCaP patient-derived xenograft mouse models (LuCaP-PDXs) which were found to be sensitive to ADT (LuCaP73 and LuCaP136;CS) or resistant (LuCaP167;CR). METHODS: [18F]DCFPyL (2-(3-{1-carboxy-5-[(6-[18F]fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid) was used to assess PSMA in vitro (saturation assays) in LuCaP tumor membrane homogenates and in vivo (imaging/biodistribution) in LuCaP-PDXs. Control and ADT-treated LuCaPs were imaged before ADT (0 days) and 2-, 7-, 14-, and 21-days post-ADT from which tumor:muscle ratios (T:Ms) were determined and concurrently tumor volumes were measured (caliper). After the 21-day imaging, biodistributions and histologic/genomic (PSMA, AR) analysis were done. RESULTS: [18F]DCFPyL exhibited high affinity for PSMA and distinguished different levels of PSMA in LuCaP tumors. Post-ADT CS LuCaP73 and LuCaP136 tumor volumes significantly decreased at day 7 or 14 respectively vs controls, whereas the CR LuCaP167 tumor volumes were minimally changed. [18F]DCFPyL imaging T:Ms were increased 3-5-fold in treated LuCaP73 tumors vs controls, while treated LuCaP136 T:Ms remained unchanged which was confirmed by day 21 biodistribution results. For treated LuCaP167, T:Ms were decreased (~ 45 %) vs controls but due to low T:M values (<2) may not be indicative of PSMA level changes. LuCaP73 tumor PSMA histologic/genomic results were comparable to imaging/biodistribution results, whereas the results for other tumor types varied. CONCLUSION: Tumor responses to ADT varied from sensitive to resistant among these LuCaP PDXs, while only the high PSMA expressing LuCaP model exhibited an increase in PSMA levels in response to ADT. These models may be useful in understanding the clinical relevance of PSMA PET responses to ADT and potentially the relationship to disease progression as it may relate to the genomic signature.

Origin of the Isomer Stability of Polymethylated DOTA Chelates Complexed with Ln3+ ions.

DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-based chelates that give only a single isomer in solution when complexed with lanthanide (Ln3+) ions is of value for studying protein dynamics and interactions via NMR. Herein, we have investigated the geometries, energetics, and electrostatic potentials of Lu complexed with DOTA (1), ring methylated M4DOTA (2), and arm methylated R-DOTMA (3) and S-DOTMA (4), as well as, both ring and arm methylated 4S-4S-M4DOTMA (5) and 4S-4R-M4DOTMA (6) at the level of M06-L/6-31+G(d)-SDD, to elucidate the origin of the isomer stability. These analyses indicate that the electrostatic repulsion between the arm methyl and the neighboring carboxylate significantly destabilizes the square antiprism (SAP) isomer of Lu-5 and the twisted square antiprism (TSAP) isomer of Lu-6, while the steric repulsion between the ring and arm methyl groups attenuates the stability of both TSAP of Lu-5 and SAP of Lu-6. To rationalize the variable temperature proton NMR spectra, the energy barriers for the inter-conversion in Lu-5 and Lu-6 via arm rotation were also calculated. The modulation of the stability and rigidity of Ln complexes via a modification of DOTA is also discussed. Our investigation will aid to design better chelates for the Ln3+ ions for its use in molecular medicine.

Comparison of Solution Properties of Polymethylated DOTA-like Lanthanide Complexes with Opposite Chirality of the Pendant Arms.

Polymethylated lanthanide 4S4R-M4DOTMA complexes, bearing the ring methyl groups oriented in the SSSS position and the arm methyl groups in the RRRR configuration, exist exclusively as the SAP [Λ(δδδδ)] isomer in solution throughout the lanthanide series. This observation is in contrast to Ln-8S-M4DOTMA, which was recently reported to adopt the SAP [Λ(δδδδ)] isomer in the early lanthanides, while the late lanthanides adopt the TSAP [Δ(δδδδ)] isomer. The methyl groups on the ring and the arm are both oriented in the SSSS configuration for Ln-8S-M4DOTMA ( Dalton Trans. 2016 , 45 , 4673 - 4687 , DOI: 10.1039/C5DT03210E ). Quantum chemical calculations for Pr- and Yb-4S4R-M4DOTMA indicate that the SAP isomer is significantly more stable. The luminescence profiles of Eu-8S-M4DOTMA and Eu-4S4R-M4DOTMA showed similar profiles signifying identical coordination environments. The hydration state, q, of the Eu(III) complexes was q = 0.91-0.95, while Tb-8S-M4DOTMA had q = 0.86. A much lower q value was obtained for Tb-4S4R-M4DOTMA (q = 0.67), which indicates an elongation of the Ln-Ow bond. At 400 MHz, the relaxivity of Gd-8S-M4DOTMA is 5.1 ± 0.1 mM-1 s-1 and 3.9 ± 0.1 mM-1 s-1 at 25 and 37 °C, respectively, whereas the relaxivity of Gd-4S4R-M4DOTMA is 4.6 ± 0.1 mM-1 s-1 at 25 °C and 3.6 ± 0.1 mM-1 s-1 at 37 °C. At 45 MHz, the relaxivity of Gd-8S-M4DOTMA is 5.4 ± 0.1 mM-1 s-1, and the relaxivity of Gd-4S4R-M4DOTMA is 4.5 ± 0.1 mM-1 s-1 at 25 °C. The temperature dependence of the 17O NMR transverse relaxation rate of the Gd complexes revealed a 7-fold increase in the bound water residence lifetime of Gd-8S-M4DOTMA (1/kex = τM = 9.0 ± 0.5 ns and 1/kex = τM = 60 ± 3 ns). The Pr(III) complex of 8S-M4DOTMA crystallized as TSAP isomer with an apical water. The presence of the apical water for the TSAP of Pr-8S-M4DOTMA was further confirmed with the observation that the fluoride ion replaces the bound water from the TSAP isomer of Pr-8S-M4DOTMA. This was shown by the disappearance of the TSAP peaks and appearance of a new set of less shifted resonances, which exchange with the SAP isomer as confirmed by NMR exchange spectroscopy (EXSY).

Immuno-PET Imaging of the Programmed Cell Death-1 Ligand (PD-L1) Using a Zirconium-89 Labeled Therapeutic Antibody, Avelumab.

OBJECTIVE: The goal is to evaluate avelumab, an anti-PD-L1 monoclonal immunoglobulin G antibody labeled with zirconium-89 in human PD-L1-expressing cancer cells and mouse xenografts for clinical translation. METHODS: [89Zr]Zr-DFO-PD-L1 monoclonal antibody (mAb) was synthesized using avelumab conjugated to desferrioxamine. In vitro binding studies and biodistribution studies were performed with PD-L1+MDA-MB231 cells and MDA-MB231 xenograft mouse models, respectively. Biodistributions were determined at 1, 2, 3, 5, and 7 days post coinjection of [89Zr]Zr-DFO-PD-L1 mAb without or with unlabeled avelumab (10, 20, 40, and 400 µg). RESULTS: [89Zr]Zr-DFO-PD-L1 mAb exhibited high affinity (Kd ∼ 0.3 nM) and detected moderate PD-L1 expression levels in MDA-MB231 cells. The spleen and lymph nodes exhibited the highest [89Zr]Zr-DFO-PD-L1 mAb uptakes in all time points, while MDA-MB231 tumor uptakes were lower but highly retained. In the unlabeled avelumab dose escalation studies, spleen tissue-muscle ratios decreased in a dose-dependent manner indicating specific [89Zr]Zr-DFO-PD-L1 mAb binding to PD-L1. In contrast, lymph node and tumor tissue-muscle ratios increased 4- to 5-fold at 20 and 40 µg avelumab doses. CONCLUSIONS: [89Zr]Zr-DFO-PD-L1 mAb exhibited specific and high affinity for PD-L1 in vitro and had target tissue uptakes correlating with PD-L1 expression levels in vivo. [89Zr]Zr-DFO-PD-L1 mAb uptake in PD-L1+tumors increased with escalating doses of avelumab.

Characterizing the magnetic susceptibility tensor of lanthanide-containing polymethylated-DOTA complexes.

Lanthanide complexes based on the DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) cage are commonly used as phase contrast agents in magnetic resonance imaging, but can also be utilized in structural NMR applications due to their ability to induce either paramagnetic relaxation enhancement or a pseudocontact shift (PCS) depending on the choice of the lanthanide. The size and sign of the PCS for any given atom is determined by its coordinates relative to the metal center, and the characteristics of the lanthanide's magnetic susceptibility tensor. Using a polymethylated DOTA tag (Ln-M8-SPy) conjugated to ubiquitin, we calculated the position of the metal center and characterized the susceptibility tensor for a number of lanthanides (dysprosium, thulium, and ytterbium) under a range of pH and temperature conditions. We found that there was a difference in temperature sensitivity for each of the complexes studied, which depended on the size of the lanthanide ion as well as the isomeric state of the cage. Using 17O-NMR, we confirmed that the temperature sensitivity of the compounds was enhanced by the presence of an apically bound water molecule. Since amide-containing lanthanide complexes are known to be pH sensitive and can be used as probes of physiological pH, we also investigated the effect of pH on the Ln-M8-SPy susceptibility tensor, but we found that the changes in this pH range (5.0-7.4) were not significant.

Analysis of the isomer ratios of polymethylated-DOTA complexes and the implications on protein structural studies.

A rigidified and symmetrical polymethylated 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) ligand bearing four SSSS methyl groups in both the tetraaza ring and the acetate arms (SSSS-SSSS-M4DOTMA) was prepared. The isomer ratio of SSSS-SSSS-M4DOTMA complexed with a series of lanthanide ions was carefully investigated using RP-HPLC and NMR. A square antiprismatic (SAP) configuration was exclusively observed for the early lanthanides, while the twisted square antiprismatic (TSAP) geometry was preferred as the lanthanide ion size decreases. The late lanthanides preferentially adopted the TSAP geometry. One of the pendant arms was modified with a pyridyl disulfide group (SSSS-SSSS-M8SPy) for cysteine attachment and displayed a similar isomer trend as the parent compound, Ln-SSSS-SSSS-M4DOTMA. Covalent attachment to the ubiquitin S57C mutant showed resonances whose intensities are in agreement with the isomeric population observed by RP-HPLC. Furthermore, the NOE experiments combined with quantum chemical calculations have unequivocally demonstrated that the SAP of Pr-SSSS-SSSS-M4DOTMA and Pr-SSSS-SSSS-M8SPy, as well as the TSAP of Yb-SSSS-SSSS-M8SPy are more stable than their corresponding isomers.

Preparation and long-term biodistribution studies of a PAMAM dendrimer G5-Gd-BnDOTA conjugate for lymphatic imaging.

AIMS: To demonstrate the use of gadolinium (Gd)-labeled dendrimers as lymphatic imaging agents and establish the long-term biodistribution (90-day) of this type of agent in mice. MATERIALS & METHODS: A G5-Gd-BnDOTA dendrimer was prepared and injected into mice and monkeys for MR lymphangiography, and long-term biodistribution of the conjugate was studied. RESULTS: Administration of G5-Gd-BnDOTA in mice demonstrated a rapid uptake in the deep lymphatic system while injection in monkeys showed enhanced internal iliac nodes, indicating its general utility for lymphatic tracking. Biodistribution studies to 90 days showed that gadolinium conjugate is slowly being eliminated from the liver and other organs. CONCLUSION: The use of G5-Gd-BnDOTA holds great promise for lymphatic imaging, but its slow clearance from the body might hamper its eventual clinical translation.

The stereochemistry of amide side chains containing carboxyl groups influences water exchange rates in EuDOTA-tetraamide complexes.

Many Eu(III) complexes formed with DOTA-tetraamide ligands (where DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have sufficiently slow water exchange kinetics to meet the slow-to-intermediate condition required to serve as chemical exchange saturation transfer (CEST) contrast agents for MRI. This class of MRI contrast agents offers an attractive platform for creating biological sensors because water exchange is exquisitely sensitive to subtle ligand stereochemistry and electronic effects. Introduction of carboxyl groups or carboxyl ethyl ester groups on the amide substituents has been shown to slow water exchange in these complexes, but less is known about the orientation or position of these side-chain groups relative to the inner-sphere Eu(III)-bound water molecule. In this study, a series of Eu(III) complexes having one or more carboxyl groups or carboxyl esters at the δ-position of the pendant amide side chains were prepared. Initial attempts to prepare optically pure EuDOTA-[(S)-Asp]4 resulted in a chemically pure ligand consisting of a mixture of stereochemical isomers. This was traced to racemization of (S)-aspartate diethyl ester during the synthetic procedure. Nevertheless, NMR studies of the Eu(III) complexes of this mixture revealed that each isomer had a different water exchange rate, differing by a factor of 2 or more. A second controlled synthesis and CEST study of EuDOTA-[(S)-Asp]4 and cis-EuDOTA-[(S)-Asp]2[(R)-Asp]2 confirmed that the water exchange rates in these diastereomeric complexes are controlled by the axial versus equatorial orientation of the carboxyl groups on the amide side chains. These observations provide new insights toward the development of even more slowly water exchanging systems which will be necessary for practical in vivo applications.

The pH sensitivity of -NH exchange in LnDOTA-tetraamide complexes varies with amide substituent.

The amide proton exchange rates in various lanthanide(III) DOTA-tetraamide complexes were investigated by CEST as a function of variable chemical structures and charges on the amide substituents. Comparisons were made between YbDOTA-(gly)(4)(-) (Yb-1), YbDOTA-(NHCH(2)PO(3))(4 (5-) (Yb-2) and YbDOTA-(NHCH(2)PO(3)Et(2))(4)(3+) (Yb-3). The general shapes of the CEST vs pH profiles were similar for the three complexes but they showed maximum CEST intensities at different pH values, pH 8.3, 8.8 and 6.9 for Yb-1, Yb-2 and Yb-3, respectively. This indicates that a more negatively charged substituent on the amide helps stabilize the partial positive charge on the amide nitrogen and consequently more base is required to catalyze proton exchange. The chemical shifts of the -NH protons in Yb-1 and Yb-2 were similar (-17 ppm) while the -NH proton in Yb-3 was at -13 ppm. This shows that the crystal field produced by the amide oxygen donor atoms in Yb-3 is substantially weaker than that in the other two complexes. In an effort to expand the useful range of pH values that might be measured using these complexes as CEST agents, the shapes of the CEST vs pH curves were also determined for two thulium(III) complexes with much larger hyperfine shifted -NH proton resonances. The ratio of CEST from -NH exchange in Tm-1 compared with CEST from -NH exchange in Tm-3 was found to be linear over an extended pH range, from 6.3 to 7.4. This demonstrates a potential advantage of using mixtures of lanthanide(III) DOTA-tetraamides for mapping tissue pH by use of ratiometric CEST imaging.

TmDOTA-tetraglycinate encapsulated liposomes as pH-sensitive LipoCEST agents.

Lanthanide DOTA-tetraglycinate (LnDOTA-(gly)4⁻) complexes contain four magnetically equivalent amide protons that exchange with protons of bulk water. The rate of this base catalyzed exchange process has been measured using chemical exchange saturation transfer (CEST) NMR techniques as a function of solution pH for various paramagnetic LnDOTA-(gly)4⁻ complexes to evaluate the effects of lanthanide ion size on this process. Complexes with Tb(III), Dy(III), Tm(III) and Yb(III) were chosen because these ions induce large hyperfine shifts in all ligand protons, including the exchanging amide protons. The magnitude of the amide proton CEST exchange signal differed for the four paramagnetic complexes in order, Yb>Tm>Tb>Dy. Although the Dy(III) complex showed the largest hyperfine shift as expected, the combination of favorable chemical shift and amide proton CEST linewidth in the Tm(III) complex was deemed most favorable for future in vivo applications where tissue magnetization effects can interfere. TmDOTA-(gly)4⁻ at various concentrations was encapsulated in the core interior of liposomes to yield lipoCEST particles for molecular imaging. The resulting nanoparticles showed less than 1% leakage of the agent from the interior over a range of temperatures and pH. The pH versus amide proton CEST curves differed for the free versus encapsulated agents over the acidic pH regions, consistent with a lower proton permeability across the liposomal bilayer for the encapsulated agent. Nevertheless, the resulting lipoCEST nanoparticles amplify the CEST sensitivity by a factor of ∼104 compared to the free, un-encapsulated agent. Such pH sensitive nano-probes could prove useful for pH mapping of liposomes targeted to tumors.

Advantages of macromolecular to nanosized chemical-exchange saturation transfer agents for MRI applications.

Chemical-exchange saturation transfer (CEST) agents are a new class of MRI contrast agents that offer a number of advantages over conventional Gd(3+) agents. Over the past few years, a variety of small-molecule CEST agents responsive to physiological conditions, such as pH and temperature, have been designed and their imaging applications have been reported. One of the major drawbacks of current small-molecule CEST agents is their relatively low sensitivity. The advantages of using macromolecular and nanosized systems with large numbers of exchangeable groups to improve contrast sensitivity are highlighted in this brief review. Although this approach has been shown to amplify contrast sensitivity, other limitations, including relatively small chemical-shift differences between the exchanging species and bulk water and less than optimal proton exchange rates, still exist. By addressing these issues, it is anticipated that CEST agents will find useful applications in the detection of specific biomarkers of disease.

Synthesis and relaxometric studies of a dendrimer-based pH-responsive MRI contrast agent.

The design of effective pH responsive MRI contrast agents is a key goal in the development of new diagnostic methods for conditions such as kidney disease and cancer. A key factor determining the effectiveness of an agent is the difference between the relaxivity of the "on" state compared to that of the "off" state. In this paper, we demonstrate that it is possible to improve the pH-responsive action of a low molecular weight agent by conjugating it to a macromolecular construct. The synthesis of a bifunctional pH responsive agent is reported. As part of that synthetic pathway we examine the Ing-Manske reaction, identifying an undesirable by-product and establishing effective conditions for promoting a clean and effective reaction. Reaction of the bifunctional pH responsive agent with a G5-PAMAM dendrimer yielded a product with an average of 96 chelates per dendrimer. The relaxivity of the dendrimer conjugate rises from 10.8 mM(-1) s(-1) (pH 9) to 24.0 mM(-1) s(-1) (pH 6) per Gd(3+) ion. This more than doubles the relaxivity pH response, Deltar(1), of our agent from just 51 % for the original low molecular weight chelate to 122 % for the dendrimer.