Current Radiopharmaceuticals - Volume 2, Issue 1, 2009

Dear Colleague Since the conception of Current Radiopharmaceuticals in January 2008, we have been inundated with a variety of research articles ranging from topics discussing the use of radiopharmaceuticals in imaging, drug discovery to targeted alpha therapy. All these, articles have been peer reviewed by the journal's editorial board. Here, I would personally like to take this opportunity to thank everyone on the journals board that has volunteered unlimited time to review many articles that we have received and the editorial staff at Bentham Science. This new journal has captured the imagination of many scientists working in nuclear medicine. The journal owes its success to the number of authors who have submitted high quality manuscripts sharing in-depth knowledge in the area of radiopharmaceuticals. The second volume in 2009 in succession of the reputation of the first volume will also comprise of three issues. One focus of the journal is to give a clinical overview of some of the articles. This was first done by Mansi et al. in volume one issue three in which he summarizes the clinical aspects of ‘Targeted Alpha Particle Therapy’. In addition, we attempted to ‘group’ articles together to constitute a thematic issue, the first issue of volume one where the guest editors Kalevi Kairemo and Kim Bergstrom compiled an issue on the topic ‘The Role of Radiopharmaceuticals in Drug Discovery’; Ganesan Vaidyanathan and Roy Larsen on ‘Targeted Alpha Particle Therapy’ continued this thematic approach. The idea behind thematic issues is to generate maximum impact to the published subject matter in that particular area of radiopharmaceuticals to drive the journals' impact factor. Finally, we have some very interesting articles and reviews, which will be published in the forthcoming issues. The journal is growing fast and so is its reputation amongst researchers in pharmaceutical companies, institutions and universities; all of this will lead to the future success of Current Radiopharmaceuticals. Kind regards

Radiosyntheses of two new N3-substituted analogues of thymidine, N3-[4-(4-(2-[18F]fluoroethyl)phenyl)butyl] thymidine ([18F]-FEPBT) and N3-[5-(4-(2-[18F]fluoroethyl)phenyl)pentyl]thymidine ([18F]-FEPPT) are reported. Synthesesof the precursor compounds, 3',5'-O-bis-tetrahydropyranyl-N3-[4-(4-(2-methanesulfonyl-ethyl)phenyl)butyl]thymidine and 3',5'-O-bis-tetrahydropyranyl-N3-[5-(4-(2-methanesulfonyl-ethyl)phenyl)pentyl]thymidine are described. Radiofluorination of these precursors was performed using K[18F]/kryptofix 2.2.2. in dry MeCN. Hydrolysis of the protecting groups followed by HPLC purification yielded the desired products [18F]-FEPBT and [18F]-FEPPT. The radiochemical yields of [18F]-FEPBT were 35%-48% decay corrected (d. c.) with an average of 44%; and those of [18F]-FEPPT were 32%-40% (d. c.) with an average of 37% in three runs per compound. Radiochemical purity was > 99% and specific activity was > 74 GBq/μmol at the end of synthesis. The synthesis time was 80-90 min from the end of bombardment (EOB).

Better tumor markers are needed for early diagnosis and staging of prostate cancer, and for monitoring therapeutic response than the currently used prostate specific antigen (PSA). Prostate specific membrane antigen (PSMA) is highly expressed on the surface of prostatic epithelial cells making it a good target for prostate cancer. In this study, mAb 3C6, specific for the extracellular epitope of PSMA, was evaluated both in vitro and in vivo for PSMA-targeting. Immunoreactivity and specificity of mAb 3C6 was evaluated by flow cytometry using prostate cell lines expressing PSMA such as LNCaP and 22Rv1 and a cell line, DU145, that expresses very little PSMA. 3C6 was conjugated with the acyclic CHX-A” DTPA chelate, radiolabeled with 111In, and its in vitro and in vivo properties were assessed. The biodistribution of the radioimmunoconjugate evaluated in athymic mice bearing xenografts of three human prostate carcinoma cell lines shows high uptake after 72 hr in LNCaP tumors (%ID/g 22.93 + 6.32) and 22Rv1 (%ID/g 10.44 + 2.32) in contrast to low uptake by the DU145 tumors (%ID/g 4.27 + 0.37). Planar γ-scintigraphic images obtained for xenografted tumor bearing mice demonstrated targeting for PSMA positive tumors suggesting possible applications in imaging and for targeted radiation therapy.

The aim of the study was to determine the usefulness of the 99mTc-HMPAO-labelled leukocytes scintigraphy in monitoring patients with inflammatory bowel disease confirmed by histology. Patients and method: 40 patients (23 boys and 17 girls) were examined originally; 23 of them were re-examined repeatedly. A routine 99mTc-HMPAO method was used on in vitro labelled leukocytes. All children had an early abdominal scintigraphic image, SPECT of the abdomen and a whole body scan. For classification purposes, the semi-quantitative analysis of Cheow et al. (2005) was used to assess the disease activity. The patients were divided into 3 groups: group I with low (0-3), group II with middle (4-6), and group III with high (7-9) scores of a pathological uptake in the abdomen. Results: 1) We found 22 patients in group I, 33 patients in group II, and 12 patients in group III by the early scintigraphic image evaluation of the abdomen. 2) On the later whole-body images we detected 10 patients in group I, 32 patients in group II and 25 patients in group III. 3) All later images had the same or higher scores than the early images. 4) The follow-up scintigraphic monitoring of children revealed regression of the disease in 10 patients, no substantial changes in 8 patients, and relapse in 5 children. Conclusion: The results indicate that this technique might play an important role in the follow-up of IBD patients and as an addition to clinical symptoms and laboratory examinations, provides good information about the current stage of disease in IBD monitoring.

The design of radioligands selective for particular GABAA receptor isoforms is challenging because of their great potential for the diagnostic imaging of neuropsychiatric diseases such as epilepsy and sleep disorders. A subtype specificity higher than that of the classical benzodiazepines makes the novel pyrazolopyrimidine indiplon an attractive candidate structure. In the present work, N-(3-[18F]fluoro-propyl)-N-{3-[3-(thiophene-2-carbonyl)-pyrazolo[1,5-a]pyrimidine- 7-yl]-phenyl}-acetamide ([18F]fluoro-propyl-indiplon; [18F]FPI) was characterized and validated as a candidate tracer for positron emission tomography imaging of α1-GABAA receptors. In vitro, receptor affinity and autoradiography of [18F]FPI were assessed in cerebellar homogenates and sagittal brain slices of rats, respectively. The regional brain and organ uptake was investigated in NMRI mice at various times after i.v. application of [18F]FPI by ex vivo autoradiography and dissection, respectively. Radiometabolites of [18F]FPI were determined in blood, brain, and urine samples by highperformance liquid chromatography. [18F]FPI binds to cerebellar receptors with high affinity (KD 2.93 nM), and the in vitro labelling pattern appears to be more selective for α1-containing GABAA receptors than [3H]flunitrazepam. Intravenous application of [18F]FPI revealed (i) a strong biotransformation, (ii) low brain-to-plasma ratios of 18F (<0.4 at all times after tracer injection), (iii) homogenous and non-specific 18F distribution in the rat brain, and (iv) high radioactivity uptake in liver and femur. Thus, despite the promising results obtained by in vitro evaluation, [18F]FPI is excluded from further development as PET tracer.

Protection of the thiol function of L,L'-EC by S-thiomethylation allowed automatic deprotection during 99mTc - chelation without the use of any additional reagents. After radiolabeling (either at pH 8.5 or > 8.5), this precursor of L,L'- EC produced the desired chelate that was compared to 99mTc L,L'-EC, the standard chelate, by HPLC, biodistribution and scintigraphic studies. This led to the development of a single vial kit for 99mTc L,L'-EC. The chromatographic and biological properties of this kit were comparable to those of the commercially available EC kits, e.g. TCK-43 and TC-IK-25. This method of chelation from S-thiomethylated precursor has been extended to prepare the corresponding technetium-99 and rhenium chelates. These chelates of the protected ligands were chromatographically (HPLC) and spectroscopically comparable to the corresponding Tc and Re chelates obtained from the unprotected precursor. The results suggest that 99mTc L,L'-EC could be prepared from S-thiomethyl EC at pH 8.5 or higher, and this method of chelation could be used for the development of single vial EC kit.

Earlier in vivo PET experiments have demonstrated the binding of the neuroprotective alkaloid vinpocetine to the peripheral benzodiazepine receptor (PBR) or, with other terminology, the TSPO (18kDa translocator protein). With the aim of demonstrating the direct binding of vinpocetine to TSPO in vitro, two different ionidated versions of the novel TSPO ligand DAA1106 were used in autoradiographic experiments on human postmortem whole hemisphere brain slices. Vinpocetine effectively blocked the binding of both [125I]desmethoxy-DAA1106 and [125I]desfluoro-DAA1106 to TSPO, the decrease in binding reaching 30 % to 64 % in various brain structures. This present findings yield further evidence to vinpocetine's direct binding to the TSPO in the human brain in vitro as well as to the possible use of its radiolabelled versions as imaging biomarkers. The results also support the usefulness of two ionidated versions of DAA1106 as biomarkers of TSPO for in vitro and in vivo studies in neurological diseases accompanied with microglia activation.

A commercial coiled-tube micro-reactor (NanoTek; Advion) was used as a convenient platform for the synthesis of [18F]fallypride in small doses (0.5-1.5 mCi) for micro-PET studies of brain dopamine subtype-2 receptors in rodents. Each radiosynthesis used low amounts (20-40 μg; 39-77 nmol) of tosylate precursor and [18F]fluoride ion (0.5-2.5 mCi). Optimization of the labeling reaction in the apparatus, with respect to the effects of precursor amount, reaction temperature, flow rate and [18F]fluoride ion to precursor ratio, was achieved rapidly and the decay-corrected radiochemical yield of [18F]fallypride (up to 88%) was reproducible. The low amounts of material used in each radiosynthesis allowed crude [18F]fallypride to be purified rapidly on an analytical-size reverse phase HPLC column, preceding formulation for intravenous injection. Scale-up of the reaction was easily achieved by continuously infusing reagent precursor solutions to obtain [18F]fallypride in much greater quantity.

The aim of this study was to compare the results of nor-cholesterol scintigraphy and those of magnetic resonance imaging (MRI) in characterizing adrenal adenomas and differentiating between hypersecreting and nonhypersecreting lesions. We studied 41 patients (18 M and 23 F, mean age 47±15 years) with hypersecreting (n=19) or non-hypersecreting (n=22) unilateral adrenal tumors; all patients underwent iodine-131 nor-cholesterol adrenal scintigraphy and MRI studies. Pathology examinations (n=26) or follow-up data (n=15) were obtained showing 34 adenomas, 2 cysts, 1 myelolipoma, 1 pheochromocytoma, 1 carcinoma and 2 metastases. Imaging studies were qualitatively evaluated and the corresponding results were classified as true-positive, true-negative, false-positive and false-negative calculating diagnostic accuracy of each test. Imaging studies were also quantitatively assessed comparing nor-cholesterol uptake and MRI signal intensity ratios (SIRs) in hypersecreting and non-hypersecreting adenomas. The diagnostic accuracies of norcholesterol scintigraphy (95%) and MRI (93%) to identify adrenal adenomas were comparable; however, while a significantly (p=0.01) higher nor-cholesterol uptake was observed in hypersecreting (n=19) adenomas compared to nonhypersecreting (n=15) lesions, no significant differences in MRI SIRs were found in this comparative analysis; furthermore, no significant difference in tumor size occurred between hypersecreting and non-hypersecreting adenomas. In conclusion, adrenal scintigraphy using nor-cholesterol and MRI are both able to accurately identify cortical adenomas; in particular, while quantitative analysis of nor-cholesterol uptake is effective to differentiate between hypersecreting and nonhypersecreting adenomas, MRI SIRs evaluation is not useful for this purpose. These findings have relevant clinical implications particularly in patients with non-hypersecreting adenomas; in such patients, laboratory data are not helpful for lesion diagnosis, while imaging findings allow tumor characterization; in particular, the lower nor-cholesterol uptake observed in non-hypersecreting adenomas might reflect the normal hormone synthesis status of these lesions and, thus, regular secretion.

Faster and more efficient approaches for radiolabeling biomolecules with short-lived 18F are in dire need. Herein we report a new 18F-labeled prosthetic group containing an acetylene function that permits the labeling of biomolecules via click chemistry. This template, propargyl 4- [18F]fluorobenzoate ([18F]PFB) was synthesized from a quaternary salt precursor in decay-corrected radiochemical yields of 58 ± 31%. Several model compounds containing an azide moiety—benzyl azide, two lysine derivatives and a transglutaminase-reactive peptide—were labeled using [18F]PFB via a click reaction in decay-corrected radiochemical yields of 88 ± 4%, 79 ± 33%, 75 ± 5%, and 37 ± 31%, respectively. Our results suggest that the novel agent [18F]PFB is a potentially useful template for the 18F-labeling of biomolecules via click chemistry.

Radiolabelled nucleosides have potential applications as cell proliferation markers and as substrates for the Herpes simplex thymidine kinase (HSV-TK) reporter gene used in gene therapy. 2'-Deoxy-2'-[18F]fluorothymidine ([18F]FT) is highly-selective for HSV-TK relative to mammalian TK and is therefore of interest as a gene therapy reporter probe. [18F]FT was prepared via the SN 2 displacement of nosyl on 3-tert-butoxycarbonyl-1-(3',5'-di-O-benzoyl-2'-O-p-nitrophenylsulfonyl- β-D-arabinofuranosyl)thymine by [18F]fluoride anion, followed by deprotection. This radiofluorination precursor was synthesized from commercially-available 5-methyluridine in 11.5 % chemical yield (6 steps; Scheme 2). Fluorination of the nosylate using cyclotron-produced [18F]fluoride as the no-carrier-added activated K[18F]F-Kryptofix222 complex in acetonitrile at 95 °C, with a reaction time of 10 min, afforded crude [18F]FT in radiochemical yields (RCY) ranging from 4.2 to 15.6 percent (9.2 ± 3.8 %; n = 20). Radio TLC of the crude radiofluorinated product showed 36 to 78 percent (59.1 ± 17.2 %; n = 20) [18F]FT, recovered with radiochemical purity of 88 to 98 percent (93.8 ± 3.2 %; n = 20). Using a GE TracerLab FX synthesis unit and a manually-loaded HPLC, synthesis and purification times of 60 ± 5 min afforded overall recovered radiochemical yields averaging 5.5 ± 1.9 % (n = 20). The identity of [18F]FT was confirmed by TLC co-development, and HPLC co-elution with authentic FT. An authentic sample of reference 2'-deoxy-2'-fluorothymidine (FT) was prepared by direct fluorination of the 2'-arabino compound 1-(3',5'-di-O-trityl-β-D-arabinofuranosyl)thymine with DAST, followed by removal of the trityl protecting groups with trifluoroacetic acid, in 4 steps from 2,2'-anhydro-1-(β-D-arabinofuranosyl)thymine, in 43 % overall chemical yield.