Macrocyclic Gd3+Complexes with Pendant Crown Ethers Designed for Binding Zwitterionic Neurotransmitters

Molecular fMRI of neurochemical signaling

Magnetic resonance imaging (MRI) is the most widely applied technique for brain-wide measurement of neural function in humans and animals. In conventional functional MRI (fMRI), brain signaling is detected indirectly, via localized activity-dependent changes in regional blood flow, oxygenation, and volume, to which MRI contrast can be readily sensitized. Although such hemodynamic fMRI methods are powerful tools for analysis of brain activity, they lack specificity for the many molecules and cell types that play functionally distinct roles in neural processing. A suite of techniques collectively known to as "molecular fMRI," addresses this limitation by permitting MRI-based detection of specific molecular processes in deep brain tissue. This review discusses how molecular fMRI is coming to be used in the study of neurochemical dynamics that mediate intercellular communication in the brain. Neurochemical molecular fMRI is a potentially powerful approach for mechanistic analysis of brain-wide function, but the techniques are still in early stages of development. Here we provide an overview of the major advances and results that have been achieved to date, as well as directions for further development.

Aromatic and heteroaromatic azacrown compounds: advantages and disadvantages of rigid macrocyclic ligands

Current approaches to the synthesis of aromatic and heteroaromatic azamacrocycles and their derivatives are summarized and systematized. The relationship between the structure of azacrown compounds and their complexation behaviour towards metal cations is analyzed. The diversity of practical applications of azamacrocyclic derivatives in medicine, biology and analytical and organic chemistry, as well as for the design of molecular devices is demonstrated.

The bibliography includes 307 references.

In-depth Study of a Novel Class of Ditopic Gadolinium(III)-based MRI Probes Sensitive to Zwitterionic Neurotransmitters

The efficacy of Gd-based low-molecular weight ditopic MRI probes on binding zwitterionic neurotransmitters (ZNTs) relies on their structural compatibility. ZNTs are challenging biomarkers for monitoring chemical neurotransmission due to their intrinsic complexity as target molecules. In this work, we focus on tuning the cyclen- and azacrown ether-based binding sites properties to increase the affinity toward ZNTs. Our approach consisted in performing structural modifications on the binding sites in terms of charge and size, followed by the affinity evaluation through T₁-weighted relaxometric titrations. We prepared and investigated six Gd³⁺ complexes with different structures and thus properties, which were found to be acetylcholine insensitive; moreover, two of them displayed considerably stronger affinity toward glutamate and glycine over hydrogencarbonate and other ZNTs. Complexes with small and non-charged or no substituents on the azacrown moiety displayed the highest affinities toward ZNTs, followed by strong decrease in longitudinal relaxivity r₁ of around 70%. In contrast, hosts with negatively charged substituents exhibited lower decrease in r₁ of nearly 30%. The thorough investigations involving relaxometric titrations, luminescence, and NMR diffusion experiments, as well as theoretical density functional theory calculations, revealed that the affinity of reported hosts toward ZNTs is greatly affected by the remote pendant on the azacrown derivative.

A light-responsive liposomal agent for MRI contrast enhancement and monitoring of cargo delivery

A liposomal MRI-probe changing relaxivity and releasing cargo upon light irradiation was developed for diagnostics and monitoring of drug delivery.

A low-molecular-weight ditopic MRI probe for ratiometric sensing of zwitterionic amino acid neurotransmitters

The capability for cooperative binding of a ditopic Gd-based responsive MRI probe was used to advance the sensitivity towards the major excitatory (Glu) and inhibitory (GABA) neurotransmitters over their physiological competitor hydrogencarbonate.

Investigations into the effects of linker length elongation on the behaviour of calcium-responsive MRI probes

The effects of subtle structural changes on the coordination behaviour and subsequent relaxometric properties of two novel calcium-responsive magnetic resonance imaging probes have been assessed via a range of physicochemical techniques.

Coordination Properties of GdDO3A-Based Model Compounds of Bioresponsive MRI Contrast Agents

We report a detailed characterization of the thermodynamic stability and dissociation kinetics of Gd³⁺ complexes with DO3A derivatives containing a (methylethylcarbamoylmethylamino)acetic acid (L¹), (methylpropylcarbamoylmethylamino)acetic acid (L²), 2-dimethylamino- N-ethylacetamide (L³), or 2-dimethylamino- N-propylacetamide (L⁴) group attached to the fourth nitrogen atom of the macrocyclic unit. These ligands are model systems of Ca²⁺- and Zn²⁺-responsive contrast agents (CA) for application in magnetic resonance imaging (MRI). The results of the potentiometric studies ( I = 0.15 M NaCl) provide stability constants with log K_GdL values in the range 13.9-14.8. The complex speciation in solution was found to be quite complicated due to the formation of protonated species at low pH, hydroxido complexes at high pH, and stable dinuclear complexes in the case of L^1,2. At neutral pH significant fractions of the complexes are protonated at the amine group of the amide side chain (log K_GdL×H = 7.2-8.1). These ligands form rather weak complexes with Mg²⁺ and Ca²⁺ but very stable complexes with Cu²⁺ (log K_CuL = 20.4-22.3) and Zn²⁺ (log K_ZnL = 15.5-17.6). Structural studies using a combination of ¹H NMR and luminescence spectroscopy show that the amide group of the ligand is coordinated to the metal ion at pH ∼8.5, while protonation of the amine group provokes the decoordination of the amide O atom and a concomitant increase in the hydration number and proton relaxivity. The dissociation of the complexes occurs mainly through a rather efficient proton-assisted pathway, which results in kinetic inertness comparable to that of nonmacrocyclic ligands such as DTPA rather than DOTA-like complexes.

Strategies for sensing neurotransmitters with responsive MRI contrast agents

A great deal of research involving multidisciplinary approaches is currently dedicated to the understanding of brain function. The complexity of physiological processes that underlie neural activity is the greatest hurdle to faster advances. Among imaging techniques, MRI has great potential to enable mapping of neural events with excellent specificity, spatiotemporal resolution and unlimited tissue penetration depth. To this end, molecular imaging approaches using neurotransmitter-sensitive MRI agents have appeared recently to study neuronal activity, along with the first successful in vivo MRI studies. Here, we review the pioneering steps in the development of molecular MRI methods that could allow functional imaging of the brain by sensing the neurotransmitter activity directly. We provide a brief overview of other imaging and analytical methods to detect neurotransmitter activity, and describe the approaches to sense neurotransmitters by means of molecular MRI agents. Based on these initial steps, further progress in probe chemistry and the emergence of innovative imaging methods to directly monitor neurotransmitters can be envisaged.

Gadolinium as an MRI contrast agent

MRI contrast is often enhanced using a contrast agent. Gd³⁺-complexes are the most widely used metallic MRI agents, and several types of Gd³⁺-based contrast agents (GBCAs) have been developed. Furthermore, recent advances in MRI technology have, in part, been driven by the development of new GBCAs. However, when designing new functional GBCAs in a small-molecular-weight or nanoparticle form for possible clinical applications, their functions are often compromised by poor pharmacokinetics and possible toxicity. Although great progress must be made in overcoming these limitations and many challenges remain, new functional GBCAs with either small-molecular-weight or nanoparticle forms offer an exciting opportunity for use in precision medicine.

Heading toward Macromolecular and Nanosized Bioresponsive MRI Probes for Successful Functional Imaging

The quest for bioresponsive or smart contrast agents (SCAs) in molecular imaging, in particular magnetic resonance imaging (MRI), is progressively increasing since they allow for the monitoring of essential biological processes on molecular and cellular levels in a dynamic fashion. These are offshoot molecules of common contrast agents that are sensitive to biochemical changes in their environment, capable of reporting on such changes by inducing MRI signal alteration. Various mechanistic approaches and different types of SCAs have been developed in order to visualize desired processes, using diverse imaging protocols and methods. To date, the most frequently exploited probes are paramagnetic molecules that change longitudinal or transverse relaxation at proton frequency, or so-called T₁- and T₂-weighted probes, respectively. Moreover, SCAs operating by the chemical exchange saturation transfer mechanism, suitable for ¹⁹F MRI or possessing hyperpolarized nuclei have also appeared in the past decade, slowly finding their role in functional imaging studies. Following these mechanistic principles, a large number of SCAs suitable for diverse targets have been reported to date. This Account condenses this exciting progress, particularly focusing on probes designed for abundant targets that are suitable for practical, in vivo utilization. To date, the greatest advancements have been certainly made in the preparation of pH sensitive probes, which usually contain protonable groups that interact with paramagnetic centers, or take advantage of supramolecular (dis)assembling to induce the MRI signal change, thereupon enabling pH mapping in vivo. In a complementary approach, a combination of metal chelating ligands for Ca²⁺ or Zn²⁺ with MR reporting units results in a wide variety of SCAs that operate with different contrast mechanisms and can be used for initial functional experiments. Finally, the first examples of molecular sensing by creating host-guest complexes to track neurotransmitter flux have also been recently reported, allowing the study of brain function in an unprecedented manner. Nevertheless, wider SCA utilization in vivo has not yet been achieved. There are a few reasons for this disparity between their nominal potential and practical usage, with one of the major reasons being the low sensitivity of the MRI technique. Subsequently, the production of detectable signal change can be achieved using higher concentrations of the bioresponsive probe; however, the biocompatibility of these probes then starts to play an important role. An elegant solution to these practical challenges has been found with the integration of multiple small-sized SCAs into macromolecular and nanosized probes. In such case, the multivalent SCAs are able to circumvent the sensitivity issue, thus enhancing the MR signal and desired contrast changes. Moreover, they prolong the probe tissue retention time, while often reducing their toxicity. Finally, with altered size and properties, they allow for exploitation of mechanisms that induce the contrast change which is not possible with small-sized SCAs. To this end, this Account also discusses the current approaches that aim to develop macromolecular and nanosized SCAs suitable for practical MRI applications. With these, further progress of this exciting field is affirmed, with remarkable results expected in the near future on both the probe preparation and their utilization in functional molecular imaging.

Water exchange reaction of a manganese catalase mimic: oxygen-17 NMR relaxometry study on (aqua)manganese(iii) in a salen scaffold and its reactions in a mildly basic medium

Water exchange of trans-[Mn^III(salen)(OH₂)₂]⁺ studied by line broadening ¹⁷OH₂ NMR discloses its mechanism as associative interchange (I_a).

Gd(3+)-Based Magnetic Resonance Imaging Contrast Agent Responsive to Zn(2+)

We report the heteroditopic ligand H5L, which contains a DO3A unit for Gd(3+) complexation connected to an NO2A moiety through a N-propylacetamide linker. The synthesis of the ligand followed a convergent route that involved the preparation of 1,4-bis(tert-butoxycarbonylmethyl)-1,4,7-triazacyclononane following the orthoamide strategy. The luminescence lifetimes of the Tb((5)D4) excited state measured for the TbL complex point to the absence of coordinated water molecules. Density functional theory calculations and (1)H NMR studies indicate that the EuL complex presents a square antiprismatic coordination in aqueous solution, where eight coordination is provided by the seven donor atoms of the DO3A unit and the amide oxygen atom of the N-propylacetamide linker. Addition of Zn(2+) to aqueous solutions of the TbL complex provokes a decrease of the emission intensity as the emission lifetime becomes shorter, which is a consequence of the coordination of a water molecule to the Tb(3+) ion upon Zn(2+) binding to the NO2A moiety. The relaxivity of the GdL complex recorded at 7 T (25 °C) increases by almost 150% in the presence of 1 equiv of Zn(2+), while Ca(2+) and Mg(2+) induced very small relaxivity changes. In vitro magnetic resonance imaging experiments confirmed the ability of GdL to provide response to the presence of Zn(2+).