Synthesis, Radiolabeling, and Characterization of Plasma Protein-Binding Ligands: Potential Tools for Modulation of the Pharmacokinetic Properties of (Radio)Pharmaceuticals

The development of (radio)pharmaceuticals with favorable pharmacokinetic profiles is crucial for allowing the optimization of the imaging or therapeutic potential and the minimization of undesired side effects. The aim of this study was, therefore, to evaluate and compare three different plasma protein binders (PPB-01, PPB-02, and PPB-03) that are potentially useful in combination with (radio)pharmaceuticals to enhance their half-life in the blood. The entities were functionalized with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator via a l-lysine and β-alanine linker moiety using solid-phase peptide chemistry and labeled with 177Lu (T1/2 = 6.65 days), a clinically established radiometal. The binding capacities of these radioligands and 177Lu-DOTA were evaluated using human plasma and solutions of human serum albumin (HSA), human α1-acid glycoprotein (α1-AGP), and human transthyretin (hTTR) by applying an ultrafiltration assay. 177Lu-DOTA-PPB-01 and 177Lu-DOTA-PPB-02 bound to a high and moderate extent to human plasma proteins (>90% and ∼70%, respectively), whereas the binding to hTTR was considered negligible (177Lu-DOTA-PPB-03 showed almost complete binding to human plasma proteins (>90%) with a high fraction bound to hTTR (∼50%). Plasma protein binding of the 177Lu-DOTA complex, which was used as a control, was not observed (177Lu-DOTA-PPB-01 and 177Lu-DOTA-PPB-02 were both displaced (>80%) from HSA by ibuprofen, specific for Sudlow's binding site II and coherent with the aromatic structures, and >80% by their respective binding entities. 177Lu-DOTA-PPB-03 was displaced from hTTR by the site-marker l-thyroxine (>60%) and by its binding entity PPB-03* (>80%). All three radioligands were investigated with regard to the in vivo blood clearance in normal mice. 177Lu-DOTA-PPB-01 showed the slowest blood clearance (T1/2,β: >15 h) followed by 177Lu-DOTA-PPB-03 (T1/2,β: ∼2.33 h) and 177Lu-DOTA-PPB-02 (T1/2,β: ∼1.14 h), which was excreted relatively fast. Our results confirmed the high affinity of the 4-(4-iodophenyl)-butyric acid entity (PPB-01) to plasma proteins, while replacement of the halogen by an ethynyl entity (PPB-02) reduced the plasma protein binding significantly. An attractive approach is the application of the transthyretin binder (PPB-03), which shows high affinity to hTTR. Future studies in our laboratory will be focused on the application of these binding entities in combination with clinically relevant targeting agents for diagnostic and therapeutic purposes in nuclear medicine.