An HPLC and UHPLC-HRMS approach to study PSMA-11 instability in aqueous solution

Abstract

Background The stability of precursors and reagents is of utmost importance for developing a robust radiolabelling method that provides high and constant radiochemical yield and radiochemical purity. While performing the QC of the [68Ga]Ga-PSMA-11 injectable solutions according to Ph. Eur. Monograph that has recently been published, a trend to the instability of the standard PSMA-11, the same used as a precursor for [68Ga]Ga-PSMA-11 radiosynthesis, has been observed. This instability led to the formation of a side product in a time-dependent manner. The formation of this compound, besides making the implementation of the Ph. Eur. analytical method more difficult, negatively influenced the radiochemical yield and the radiochemical purity by increasing gallium-68 in colloidal and ionic forms. Results The nature of the side product was investigated by adding chelators, such as EDTA, to PSMA-11 solutions and using the combination of UHPLC-HRMS. The results led to the definition of the side product structure, as natFe-PSMA-11, from the combination of the high-affinity chelator HBED-CC, present in the molecule of PSMA-11, and environmental Fe (III). Conclusions Strategies to reduce the risk of low radiolabeling yields and to increase the stability of the PSMA-11 in an aqueous solution were also discussed.

Radiolabeled PSMA Inhibitors

“…Iudicello et al showed that PSMA-11, which is used as a reference standard and precursor for the synthesis of [ 68 Ga]Ga-PSMA-11, when dissolved in acidic aqueous solution forms a nat Fe-PSMA-11 complex (a side product with two diastereomeric forms similar to the ones previously reported for [ 68 Ga]Ga-PSMA-11) resulting thus, in lower radiochemical yield, chemical purity and radiochemical purity [ 91 , 92 ]. As a consequence, for storage of the PSMA-11 solution, EDTA was added to transchelate the Fe iron from the HBED-CC chelator.…”

Optimization of Precursor Preparation in PSMA-11 Radiolabeling to Obtain a Highly Reproducible Radiochemical Yield

“…The PSMA (prostate-specific membrane antigen) conjugated to HBED-CC (N,N'-bis [2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid), better known as PSMA-11, is one of the most widespread precursors for PET imaging of prostate cancer after radiolabeling with gallium-68 because the PSMA is highly expressed on most prostate cancer (PCa) cells (Figure 1). Stability testing for the PSMA-11 has provided evidence that the quality of precursor in aqueous solution varies with time due to the presence of Fe (III) in the precursor material and the consequent interaction between HBED-CC and Fe (III) already at room temperature [4].…”

“…Nevertheless, if a monograph has been published such as e.g., for [ 68 Ga]Ga-PSMA-11, the maximum amount of precursor (ligand) to use in the synthesis has already been defined. Stability testing for the PSMA-11 has provided evidence that the quality of precursor in aqueous solution varies with time due to the presence of Fe (III) in the precursor material and the consequent interaction between HBED-CC and Fe (III) already at room temperature [4].…”

“…Stability testing for the PSMA-11 has provided evidence that the quality of precursor in aqueous solution varies with time due to the presence of Fe (III) in the precursor material and the consequent interaction between HBED-CC and Fe (III) already at room temperature [ 4 ].…”

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

“…A recent paper describes a series of investigations after observing that both the precursor and its reference standard of [ 68 Ga]Ga-PSMA-11 are unstable in aqueous solutions (Iudicello et al 2021 ). The authors demonstrate that contamination with the omnipresent Fe(III) ion during radiosynthesis and during the synthesis of the precursor, can contribute to low radiolabelling yield and impurities in the radiopharmaceutical, and cause difficulties with the pharmacopoeial analytical method for radiochemical purity.…”