A low-molecular-weight (18) F-labeled tetrazine derivative was developed as a highly versatile tool for bioorthogonal PET imaging. Prosthetic groups and undesired carrying of (18) F through additional steps were evaded by direct (18) F-fluorination of an appropriate tetrazine precursor. Reaction kinetics of the cycloaddition with trans-cyclooctenes were investigated by applying quantum chemical calculations and stopped-flow measurements in human plasma; the results indicated that the labeled tetrazine is suitable as a bioorthogonal probe for the imaging of dienophile-tagged (bio)molecules. In vitro and in vivo investigations revealed high stability and PET/MRI in mice showed fast homogeneous biodistribution of the (18) F-labeled tetrazine that also passes the blood-brain barrier. An in vivo click experiment confirmed the bioorthogonal behavior of this novel tetrazine probe. Due to favorable chemical and pharmacokinetic properties this bioorthogonal agent should find application in bioimaging and biomedical research.

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A low-molecular-weight tetrazine labeled with the short-lived positron emitter carbon-11 was developed as a bioorthogonal PET probe for pretargeted imaging. A method for efficient and fast synthesis of this imaging agent is presented using radiolabeling of a readily available precursor. High reactivity with trans-cyclooctenes was observed and in vivo investigations including PET/MR scanning showed homogeneous biodistribution, good metabolic stability, and rapid excretion in naive mice. These properties are key to the success of bioorthogonal (11)C-PET imaging, which has been shown in a simple pretargeting experiment using TCO-modified mesoporous silica nanoparticles. Overall, this (11)C-labeled tetrazine represents a highly versatile and advantageous chemical tool for bioorthogonal PET imaging and enables pretargeting approaches using carbon-11 for the first time.

In the present study, the influence of stress from handling and transport on some frequently examined blood parameters of racing pigeons was evaluated. After 3 hr, there was a highly significant (P < 0.01) increase in the number as well as in the percentage of heterophils and decrease of lymphocytes. In clinical chemistries, increases of creatine kinase and glucose and a decrease of uric acid were observed. There was a mean decrease of the total white blood count of >15% that was less significant (P < 0.05). Changes in lactate dehydrogenase, basophils, and monocytes did not prove to be significant; eosinophils, aspartate aminotransferase, total protein, and the packed cell volume were not influenced by stress.

The adenosine triphosphate-binding cassette transporter P-glycoprotein (ABCB1/Abcb1a) restricts at the blood–brain barrier (BBB) brain distribution of many drugs. ABCB1 may be involved in drug–drug interactions (DDIs) at the BBB, which may lead to changes in brain distribution and central nervous system side effects of drugs. Positron emission tomography (PET) with the ABCB1 substrates (R)-[11C]verapamil and [11C]-N-desmethyl-loperamide and the ABCB1 inhibitor tariquidar has allowed direct comparison of ABCB1-mediated DDIs at the rodent and human BBB. In this work we evaluated different factors which could influence the magnitude of the interaction between tariquidar and (R)-[11C]verapamil or [11C]-N-desmethyl-loperamide at the BBB and thereby contribute to previously observed species differences between rodents and humans. We performed in vitro transport experiments with [3H]verapamil and [3H]-N-desmethyl-loperamide in ABCB1 and Abcb1a overexpressing cell lines. Moreover we conducted in vivo PET experiments and biodistribution studies with (R)-[11C]verapamil and [11C]-N-desmethyl-loperamide in wild-type mice without and with tariquidar pretreatment and in homozygous Abcb1a/1b(−/−) and heterozygous Abcb1a/1b(+/−) mice. We found no differences for in vitro transport of [3H]verapamil and [3H]-N-desmethyl-loperamide by ABCB1 and Abcb1a and its inhibition by tariquidar. [3H]-N-Desmethyl-loperamide was transported with a 5 to 9 times higher transport ratio than [3H]verapamil in ABCB1- and Abcb1a-transfected cells. In vivo, brain radioactivity concentrations were lower for [11C]-N-desmethyl-loperamide than for (R)-[11C]verapamil. Both radiotracers showed tariquidar dose dependent increases in brain distribution with tariquidar half-maximum inhibitory concentrations (IC50) of 1052 nM (95% confidence interval CI: 930–1189) for (R)-[11C]verapamil and 1329 nM (95% CI: 980–1801) for [11C]-N-desmethyl-loperamide. In homozygous Abcb1a/1b(−/−) mice brain radioactivity distribution was increased by 3.9- and 2.8-fold and in heterozygous Abcb1a/1b(+/−) mice by 1.5- and 1.1-fold, for (R)-[11C]verapamil and [11C]-N-desmethyl-loperamide, respectively, as compared with wild-type mice. For both radiotracers radiolabeled metabolites were detected in plasma and brain. When brain and plasma radioactivity concentrations were corrected for radiolabeled metabolites, brain distribution of (R)-[11C]verapamil and [11C]-N-desmethyl-loperamide was increased in tariquidar (15 mg/kg) treated animals by 14.1- and 18.3-fold, respectively, as compared with vehicle group. Isoflurane anesthesia altered [11C]-N-desmethyl-loperamide but not (R)-[11C]verapamil metabolism, and this had a direct effect on the magnitude of the increase in brain distribution following ABCB1 inhibition. Our data furthermore suggest that in the absence of ABCB1 function brain distribution of [11C]-N-desmethyl-loperamide but not (R)-[11C]verapamil may depend on cerebral blood flow. In conclusion, we have identified a number of important factors, i.e., substrate affinity to ABCB1...

Breast cancer resistance protein (BCRP) is the most abundant multidrug efflux transporter at the human blood–brain barrier (BBB), restricting brain distribution of various drugs. In this study, we developed a positron emission tomography (PET) protocol to visualize Bcrp function at the murine BBB, based on the dual P-glycoprotein (P-gp)/Bcrp substrate radiotracer [11C]tariquidar in combination with the Bcrp inhibitor Ko143. To eliminate the contribution of P-gp efflux to [11C]tariquidar brain distribution, we studied mice in which P-gp was genetically knocked out ( Mdri1a/b(−/−) mice) or chemically knocked out by pretreatment with cold tariquidar. We found that [11C]tariquidar brain uptake increased dose dependency after administration of escalating doses of Ko143, both in Mdr1a/b(−/−) mice and in tariquidar pretreated wild-type mice. After 15 mg/kg Ko143, the maximum increase in [11C]tariquidar brain uptake relative to baseline scans was 6.3-fold in Mdr1a/bf(−/−) mice with a half-maximum effect dose of 4.98 mg/kg and 3.6-fold in tariquidar (8 mg/kg) pretreated wild-type mice, suggesting that the presented protocol is sensitive to visualize a range of different functional Bcrp activities at the murine BBB. We expect that this protocol can be translated to the clinic, because tariquidar can be safely administered to humans at doses that completely inhibit cerebral P-gp.

P-glycoprotein (P-gp, ABCB1) is an efflux transporter at the blood–brain barrier (BBB), which mediates clearance of beta-amyloid (Aβ) from brain into blood. We used ( R)-[11C]verapamil PET in combination with partial P-gp inhibition with tariquidar to measure cerebral P-gp function in a beta-amyloidosis mouse model (APPtg) and in control mice at three different ages (50, 200 and 380 days). Following tariquidar pre-treatment (4 mg/kg), whole brain-to-plasma radioactivity concentration ratios ( Kp,brain) were significantly higher in APPtg than in wild-type mice aged 50 days, pointing to decreased cerebral P-gp function. Moreover, we found an age-dependent decrease in cerebral P-gp function in both wild-type and APPtg mice of up to −50%. Alterations in P-gp function were more pronounced in Aβ-rich brain regions (hippocampus, cortex) than in a control region with negligible Aβ load (cerebellum). PET results were confirmed by immunohistochemical staining of P-gp in brain microvessels. Our results confirm previous findings of reduced P-gp function in Alzheimer’s disease mouse models and show that our PET protocol possesses adequate sensitivity to measure these functional changes in vivo. Our PET protocol may find use in clinical studies to test the efficacy of drugs to induce P-gp function at the human BBB to enhance Aβ clearance.

Purpose: Multidrug resistance-associated proteins (MRPs) mediate the hepatobiliary and renal excretion of many drugs and drug conjugates. The positron emission tomography (PET) tracer 6-bromo-7-[11 C]methylpurine is rapidly converted in tissues by glutathione-S-transferases into its glutathione conjugate, and has been used to measure the activity of Abcc1 in the brain and the lungs of mice. Aim of this work was to investigate if the activity of MRPs in excretory organs can be measured with 6-bromo-7-[ 11 C]methylpurine.Procedures: We performed PET scans with 6-bromo-7-[ 11 C]methylpurine in groups of wild-type, Abcc4(−/−) and Abcc1 (−/−) mice, with and without pre-treatment with the prototypical MRP inhibitor MK571.Results: 6-Bromo-7-[ 11 C]methylpurine-derived radioactivity predominantly underwent renal excretion. In blood, MK571 treatment led to a significant increase in the AUC and a decrease in the elimination rate constant of radioactivity (k elimination,blood ). In the kidneys, there were significant decreases in the rate constant for radioactivity uptake from the blood (k uptake,kidney ), k elimination,kidney , and the rate constant for tubular secretion of radioactivity (k urine ). Experiments in Abcc4 (−/−) mice indicated that Abcc4 contributed to renal excretion of 6-bromo-7-[ 11 C]methylpurine-derived radioactivity.Conclusions: Our data suggest that 6-bromo-7-[ 11 C]methylpurine may be useful to assess the activity of MRPs in the kidneys as well as in other organs (brain, lungs), although further work is needed to identify the MRP subtypes involved in the disposition of 6-bromo-7-[ 11 C]methylpurinederived radioactivity.Electronic supplementary material The online version of this article (https://