Improving the Pharmacological Profile with Albumin Binders: PSMA-based Radiohybrid Ligands Labelled with Iodine-123 and Actinium-225 for Targeted Alpha Therapy

Macropa-PSMA-ligand conjugates used for targeted alpha-therapy (TAT) with actinium-225 were furnished with an albumin binder to improve their pharmacological behaviour in vivo. [1,2] Additionally, the introduction of an iodine-containing albumin binder provides the basis for the development of a novel theranostic radioconjugate pair using the radiohybrid approach. This involves the complexation of the alpha emitter actinium-225 (half-life: 9.9 d) and the introduction of the easily accessible SPECT-compatible radiohalogen iodine-123 into the same molecule. Advantageously, its ideal physical half-life of 13.2 h and mild radioiodination conditions allows the imaging of longer circulating radioconjugates. The PSMA-binding motif based on the PSMA-617 structure was prepared by a multi-step peptide synthesis followed by subsequent conjugation with the macropa chelator by Cu-catalysed azide-alkyne (CuAAC) click chemistry. The labelling precursor for the introduction of iodine-123 was synthesized by replacing the 4-(p-iodophenyl)butyrate by a trimethylstannyl group to enable labelling by electrophilic aromatic substitution reaction. To determine the binding affinity, LNCaP cells were incubated with the nonradioactive conjugates (with and without nonradioactive lanthanum (La) as congener for actinium-225) in a competition assay and spiked with the radioconjugate [133La]La-PSMA-617. The influence of the complexing agent and the metal ion loading on the binding affinity was evaluated. Two radioconjugates were developed: [123I]I-mcp-M-alb-PSMA with one PSMA-binding motif was prepared from the stannyl precursor (DMSO, Iodogen, 20 min, rt, RCY: 49%) and [123I]I-mcp-D-alb-PSMA with two PSMA-binding motifs was labelled under the same conditions with the exception of adding nonradioactive iodine after completion of radiolabelling to iodinate the second stannyl group (RCY: 20%). Both radioconjugates were purified by HPLC. Beneficially, no influence of chelator loading on cell binding was observed in vitro. The cell binding is comparable across the analogues (mcp-M-alb-PSMA: Ki = 8.46 nM (7.05 - 10.14), La-mcp-M-alb-PSMA: Ki = 8.46 nM (6.72 - 10.66) / mcp-D-alb-PSMA: Ki = 2.35 nM (2.03 - 2.71), La-mcp-D-alb-PSMA: Ki = 2.21 nM (1.64 - 2.96)). Preliminary small animal SPECT imaging with tumor-bearing mice was executed pointing out a biodistribution of [123I]I-mcp-M-alb-PSMA which is comparable to [225Ac]Ac-mcp-M-alb-PSMA. The synthesis of the tin precursors and subsequent radiolabelling with iodine-123 provided two new radioconjugates which act as diagnostic counterparts to the corresponding actinium-225 radioconjugates. The introduction of iodine-123, with or without metal ion loading of the macropa chelator, did not alter the PSMA binding affinity in vitro. The addition of the albumin binding domain opens up a new approach to use iodine-123 in combination with actinium-225 as new radiohybrid pair for the development of hybrid radiopharmaceuticals within the theranostic concept.