Frank Roesch

The Renaissance of the 68Ge/68Ga Radionuclide Generator Initiates New Developments in 68Ga Radiopharmaceutical Chemistry

⁶⁸Ge/⁶⁸Ga radionuclide generators have been investigated for almost fifty years now, since the cyclotron-independent availability of positron emitting ⁶⁸Ga via the ⁶⁸Ge/⁶⁸Ga system had always attracted researches working in basic nuclear chemistry as well as radiopharmaceutical chemistry. However, it took decades and generations of research (and researchers) to finally approach a reliable level of ⁶⁸Ge/⁶⁸Ga generator designs, adequate to the modern requirements of radiometal labeling chemistry. ⁶⁸Ga radiopharmacy now is awaking from a sort of hibernation. The exciting perspective for the ⁶⁸Ge/⁶⁸Ga generator, now - more than ever, asks for systematic chemical, radiochemical, technological and radiopharmaceutical efforts, to guarantee reliable, highly-efficient and medically approved ⁶⁸Ge/⁶⁸Ga generator systems. The expected future broad clinical impact of ⁶⁸Ga-labelled radiopharmaceuticals - beyond the ⁶⁸Ga-DOTA-octreotide derivatives - for imaging tumors and many organs, on the other hand, identifies the development of sophisticated Ga(III) chelating structures to be a key factor. Today, open chain complexing agents have almost completely been displaced by macrocyclic DOTA and NOTA-derived conjugates. Structures of chelating moieties are being optimized in terms of thermodynamic stability and kinetic inertness, in terms of labeling efficacies at different, even acidic pH, and in terms of synthetic options towards bifunctionality, directed to sophisticated covalent coupling strategies to a variety of biologically relevant targeting vectors. Today, one may expect that the ⁶⁸Ge/⁶⁸Ga radionuclide generator systems could contribute to and facilitate the clinical impact of nuclear medicine diagnoses for PET in a dimension comparable to the established ⁹⁹Mo/⁹⁹(m)Tc generator system for SPECT.

Validation of 68Ge/68Ga generator processing by chemical purification for routine clinical application of 68Ga-DOTATOC

INTRODUCTION Imaging of somatostatin receptor expressing tumours has been greatly enhanced by the use of (68)Ga-DOTATOC and PET/CT. METHODS In this work, a purification method for the (68)Ge/(68)Ga generator eluate and a method to produce (68)Ga-DOTATOC suitable for clinical use were evaluated. The generator eluate was purified and concentrated on a cation-exchange cartridge in HCl/acetone media. The efficacy of this procedure in eliminating metal impurities from the (68)Ga solution was investigated by ICP-MS. The radiotracer quality was evaluated by radio-TLC, GC and gamma-ray spectrometry. RESULTS (68)Ga-DOTATOC preparations (n=33) were carried out with a mean synthesis yield of 59.3+/-2.8% (not corrected for decay) and a batch activity ranging from 555 to 296 MBq. The radiochemical and radionuclidic purity were >98% and 99.9999%, respectively. With this purification process, >95% of the Fe(III), Zn(II) and Mn(II) were eliminated from the solution. CONCLUSIONS (68)Ga-DOTATOC produced with this method can be efficiently used in nuclear medicine departments for PET evaluations.

Scandium-44: Benefits of a Long-Lived PET Radionuclide Available from the 44 Ti/ 44 Sc Generator System

(44)Ti/(44)Sc radionuclide generators are of interest for molecular imaging. The 3.97 hours half-life of (44)Sc and its high positron branching of 94.27% may stimulate the application of (44)Sc-labeled PET radiopharmaceuticals. This review describes the current status of (44)Ti production, (44)Ti/(44)Sc radionuclide generator development, post-processing of generator eluates towards medical application, identification of ligands adequate to Sc(III) co-ordination chemistry, proof-of-principle labeling of (44)Sc-DOTA-octreotides, investigation of in vitro and in vivo parameters, and initial applications for molecular imaging - both in small animals and humans.

Maturation of a Key Resource - The Germanium-68/Gallium-68 Generator: Development and New Insights

(68)Ge/(68)Ga radionuclide generators have been investigated for almost fifty years, since the cyclotron-independent availability of positron emitting (68)Ga via the (68)Ge/(68)Ga system had always attracted researches working in basic nuclear chemistry as well as radiopharmaceutical chemistry. However, it took decades and generations of research (and researchers) to finally reach a level of (68)Ge/(68)Ga radionuclide generator designs adequate to the modern requirements of radiometal labelling chemistry. Nevertheless, most of the existing commercial generator systems address aspects of (68)Ge breakthrough and safe synthesis of (68)Ga radiopharmaceuticals by adopting eluate post-processing technologies. Among the strategies to purify (68)Ga eluates, the cation exchange based version is relevant in terms of purification efficiency. In addition, it offers more options towards further developments of (68)Ga radiopharmaceuticals. Today, one may expect that the (68)Ge/(68)Ga radionuclide generator systems could contribute to the clinical impact of nuclear medicine diagnoses for PET similar to the established (99)Mo/(99m)Tc generator system for SPECT. The exciting perspective for the (68)Ge/(68)Ga radionuclide generator system, in turn, asks for systematic chemical, radiochemical, technological and radiopharmaceutical efforts, to guarantee reliable, highly-efficient and medically approved (68)Ge/(68)Ga generator systems.

Radiolabeling of DOTATOC with the long-lived positron emitter 44Sc

The positron-emitting radionuclide (44)Sc with a half-life of 3.97 h and a β(+) branching of 94.3% is of potential interest for clinical PET. As so far it is available from a (44)Ti/(44)Sc generator in Mainz, where long-lived (44)Ti decays to no-carrier-added (nca) (44)Sc. The (44)Sc is a trivalent metal cation and should be suitable for complexation with many well established bifunctional chelators conjugated to peptides or other molecular targeting vectors. Thus, the aim of this work was to investigate the potential of (44)Sc for labeling of DOTA-conjugated peptides. DOTA-D-Phe(1)-Tyr(3)-octreotide (DOTATOC) was used as a model molecule to study and optimize labeling procedure. Reaction parameters such as buffer conditions, concentration of peptide, pH range, reaction temperature and time were optimized. Addition of 21 nmol of DOTATOC to (44)Sc in ammonium acetate buffer pH 4.0 provided labeling yields >98% within 25 min of heating in an oil-bath at 95°C. This time can be reduced to 3 min only by applying microwave supported heating. (44)Sc-DOTATOC was found to be stable in 0.9% NaCl, PBS pH 7.4, fetal calf and human serums, and also in the presence of competing metal cations (Fe(3+), Ca(2+), Cu(2+), Mg(2+)), as well as other ligand competitors, like EDTA and DTPA, even after almost 25 h incubation at 37°C. Present study shows that nca (44)Sc forms stable complexes with the macrocyclic ligand DOTA and that (44)Sc-DOTATOC and analog targeting vectors may be synthesized for further preclinical and clinical investigations.

44Sc-DOTA-BN[2-14]NH2 in comparison to 68Ga-DOTA-BN[2-14]NH2 in pre-clinical investigation. Is 44Sc a potential radionuclide for PET?

AIM In the present study we demonstrate the in vitro and in vivo comparison of the (44)Sc and (68)Ga labeled DOTA-BN[2-14]NH(2). (44)Sc is a positron emitter with a half life of 3.92 h. Hence it could be used for PET imaging with ligands requiring longer observation time than in the case of (68)Ga. METHODS The binding affinity of (nat)Sc-DOTA-BN[2-14]NH(2) and (nat)Ga-DOTA-BN[2-14]NH(2) to GRP receptors was studied in competition to [(125)I-Tyr(4)]-Bombesin in the human prostate cancer cell line PC-3. A preliminary biodistribution in normal rats was performed, while first microPET images were assessed in male Copenhagen rats bearing the androgen-independent Dunning R-3327-AT-1 prostate cancer tumor. RESULTS The affinity to GRP receptors in the PC-3 cell line was higher for (nat)Ga-DOTA-BN[2-14]NH(2) (IC(50)(nM)=0.85 ± 0.06) than that of (nat)Sc-DOTA-BN[2-14]NH(2) (IC(50) (nM)=6.49 ± 0.13). The internalization rate of (68)Ga labeled DOTA-BN[2-14]NH(2) was slower than that of (44)Sc, but their final internalization percents were comparable. (68)Ga-DOTA-BN[2-14]NH(2) was externalized faster than (44)Sc-DOTA-BN[2-14]NH(2). The biodistribution of (44)Sc-DOTA-BN[2-14]NH(2) and (68)Ga-DOTA-BN[2-14]NH(2) in normal rats revealed a higher uptake in target organs and tissues of the first one while both excreted mainly through urinary tract. In microPET images both tracers were accumulated in the tumor with similar uptake patterns. CONCLUSIONS Despite the differences in the receptor affinity both the (68)Ga- and the (44)Sc-labeled DOTA-BN[2-14]NH(2) tracers showed comparable distribution and similar time constants of uptake and elimination. Moreover no differences in tumor accumulation (neither in the overall uptake nor in the dynamics) were observed from the microPet imaging. From that perspective the use of either (44)Sc or (68)Ga for detecting tumors with GRP receptors is equivalent.

Potential use of 68Ga-apo-transferrin as a PET imaging agent for detecting Staphylococcus aureus infection

INTRODUCTION (67)Ga citrate has been extensively used to detect infection and inflammation since 1971. However, its clinical utility is compromised due to several limitations. The present project explored whether (68)Ga-apo-transferrin ((68)Ga-TF), when prepared in vitro, is a useful agent for positron emission tomography (PET) imaging of bacterial infection. METHODS An infection was induced in male Wistar rats by injecting 5 × 10(5) CFU units of Staphyococcus aureus in the right thigh muscle. (68)Ga-TF was synthesized by mixing (68)GaCl(3) with apo-transferrin (TF, 2 mg) in sodium carbonate (0.1 M, pH 7.0) and incubating at 40 °C for 1 h. Animals were injected with 10-15 MBq of (68)Ga-TF containing approximately 0.2 mg TF and imaged at different time intervals using Siemens Biograph PET-CT. RESULTS When (68)Ga-TF were injected in the infected rats, the infection lesion was detectable within 20 min post injection. The biodistribution showed the uptake at the lesion increased with time as shown by significantly increased standard uptake values for up to 4 h post injection. There was a considerable decrease in the background activity during the same period of study, giving higher target-to-muscle ratios. Blood pool activity at 3 h post injection was insignificant. (68)GaCl(3) (when not conjugated to TF) did not localize at the infection lesion up to 120 min post injection. CONCLUSION The preliminary results suggest that (68)Ga-TF is capable of detecting S. aureus infection in the rat model, within an hour after intravenous injection.

Approaching ‘Kit-Type’ Labelling with 68Ga: The DATA Chelators

The DATA chelators are a novel class of tri‐anionic ligands based on 6‐amino‐1,4‐diazepine‐triacetic acid, which have been introduced recently for the chelation of 68Ga. Compared with macrocyclic chelators based on the cyclen scaffold (i.e., DOTA, DO3A, and DO2A derivatives), DATA chelators undergo quantitative radiolabelling more rapidly and under milder conditions. In this study, a systematic evaluation of the labelling of four DATA chelators—DATAM, DATAP, DATAPh, and DATAPPh—with 68Ga is presented. The results highlight the extraordinary potential of this new class of chelators for application in molecular imaging using 68Ga positron emission tomography (PET).

[68Ga]Ga-DO2A-(OBu-L-tyr)2: Synthesis, 68Ga-radiolabeling and in vitro studies of a novel 68Ga-DO2A-tyrosine conjugate as potential tumor tracer for PET

The synthesis, (68)Ga-labeling and in vitro study of the novel tyrosine chelate derivative [(68)Ga]Ga-1,4,7,10-tetraazacyclododecane-1,7-diacetic acid-4,10-di-(O-butyl)-l-tyrosine ([(68)Ga]Ga-DO(2)A-(OBu-l-tyr)(2)) as a potential tracer for imaging tumor metabolism by positron emission tomography (PET) is presented. This approach combines the biological amino acid transporter targeting properties of l-tyrosine with the outstanding availability of (68)Ga(III) via the (68)Ge/(68)Ga generator. In vitro studies utilizing the F98-glioblastoma cell line revealed specific uptake of [(68)Ga]Ga-DO2A-(OBu-l-tyr)(2) that was comparable to that of the reference O-(2-[(18)F]fluoroethyl)-l-tyrosine (FET). These promising results indicate a high potential of [(68)Ga]Ga-DO(2)A-(OBu-l-tyr)(2) for molecular imaging of tumor-driven amino acid uptake by PET.

A methodical 68Ga-labelling study of DO2A-(butyl-L-tyrosine)2 with cation-exchanger post-processed 68Ga: practical aspects of radiolabelling.

Positron emission tomography (PET) with (68)Ga is a fast-growing field in molecular imaging, both in research and in clinical routine. The availability of (68)Ga via the (68)Ge/(68)Ga radionuclide generator facilitates the development and production of radiopharmaceuticals independent of a cyclotron. The presented work shows a complete (68) Ga labelling study exemplified on [(68)Ga]DO2A-(butyl-L-tyrosine)(2), a potential tumour tracer for PET. A methodical sequence is followed to optimize the (68)Ga-labelling reaction. Practical aspects are described and the different parameters contributing to the labelling yield are demonstrated. The influence of temperature, time, amount of labelling precursor and pH value on the radiochemical yields is demonstrated. A conventional heating method is compared with microwave irradiation as an alternative labelling method. Finally, purification of (68)Ga-labelled compounds via solid-phase extraction and quality control is shown. The procedure described in this manuscript may serve as a guideline for optimizing (68)Ga labelling reactions.