No abstract

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 Lu (T = 6.65 days), a clinically established radiometal. The binding capacities of these radioligands and Lu-DOTA were evaluated using human plasma and solutions of human serum albumin (HSA), human α-acid glycoprotein (α-AGP), and human transthyretin (hTTR) by applying an ultrafiltration assay. Lu-DOTA-PPB-01 andLu-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 (<10%). Lu-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 Lu-DOTA complex, which was used as a control, was not observed (<1%). Lu-DOTA-PPB-01 andLu-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. Lu-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. Lu-DOTA-PPB-01 showed the slowest blood clearance (T: >15 h) followed by Lu-DOTA-PPB-03 (T: ∼2.33 h) and Lu-DOTA-PPB-02 (T: ∼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.

Purpose The β¯-emitting terbium-161 also emits conversion and Auger electrons, which are believed to be effective in killing single cancer cells. Terbium-161 was applied with somatostatin receptor (SSTR) agonists that localize in the cytoplasm (DOTATOC) and cellular nucleus (DOTATOC-NLS) or with a SSTR antagonist that localizes at the cell membrane (DOTA-LM3). The aim was to identify the most favorable peptide/terbium-161 combination for the treatment of neuroendocrine neoplasms (NENs). Methods The capability of the 161Tb- and 177Lu-labeled somatostatin (SST) analogues to reduce viability and survival of SSTR-positive AR42J tumor cells was investigated in vitro. The radiopeptides’ tissue distribution profiles were assessed in tumor-bearing mice. The efficacy of terbium-161 compared to lutetium-177 was investigated in therapy studies in mice using DOTATOC or DOTA-LM3, respectively. Results In vitro, [161Tb]Tb-DOTA-LM3 was 102-fold more potent than [177Lu]Lu-DOTA-LM3; however, 161Tb-labeled DOTATOC and DOTATOC-NLS were only 4- to fivefold more effective inhibiting tumor cell viability than their 177Lu-labeled counterparts. This result was confirmed in vivo and demonstrated that [161Tb]Tb-DOTA-LM3 was significantly more effective in delaying tumor growth than [177Lu]Lu-DOTA-LM3, thereby, prolonging survival of the mice. A therapeutic advantage of terbium-161 over lutetium-177 was also manifest when applied with DOTATOC. Since the nuclear localizing sequence (NLS) compromised the in vivo tissue distribution of DOTATOC-NLS, it was not used for therapy. Conclusion The use of membrane-localizing DOTA-LM3 was beneficial and profited from the short-ranged electrons emitted by terbium-161. Based on these preclinical data, [161Tb]Tb-DOTA-LM3 may outperform the clinically employed [177Lu]Lu-DOTATOC for the treatment of patients with NENs.

The decay of terbium-161 results in the emission of β¯-particles as well as conversion and Auger electrons, which makes terbium-161 interesting for therapeutic purposes. The aim of this study was to use dual-isotope SPECT imaging in order to demonstrate visually that terbium-161 and lutetium-177 are interchangeable without compromising the pharmacokinetic profile of the radiopharmaceutical. The 161Tb- and 177Lu-labeled somatostatin (SST) analogues DOTATOC (agonist) and DOTA-LM3 (antagonist) were tested in vitro to demonstrate equal properties regarding distribution coefficients and cell uptake into SST receptor-positive AR42J tumor cells. The radiopeptides were further investigated in AR42J tumor-bearing nude mice using the method of dual-isotope (terbium-161/lutetium-177) SPECT/CT imaging to enable the visualization of their distribution profiles in the same animal. Equal pharmacokinetic profiles were demonstrated for either of the two peptides, irrespective of whether it was labeled with terbium-161 or lutetium-177. Moreover, the visualization of the sub-organ distribution confirmed similar behavior of 161Tb- and 177Lu-labeled SST analogues. The data were verified in quantitative biodistribution studies using either type of peptide labeled with terbium-161 or lutetium-177. While the radionuclide did not have an impact on the organ distribution, this study confirmed previous data of a considerably higher tumor uptake of radiolabeled DOTA-LM3 as compared to the radiolabeled DOTATOC.

Lu-Ibu-DAB-PSMA, a radioligand modified with ibuprofen as the albumin binder, showed higher accumulation in PSMA-positive tumors of mice than the clinically used [ 177 Lu]Lu-PSMA-617 but lower retention in non-targeted tissues than previously developed albumin-binding PSMA radioligands. The aim of this study was to investigate whether the stereochemistry of the incorporated ibuprofen affects the radioligand's in vitro and in vivo properties and to select the more favorable radioligand for further development. For this purpose, SibuDAB and RibuDAB containing (S)-and (R)-ibuprofen, respectively, were synthesized and labeled with lutetium-177. In vitro, the two isomers had similar properties; however, [ 177 Lu]Lu-SibuDAB showed increased binding to mouse and human plasma proteins (91 ± 1 and 88 ± 2%, respectively) compared to [ 177 Lu]Lu-RibuDAB (75 ± 2 and 79 ± 2%, respectively). In vivo, [ 177 Lu]Lu-SibuDAB was metabolically more stable than [ 177 Lu]Lu-RibuDAB with ∼90 vs ∼67% intact radioligand detected in the blood at 4 h post injection (p.i.). In line with the lower albumin-binding affinity, the blood clearance of [ 177 Lu]Lu-RibuDAB in mice was considerably faster [27% of injected activity (% IA), 1 h p.i.] than for [ 177 Lu]Lu-SibuDAB (50% IA, 1 h p.i.). Time-dependent biodistribution studies performed in tumor-bearing athymic nude mice showed high PSMA-specific tumor uptake for both isomers. A twofold increased area under the curve (AUC 0→8d ) of the blood retention was determined for [ 177 Lu]Lu-SibuDAB as compared to [ 177 Lu]Lu-RibuDAB, whereas the kidney AUC 0→8d value of [ 177 Lu]Lu-SibuDAB was only half as high as for [ 177 Lu]Lu-RibuDAB. As a result, a more favorable tumor-to-kidney AUC 0→8d ratio was obtained for [ 177 Lu]Lu-SibuDAB, which was also visualized on SPECT/CT images. Based on its improved kidney clearance and higher metabolic stability, [ 177 Lu]Lu-SibuDAB was selected as the more favorable radioligand. Therapy studies performed with [ 177 Lu]Lu-SibuDAB (5 MBq/mouse) demonstrated the anticipated therapeutic superiority over the current gold-standard [ 177 Lu]Lu-PSMA-617 (5 MBq/mouse). The significantly increased survival time of mice treated with [ 177 Lu]Lu-SibuDAB as compared to those injected with [ 177 Lu]Lu-PSMA-617 justifies further development of this novel radioligand toward clinical application.