ABC transporters protect the brain by transporting neurotoxic compounds from the brain back into the blood. P-glycoprotein (P-gp) is the most investigated ABC (efflux) transporter, as it is implicated in neurodegenerative diseases such as Alzheimer's disease. Altered function of P-gp can be studied in vivo, using Positron Emission Tomography (PET). To date, several radiopharmaceuticals have been developed to image P-gp function in vivo. So far, attempts to image expression levels of P-gp using radiolabeled P-gp inhibitors have not been successful. Improved knowledge of compound behavior toward P-gp from in vitro studies should increase predictability of in vivo outcome.

P-glycoprotein is an efflux transporter located in the blood–brain barrier. (R)-[11C]Verapamil is widely used as a PET tracer to investigate its function in patients with epilepsy, Alzheimer’s disease, and other neurodegenerative diseases. Currently it is not possible to use this successful tracer in clinics without a cyclotron, because of the short half-life of carbon-11. We developed two new fluorine-18 labeled (R)-verapamil analogs, with the benefit of a longer half-life. The synthesis of (R)-N-[18F]fluoroethylverapamil ([18F]1) and (R)-O-[18F]fluoroethylnorverapamil ([18F]2) has been described. [18F]1 was obtained in reaction of (R)-norverapamil with the volatile [18F]fluoroethyltriflate acquired from bromoethyltosylate and a silver trilate column with a radiochemical yield of 2.7% ± 1.2%. [18F]2 was radiolabeled by direct fluorination of precursor 13 and required final Boc-deprotection with TFA resulting in a radiochemical yield of 17.2% ± 9.9%. Both tracers, [18F]1 and [18F]2, were administered to Wistar rats, and blood plasma and brain samples were analyzed for metabolic stability. Using [18F]1 and [18F]2, PET scans were performed in Wistar rats at baseline and after blocking with tariquidar, showing a 3.6- and 2.4-fold increase in brain uptake in the blocked rats, respectively. In addition, for both [18F]1 and [18F]2, PET scans in Mdr1a/b(−/−), Bcrp1(−/−), and WT mice were acquired, in which [18F]2 showed a more specific brain uptake in Mdr1a/b(−/−) mice and no increased signal in Bcrp1(−/−) mice. [18F]2 was selected as the best performing tracer and should be evaluated further in clinical studies.

The striking similar in vivo behavior of [F]3-d to that of (R)-[C]verapamil, and its improved metabolic stability compared with the other fluorine-18 labeled tracers synthesized, supports the potential clinical translation of [F]3-d as a PET radiopharmaceutical for P-gp evaluation.

Positron emission tomography (PET) imaging of P-glycoprotein (P-gp) in the blood-brain barrier can be important in neurological diseases where P-gp is affected, such as Alzheimer´s disease. Radiotracers used in the imaging studies are present at very small, nanomolar, concentration, whereas in vitro assays where these tracers are characterized, are usually performed at micromolar concentration, causing often discrepant in vivo and in vitro data. We had in vivo rodent PET data of [11C]verapamil, (R)-N-[18F]fluoroethylverapamil, (R)-O-[18F]fluoroethyl-norverapamil, [18F]MC225 and [18F]MC224 and we included also two new molecules [18F]MC198 and [18F]KE64 in this study. To improve the predictive value of in vitro assays, we labeled all the tracers with tritium and performed bidirectional substrate transport assay in MDCKII-MDR1 cells at three different concentrations (0.01, 1 and 50 µM) and also inhibition assay with P-gp inhibitors. As a comparison, we used non-radioactive molecules in transport assay in Caco-2 cells at a concentration of 10 µM and in calcein-AM inhibition assay in MDCKII-MDR1 cells. All the P-gp substrates were transported dose-dependently. At the highest concentration (50 µM), P-gp was saturated in a similar way as after treatment with P-gp inhibitors. Best in vivo correlation was obtained with the bidirectional transport assay at a concentration of 0.01 µM. One micromolar concentration in a transport assay or calcein-AM assay alone is not sufficient for correct in vivo prediction of substrate P-gp PET ligands.

Table of contentsOP03 Selective extraction of medically-related radionuclides from proton-irradiated thorium targetsV. Radchenko, J.W. Engle, C. Roy, J. Griswold, M.F. Nortier, E.R. Birnbaum, M. Brugh, S. Mirzadeh, K. D. John, M.E. FassbenderOP04 Comparison of [68Ga]FSC(succ-RGD)3 and [68Ga]NODAGA-RGD for PET imaging of αvβ3 integrin expressionChuangyan Zhai, Gerben M. Franssen, Milos Petrik, Peter Laverman, Clemens DecristoforoOP05 A new NPY-Y1R targeting peptide for breast cancer PET imagingAit-Mohand Samia, Dumulon-Perreault Véronique, Guérin BrigitteOP06 The influence of multivalency on CCK 2 receptor targetingD. Summer, A. Kroess, C. Rangger, H. Haas, P. Laverman, F. Gerben, E. von Guggenberg, C.DecristoforoOP07 SPECT Imaging of αvβ3 Expression by [99mTc(N)PNP43]- Bifunctional Chimeric RGD Peptide not Cross-Reacting with αvβ5Cristina Bolzati, Nicola Salvarese, Fiorenzo Refosco, Laura Meléndez-Alafort, Debora Carpanese, Antonio Rosato, Michele Saviano, Annarita Del Gatto, Daniela Comegna, Laura ZaccaroOP09 New dienophiles for the inverse-electron-demand Diels-Alder reaction and for pretargeted PET imagingEmilie Billaud, Muneer Ahamed, Frederik Cleeren, Elnaz Shahbazali, Tim Noël, Volker Hessel, Alfons Verbruggen and Guy BormansOP10 New complexing agent for Al18F-labelling of heat-sensitive biomolecules: Synthesis and preclinical evaluation of Al18F-RESCA1-HASCleeren F, Lecina J, Koole M, Verbruggen A and Bormans GOP11 A novel versatile precursor efficient for F-18 radiolabelling via click-chemistryB. Lugatoa, S. Stucchia, E.A. Turollaa, L. Giulianoa, S.Toddea, P. FerraboschibOP12 A general applicable method to quantify unidentified UV impurities in radiopharmaceuticalsR.P. Klok, M.P.J. Mooijer, N.H. Hendrikse, A.D. WindhorstOP13 Development of [18F]Fluoro-C-glycosides to radiolabel peptidesCollet C., Petry N., Chrétien F., Karcher G., Pellegrini-Moïse N., Lamandé-Langle S.OP14 A Microfluidic Approach for the 68Ga-labeling of PSMAHBED-CC and NODAGA-RGDSarah Pfaff, Cecile Philippe, Markus Mitterhauser, Marcus Hacker, Wolfgang WadsakOP16 Surprising reactivity of astatine in the nucleophilic substitution of aryliodonium salts: application to the radiolabeling of antibodiesFrançois Guérard, Yong-Sok Lee, Sébastien Gouard, Kwamena Baidoo, Cyrille Alliot, Michel Chérel, Martin W. Brechbiel, Jean-François GestinOP17 64Cu-NOTA-pertuzumab F(ab')2 fragments, a second-generation probe for PET imaging of the response of HER2-positive breast cancer to trastuzumab (Herceptin)Lam K, Chan C, Reilly RMOP18 Development of radiohalogenated analogues of a avb6-specific peptide for high LET particle emitter targeted radionuclide therapy of cancerSalomé Paillas, John Marshall, Jean-Pierre Pouget, Jane SosabowskiOP19 Ligand Specific Efficiency (LSE) as a guide in tracer optimizationEmmanuelle Briard, Yves P. Auberson, John Reilly, Mark Healy, David SykesOP23 The radiosynthesis of an 18F-labeled triglyceride, developed to visualize and quantify brown adipose tissue activityAndreas Paulus, Wouter van Marken Lichtenbelt,Felix Mottaghy,...