Bifunctional Cyclam-Based Ligands with Phosphorus Acid Pendant Moieties for Radiocopper Separation: Thermodynamic and Kinetic Studies

Nickel-Catalyzed Defluorophosphonylation of Aryl Fluorides

A Ni-catalyzed cross-coupling reaction between aryl fluorides and dialkyl phosphonates [HP(O)(OR)₂] (R = secondary alkyl groups) in the presence of potassium tert-butoxide as a base is reported. The reaction converted various aryl fluorides into the corresponding aryl phosphonates even when electron-donating substituents were present on the aromatic ring. The combined experimental and computational studies suggested Ni-K⁺ cooperative action of a Ni(0) complex chelated with a strongly electron-donating ion-bridged dimeric phosphite ligand system [P(OR)₂O^-K⁺]₂ that facilitates turnover-limiting C-F bond oxidative addition of aryl fluorides.

Phosphonate Chelators for Medicinal Metal Ions

A family of phosphonate-bearing chelators was synthesized to study their potential in metal-based (radio)pharmaceuticals. Three ligands (H₆phospa, H₆dipedpa, H₆eppy; structures illustrated in manuscript) were fully characterized, including X-ray crystallographic structures of H₆phospa and H₆dipedpa. NMR spectroscopy techniques were used to confirm the complexation of each ligand with selected trivalent metal ions. These methods were particularly useful in discerning structural information for Sc³⁺ and La³⁺ complexes. Solution studies were conducted to evaluate the complex stability of 15 metal complexes. As a general trend, H₆phospa was noted to form the most stable complexes, and H₆eppy associated with the least stable complexes. Moreover, In³⁺ complexes were determined to be the most stable, and complexes with La³⁺ were the least stable, across all metals. Density functional theory (DFT) was employed to calculate structures of H₆phospa and H₆dipedpa complexes with La³⁺ and Sc³⁺. A comparison of experimental ¹H NMR spectra with calculated ¹H NMR spectra using DFT-optimized structures was used as a method of structure validation. It was noted that theoretical NMR spectra were very sensitive to a number of variables, such as ligand configuration, protonation state, and the number/orientation of explicit water molecules. In general, the inclusion of an explicit second shell of water molecules qualitatively improved the agreement between theoretical and experimental NMR spectra versus a polarizable continuum solvent model alone. Formation constants were also calculated from DFT results using potential-energy optimized structures. Strong dependence of molecular free energies on explicit water molecule number, water molecule configuration, and protonation state was observed, highlighting the need for dynamic data in accurate first-principles calculations of metal-ligand stability constants.

Cross-Bridged Cyclam with Phosphonate and Phosphinate Pendant Arms: Chelators for Copper Radioisotopes with Fast Complexation

Cross-bridged cyclam derivatives bearing two phosphonate (H₄L¹), bis(phosphinate) (H₄L²), or phosphinate (H₂L³) pendant arms were synthesized and studied with respect to their application as copper radioisotope carriers in nuclear medicine. The ligands show high macrocycle basicity (pK₁ > 14) and high Cu(II) complex stability (log K = 20-24). The complexation and dissociation kinetics of the Cu(II) complexes were studied by ultraviolet-visible spectroscopy. Phosphonate Cu(II)-H₄L¹ and bis(phosphinate) Cu(II)-H₄L² complexes form very quickly, reaching quantitative formation within 1 s at pH ∼6 and millimolar concentrations. Conversely, the formation of the phosphinate complex Cu(II)-H₂L³ is much slower (9 min at pH ∼6) due to the low stability of the out-of-cage reaction intermediate. All studied complexes are highly kinetically inert, showing half-lives of 120, 11, and 111 h for Cu(II)-H₄L¹, Cu(II)-H₄L², and Cu(II)-H₂L³ complexes, respectively, in 1 M HClO₄ at 90 °C. The high thermodynamic stability, fast formation, and extreme kinetic inertness of Cu(II) complexes indicate that phosphonate and bis(phosphinate) derivatives are promising ligands for nuclear medicine.

Rapid Thermodynamically Stable Complex Formation of [nat/111In]In3+, [nat/90Y]Y3+, and [nat/177Lu]Lu3+ with H6dappa

A phosphinate-bearing picolinic acid-based chelating ligand (H₆dappa) was synthesized and characterized to assess its potential as a bifunctional chelator (BFC) for inorganic radiopharmaceuticals. Nuclear magnetic resonance (NMR) spectroscopy was employed to investigate the chelator coordination chemistry with a variety of nonradioactive trivalent metal ions (In³⁺, Lu³⁺, Y³⁺, Sc³⁺, La³⁺, Bi³⁺). Density functional theory (DFT) calculations explored the coordination environments of aforementioned metal complexes. The thermodynamic stability of H₆dappa with four metal ions (In³⁺, Lu³⁺, Y³⁺, Sc³⁺) was deeply investigated via potentiometric and spectrophotometric (UV-vis) titrations, employing a combination of acidic in-batch, joint potentiometric/spectrophotometric, and ligand-ligand competition titrations; high stability constants and pM values were calculated for all four metal complexes. Radiolabeling conditions for three clinically relevant radiometal ions were optimized ([¹¹¹In]In³⁺, [¹⁷⁷Lu]Lu³⁺, [⁹⁰Y]Y³⁺), and the serum stability of [¹¹¹In][In(dappa)]^3- was studied. Through concentration-, time-, temperature-, and pH-dependent labeling experiments, it was determined that H₆dappa radiolabels most effectively at near-physiological pH for all radiometal ions. Furthermore, very rapid radiolabeling at ambient temperature was observed, as maximal radiolabeling was achieved in less than 1 min. Molar activities of 29.8 GBq/μmol and 28.2 GBq/μmol were achieved for [¹¹¹In]In³⁺ and [¹⁷⁷Lu]Lu³⁺, respectively. For H₆dappa, high thermodynamic stability did not correlate with kinetic inertness-lability was observed in serum stability studies, suggesting that its metal complexes might not be suitable as a BFC in radiopharmaceuticals.

Improved Conjugation, 64-Cu Radiolabeling, in Vivo Stability, and Imaging Using Nonprotected Bifunctional Macrocyclic Ligands: Bis(Phosphinate) Cyclam (BPC) Chelators

Bifunctional derivatives of bis(phosphinate)-bearing cyclam (BPC) chelators bearing a carboxylate, amine, isothiocyanate, azide, or cyclooctyne in the BP side chain were synthesized. Conjugations required no protection of phosphinate or ring secondary amine groups. The ring amines were not reactive (proton protected) at pH < ∼8. For isothiocyanate coupling, oligopeptide N-terminal α-amines were more suitable than alkyl amines, e.g., Lys ω-amine (p K_a ∼7.5-8.5 and ∼10-11, respectively) due to lower basicity. The Cu-64 labeling was efficient at room temperature (specific activity ∼100 GBq/μmol; 25 °C, pH 6.2, ∼100 ligand equiv, 10 min). A representative Cu-64-BPC was tested in vivo showing fast clearance and no nonspecific radioactivity deposition. The monoclonal anti-PSCA antibody 7F5 conjugates with thiocyanate BPC derivative or NODAGA were radiolabeled and studied in PC3-PSCA tumor bearing mice by PET. The radiolabeled BPC conjugate was accumulated in the prostate tumor with a low off-target uptake, unlike Cu-64-labeled NODAGA-antibody conjugate. The BPC chelators have a great potential for theranostic applications of the Cu-64/Cu-67 matched pair.

Formation and decomplexation kinetics of copper(ii) complexes with cyclen derivatives having mixed carboxylate and phosphonate pendant arms

The kinetic properties of Cu(ii) complexes of H4dota and its analogues with one (H5do3ap), two in the 1,7-position (trans-H6do2a2p), three (H7doa3p) and four (H8dotp) phosphonic acid pendant arms were investigated. The formation of a Cu(ii) complex with H4dota, trans-H6do2a2p and H8dotp at a slightly acidic pH is faster for the phosphonic acid derivatives than for H4dota, but with no simple dependence on the number of -CH2PO3H2 substituents (trans-H6do2a2p > H8dotp > H4dota; pH 4-6). Relative differences in the reactivity among the differently protonated species (HnL(x-)) of the same ligand are successively decreased with the more phosphonic acid groups in the ligand. The faster complexation is probably caused by the higher ability of phosphonates to bind the metal ion and/or to assist in the transfer of protons from the ring amine groups to the bulk water. The acid-assisted decomplexation kinetics of the complexes was followed in highly acidic solutions ([H(+)] = 0.01-5 M) and at different temperatures (15-70 °C) to determine the activation parameters of the reaction. The kinetic inertness of the Cu(ii) complexes follows the order: H4dota > H5do3ap > trans-H6do2a2p > H7doa3p > H8dotp. To obtain information on the influence of additional pendant arms, analogous data were obtained for trans-H2do2a. The ligand is less reactive than H4dota, but the kinetic inertness of its Cu(ii) complex is similar to that of the H4dota complex. As it was considered that the published thermodynamics data on the Cu(ii)-H8dotp system are probably incorrect, the system was re-investigated. It showed a very high stability for the [Cu(dotp)](6-) species and the easy formation of several Cu2L species in the presence of an excess of the metal ion. Also, the structure of the (H6doa3p)(-) anion in the solid state was determined. These experimental data demonstrate that the substitution of acetic acid pendant arms by methylphosphonic acid ones in H4dota-like ligands increases the rate of complexation but significantly decreases the kinetic inertness of the Cu(ii) complexes.

Optimization of the selectivity and rate of copper radioisotope complexation: formation and dissociation kinetic studies of 1,4,8-trimethylcyclam-based ligands with different coordinating pendant arms

Influence of coordinating pendant arm character on selectivity and rate of copper(ii) complexation was investigated to optimize ligands for radiomedicinal use.

In vitro evaluation of the monoclonal antibody 64Cu-IgG M75 against human carbonic anhydrase IX and its in vivo imaging

Specific oncology diagnostics requires new types of the selective radiopharmaceuticals, particularly those suitable for the molecular PET imaging. The aim of this work is to present a new, specific PET-immunodiagnostic radiopharmaceutical based on the monoclonal antibody IgG M75 targeting human carbonic anhydrase IX labelled with ⁶⁴Cu (T_½ = 12.70h) and its in vitro and in vivo evaluation. The antibody IgG M75 was conjugated with a non-commercial copper-specific chelator "phosphinate" and then labelled with the positron emitter ⁶⁴Cu. Stability of the labelled conjugated was tested in human serum. The immunoreactivity of the labelled conjugate was evaluated in vitro on a suitable cell cultures of the colorectal carcinoma (HT-29) and its imaging properties were estimated in vivo on a mouse model with inoculated colorectal carcinoma HT-29 imaged on a µPET/CT. The tested radioimmunoconjugate was obtained in a specific activity of 0.25-0.5 MBq/µg. In vitro uptake experiments revealed specific binding to the HT-29 cells (45 ± 2.8% of the total added activity) and the measured K_D value was found to be 9.2nM. Imaging clearly demonstrated significant uptake of the labelled monoclonal antibody in the tumour at 18h post administration. The radioimmunoconjugate ⁶⁴Cu-PS-IgG M75 seems to be a suitable candidate for PET diagnostics of hypoxic tumours expressing human carbonic anhydrase IX.

Seven years of radionuclide laboratory at IMC - important achievements

For many important research topics in polymer science the use of radionuclides brings significant benefits concerning nanotechnology, polymer drug delivery systems, tissue engineering etc. This contribution describes important achievements of the radionuclide laboratory at Institute of Macromolecular Chemistry of the Academy of Sciences of the Czech Republic (IMC) in the area of polymers for biomedical applications. Particular emphasis will be given to water-soluble polymer carriers of radionuclides, thermoresponsive polymer radionuclide carriers, thermoresponsive polymers for local brachytherapy, polymer scaffolds modified with (radiolabeled) peptides and polymer copper chelators for the therapy of Wilson´s disease.

A shortcut to high-affinity Ga-68 and Cu-64 radiopharmaceuticals: one-pot click chemistry trimerisation on the TRAP platform

Due to its 3 carbonic acid groups being available for bioconjugation, the TRAP chelator (1,4,7-triazacyclononane-1,4,7-tris(methylene(2-carboxyethylphosphinic acid))) is chosen for the synthesis of trimeric bioconjugates for radiolabelling. We optimized a protocol for bio-orthogonal TRAP conjugation via Cu(I)-catalyzed Huisgen-cycloaddition of terminal azides and alkynes (CuAAC), including a detailed investigation of kinetic properties of Cu(II)-TRAP complexes. TRAP building blocks for CuAAC, TRAP(alkyne)3 and TRAP(azide)3 were obtained by amide coupling of propargylamine/3-azidopropyl-1-amine, respectively. For Cu(II) complexes of neat and triply amide-functionalized TRAP, the equilibrium properties as well as pseudo-first-order Cu(II)-transchelation, using 10 to 30 eq. of NOTA and EDTA, were studied by UV-spectrophotometry. Dissociation of any Cu(II)-TRAP species was found to be independent on the nature or excess of a competing chelator, confirming a proton-driven two-step mechanism. The respective thermodynamic stability constants (log K(ML): 19.1 and 17.6) and dissociation rates (k: 38 × 10(-6) and 7 × 10(-6) s(-1), 298 K, pH 4) show that the Cu(II) complex of the TRAP-conjugate possesses lower thermodynamic stability but higher kinetic inertness. At pH 2-3, its demetallation with NOTA was complete within several hours/days at room temperature, respectively, enabling facile Cu(II) removal after click coupling by direct addition of NOTA trihydrochloride to the CuAAC reaction mixture. Notwithstanding this, an extrapolated dissociation half life of >100 h at 37 °C and pH 7 confirms the suitability of TRAP-bioconjugates for application in Cu-64 PET (cf. t(1/2)(Cu-64) = 12.7 h). To showcase advantages of the method, TRAP(DUPA-Pep)3, a trimer of the PSMA inhibitor DUPA-Pep, was synthesized using 1 eq. TRAP(alkyne)3, 3.3 eq. DUPA-Pep-azide, 10 eq. Na ascorbate, and 1.2 eq. Cu(II)-acetate. Its PSMA affinity (IC50), determined by the competition assay on LNCaP cells, was 18-times higher than that of the corresponding DOTAGA monomer (IC50: 2 ± 0.1 vs. 36 ± 4 nM), resulting in markedly improved contrast in Ga-68-PET imaging. In conclusion, the kinetic inertness profile of Cu(II)-TRAP conjugates allows for simple Cu(II) removal after click functionalisation by means of transchelation, but also confirms their suitability for Cu-64-PET as demonstrated previously (Dalton Trans., 2012, 41, 13803).