Stable and Inert Yttrium(III) Complexes with Pyclen-Based Ligands

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Stable and Inert Yttrium(III) Complexes with Pyclen-Based Ligands Bearing Pendant Picolinate Arms: Toward New Pharmaceuticals for β‑Radiotherapy Mariane Le Fur,† Maryline Beyler,† Enikő Molnár,‡ Olivier Fougère,§ David Esteban-Gómez,∥ Gyula Tircsó,‡ Carlos Platas-Iglesias,∥ Nicolas Lepareur,⊥ Olivier Rousseaux,*,§ and Raphael̈ Tripier*,† †

Université de Bretagne Occidentale, UMR-CNRS 6521, IBSAM, UFR des Sciences et Techniques, 6 avenue Victor le Gorgeu, C.S. 93837, 29238 BREST Cedex 3, France ‡ Department of Inorganic and Analytical Chemistry, Faculty of Science and Technology, University of Debrecen, Egyetem tér 1, H-4032 Debrecen, Hungary § Groupe Guerbet, Centre de Recherche d’Aulnay-sous-Bois, BP 57400, 95943 Roissy CdG Cedex, France ∥ Departamento de Química, Facultade de Ciencias & Centro de Investigaciones Científicas Avanzadas (CICA), Universidade da Coruña, 15071 A Coruña, Spain ⊥ Département de Médecine Nucléaire, Centre Eugène Marquis, INSERM U1241, Avenue de la Bataille Flandres, Dunkerque CS 44229, 35042 Rennes, Cedex, France S Supporting Information *

ABSTRACT: 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).

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INTRODUCTION Yttrium is a group 3 element that has a chemical behavior similar to that of lanthanides and historically belongs to the rare-earth elements together with scandium and the lanthanum. As the latter, it exists in the form of a trivalent ion that preferentially interacts with hard donor atoms such as negatively charged oxygen atoms and amine nitrogen atoms to form eight- or nine-coordinate chelates.1 Yttrium possesses a particularly interesting isotope, 90Y, which is a pure β− emitter with a half-life of 64.2 h and a decay energy of 2.28 MeV. Yttrium-90 disintegrates into the stable 90 Zr isotope and can be produced both by neutron activation from stable 89Y or from a 90Sr/90Y generator.2 The interest in yttrium(III) is also justified by the existence of the β+ emitter 86 Y suitable for PET imaging. Despite its interesting properties (t1/2 = 14.2 h, Eβ+ = 1.2 MeV), this isotope is so far not approved for human applications contrary to 90Y. The best© XXXX American Chemical Society

known yttrium-90 radiopharmaceutical used in clinical practice is Zevalin. It was the first radio-immunotherapy treatment to be approved by the U.S. Food and Drug Administration (FDA) agency for the treatment of non-Hodgkin lymphoma (NHL). This drug uses the monoclonal antibody ibritumomab in conjunction with the chelator tiuxetan, a modified version of DTPA. 3 Yttrium-90 chelates are also widely used in combination with somatostatin-like peptides (peptide inhibiting growth hormones or gastrointestinal hormones). The somatostatin analogues, DOTA-TOC and DOTA-TATE, have been radiolabeled with numerous radionuclides including 111In, 177 Lu, and 90Y.4 The 90Y-DOTA-TATE radiopharmaceutical was proven to be efficient in the treatment of brain tumors.5 However, even if clinically approved, the currently used Received: November 20, 2017

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DOI: 10.1021/acs.inorgchem.7b02953 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

complexation of lanthanide ions, then for radiolabeling with β+ (68Ga, 64Cu) and γ (67Ga, 111In) emitters used for PET and SPECT imaging.8 More recently PCTMB was proposed as an interesting alternative to DOTA as a 90Y chelator for applications where neutral and lipophilic derivatives are required.9 In a recent work,10 we reported the synthesis of two regioisomers of the monopicolinate diacetate pyclen derivatives L1 and L2, which were shown to form very stable and inert eightcoordinated Y3+ complexes (Chart 1). We proved that a different arrangement of the donor atoms around the metal center leads to very different complexation properties that could be explained by the capping bond effect.11 In fact, the eight-coordination environment of Y3+ offered by L1 or L2 is completed by the binding of a water molecule, which in the case of YL2 occupies a capping position in the tricapped trigonal prismatic coordination polyhedron. In the case of YL1 a capping position is occupied by an oxygen atom of a carboxylate group, causing a lower stability compared to YL2. Considering these encouraging results, we thought judicious to synthesize the nine-coordinated Y3+ complexes obtained with the two regio-isomers of the dipicolinate monoacetate pyclen derivatives L3 and L4 (Chart 1). The potentially nonadentate character of these ligands is expected to fulfill the metal coordination environment avoiding the presence of coordinated water molecules, therefore increasing the stability of the complexes to approach that of Y(DOTA)− (Chart 1). More interestingly, we wanted to verify if the two regio-isomers offering a saturated coordination environment to yttrium(III) lead to complexes with different stability properties. Thus, in this paper we report the synthesis of L3 and L4 and a complete physico-chemical study of the solid state and in solution structures of their yttrium(III) complexes, as well as encouraging radiolabeling experiments.

yttrium-chelates still show some limitations. For instance, the acyclic DTPA chelator is known to form complexes with low stability and kinetic inertness compared to macrocyclic analogues such as DOTA derivatives. However, the latter suffer from relatively low complexation kinetics, which is often the counterpart of high kinetic inertness.6 Thus, finding chelators that bind rapidly the β−-emitter and form a radiocomplex presenting both high thermodynamic stability and kinetic inertness for safe use in vivo remains a major challenge for coordination chemists. The efficiency of N-functionalized polyazamacrocycles for the coordination of large cations such as lanthanide ions is well established.7 However, while some of them are limited by slow complexation kinetics (i.e., DOTA), some others stand out for their fast formation process, as it is especially the case of pyclen derivatives. Pyclen (3,6,9,15-tetraazabicyclo[9.3.1]pentadeca1(15),11,13-triene) is a 12-membered tetraaza-macrocyclic ligand that incorporates a pyridine core within the macrocyclic ring. The triaminocarboxylate derivative PCTA (3,6,9,15tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9,-triacetate, Chart 1) forms Ln3+ (including Y3+) complexes much Chart 1. Ligands Discussed in This Work

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RESULTS AND DISCUSSION Synthesis. The synthesis of the dipicolinate pyclen ligands L3 and L4 followed the strategies recently developed for their monopicolinate analogues L1 and L2.10 The synthesis of L3 (Scheme 1) started from N3-pyclen-Boc (1), which was previously reported by Siauge et al.12 The two picolinate moieties were introduced on the two amine functions of 1 adjacent to the pyridyl ring to give compound 2. Removal of the protecting Boc group of 2 was performed in MeOH with the presence of concentrated sulfuric acid to avoid the hydrolysis of the methyl ester of the picolinate unit during this step. Alkylation of 3 with tert-butylbromoacetate gave compound 4, while a final acid hydrolysis in hydrochloric acid (6 M) led to the HCl salt of ligand L3. As for its cis-diacetate analogue L2, the synthesis of the cisdipicolinate pyclen required the protection of the two adjacent secondary amines. This was achieved by reaction of pyclen with diethyl oxalate to afford pyclen oxalate 5, as already described (Scheme 2).10,13 Compound 5 was purified by recrystallization from an ethyl acetate/dichloromethane mixture, which yielded single crystals suitable for X-ray diffraction (Figure 1). The crystal data confirm the formation of a six-membered piperazine-2,3-dione ring, leaving only one amine nitrogen atom for subsequent alkylation. Reaction of pyclen-oxalate 5 with tert-butylbromoacetate afforded 6 with quantitative yield. Removal of the oxalamide bridge was directly achieved under esterification conditions (MeOH/H2SO4) in order to retain the ester protection of the acetate pendant arm. This strategy

faster than other macrocyclic ligands of similar ring size.4 These uncommon properties of pyclen derivatives may be related to the presence of the aromatic moiety that confers to the macrocyclic backbone an important rigidity that constrains the overall structure. The nbutyl phosphonate ester derivative of PCTA (PCTMB) was also widely studied because of its interesting properties for the selective targeting of cancer. Within this context PCTMB was investigated for the B

DOI: 10.1021/acs.inorgchem.7b02953 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of trans-Dipicolinate Pyclen L3

Scheme 2. Synthesis of cis-Dipicolinate Pyclen L4

allowed the formation of compound 7 with a quantitative yield and represents a major improvement compared to the two-step procedure used for the cis-diacetate analogue L1.10 The two secondary amine functions were alkylated using 2 equiv of methyl-6-(chloromethyl)picolinate to yield compound 8. Finally, the ligand L4 was obtained as the HCl salt after hydrolysis of the ester moieties in hydrochloric acid (6 M). The yttrium(III) complexes of L3 and L4 were synthesized by adding about 1.7 equiv of yttrium salt to an aqueous solution of the ligand while maintaining the pH of the solution around 5. The resulting complexes were purified by semipreparative HPLC on a reverse-phase C18 column and isolated in very good yields (∼90%). X-ray Crystal Structures of YL3 and YL4. The slow evaporation of aqueous solution of YL3 and YL4 led to the formation of single crystals suitable for X-ray diffraction analysis. The structures of the two complexes are shown in Figure 2, while the bond distances of the coordination sphere of the metal ion are compiled in Table 1. In both complexes the

Figure 1. ORTEP view of the structure of 5 (30% ellipsoid probability).

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DOI: 10.1021/acs.inorgchem.7b02953 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. View of the crystal structures of YL3 (left) and YL4 (right) with atom labeling; hydrogen atoms and water molecules are omitted for clarity. The ORTEP plot is at the 30% probability level.

centrosymmetrically related Δ(δλδλ) and Λ(δλδλ) enantiomers. The YL3 and YL4 complexes present rather similar distances of the metal coordination environment. The YL3 complex possesses a rather long Y−O distance (Y(1)−O(5): 2.398 Å) and short Y−N distances involving pyridyl nitrogen atoms (2.458−2.486 Å). The opposite trend is observed for YL4, which shows Y−Npyridyl distances in the range 2.465−2.548 Å and Y−O distances ≤2.361 Å. The Y−N distances involving donor atoms of the pyclen unit are similar to those reported previously for Y3+ pyclen-based complexes.9 The distances to N atoms of the pyridine units are similar to those observed for other nine-coordinated Y3+ complexes containing pyridine units.16,17 The distances to the amine donor atoms of the macrocycle and the oxygen atoms of the carboxylate groups are also within the usual range observed for complexes with polyaminocarboxylate ligands.18−20 The coordination polyhedron around the Y3+ ions in YL3 and YL4 was assessed performing shape measures with the aid of the SHAPE program.21−25 This analysis indicated that the metal coordination environments can be best described as a muffin, an irregular 1:5:3 polyhedron containing three vertexes related by a 2-fold pseudosymmetry axis, another set of five vertexes related by a 5-fold pseudosymmetry axis and the remaining vertex sitting on the latter axis (Figure 2).25 Structure of the Complexes in Solution. The structure of the YL3 and YL4 complexes in solution was investigated using 1H, 13C, and 89Y NMR spectroscopies in D2O solution.

Table 1. Bond Lengths (Å) of the Metal Coordination Environment in YL3 and YL4 Complexesa Y(1)−N(1) Y(1)−N(2) Y(1)−N(3) Y(1)−N(4) Y(1)−N(5) Y(1)−N(6) Y(1)−O(1) Y(1)−O(3) Y(1)−O(5) a

YL3

YL4

2.606(3) 2.630(3) 2.614(3) 2.486(2) 2.458(2) 2.476(2) 2.327(2) 2.315(2) 2.398(2)

2.603(2) 2.645(2) 2.575(3) 2.548(2) 2.465(2) 2.535(2) 2.3074(18) 2.361(2) 2.338(2)

See Figure 2 for labeling.

yttrium(III) ion is coordinated by the six nitrogen atoms of the ligand and three carboxylate oxygen atoms. This results in nine coordination around the metal ion. The macrocyclic pyclen unit adopts a [4242] conformation, in contrast to the [3333] conformation adopted by cyclen derivatives.14 As a result, the four five-membered chelate rings formed by the coordination of the pyclen moiety adopt (δλδλ) conformation, which is characterized by the presence of a mirror plane that makes pyclen achiral.5 Thus, the only source of chirality in YL3 and YL4 is related to the layout of the pendant arms, which generates two enantiomers that can be denoted as Δ and Λ.15 A close inspection of the crystal data shows that crystals of YL3 and YL4 contain the two D

DOI: 10.1021/acs.inorgchem.7b02953 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. 1H NMR spectra (D2O, 298 K, 300 MHz) of L4 (bottom) and the YL4 complex (top).

Table 2. Experimental and Calculated 89Y NMR Chemical Shifts, Number, and Type of Coordinating Atoms YL1 YL2 YL3 YL4 YPCTA

δexp

δcalca

δcalcb

nNam

nNpy

nNpy(pyclen)c

nOc

nOw

116.4 131.4 147.8 151.1 95.9

97.7 97.7 119.6 119.6 75.8

115.3 115.3 137.2 137.2 93.4

3 3 3 3 3

1 1 2 2 0

1 1 1 1 1

3 3 3 3 3

1 1 0 0 2

a c

Calculated with eq 1 using SNam = 68.1, SNpy = 85.7, SOc = 94.0, and SOw = 107.6 ppm. bCalculated assuming that SNpy(pyclen) = 68.1 ppm, see text. Number of pyridyl donors atom of the pyclen unit.

The 1H and 13C NMR spectra were assigned on the basis of 2D 1 H−1H COSY, 1H−13C HMBC and 1H−13C HMQC spectra (Figures S1−S20, Tables S1−S4). The 1H NMR spectra of L4 and YL4 are compared in Figure 3. The spectrum of the free ligand presents relatively broad signals for the aliphatic protons. After complexation with Y3+ the aliphatic proton nuclei become diastereotopic, so that each CH2 unit gives two proton signals, as illustrated with methylene protons 8 and 24 (Figure 3). The sharpness of the 1H signals and the diastereotopic character of the aliphatic proton signals indicate that only one diastereoisomer is present in solution, with no fluxional behavior within the NMR time scale. A similar situation is observed for the YL3 complex. The structure of the complexes in solution was further investigated by measuring the 89Y NMR shifts (Table 2). The chemical shifts of YL1, YL2, YL3, and YL4 were obtained by using 1H,89Y-HMQC NMR experiments to overcome the long acquisition times required for traditional 89Y NMR, as previously described.26,27 The 2D NMR spectra of the four complexes are illustrated in the Supporting Information (Figures S21−S24). A recent report showed that 89Y NMR of Y3+ complexes with polyaminocarboxylate ligands can provide useful informations on the structure of Y3+ complexes in solution.27 In fact, a correlation between the 89Y chemical shift and the chemical structure of the complexes could be established by the following empirical eq 1:

oxygen atom of water molecules. Sx represents the shielding constant of each donor atom (SNam = 68.1, SNpy = 85.7, SOc = 94.0, and SOw = 107.6 ppm) and A = 863 ppm. The use of this expression provides calculated 89Y chemical shifts that deviate from the experimental values by 19−34 ppm, with a systematic deviation of the calculated values toward higher fields. A similar situation is observed for YPCTA, which was investigated in this work for comparative purposes (Table 2). These results suggest that some donor atoms provide a smaller shielding constant than that estimated previously. A close inspection of the X-ray structures of YL3 and YL4 shows that the Y−N distance involving the pyridyl N atom of the macrocycle is significantly longer than the average distances involving the N atoms of the picolinate groups. We thus hypothesized that the N atom of the pyclen fragment provides a smaller shielding contribution to the overall 89Y NMR shift than the N atoms of the picolinate groups. Indeed, using a shielding constant of 68.1 ppm (the value corresponding to amine N atoms) for all the N donor atoms of the macrocyclic fragment gives calculated shifts in nice agreement with the experiment, with deviations