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

Abstract

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).