Exploring the Limits of Ligand Rigidification in Transition Metal Complexes with Mono-N-Functionalized Pyclen Derivatives.

We report the synthesis of a new family of side-bridged pyclen ligands. The incorporation of an ethylene bridge between two adjacent nitrogen atoms was reached from the pyclen-oxalate precursor described previously. Three new side-bridged pyclen macrocycles, Hsb-3-pc1a, sb-3-pc1py, and Hsb-3-pc1pa, were obtained with the aim to assess their coordination properties toward Cu2+ and Zn2+ ions. We also prepared their nonreinforced analogues H3-pc1a, 3-pc1py, and H3-pc1pa as comparative benchmarks. The two series of ligands were characterized and their coordination properties were investigated in detail. The Zn2+ and Cu2+ complexes with the nonside-bridged series H3-pc1a, 3-pc1py, and H3-pc1pa were successfully isolated and their structures were assessed by X-ray diffraction studies. In the case of the side-bridged family, the synthesis of the complexes was far more difficult and, in some cases, unsuccessful. The results of our studies demonstrate that this difficulty is related to the extreme stiffening and basicity of such side-bridged pyclens.

Versatile Macrocyclic Platform for the Complexation of [natY/90Y]Yttrium and Lanthanide Ions.

We report a macrocyclic ligand (H3L6) based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing three acetate pendant arms and a benzyl group attached to the fourth nitrogen atom of the macrocycle. The X-ray structures of the YL6 and TbL6 complexes reveal nine coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and three oxygen atoms of the carboxylate pendants. A combination of NMR spectroscopic studies (1H, 13C, and 89Y) and DFT calculations indicated that the structure of the YL6 complex in the solid state is maintained in an aqueous solution. The detailed study of the emission spectra of the EuL6 and TbL6 complexes revealed Ln3+-centered emission with quantum yields of 7.0 and 60%, respectively. Emission lifetime measurements indicate that the ligand offers good protection of the metal ions from surrounding water molecules, preventing the coordination of water molecules. The YL6 complex is remarkably inert with respect to complex dissociation, with a lifetime of 1.7 h in 1 M HCl. On the other hand, complex formation is fast (∼1 min at pH 5.4, 2 × 10-5 M). Studies using the 90Y-nuclide confirmed fast radiolabeling since [90Y]YL6 is nearly quantitatively formed (radiochemical yield (RCY) > 95) in a short time over a broad range of pH values from ca. 2.4 to 9.0. Challenging experiments in the presence of excess ethylenediaminetetraacetic acid (EDTA) and in human serum revealed good stability of the [90Y]YL6 complex. All of these experiments combined suggest the potential application of H3L6 derivatives as Y-based radiopharmaceuticals.

Tuning the lipophilic nature of pyclen-based 90Y3+ radiopharmaceuticals for ß-radiotherapy.

Pyclen-dipicolinate chelates proved to be very efficient chelators for the radiolabeling with ß--emitters such as 90Y. In this study, a pyclen-dipicolinate ligand functionalized with additional C12 alkyl chains was synthesized. The radiolabeling with 90Y proved that the addition of saturated carbon chains does not affect the efficiency of the radiolabeling, whereas a notable increase in lipophilicity of the resulting 90Y radiocomplex was observed. As a result, the compound could be extracted in Lipiodol® and encapsulated in biodegrable pegylated poly(malic acid) nanoparticles demonstrating the potential of lipophilic pyclen-dipicolinate derivatives as platforms for the design of radiopharmaceuticals for the treatment of liver or brain cancers by internal radiotherapy.

Pyclen-Based Ligands Bearing Pendant Picolinate Arms for Gadolinium Complexation.

We report the synthesis of two pyclen-based regioisomer ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with picolinic acid pendant arms either at positions 3,9-pc2pa (L5) or 3,6-pc2pa (L6) of the macrocyclic fragment. The ligands were prepared by the regiospecific protection of one of the amine nitrogen atoms of the macrocycle using Boc and Alloc protecting groups, respectively. The X-ray structure of the Gd(III) complex of L5 contains trinuclear [(GdL5)3(H2O)3]3+ entities in which the monomeric units are joined by µ2-η1:η1-carboxylate groups. However, the 1H and 89Y NMR spectra of its Y(III) analogue support the formation of monomeric complexes in solution. The Tb(III) complexes are highly luminescent, with emission quantum yields of up to 28% for [TbL5]+. The luminescence lifetimes recorded in H2O and D2O solutions indicate the presence of a water molecule coordinated to the metal ion, as also evidenced by the 1H relaxivities measured for the Gd(III) analogues. The Gd(III) complexes present very different exchange rates of the coordinated water molecule (kex298 = 87.1 × 106 and 1.06 × 106 s-1 for [GdL5]+ and [GdL6]+, respectively). The very high water exchange rate of [GdL5]+ is associated with the steric hindrance originating from the coordination of the ligand around the water binding site, which favors a dissociatively activated water exchange process. The Gd(III) complexes present rather high thermodynamic stabilities (log KGdL = 20.47 and 19.77 for [GdL5]+ and [GdL6]+, respectively). Furthermore, these complexes are remarkably inert with respect to their acid-assisted dissociation, in particular the complex of L5.

Expanding the Scope of Pyclen-Picolinate Lanthanide Chelates to Potential Theranostic Applications.

A family of three picolinate pyclen-based ligands, previously investigated for the complexation of Y3+ and some lanthanide ions (Gd3+, Eu3+), was studied with 161Tb and 177Lu in view of potential radiotherapeutic applications. The set of six Tb3+ and Lu3+ complexes was synthesized and fully characterized. The coordination properties in the solid state and in solution were thoroughly studied. Potentiometric titrations, supported by density functional theory (DFT) calculations, showed the very high stability constants of the Tb3+ and Lu3+ complexes, associated with remarkable kinetic inertness for up to 30 days in 1 M HCl. A complete radiolabeling study performed with both 161Tb and 177Lu radionuclides, including experiments with regard to the stability with and without scavengers and kinetic inertness using challenging agents, proved that the ligands could reasonably compete with the DOTA analogue. To conclude, the potential of using the same ligand for both radiotherapy and optical imaging was highlighted for the first time.

Expanding the Family of Pyclen-Based Ligands Bearing Pendant Picolinate Arms for Lanthanide Complexation.

We report a detailed characterization of lanthanide complexes with two azaligands based on the pyclen macrocycle containing two picolinate and one acetate pendant arms. The two picolinate arms are attached to either opposite (L3) or adjacent (L4) amine nitrogen atoms of the macrocyclic platform. The X-ray structures of the Yb3+ complexes show nine coordination of the ligand to the metal ion, a situation that is also observed for EuL4 in the solid state. The EuL3 complex forms centrosymmetric dimers in the solid state joined by µ2-η1:η1 carboxylate groups, which results in 10-coordinate Eu3+ ions. The emission spectra of EuL3 measured in H2O and D2O solution reveal the presence of a hydration equilibrium involving a nine-coordinate species lacking inner-sphere water molecules and a monohydrated 10-coordinate species. The Eu3+ complexes present modest emission quantum yields in buffered aqueous solution (Φ = 16 and 22% for EuL3 and EuL4, 0.1 M tris buffer, pH 7.4), while the corresponding Tb3+ complexes present very high emission quantum yields under the same conditions (∼90%). 1H NMR studies show that the complexes of L3 present a fluxional behavior in D2O solution, while those of L4 are more rigid. The analysis of the Yb3+-induced NMR shifts of YbL4 indicates that the complex presents a structure in solution similar to that observed in the solid state. The Gd3+ complexes present very high thermodynamic stability constants (log KGdL = 23.56(2) and 23.44(2) for GdL3 and GdL4, respectively). The corresponding pGd values (pGd = -log[Gd3+]free with cL = 1 × 10-5 M and cGd = 1 × 10-6 at pH 7.4) of 20.69 (GdL3) and 21.83 (GdL4) are higher than that of Gd(dota)- (pGd = 19.21). The Gd3+ complexes also show remarkable inertness with respect to their proton-assisted dissociation, with dissociation half-life times of 50 min (GdL3) and 20 h (GdL4) in 1 M HCl.

Stable and Inert Yttrium(III) Complexes with Pyclen-Based Ligands Bearing Pendant Picolinate Arms: Toward New Pharmaceuticals for ß-Radiotherapy.

We report the synthesis of two azaligands based on the pyclen macrocyclic platform containing two picolinate and one acetate pendant arms. The two ligands differ in the relative positions of the pendant functions, which are arranged either in a symmetrical (L3) or nonsymmetrical (L4) fashion. The complexation properties of the ligands toward natY3+ and 90Y3+ were investigated to assess their potential as chelating units for radiopharmaceutical applications. The X-ray structures of the YL3 and YL4 complexes show nonadentate binding of the ligand to the metal ions. A multinuclear 1H, 13C, and 89Y NMR study shows that the complexes present a structure in solution similar to that observed in the solid state. The two complexes present very high thermodynamic stability constants (log KYL = 23.36(2) and 23.07(2) for YL3 and YL4, respectively). The complexes also show a remarkable inertness with respect to their proton-assisted dissociation, especially YL4. 90Y radiolabeling was proved to be efficient under mild conditions. The 90YL3 and 90YL4 radiochelates were found to be more stable than 90Y(DOTA).

A Coordination Chemistry Approach to Fine-Tune the Physicochemical Parameters of Lanthanide Complexes Relevant to Medical Applications.

The geometric features of two pyclen-based ligands possessing identical donor atoms but different site organization have a profound impact in their complexation properties toward lanthanide ions. The ligand containing two acetate groups and a picolinate arm arranged in a symmetrical fashion (L1) forms a Gd3+ complex being two orders of magnitude less stable than its dissymmetric analogue GdL2. Besides, GdL1 experiences a much faster dissociation following the acid-catalyzed mechanism than GdL2. On the contrary, GdL1 exhibits a lower exchange rate of the coordinated water molecule compared to GdL2. These very different properties are related to different strengths of the Gd-ligand bonds associated to steric effects, which hinder the coordination of a water molecule in GdL2 and the binding of acetate groups in GdL1.

The role of the capping bond effect on pyclen natY3+/90Y3+ chelates: full control of the regiospecific N-functionalization makes the difference.

Thanks to a smart regiospecific N-functionalization, a pyclen based ligand bearing one picolinate and two acetate arms organized in a dissymmetric manner was synthesized for Y3+ complexation, and compared to its symmetric analogue. The nature of the capping bonds around the metal coordination environment has a dramatic effect on the properties of the chelate, the natY3+ and 90Y3+ dissymmetric derivatives presenting enhanced thermodynamic stability and kinetic inertness.

Pyclen Tri-n-butylphosphonate Ester as Potential Chelator for Targeted Radiotherapy: From Yttrium(III) Complexation to (90)Y Radiolabeling.

The Y(3+) complex of PCTMB, the tri-n-butyl phosphonate ester of pyclen (3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene), was synthesized as well as its Ho(3+) and Lu(3+) analogues. X-ray diffraction analyses revealed isomorphous dimeric M2(PCTMB)2·9H2O (M = Y, Ho, Lu) structures that crystallize in the centrosymmetric P1̅ triclinic space group. (1)H NMR and UV studies in aqueous solutions indicated that Y(3+) complexation is fast, being quantitative in 167 min at pH 3.8 and in 13 min at pH 5.5 (25 °C, acetate buffer, I = 0.150 M, [Y(3+)] = [PCTMB] = 0.2 mM). (1)H NMR DOSY and photon correlation spectroscopy experiments evidenced the formation of aggregates in chloroform with a bimodal distribution that changes slightly with concentration (11-24 and 240-258 nm). The behavior of the acid-assisted dissociation of the complex of Y(3+) with PCTMB was studied under pseudo-first-order conditions, and the half-life of the [Y(PCTMB)] complex in 0.5 M HCl at 25 °C was found to be 37 min, a value that decreases to 2.6 min in 5 M HCl. The Y(3+) complex of PCTMB is thermodynamically very stable, with a stability constant of log KY-PCTMB = 19.49 and pY = 16.7 measured by potentiometry. (90)Y complexation studies revealed fast radiolabeling kinetics; optimal radiolabeling conditions were obtained for (90)Y in acetate medium, PCTMB at 10(-4) to 10(-2) M in acetate buffer pH = 4.75, 15 min at 45-60 °C. In vitro stability studies in human serum showed that [(90)Y(PCTMB)] is quite stable, with about 90% of the activity still in the form of the radiotracer at 24 h and 80% from 48 h to 72 h. A comparison with other ligands such as PCTA, DOTA, and DTPA already used for in vivo application shows that [(90)Y(PCTMB)] is an interesting lipophilic and neutral analogue of these reference chelates for therapeutic applications in aqueous and nonaqueous media.