Glutathione-Mediated Metabolism of Technetium-99m SNS/S Mixed Ligand Complexes: A Proposed Mechanism of Brain Retention

Two series of [99mTc](SNS/S) mixed ligand complexes each carrying the N-diethylaminoethyl or the N-ethyl-substituted bis(2-mercaptoethyl)amine ligand (SNS) are produced at tracer level using tin chloride as reductant and glucoheptonate as transfer ligand. The identity of [99mTc](SNS/S) complexes is established by high-performance liquid chromatographic (HPLC) comparison with authentic rhenium samples. The para substituent R on the phenylthiolate coligand (S) ranges from electron-donating (−NH2) to electron-withdrawing (−NO2) groups, to study complex stability against nucleophiles as a result of N- and R-substitution. The relative resistance of [99mTc](SNS/S) complexes against nucleophilic attack of glutathione (GSH), a native nucleophilic thiol of 2 mM intracerebral concentration, is investigated in vitro by HPLC. The reaction of [99mTc](SNS/S) complexes with GSH is reversible and advances via substitution of the monothiolate ligand by GS- and concomitant formation of the hydrophilic [99mTc](SNS/GS) daughter compound. The N-diethylaminoethyl complexes are found to be more reactive against GSH as compared to the N-ethyl ones. Complex reactivity as a result of R-substitution follows the sequence −NO2 ≫ −H > −NH2. These in vitro findings correlate well with in vivo distribution data in mice. Thus, brain retention parallels complex susceptibility to GSH attack. Furthermore, isolation of the hydrophilic [99mTc](SNS/GS) metabolite from biological fluids and brain homogenates provides additional evidence that the brain retention mechanism of [99mTc](SNS/S) complexes is GSH-mediated

Radiometal-Dependent Biological Profile of the Radiolabeled Gastrin-Releasing Peptide Receptor Antagonist SB3 in Cancer Theranostics: Metabolic and Biodistribution Patterns Defined by Neprilysin

Recent advances in oncology involve the use of diagnostic/therapeutic radionuclide-carrier pairs that target cancer cells, offering exciting opportunities for personalized patient treatment. Theranostic gastrin-releasing peptide receptor (GRPR)-directed radiopeptides have been proposed for the management of GRPR-expressing prostate and breast cancers. We have recently introduced the PET tracer ⁶⁸Ga-SB3 (SB3, DOTA-<i>p</i>-aminomethylaniline-diglycolic acid-DPhe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt), a receptor-radioantagonist that enables the visualization of GRPR-positive lesions in humans. Aiming to fully assess the theranostic potential of SB3, we herein report on the impact of switching ⁶⁸Ga to ¹¹¹In/¹⁷⁷Lu-label on the biological properties of resulting radiopeptides. Notably, the bioavailability of ¹¹¹In/¹⁷⁷Lu-SB3 in mice drastically deteriorated compared with metabolically robust ⁶⁸Ga-SB3, and as a result led to poorer ¹¹¹In/¹⁷⁷Lu-SB3 uptake in GRPR-positive PC-3 xenografts. The peptide cleavage sites were identified by chromatographic comparison of blood samples from mice intravenously receiving ¹¹¹In/¹⁷⁷Lu-SB3 with each of newly synthesized ¹¹¹In/¹⁷⁷Lu-SB3-fragments. Coinjection of the radioconjugates with the neprilysin (NEP)-inhibitor phosphoramidon led to full stabilization of ¹¹¹In/¹⁷⁷Lu-SB3 in peripheral mouse blood and resulted in markedly enhanced radiolabel uptake in the PC-3 tumors. In conclusion, in situ NEP-inhibition led to indistinguishable ⁶⁸Ga/¹¹¹In/¹⁷⁷Lu-SB3 profiles in mice emphasizing the theranostic prospects of SB3 for clinical use

New Gastrin Releasing Peptide Receptor-Directed [^99mTc]Demobesin 1 Mimics: Synthesis and Comparative Evaluation

We have previously reported on the gastrin releasing peptide receptor (GRPR) antagonist [^99mTc]<b>1</b>, ([^99mTc]­demobesin 1, ^99mTc-[N₄′-diglycolate-dPhe⁶,Leu-NHEt¹³]­BBN­(6–13)). [^99mTc]<b>1</b> has shown superior biological profile compared to analogous agonist-based ^99mTc-radioligands. We herein present a small library of [^99mTc]<b>1</b> mimics generated after structural modifications in (a) the linker ([^99mTc]<b>2</b>, [^99mTc]<b>3</b>, [^99mTc]<b>4</b>), (b) the peptide chain ([^99mTc]<b>5</b>, [^99mTc]<b>6</b>), and (c) the C-terminus ([^99mTc]<b>7</b> or [^99mTc]<b>8</b>). The effects of above modifications on the biological properties of analogs were studied in PC-3 cells and tumor-bearing SCID mice. All analogs showed subnanomolar affinity for the human GRPR, while most receptor-affine <b>4</b> and <b>8</b> behaved as potent GRPR antagonists in a functional internalization assay. In mice bearing PC-3 tumors, [^99mTc]<b>1</b>–[^99mTc]<b>6</b> exhibited GRPR-specific tumor uptake, rapidly clearing from normal tissues. [^99mTc]<b>4</b> displayed the highest tumor uptake (28.8 ± 4.1%ID/g at 1 h pi), which remained high even after 24 h pi (16.3 ± 1.8%ID/g), well surpassing that of [^99mTc]<b>1</b> (5.4 ± 0.7%ID/g at 24 h pi)

New Gastrin Releasing Peptide Receptor-Directed [^99mTc]Demobesin 1 Mimics: Synthesis and Comparative Evaluation

We have previously reported on the gastrin releasing peptide receptor (GRPR) antagonist [^99mTc]<b>1</b>, ([^99mTc]­demobesin 1, ^99mTc-[N₄′-diglycolate-dPhe⁶,Leu-NHEt¹³]­BBN­(6–13)). [^99mTc]<b>1</b> has shown superior biological profile compared to analogous agonist-based ^99mTc-radioligands. We herein present a small library of [^99mTc]<b>1</b> mimics generated after structural modifications in (a) the linker ([^99mTc]<b>2</b>, [^99mTc]<b>3</b>, [^99mTc]<b>4</b>), (b) the peptide chain ([^99mTc]<b>5</b>, [^99mTc]<b>6</b>), and (c) the C-terminus ([^99mTc]<b>7</b> or [^99mTc]<b>8</b>). The effects of above modifications on the biological properties of analogs were studied in PC-3 cells and tumor-bearing SCID mice. All analogs showed subnanomolar affinity for the human GRPR, while most receptor-affine <b>4</b> and <b>8</b> behaved as potent GRPR antagonists in a functional internalization assay. In mice bearing PC-3 tumors, [^99mTc]<b>1</b>–[^99mTc]<b>6</b> exhibited GRPR-specific tumor uptake, rapidly clearing from normal tissues. [^99mTc]<b>4</b> displayed the highest tumor uptake (28.8 ± 4.1%ID/g at 1 h pi), which remained high even after 24 h pi (16.3 ± 1.8%ID/g), well surpassing that of [^99mTc]<b>1</b> (5.4 ± 0.7%ID/g at 24 h pi)