Automated Synthesis of 68Ga-Labeled DOTA-MGS8 and Preclinical Characterization of Cholecystokinin-2 Receptor Targeting

[68Ga]Ga-FAPI-46 synthesis on a GAIA® module system: Thorough study of the automated radiolabeling reaction conditions

The influence of several parameters involved in the 68Ga radiolabeling of FAPI-46 was studied at the scale of the automated reaction. Among the buffers tested, HEPES 0.3 M pH 4 allowed both high radiochemical purity (RCP) and radiochemical yield (RCY), without prepurification of 68Ga but after final purification of [68Ga]Ga-FAPI-46 on a C18 cartridge. A longer reaction time did not show significant benefit on the RCP, while higher loads of FAPI-46 and gentisic acid as anti-radiolysis compound allowed better RCY.

Validation of a size exclusion method for concomitant purification and formulation of peptide radiopharmaceuticals

BACKGROUND

Both in clinical routine and in preclinical research, the established standard procedure for the final purification of radiometal-labeled peptide radiopharmaceuticals is cartridge-based reversed-phase (RP) solid phase extraction (SPE). It allows the rapid and quantitative separation of the radiolabeled peptide from hydrophilic impurities and easy integration into automated synthesis procedures. However, product elution from RP cartridges necessitates the use of organic solvents and product recovery is sometimes limited. Thus, an alternative purification method based on commercially available size exclusion cartridges was investigated.

RESULTS

Since most peptide radiopharmaceuticals have a molecular weight > 1 kDa, Sephadex G10 cartridges with a molecular size cut-off of 700 Da were used for the final purification of a broad palette of 68Ga-, 64Cu- and 99mTc-labeled experimental peptide radiotracers as well as the clinically relevant ligand PSMA-617. Results (radiochemical purity (RCP, determined by ITLC), recovery from the solid support) were compared to the respective standard RP-SPE method. Generally, retention of unreacted 68Ga, 64Cu and 99mTc salts on the G10 cartridges was quantitative up to the specified elution volume (1.2 mL) for 68Ga and 99mTc and 99.6% for 64Cu. Even at increased elution volumes of 1.5-2 mL, RCPs of the eluted 68Ga- and 99mTc -radiopeptides were > 99%. For all peptides with a molecular weight ≥ 2 kDa, product recovery from the G10 cartridges was consistently > 85% upon respective adjustment of the elution volume. Product recovery was lowest for [68Ga]Ga-PSMA-617 (67%, 1.2 mL to 84%, 2 mL). The pH of the final product solution was found to be volume-dependent (1.2 mL: pH 6.3; 1.5 mL: pH 5.9; 2 mL: pH 5.5). Notably, the G10 cartridges were reused up to 20 times without compromising performance, and implementation of the method in an automated radiosynthesis procedure was successful.

CONCLUSIONS

Overall, size exclusion purification yielded all peptide radiopharmaceuticals in excellent radiochemical purities (> 99%) in saline within 10-12 min. Although product recovery is marginally inferior to classical SPE purifications, this method has the advantage of completely avoiding organic solvents and representing a cost-effective, easy-to-implement purification approach for automated radiotracer synthesis.

"One Method to Label Them All": A Single Fully Automated Protocol for GMP-Compliant 68Ga Radiolabeling of PSMA-11, Transposable to PSMA-I&T and PSMA-617

BACKGROUND

Prostate-specific membrane antigen (PSMA) is an ideal target for molecular imaging and targeted radionuclide therapy in prostate cancer. Consequently, various PSMA ligands were developed. Some of these molecules are functionalized with a chelator that can host radiometals, such as 68Ga for PET imaging. The 68Ga radiolabeling step benefits from process automation, making it more robust and reducing radiation exposure.

OBJECTIVE

To design a single automated radiolabeling protocol for the GMP-compliant preparation of [68Ga]Ga-PSMA-11, transposable to the production of [68Ga]Ga-PSMA-617 and [68Ga]Ga-PSMA-I&T.

METHODS

A GAIA® synthesis module and a GALLIAD® generator were used. Radio-TLC and radio-HPLC methods were validated for radiochemical purity (RCP) determination. Three [68Ga]Ga-PSMA-11 validation batches were produced and thoroughly tested for appearance and pH, radionuclide identity and purity, RCP, stability, residual solvent and sterility. Minimal modifications were made to the reagents and disposables for optimal application to other PSMA ligands.

RESULTS

[68Ga]Ga-PSMA-11 for clinical application was produced in 27 min. The 3 validation batches met the quality criteria expected by the European Pharmacopoeia to allow routine production. For optimal transposition to PSMA-617, the solid phase extraction cartridge was changed to improve purification of the radiolabeled product. For application to PSMA-I&T, the buffer solution initially used was replaced by HEPES 2.7 M to achieve good radiochemical yields. Residual HEPES content was checked in the final product and was below the Ph. Eur. threshold.

CONCLUSION

A single automated radiolabeling method on the GAIA® module was developed and implemented for 68Ga radiolabeling of 3 PSMA ligands, with slight adjustments for each molecule.

Automated Synthesis of [68Ga]Ga-FAPI-46 on a Scintomics GRP Synthesizer

[68Ga]Ga-FAPI-46 is a radiolabelled fibroblast activation protein inhibitor that selectively binds to fibroblast activation protein (FAP), which is overexpressed by cancer-associated fibroblasts (CAFs) in the tumour microenvironment. In recent years, radiolabelled FAP inhibitors (FAPIs) are becoming increasingly important in cancer diagnostics and also for targeted radionuclide therapy. Because of the increasing demand for radiolabelled FAPIs, automating the synthesis of these compounds is of great interest. In this work, we present a newly programmed automatic synthesis process of [68Ga]Ga-FAPI-46 on a Scintomics GRP module using two Galli Ad generators as a radionuclide source. Dedicated cassettes for the labelling of 68Ga-peptides were used without any modifications. The generators were connected via a three-way valve to the module and eluted automatically over a strong cation exchange (SCX) cartridge by using the vacuum pump of the synthesis module, eliminating the need to transfer the eluates into a separate vial. After a reaction step in HEPES buffer, the compound was purified by solid-phase extraction (SPE) over a Sep-Pak Light C18 cartridge. The evaluation of 10 routine syntheses of [68Ga]Ga-FAPI-46 resulted in a radiochemical yield of 72.6 ± 4.9%. The radiochemical purity was 97.6 ± 0.3%, and the amount of free gallium-68 and colloid was <2%. The final product fulfilled the quality criteria, which were adapted from relevant monographs of the European Pharmacopoeia (Ph. Eur.). This work presents the successful preparation of multiple doses of [68Ga]Ga-FAPI-46 in a GMP-compliant automated process for clinical use.

Investigation of the structure-activity relationship at the N-terminal part of minigastrin analogs

BACKGROUND

Over the last years, several strategies have been reported to improve the metabolic stability of minigastrin analogs. However, currently applied compounds still reveal limited in vitro and in vivo stability. We thus performed a glycine scan at the N-terminus of DOTA-MGS5 (DOTA-D-Glu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1-Nal) to systematically analyze the peptide structure. We substituted N-terminal amino acids by simple PEG spacers and investigated in vitro stability in human serum. Furthermore, we evaluated different modifications on its tetrapeptide binding sequence (H-Trp-(N-Me)Nle-Asp-1-Nal-NH₂).

RESULTS

Affinity data of all glycine scan peptides were found to be in a low nanomolar range (4.2-8.5 nM). However, a truncated compound lacking the D-γ-Glu-Ala-Tyr sequence revealed a significant loss in CCK-2R affinity. Substitution of the D-γ-Glu-Ala-Tyr-Gly sequence of DOTA-γ-MGS5 (DOTA- D-γ-Glu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1-Nal-NH₂) by polyethylene glycol (PEG) spacers of different length exhibited only a minor influence on CCK-2R affinity and lipophilicity. However, in vitro stability of the PEG-containing compounds was significantly decreased. In addition, we confirmed that the tetrapeptide sequence H-Trp-Asp-(N-Me)Nle-1-Nal-NH₂ is indeed sufficient for high CCK-2R affinity.

CONCLUSION

We could demonstrate that a substitution of D-γ-Glu-Ala-Tyr-Gly by PEG spacers simplified the peptide structure of DOTA-MGS5 while high CCK-2R affinity and favorable lipophilicity were maintained. Nevertheless, further optimization with regard to metabolic stability must be carried out for these minigastrin analogs.