Frank Rösch

Processing of Generator-Produced 68Ga for Medical Application

The 68Ge/68Ga generator provides an excellent source of positron-emitting 68Ga. However, newly available “ionic” 68Ge/68Ga radionuclide generators are not necessarily optimized for the synthesis of 68Ga-labeled radiopharmaceuticals. The eluates have rather large volumes, a high concentration of H+ (pH of 1), a breakthrough of 68Ge, increasing with time or frequency of use, and impurities such as stable Zn(II) generated by the decay of 68Ga, Ti(IV) as a constituent of the column material, and Fe(III) as a general impurity. Methods: We have developed an efficient route for the processing of generator-derived 68Ga eluates, including the labeling and purification of biomolecules. Preconcentration and purification of the initial generator eluate are performed using a miniaturized column with organic cation-exchanger resin and hydrochloric acid/acetone eluent. The purified fraction was used for the labeling of nanomolar amounts of octreotide derivatives either in pure aqueous solution or in buffers. Results: Using the generator post-eluate processing system, >97% of the initially eluated 68Ga activity was obtained within 4 min as a 0.4-mL volume of a hydrochloric acid/acetone fraction. The initial amount of 68Ge(IV) was decreased by a factor of 104, whereas initial amounts of Zn(II), Ti(IV), and Fe(III) were reduced by factors of 105, 102, and 10, respectively. The processed 68Ga fraction was directly transferred to solutions containing labeling precursors—for example, DOTA-dPhe1-Tyr3-octreotide (DOTATOC) (DOTA = 1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′′-tetraacetic acid). Labeling yields of >95% were achieved within 10 min. Overall yields reached 70% at 20 min after generator elution relative to the eluted 68Ga activity, not corrected for decay. Specific activities of 68Ga-DOTATOC were 50 MBq/nmol using a standard protocol, reaching 450 MBq/nmol under optimized conditions. Conclusion: Processing on a cation-exchanger in hydrochloric acid/acetone media represents an efficient strategy for the concentration and purification of generator-derived 68Ga(III) eluates. The developed scheme guarantees high yields and safe preparation of injectable 68Ga-labeled radiopharmaceuticals for routine application and is easy to automate. Thus, it is being successfully used in clinical environments and might contribute to a new direction for clinical PET, which could benefit significantly from the easy and safe availability of the radionuclide generator-derived metallic positron-emitter 68Ga.

Association of Low Striatal Dopamine D2 Receptor Availability With Nicotine Dependence Similar to That Seen With Other Drugs of Abuse

OBJECTIVE All drugs of abuse induce a phasic dopamine release within the striatum that does not undergo habituation. Prolonged substance consumption impairs the natural function of the mesolimbic dopamine system, as shown by a decrease in the availability of striatal dopamine 2 (D(2)) receptors in patients suffering from cocaine, heroin, amphetamine, and alcohol dependence. However, it is unclear whether similar changes can also be observed in heavy-smoking nicotine-dependent smokers. METHOD In vivo D(2)/D(3) receptor availability was determined with [ (18)F]fallypride positron emission tomography in 17 heavy-smoking nicotine-dependent subjects and in 21 age-matched never-smoking comparison subjects. The smokers were scanned twice: first, during a period of usual consumption and second, 24 hours after smoking cessation. RESULTS Independent of the withdrawal status, the nicotine-dependent smokers displayed significantly less availability of D(2)/D(3) receptors within the bilateral putamen functionally covering parts of the dorsal striatum, as compared to the never-smoking subjects. Nicotine craving under the consumption condition correlated positively with D(2)/D(3) receptor availability within the ventral striatum but negatively with D(2)/D(3) receptor availability within the anterior cingulate and inferior temporal cortex. CONCLUSIONS Similar to other types of substance abuse, nicotine dependence is associated with low availability of dorsal striatal D(2)/D(3) receptors. In contrast to previous findings on abstinent alcohol-dependent patients, nicotine craving seems to be maintained by a region-specific shift in D(2)/D(3) receptor availabilities, with higher availability within the ventral striatum but lower availability within the anterior cingulate and inferior temporal cortex.

Brain and Plasma Pharmacokinetics of Aripiprazole in Patients With Schizophrenia: An [ 18 F]Fallypride PET Study

OBJECTIVE Aripiprazole at clinically effective doses occupies some 90% of striatal dopamine 2 and 3 (D(2)/D(3)) receptors. In order to further characterize its extrastriatal and time-dependent binding characteristics, the authors conducted positron emission tomography (PET) studies with the D(2)/D(3) antagonist [(18)F]fallypride at varying time points after the last aripiprazole administration in patients with schizophrenia. METHOD Sixteen inpatients with a DSM-IV diagnosis of schizophrenia or schizoaffective disorder receiving treatment with aripiprazole underwent an [(18)F]fallypride PET scan. Receptor occupancy was calculated as the percentage reduction in binding potential relative to unblocked values measured in eight age-matched, medication-free patients with schizophrenia. In addition, aripiprazole serum concentrations were determined as part of a routine therapeutic drug monitoring program in a large group of patients (N=128) treated with aripiprazole. RESULTS Mean dopamine D(2)/D(3) receptor occupancy was high in all brain regions investigated, with no binding difference across brain regions. Nonlinear regression analysis revealed maximum attainable receptor occupancy (E(max)) values close to saturation. The values for serum concentration predicted to provide 50% of E(max) (EC(50)) were in the range of 5-10 ng/ml in all brain regions. The D(2)/D(3) receptors were completely saturated when serum aripiprazole concentration exceeded 100-150 ng/ml. The mean concentration in the large clinical patient sample was 228 ng/ml (SD=142). CONCLUSIONS Because of its high affinity for D(2)/D(3) receptors and its long elimination half-life, aripiprazole at clinical doses occupies a high fraction of its target receptor everywhere in the brain. Its dissociation from those receptors is very slow, such that the authors calculate from the results that in patients with serum aripiprazole concentrations in the range typical for clinical practice, D(2)/D(3) receptors must remain nearly saturated for as long as 1 week after the last dose.

Radioactive Labeling of Defined HPMA-Based Polymeric Structures Using [18F]FETos for In Vivo Imaging by Positron Emission Tomography

During the last decades polymer-based nanomedicine has turned out to be a promising tool in modern pharmaceutics. The following article describes the synthesis of well-defined random and block copolymers by RAFT polymerization with potential medical application. The polymers have been labeled with the positron-emitting nuclide fluorine-18. The polymeric structures are based on the biocompatible N-(2-hydroxypropyl)-methacrylamide (HPMA). To achieve these structures, functional reactive ester polymers with a molecular weight within the range of 25,000-110,000 g/mol were aminolyzed by 2-hydroxypropylamine and tyramine (3%) to form (18)F-labelable HPMA-polymer precursors. The labeling procedure of the phenolic tyramine moieties via the secondary labeling synthon 2-[(18)F]fluoroethyl-1-tosylate ([(18)F]FETos) provided radiochemical fluoroalkylation yields of ∼80% for block copolymers and >50% for random polymer architectures within a synthesis time of 10 min and a reaction temperature of 120 °C. Total synthesis time including synthon synthesis, (18)F-labeling, and final purification via size exclusion chromatography took less than 90 min and yielded stable (18)F-labeled HPMA structures in isotonic buffer solution. Any decomposition could be detected within 2 h. To determine the in vivo fate of (18)F-labeled HPMA polymers, preliminary small animal positron emission tomography (PET) experiments were performed in healthy rats, demonstrating the renal clearance of low molecular weight polymers. Furthermore, low metabolism rates could be detected in urine as well as in the blood. Thus, we expect this new strategy for radioactive labeling of polymers as a promising approach for in vivo PET studies.

The Striatal and Extrastriatal D2/D3 Receptor-Binding Profile of Clozapine in Patients with Schizophrenia

Positron emission tomography (PET) studies reveal that clozapine at clinically used doses occupies less than 60% of D2/D3 dopamine receptors in human striatum. Here, the occupancy of D2/D3 dopamine receptors by clozapine in patients with schizophrenia was determined to test the hypothesis that clozapine binds preferentially to extrastriatal dopamine receptors. A total of 15 clozapine-treated inpatients with schizophrenia underwent a [18F]fallypride PET scan. Receptor occupancy was calculated as percent reduction in binding potential relative to unblocked values measured in seven normal volunteers. Mean D2/D3 receptor occupancy was statistically significantly higher in cortical (inferior temporal cortex 55%) than in striatal regions (putamen 36%, caudate 43%, p<0.005). While the maximum attainable receptor occupancy Emax approached 100% both in the striatum and cortex, the plasma concentration at 50% of Emax (ED50) was much higher in the putamen (950 ng/ml) than in the inferior temporal cortex (333 ng/ml). Clozapine binds preferentially to cortical D2/D3 receptors over a wide range of plasma concentrations. This selectivity is lost at extremely high plasma levels. Occupancy of cortical receptors approaches 60% with plasma clozapine in the range 350–400 ng/ml, which corresponds to the threshold for antipsychotic efficacy of clozapine. Extrastriatal binding of clozapine may be more relevant to its antipsychotic actions than striatal. However, further studies with an intraindividual comparison of untreated vs treated state are desirable to confirm this finding.

Vascular imaging of solid tumors in rats with a radioactive arsenic-labeled antibody that binds exposed phosphatidylserine.

Purpose: We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of vascular endothelial cells in tumors, probably in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a chimeric monoclonal antibody that binds phosphatidylserine could be labeled with radioactive arsenic isotopes and used for molecular imaging of solid tumors in rats. Experimental Design: Bavituximab was labeled with 74As (β+, T1/2 17.8 days) or 77As (β−, T1/2 1.6 days) using a novel procedure. The radionuclides of arsenic were selected because their long half-lives are consistent with the long biological half lives of antibodies in vivo and because their chemistry permits stable attachment to antibodies. The radiolabeled antibodies were tested for the ability to image subcutaneous Dunning prostate R3227-AT1 tumors in rats. Results: Clear images of the tumors were obtained using planar γ-scintigraphy and positron emission tomography. Biodistribution studies confirmed the specific localization of bavituximab to the tumors. The tumor-to-liver ratio 72 h after injection was 22 for bavituximab compared with 1.5 for an isotype-matched control chimeric antibody of irrelevant specificity. Immunohistochemical studies showed that the bavituximab was labeling the tumor vascular endothelium. Conclusions: These results show that radioarsenic-labeled bavituximab has potential as a new tool for imaging the vasculature of solid tumors.

Decreased Dopamine D2/D3‐Receptor Binding in Temporal Lobe Epilepsy: An [18F]Fallypride PET Study

Summary:  Purpose: Although animal data are suggestive, evidence for an alteration of the extrastriatal dopaminergic system in human focal epilepsy is missing.

Production of the Positron Emitting Radioisotope 86Y for Nuclear Medical Application

The production of 86Y via the 86Sr(p,n)-reaction was studied. Samples of 96.3% enriched 86SrCO3 were irradiated using a 4 π water-cooled target system at nearly optimum proton energy ranges (14 → 10 MeV) at beam currents of 3–8 μA. Thick target yields calculated from the measured excitation functions were compared with results from production runs. A chemical separation procedure including the recovery of the enriched target material was developed. Activities of about 1.5 GBq (40 mCi) 86Y per batch with high radionuclidic and radiochemical purity were achieved and [86Y]citrate was prepared for pharmacokinetic studies using PET.

Radiation doses of yttrium-90 citrate and yttrium-90 EDTMP as determined via analogous yttrium-86 complexes and positron emission tomography

Yttrium-90 is used for palliative therapy for the treatment of skeletal metastases, but because it is a pure β- emitter, data on the pharmacokinetics and radiation doses to metastases and unaffected organs are lacking. To obtain such data, the present study employed yttrium-86 as a substitute for90Y, with detection by positron emission tomography (PET). The study compared the properties of two different86Y complexes —86y-citrate and86Y -ethylene diamine tetramethylene phosphonate (EDTMP) — in ten patients with prostatic cancer who had developed multiple bone metastases (the ten patients being divided into two groups of five). Early dynamics were measured up to 1 h post injection (p.i.) over the liver region, followed by subsequent whole-body PET scans up to 3 days p.i. Absolute uptake data were determined for normal bone, bone metastases, liver and kidney. Radiation doses were calculated according to the MIRD recommendations. Based on the pharmacokinetic measurements of the distribution of the86Y complexes, it was possible to calculate radiation doses for the bone metastases and the red bone marrow delivered by complexes containing90Y. In 1 cm3 of bone metastasis, doses of 26±11 mGy/MBq and 18±2 mGy/MBq were determined per MBq of injected90Y- citrate and90Y- EDTMP, respectively. The doses to the bone marrow were 2.5±0.4 mGy/MBq for90Y- citrate and 1.8±0.6 mGy/MBq for90Y-EDTMP.86Y and PET provide quantitative information applicable to the clinical use of90Y. This method may also be useful for the design of other90Y radiopharmaceuticals and for planning radiotherapy dosages.

PET/CT imaging of osteoblastic bone metastases with 68Ga-bisphosphonates: first human study

Bisphosphonates are well established ligands for Tc for planar and SPECT/CT imaging of osteoblastic metastases. A novelDOTA-based bisphosphonate, (4-{[bis-(phosphonomethyl)) carbamoyl]methyl}-7,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododec-1-yl)acetic acid (BPAMD) [1, 2], was labelled using the Ge/Ga generator-derived positron emitter Ga [3], resulting in the PET tracer [Ga]BPAMD. [Ga]BPAMD was injected i.v. into a patient with known extensive bone metastases of prostate cancer. [Ga]BPAMD [maximum intensity projection (MIP) 50 min post-injection (p.i.), 462 MBq] revealed intense accumulation in multiple osteoblastic lesions in the central skeleton, ribs and proximal extremities: a = coronal PET, b = sagittal PET/CT. For comparison, c shows F-fluoride PET (sagittal, MIP 90 min p.i., 270 MBq). Metastases were detected in the whole skeleton with a maximal standardized uptake value (SUVmax) of 77.1 and 62.1 in the 10th thoracic and L2 vertebra vs 39.1 and 39.2 for F-fluoride, respectively. The advantages of this new bone imaging PET tracer are the very high target to soft tissue ratios and fast clearance, its ease of use and the generator-based availability of Ga which becomes especially important in these days of Tc shortage. While F-fluoride is adsorbed on bone surface and is related to blood flow, the bisphosphonate [Ga]BPAMD is taken up also by osteoclasts reflecting the farnesyl diphosphate synthase enzyme dynamics in the HMG-CoA reductase pathway. Since this pathway is mainly responsible for the osteoclastic bone destruction, [Ga]BPAMD may be superior in osteoclastic lesions. Finally, [Ga]BPAMD seems to be an ideal PET/CT tracer to plan and monitor bisphosphonate therapy in several bone disorders like osteoporosis, osteitis deformans, bone metastases, multiple myeloma, osteogenesis imperfecta, etc. and also to monitor radionuclide therapy for palliation of bone pain.