Radiochemical and analytical aspects of inter-institutional quality control measurements on radiopharmaceuticals.

BACKGROUND: Clinically applied radiopharmaceuticals have to meet quality release criteria like a high radiochemical yield and radiochemical purity. Many radiopharmaceuticals do not have marketing authorization and have no dedicated monograph within the European pharmacopeia, therefore general monographs on quality control have to be applied for clinical applications. These criteria require standardization and validation in labeling and preparation, including QC measurements according to well-defined standard operation procedures. QC measurements however, are often based on detection techniques specific for a certain LC-system. Multi-institutional research and development of new radiopharmaceuticals lead to an increase in multicenter trials. Although all institutes' radiopharmacies are using the same standardized labeling and operation procedures, they often use different LC and radiodetection systems. Here we present a comparison of QC assessments for 3 radiopharmaceuticals with focus on the interpretation of chromatograms, data-output and potential differences in local practical performances of QC on (U)HPLC. METHODS: QC assessments for [111In]In-CCK, [68Ga]Ga-Bombesin and [177Lu]Lu-PSMA analogs were compared. Two of the radiopharmaceutical QC assessments were also applied in other institutes using their own HPLC-systems and concordant software. Data from the HPLC-injections and measurements is processed and summarized in chromatograms, based on a variety of smoothing algorithms for which different software programs are applied. Described radiopeptides were labeled and analyzed according their standardized labeling and operation procedures. RESULTS: Integration of main peaks on chromatograms resulted in a range of RCP, depending on the smoothing algorithm used. [111In]In-CCK(A), 68Ga-Bombesin(B) and [177Lu]Lu-PSMA(C) analogs had a RCP range of 88%-96%(A), 89-95%(B) and 92-99%(C) respectively. Important factors affecting final RCP value were site specific background radiation-levels, intrinsic system properties such as noise and sensitivity, personal interpretation e.g. peak-tailing and smoothing algorithms. CONCLUSION: Measurement of RCP shows a strong method- and system-dependency, even when parameters are validated, standardized and SOP are followed. Release criteria are frequently based on RCP data from one central location. The lack of inter inter institutional validation and standardization in RCP determination makes the results therefore rather arbitrary. For multicenter trials, we recommend to compare locally determined RCP under validated and standardized conditions of in-line activity detection between institutes for each radiopharmaceutical.

Semi-automated system for concentrating 68Ga-eluate to obtain high molar and volume concentration of 68Ga-Radiopharmaca for preclinical applications.

INTRODUCTION: 68Ga-radiopharmaceuticals are common in the field of Nuclear Medicine to visualize receptor-mediated processes. In contrast to straightforward labeling procedures for clinical applications, preclinical in vitro and in vivo applications are hampered for reasons like e.g. volume restriction, activity concentration, molar activity and osmolality. Therefore, we developed a semi-automatic system specifically to overcome these problems. A difficulty appeared unexpectedly, as intrinsic trace metals derived from eluate (Zn, Fe and Cu) are concentrated as well in amounts that influence radiochemical yield and thus lower molar activity. METHODS: To purify Gallium-68 and to reduce the high elution volume of a 68Ga-generator, a NaCl-based method using a column containing PS-H+ was implemented in a low volume PEEK system. Influence on reducing osmolality, acidity and the amount of PS-H+ resin (15-50 mg) was investigated. [68Ga]Ga was desorbed from the PS-H+ resin with acidified 2-5 M NaCl (containing 0.05 M of HCl) and 68Ga-activity was collected. DOTA-TATE was used as a peptide model. All buffers and additives used for labeling were mixed with Chelex 100 (~1 g/50 mL) for >144 h and eventually filtered using a 0.22 µm filter (Millipore). Quantification of metals was performed after labeling by HPLC (UV). RESULTS: Gallium-68 activity could be desorbed from PS-H+ cation column with 3 M NaCl, and >60% (120-180 MBq) of [68Ga]Ga was collected in <0.3 mL. Taking into account the used amount of 68Ga-eluate, buffer and other excipients, the overall amount of trace metal per labeling was <1.5 nmol. DOTA-TATE could be labeled with [68Ga]Ga with high radiochemical yield, >99% (ITLC), and a radiochemical purity of >95% (HPLC). CONCLUSION: With the here described concentration system and metal purification technique, a low activity containing 68Ga-generator can be used to label DOTA-peptide in preclinical applicable amounts >60 MBq/nmol (40-60 MBq/0.1 mL) and within 20 min.

Overview of Development and Formulation of ¹77Lu-DOTA-TATE for PRRT.

Peptide receptor radionuclide therapy (PRRT) using radiolabeled somatostatin analogs has become an established procedure for the treatment of patients suffering from inoperable neuroendocrine cancers over-expressing somatostatin receptors. Success of PRRT depends on the availability of the radiolabeled peptide with adequately high specific activity, so that required therapeutic efficacy can be achieved without saturating the limited number of receptors available on the target lesions. Specific activity of the radionuclide and the radiolabeled somatostatin analog are therefore an important parameters. Although these analogs have been investigated and improved, and successfully applied for PRRT for more than 15 years, there are still many possibilities for further improvements that fully exploit PRRT with 177Lu-DOTA-TATE. The here summarized data presented herein on increased knowledge of the components of 177Lu-DOTA-TATE (especially the purity of 177Lu and specific activity of 177Lu) and the reaction kinetics during labeling 177Lu-DOTA-TATE clearly show that the peptide dose and dose in GBq can be varied. Here we present an overview of the development, formulation and optimisation of 177Lu-DOTA-TATE, mainly addressing radiochemical parameters.

Alternative method to determine Specific Activity of (177)Lu by HPLC.

UNLABELLED: Peptide Receptor Radionuclide Therapy (PRRT) with (177)Lu-DOTA-peptides requires (177)Lu with high specific activity (SA) and values >740 GBq (177)Lu per mg Lu to maximise the atom% of (177)Lu over total Lu. Vendors provide SA values which are based on activity and mass of the target, whereas due to "burn-up" of target, these SA values are not accurate. For a radiochemist the SA of (177)Lu is of interest prior to radiolabeling. An alternative method to determine SA was developed by HPLC, which includes a metal titration of a known amount of DOTA-peptide with a known amount of activity ((177)Lu), and a unknown amount of metal ((177+nat)Lu). Based on an HPLC separation of radiometal-DOTA-peptide and DOTA-peptide, and the concordant ratio of these components the metal content ((177+nat)Lu) can be calculated, and eventually the SA of (177)Lu can be accurately determined. These experimentally determined SA values exceeded the estimated values provided by vendors by 27 ± 16%, (range 6-73 %). The deviation of SA values for samples from the same Lu batch was <2% (n ≥ 10). IN CONCLUSION: the SA of (177)Lu is apparently often higher as stated by vendors in comparison to the experimentally determined actual values. For this reason, the SA of (177)Lu-DOTA-TATE and other Lu-DOTA-peptides could be increased accordingly.

Application of single-vial ready-for-use formulation of 111In- or 177Lu-labelled somatostatin analogs.

For the sake of safety it would be desirable to store and transport the ready-for-use liquid formulation (diagnostics and therapeutics) of radiolabelled peptides. The use of ethanol, in combination with a mixture of gentisic- and ascorbic acid, has superior effects on stabilizing radiolabelled somatostatin analogs. As a consequence, (111)In- and (177)Lu-labelled somatostatin analogs can be stored and transported in a single-vial ready-for-use liquid formulation up to 7 days after radiolabelling.

Effectiveness of quenchers to reduce radiolysis of (111)In- or (177)Lu-labelled methionine-containing regulatory peptides. Maintaining radiochemical purity as measured by HPLC.

An overview how to measure and to quantify radiolysis by the addition of quenchers and to maintain Radio-Chemical Purity (RCP) of vulnerable methionine-containing regulatory peptides is presented. High RCP was only achieved with a combination of quenchers. However, quantification of RCP is not standardized, and therefore comparison of radiolabelling and RCP of regulatory peptides between different HPLC-systems and between laboratories is cumbersome. Therefore we suggest a set of standardized requirements to quantify RCP by HPLC for radiolabelled DTPA- or DOTA-peptides. Moreover, a dosimetry model was developed to calculate the doses in the reaction vials during radiolabelling and storage of the radiopeptides, and to predict RCP in the presence and absence of quenchers. RCP was measured by HPLC, and a relation between radiation dose and radiolysis of RCP was established. The here described quenchers are tested individually as ƒ(concentration) to investigate efficacy to reduce radiolysis of radiolabelled methionine-containing regulatory peptides.