Peptide-Conjugated MRI Probe Targeted to Netrin-1, a Novel Metastatic Breast Cancer Biomarker.

Despite significant progress in cancer imaging and treatment over the years, early diagnosis and metastasis detection remain a challenge. Molecular magnetic resonance imaging (MRI), with its high resolution, can be well adapted to fulfill this need, requiring the design of contrast agents which target specific tumor biomarkers. Netrin-1 is an extracellular protein overexpressed in metastatic breast cancer and implicated in tumor progression and the appearance of metastasis. This study focuses on the design and preclinical evaluation of a novel Netrin-1-specific peptide-based MRI probe, GdDOTA-KKTHDAVR (Gd-K), to visualize metastatic breast cancer. The targeting peptide sequence was identified based on the X-ray structure of the complex between Netrin-1 and its transmembrane receptor DCC. Molecular docking simulations support the probe design. In vitro studies evidenced submicromolar affinity of Gd-K for Netrin-1 (KD = 0.29 µM) and good MRI efficacy (proton relaxivity, r1 = 4.75 mM-1 s-1 at 9.4 T, 37 °C). In vivo MRI studies in a murine model of triple-negative metastatic breast cancer revealed successful tumor visualization at earlier stages of tumor development (smaller tumor volume). Excellent signal enhancement, 120% at 2 min and 70% up to 35 min post injection, was achieved (0.2 mmol/kg injected dose), representing a reasonable imaging time window and a superior contrast enhancement in the tumor as compared to Dotarem injection.

Lanthanide-Based Probes for Imaging Detection of Enzyme Activities by NIR Luminescence, T1- and ParaCEST MRI.

Applying a single molecular probe to monitor enzymatic activities in multiple, complementary imaging modalities is highly desirable to ascertain detection and to avoid the complexity associated with the use of agents of different chemical entities. We demonstrate here the versatility of lanthanide (Ln3+) complexes with respect to their optical and magnetic properties and their potential for enzymatic detection in NIR luminescence, CEST and T1 MR imaging, controlled by the nature of the Ln3+ ion, while using a unique chelator. Based on X-ray structural, photophysical, and solution NMR investigations of a family of Ln3+ DO3A-pyridine model complexes, we could rationalize the luminescence (Eu3+, Yb3+), CEST (Yb3+) and relaxation (Gd3+) properties and their variations between carbamate and amine derivatives. This allowed the design of L n L G a l 5 ${{{\bf L n L}}_{{\bf G a l}}^{5}}$ probes which undergo enzyme-mediated changes detectable in NIR luminescence, CEST and T1-weighted MRI, respectively governed by variations in their absorption energy, in their exchanging proton pool and in their size, thus relaxation efficacy. We demonstrate that these properties can be exploited for the visualization of ß-galactosidase activity in phantom samples by different imaging modalities: NIR optical imaging, CEST and T1-weighted MRI.

Zinc-sensitive MRI contrast agents: importance of local probe accumulation in zinc-rich tissues.

We present the in vitro characterisation of a Gd3+-based contrast agent that responds to Zn2+ upon interaction with Human Serum Albumin. We show that the contradictory in vivo behaviour is related to Gd3+-accumulation in Zn-rich tissues. This highlights the importance of the biodistribution of such contrast agents.

Fluorinated Mn(III)/(II)-Porphyrin with Redox-Responsive 1 H and 19 F Relaxation Properties.

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF3 ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual 1 H relaxivity and 19 F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d6 /H2 O evidenced that the 1 H relaxometric and 19 F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (dMn-F =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on 19 F T1 and T2 relaxation times. Mn-TPP-p-CF3 has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF3 in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and 19 F NMR. The broad 1 H and 19 F NMR signals of Mn(III)-TPP-p-CF3 disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened 19 F NMR signal from Mn(II)-TPP-p-CF3 . Phantom 19 F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF3 , retrieved upon complete reoxidation in air within ~24 h. 1 H NMRD curves of the Mn(III)/(II)-TPP-p-CF3 chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins.

A phosphorylated zinc finger peptide bearing a gadolinium complex for zinc detection by MRI.

Two zinc finger peptides, namely ZFQDLn and ZFQELn (Ln = Tb or Gd), with an appended Ln3+ chelate and a phosphoserine able to coordinate the Ln3+ ion are presented. The two peptides differ by the amino acid anchorage of the chelate, either aspartate (D) or glutamate (E). Both peptides are able to bind Zn2+ and adopt the ßßα fold. Interestingly, ZFQETb shows a decrease in sensitized Tb3+ luminescence upon Zn2+ binding whereas ZFQDTb does not. The luminescence change upon Zn2+ binding is attributed to a change in hydration number (q) of the Tb3+ ion due to the decoordination of the phosphoserine from the Ln3+ ion upon Zn2+ binding and peptide folding. This process is highly sensitive to the length of the linker between the Ln chelate and the peptidic backbone. The magnetic properties of the gadolinium analogue ZFQEGd were studied. An impressive relaxivity increase of 140% is observed at 60 MHz and 25 °C upon Zn2+ binding. These changes can be attributed to a combined increase effect of the hydration number of Gd3+ and of the rigidity of the system upon Zn2+ binding. Phantom MR images at 9.4 T show a clear signal enhancement in the presence of Zn2+. These zinc finger peptides offer a unique platform to design such Zn-responsive probes.

Water-Soluble Gd(III)-Porphyrin Complexes Capable of Both Photosensitization and Relaxation Enhancement.

With the aim of developing more stable Gd(III)-porphyrin complexes, two types of ligands 1 and 2 with carboxylic acid anchors were synthesized. Due to the N-substituted pyridyl cation attached to the porphyrin core, these porphyrin ligands were highly water-soluble and formed the corresponding Gd(III) chelates, Gd-1 and Gd-2. Gd-1 was sufficiently stable in neutral buffer, presumably due to the preferred conformation of the carboxylate-terminated anchors connected to nitrogen in the meta position of the pyridyl group helping to stabilize Gd(III) complexation by the porphyrin center. 1H NMRD (nuclear magnetic relaxation dispersion) measurements on Gd-1 revealed high longitudinal water proton relaxivity (r1 = 21.2 mM-1 s-1 at 60 MHz and 25 °C), which originates from slow rotational motion resulting from aggregation in aqueous solution. Under visible light irradiation, Gd-1 showed extensive photoinduced DNA cleavage in line with efficient photoinduced singlet oxygen generation. Cell-based assays revealed no significant dark cytotoxicity of Gd-1, while it showed sufficient photocytotoxicity on cancer cell lines under visible light irradiation. These results indicate the potential of this Gd(III)-porphyrin complex (Gd-1) as a core for the development of bifunctional systems acting as an efficient photodynamic therapy photosensitizer (PDT-PS) with magnetic resonance imaging (MRI) detection capabilities.

Mn2+ Bispidine Complex Combining Exceptional Stability, Inertness, and MRI Efficiency.

As an essential metal ion and an efficient relaxation agent, Mn2+ holds a great promise to replace Gd3+ in magnetic resonance imaging (MRI) contrast agent applications, if its stable and inert complexation can be achieved. Toward this goal, four pyridine and one carboxylate pendants have been introduced in coordinating positions on the bispidine platform to yield ligand L3. Thanks to its rigid and preorganized structure and perfect size match for Mn2+, L3 provides remarkably high thermodynamic stability (log KMnL = 19.47), selectivity over the major biological competitor Zn2+ (log(KMnL/KZnL) = 4.4), and kinetic inertness. Solid-state X-ray data show that [MnL3(MeOH)](OTf)2 has an unusual eight-coordinate structure with a coordinated solvent molecule, in contrast to the six-coordinate structure of [ZnL3](OTf), underlining that the coordination cavity is perfectly adapted for Mn2+, while it is too large for Zn2+. In aqueous solution, 17O NMR data evidence one inner sphere water and dissociatively activated water exchange (kex298 = 13.5 × 107 s-1) for MnL3. Its water proton relaxivity (r1 = 4.44 mM-1 s-1 at 25 °C, 20 MHz) is about 30% higher than values for typical monohydrated Mn2+ complexes, which is related to its larger molecular size; its relaxation efficiency is similar to that of clinically used Gd3+-based agents. In vivo MRI experiments realized in control mice at 0.02 mmol/kg injected dose indicate good signal enhancement in the kidneys and fast renal clearance. Taken together, MnL3 is the first chelate that combines such excellent stability, selectivity, inertness and relaxation properties, all of primary importance for MRI use.

GdIII and GaIII complexes with a new tris-3,4-HOPO ligand as new imaging probes: complex stability, magnetic properties and biodistribution.

The development of metal-based multimodal imaging probes is a highly challenging field in coordination chemistry. In this context, we have developed a bifunctional hexadentate tripodal ligand (H3L2) with three 3,4-HOPO moieties attached to a flexible tetrahedral carbon bearing a functionalizable nitro group. Complexes formed with different metal ions have potential interest for diagnostic applications, namely magnetic resonance imaging (MRI) and positron emission tomography (PET). The capacity of the ligand to coordinate GdIII and GaIII was studied and the thermodynamic stability constants of the respective complexes were determined by potentiometry and spectrophotometry. The ligand forms stable 1 : 1 ML complexes though with considerably higher affinity for GaIII than for GdIII (pGa = 26.2 and pGd = 14.3 at pH 7). The molecular dynamics simulations of the GdIII complex indicate that two water molecules can coordinate the metal ion, thus providing efficient paramagnetic enhancement of water proton relaxation. The relaxation and the water exchange properties of the GdIII chelate, assessed by a combined 17O NMR and 1H NMRD study, showed associative activated water exchange with a relatively low rate constant, k298ex = (0.82 ± 0.11) × 107 s-1, and some aggregation tendency. Biodistribution studies of the 67Ga-L2 complex suggested good in vivo stability and quick renal clearance. Further anchoring of this ligand with specific biotargeting moieties might open future prospectives for applications of labelled conjugates in both MRI and 68Ga-PET diagnostic imaging.

Doxorubicin-Sensitized Luminescence of NIR-Emitting Ytterbium Liposomes: Towards Direct Monitoring of Drug Release.

Drug-loaded liposomes are typical examples of nanomedicines. We show here that doxorubicin, the anti-cancer agent in the liposomal drug Doxil, can sensitize Ytterbium (Yb3+ ) and generate its near-infrared (NIR) emission. When doxorubicin and amphiphilic Yb3+ chelates are incorporated into liposomes, the sensitized emission of Yb3+ is dependent on the integrity of the particles, which can be used to monitor drug release. We also established the first demonstration that the NIR Yb3+ emission signal is observable in living mice following intratumoral injection of the Yb3+ -doxorubicin-liposomes, using a commercial macroscopic setup equipped with a NIR camera.

Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference.

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A2-) or piperidineacetamide (tO2DO2AMPip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X-ray structure of [Mn(tO2DO2A)(H2O)]·2H2O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A2- ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H2O)] complex occurs through both proton- and metal-assisted pathways, while the [Mn(tO2DO2AMPip)(H2O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A2- is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using 17O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and 1H nuclear magnetic relaxation dispersion (NMRD) profiles.

Concentration-Dependent Interactions of Amphiphilic PiB Derivative Metal Complexes with Amyloid Peptides Aß and Amylin*.

Metal chelates targeted to amyloid peptides are widely explored as diagnostic tools or therapeutic agents. The attachment of a metal complex to amyloid recognition units typically leads to a decrease in peptide affinity. We show here that by separating a macrocyclic GdL chelate and a PiB targeting unit with a long hydrophobic C10 linker, it is possible to attain nanomolar affinities for both Aß1-40 (Kd =4.4 nm) and amylin (Kd =4.5 nm), implicated, respectively in Alzheimer's disease and diabetes. The Scatchard analysis of surface plasmon resonance data obtained for a series of amphiphilic, PiB derivative GdL complexes indicate that their Aß1-40 or amylin binding affinity varies with their concentration, thus micellar aggregation state. The GdL chelates also affect peptide aggregation kinetics, as probed by thioflavin-T fluorescence assays. A 2D NMR study allowed identifying that the hydrophilic region of Aß1-40 is involved in the interaction between the monomer peptide and the Gd3+ complex. Finally, ex vivo biodistribution experiments were conducted in healthy mice by using 111 In labeled analogues. Their pancreatic uptake, ∼3 %ID g-1 , is promising to envisage amylin imaging in diabetic animals.

Stability, relaxometric and computational studies on Mn2+ complexes with ligands containing a cyclobutane scaffold.

The stability constants of Mn2+ complexes with ligands containing a trans-1,2-cyclobutanediamine spacer functionalized with picolinate and/or carboxylate functions were determined using potentiometric titrations (25 °C, 0.1 M KCl). The stability constant of the complex with a hexadentate ligand containing four acetate groups (L14-, log KMnL = 10.26) is improved upon replacing one (L24-, log KMnL = 14.71) or two (L34-, log KMnL = 15.81) carboxylate groups with picolinates. The [Mn(L1)]2- complex contains a water molecule coordinated to the metal ion in aqueous solutions, as evidenced by 1H NMRD studies and 17O chemical shifts and transverse relaxation rates. The 1H relaxivities determined at 60 MHz (3.3 and 2.4 mM-1 s-1 at 25 and 37 °C, respectively) are comparable to those of monohydrated complexes such as [Mn(edta)]2-. The exchange rate of the inner-sphere water molecule (k = 248 × 106 s-1) is slightly lower than that of the edta4- analogue. DFT calculations (M11/def2-TZVP) suggest that the water exchange reaction follows a dissociatively activated mechanism, providing activation parameters in reasonably good agreement with the experimental data. DFT calculations also show that the 17O hyperfine coupling constant A/ℏ is affected slightly by changes in the Mn-Owater distance and the orientation of the water molecule with respect to the Mn-O vector.

Concentration-Dependent Interactions of Amphiphilic PiB Derivative Metal Complexes with Amyloid Peptides Aß and Amylin*.

Invited for the cover of this issue are Jean-François Morfin and Éva Tóth at the CNRS in Orléans, and their collaborators from University of Debrecen, University of Coimbra and Université de Toulouse. The image depicts that when an amphiphilic compound is intravenously injected, monomer, pre-micellar and micellar forms can co-exist in the blood and have different affinities for amyloid peptides. Read the full text of the article at 10.1002/chem.202004000.

Synthesis and In Vitro Studies of a Gd(DOTA)-Porphyrin Conjugate for Combined MRI and Photodynamic Treatment.

With the aim of developing new molecular theranostic agents, a π-extended Zn(II) porphyrin as photosensitizer for photodynamic therapy (PDT) linked to two GdDOTA-type complexes for magnetic resonance imaging (MRI) detection was synthesized. The relaxivity studies revealed a much higher relaxivity value per Gd ion for this medium sized molecule (19.32 mM-1 s-1 at 20 MHz and 298 K) compared to clinical contrast agents-a value which strongly increases in the presence of bovine serum albumin, reaching 25.22 mM-1 s-1. Moreover, the photophysical studies showed the strong ability of the molecule to absorb light in the deep red (670 nm, ε ≈ 60000 M-1 cm-1) and in the near-infrared following two-photon excitation (920 nm, σ2 ≈ 650 GM). The conjugate is also able to generate singlet oxygen, with a quantum yield of 0.58 in DMSO. Promising results were obtained in cellular studies, demonstrating that the conjugate is internalized in HeLa cells at micromolar concentration and leads to 70% of cell death following 30 min irradiation at 660 nm. These results confirm the potential of the designed molecule as an imaging and therapeutic agent.

Unprecedented Kinetic Inertness for a Mn2+ -Bispidine Chelate: A Novel Structural Entry for Mn2+ -Based Imaging Agents.

The search for more biocompatible alternatives to Gd3+ -based MRI agents, and the interest in 52 Mn for PET imaging call for ligands that form inert Mn2+ chelates. Given the labile nature of Mn2+ , high inertness is challenging to achieve. The strongly preorganized structure of the 2,4-pyridyl-disubstituted bispidol ligand L1 endows its Mn2+ complex with exceptional kinetic inertness. Indeed, MnL1 did not show any dissociation for 140 days in the presence of 50 equiv. of Zn2+ (37 °C, pH 6), while recently reported potential MRI agents MnPyC3A and MnPC2A-EA have dissociation half-lives of 0.285 h and 54.4 h under similar conditions. In addition, the relaxivity of MnL1 (4.28 mm-1 s-1 at 25 °C, 20 MHz) is remarkable for a monohydrated, small Mn2+ chelate. In vivo MRI experiments in mice and determination of the tissue Mn content evidence rapid renal clearance of MnL1 . Additionally, L1 could be radiolabeled with 52 Mn and the complex revealed good stability in biological media.

Dual Imaging Gold Nanoplatforms for Targeted Radiotheranostics.

Gold nanoparticles (AuNPs) are interesting for the design of new cancer theranostic tools, mainly due to their biocompatibility, easy molecular vectorization, and good biological half-life. Herein, we report a gold nanoparticle platform as a bimodal imaging probe, capable of coordinating Gd3+ for Magnetic Resonance Imaging (MRI) and 67Ga3+ for Single Photon Emission Computed Tomography (SPECT) imaging. Our AuNPs carry a bombesin analogue with affinity towards the gastrin releasing peptide receptor (GRPr), overexpressed in a variety of human cancer cells, namely PC3 prostate cancer cells. The potential of these multimodal imaging nanoconstructs was thoroughly investigated by the assessment of their magnetic properties, in vitro cellular uptake, biodistribution, and radiosensitisation assays. The relaxometric properties predict a potential T1- and T2- MRI application. The promising in vitro cellular uptake of 67Ga/Gd-based bombesin containing particles was confirmed through biodistribution studies in tumor bearing mice, indicating their integrity and ability to target the GRPr. Radiosensitization studies revealed the therapeutic potential of the nanoparticles. Moreover, the DOTA chelating unit moiety versatility gives a high theranostic potential through the coordination of other therapeutically interesting radiometals. Altogether, our nanoparticles are interesting nanomaterial for theranostic application and as bimodal T1- and T2- MRI / SPECT imaging probes.

Aza-BODIPY Platform: Toward an Efficient Water-Soluble Bimodal Imaging Probe for MRI and Near-Infrared Fluorescence.

In this study, an original aza-BODIPY system comprising two Gd3+ complexes has been designed and synthesized for magnetic resonance imaging/optical imaging applications, by functionalization of the boron center. This strategy enabled the obtainment of a positively charged bimodal probe, which displays an increased water solubility, optimized photophysical properties in the near-infrared region, and very promising relaxometric properties. The absorption and emission wavelengths are 705 and 741 nm, respectively, with a quantum yield of around 10% in aqueous media. Moreover, the system does not produce singlet oxygen upon excitation, which would be toxic for tissues. The relaxivity obtained is high at intermediate fields (16.1 mM-1 s-1 at 20 MHz and 310 K) and competes with that of bigger or more rigid systems. A full relaxometric and 17O NMR study and fitting of the data using the Lipari-Szabo approach showed that this high relaxivity can be explained by the size of the system and the presence of some small aggregates. These optimized photophysical and relaxometric properties highlight the potential use of such systems for future bimodal imaging studies.

Gadolinium Complexes of Highly Rigid, Open-Chain Ligands Containing a Cyclobutane Ring in the Backbone: Decreasing Ligand Denticity Might Enhance Kinetic Inertness.

In an effort to explore novel ligand scaffolds for stable and inert lanthanide complexation in magnetic resonance imaging contrast agent research, three chiral ligands containing a highly rigid (1S,2S)-1,2-cyclobutanediamine spacer and different number of acetate and picolinate groups were efficiently synthesized. Potentiometric studies show comparable thermodynamic stability for the Gd3+ complexes formed with either the octadentate (L3)4- bearing two acetate or two picolinate groups or the heptadentate (L2)4- analogue bearing one picolinate and three acetate groups (log KGdL = 17.41 and 18.00 for [Gd(L2)]- and [Gd(L3)]-, respectively). In contrast, their dissociation kinetics is revealed to be very different: the monohydrated [Gd(L3)]- is considerably more labile, as a result of the significant kinetic activity of the protonated picolinate function, as compared to the bishydrated [Gd(L2)]-. This constitutes an uncommon example in which lowering ligand denticity results in a remarkable increase in kinetic inertness. Another interesting observation is that the rigid ligand backbone induces an unusually strong contribution of the spontaneous dissociation to the overall decomplexation process. Thanks to the presence of two inner-sphere water molecules, [Gd(L2)]- is endowed with high relaxivity (r1 = 7.9 mM-1 s-1 at 20 MHz, 25 °C), which is retained in the presence of large excess of endogenous anions, excluding ternary complex formation. The water exchange rate is similar for [Gd(L3)]- and [Gd(L2)]-, while it is 1 order of magnitude higher for the trishydrated tetraacetate analogue [Gd(L1)]- (kex298 = 8.1, 10, and 127 × 106 s-1, respectively). A structural analysis via density functional theory calculations suggests that the large bite angle imposed by the rigid (1S,2S)-1,2-cyclobutanediamine spacer could allow the design of ligands based on this scaffold with suitable properties for the coordination of larger metal ions with biomedical applications.

A biocompatible redox MRI probe based on a Mn(ii)/Mn(iii) porphyrin.

For the development of redox responsive MRI probes based on the MnIII/MnII couple, stable complexation of both reduced and oxidized forms of the metal ion and appropriate tuning of the redox potential in the biologically relevant range are key elements. The water soluble fluorinated Mn-porphyrin derivative Mn-3 satisfies both requirements. In aqueous solutions, it can reversibly switch between MnIII/MnII oxidation states. In the presence of ascorbic acid or ß-mercaptoethanol, the MnIII form undergoes reduction, which is slowly but fully reversed in the presence of air oxygen. A UV-Vis kinetic study of MnIII/MnII reduction under oxygen-free conditions yielded second-order rate constants, k2, of 46.1 M-1 s-1 and 13.8 M-1 s-1 for the reaction with ascorbic acid and ß-mercaptoethanol, respectively. This could correspond, in the absence of oxygen, to a half-life of a few minutes in blood plasma and a few seconds in circulating immune cells where ascorbic acid reaches 20-40 µM and a few mM concentrations, respectively. In contrast to expectations based on the redox potential, reduction with glutathione or cysteine does not occur. It is prevented by the coordination of the glutathione carboxylate group(s) to MnIII in the axial position, as was evidenced by NMR data. Therefore, MnIII-3 acts as an ascorbate specific turn-on MRI probe, which in turn can be re-oxidized by oxygen. The relaxivity increase from the oxidized to the reduced form is considerably improved at medium frequencies (up to 80 MHz) with respect to the previously studied Mn-TPPS4 analogues; at 20 MHz, it amounts to 150%. No in vitro cytotoxicity is detectable for Mn-3 in the typical MRI concentration range. Finally, 19F NMR resonances of MnIII-3 are relatively sharp which could open further opportunities to exploit such complexes as paramagnetic 19F NMR probes.

A Porphyrin Dimer-GdDOTA Conjugate as a Theranostic Agent for One- and Two-Photon Photodynamic Therapy and MRI.

A molecular theranostic agent designed for photodynamic therapy (PDT) treatment in the near-infrared and for imaging tissue tumors with magnetic resonance imaging (MRI) is reported. It consists of a linear π-conjugated Zn(II) porphyrin dimer linked at each extremity to a GdDOTA-type complex. This agent has shown very promising potential for PDT applications with good singlet oxygen generation in DMSO and high linear absorption in the near-infrared (λmax = 746 nm, ε ≈ 105 M-1 cm-1). Moreover, this molecule has a propensity for two-photon excited PDT with high two-photon cross sections (∼8000 GM in 880-930 nm range), which should allow for deeper tumor treatments and higher spatial precision as compared to conventional one-photon PDT. Regarding the MRI contrast agent properties, the molecule has shown superior relaxivity (14.4 mM-1 s-1 at 40 MHz, 298 K) in comparison to clinical contrast agents and the ability to be internalized in cells, thanks to its amphiphilic character. Irradiation of HeLa cells using either one-photon (740 nm) or two-photon excitation (910 nm) has led in both cases to important cell death.