Synthesis and preliminary evaluation of 18F- or 11C-labeled bicyclic nucleoside analogues as potential probes for imaging varicella-zoster virus thymidine kinase gene expression using positron emission tomography

Two radiolabeled bicyclic nucleoside analogues (BCNAs) were synthesized, namely 3-(2'-deoxy-beta-d-ribofuranosyl)-6-(3-[18F]fluoroethoxyphenyl)-2,3-dihydrofuro[2,3-d]pyrimidin-2-one ([18F]-2) and 3-(2'-deoxy-beta-d-ribofuranosyl)-6-(3-[11C]methoxyphenyl)-2,3-dihydrofuro[2,3-d]pyrimidin-2-one ([11C]-3), and evaluated as PET reporter probes for varicella-zoster virus thymidine kinase (VZV-tk) gene expression imaging in brain. [18F]-2 and [11C]-3 were synthesized starting from phenol precursor 1. The phenol precursor 1 was converted to stable as well as to radiolabeled compounds 2 and 3 using (19/18)FCH(2)CH(2)Br or (12/11)CH(3)I as alkylating agent. In vitro evaluation of [18F]-2 and [11C]-3 in 293T cells showed a 4.5 and 53-fold higher uptake, respectively, into VZV-tk gene-transduced cells compared to control cells. However, biodistribution studies in mice demonstrated low uptake of these tracers in the brain. RP-HPLC analysis of plasma and urine samples of mice injected with [11C]-3 revealed that this tracer is very stable in vivo. These data warrant further evaluation of these tracers as noninvasive imaging agents for VZV infection and VZV-tk reporter gene expression in vivo.

Technetium-99m labelled integrated tropane–BAT as a potential dopamine transporter tracer

To reduce the molecular weight of 99mTc-labelled tropanes with the aim to enhance the passage over the blood-brain barrier, a so-called integrated tropane-BAT construct was developed. For this purpose a mercaptoethyl substituent was attached to the amine nitrogen atom of a nortropane precursor and the methyl carboxylate in 2beta-position was converted to a 2-mercaptoethylaminomethylene substituent. This integrated tropane-BAT construct could be labelled efficiently (85-90%) with technetium-99m. Results of LC-MS analysis of the tracer agent support the assumed structure. Biodistribution studies in normal rats (n=3) showed a slightly higher brain uptake for the new tracer agents as compared to 99mTc-TRODAT-1. These results indicate that further biological evaluation of the integrated 99mTc-tropane-BAT is warranted.

Synthesis of 18F-fluoroalkyl-beta-D-glucosides and their evaluation as tracers for sodium-dependent glucose transporters

Three omega-(18)F-fluoro-n-alkyl-beta-D-glucosides (alkyl = ethyl (5a)), n-butyl (5b), and n-octyl (5c)) were synthesized and evaluated as potential substrates for the sodium/D-glucose cotransporter SGLT1.

METHODS

The ligands were prepared by (18)F-fluoride displacement of the corresponding tetraacetyl-protected tosylate alkylglucoside precursors in CH(3)CN, followed by hydrolysis of the protective acetate esters with NaOMe/MeOH. Transport of the nonradioactive analogs 5a, 5b, and 5c by the human sodium-D-glucose cotransporter hSGLT1 was characterized in vitro in oocytes of Xenopus laevis that expressed hSGLT1. The biodistribution of the tracers was determined in mice and the presence of metabolites in the blood was investigated. Compound 5a was also evaluated in mice pretreated with phlorizin. The intrarenal distribution of 5a in mice kidney was visualized using autoradiography.

RESULTS

The radiochemical yield of 5a, 5b, and 5c was in the range of 8%-15% (end of bombardment) with a total synthesis time of 90 min. The in vitro evaluation after expression of the hSGLT1 showed that 2'-fluoroethyl-beta-D-glucoside (5a) was transported with similar Michaelis-Menten K(m) and V(max) (maximum velocity) values as compared with methyl-alpha-D-glucopyranoside (alpha MDG). The more lipophilic compounds 5b and 5c were not transported but inhibited transport of alpha MDG. In vivo tissue distribution in mice revealed that 5a, 5b, and 5c were cleared mainly by the renal pathway and that 5a showed a significantly higher accumulation in the kidneys and a slower renal excretion as compared with 5b and 5c. Compound 5a was retained mainly in the renal outer medulla containing S3 segments of proximal tubules and the accumulation could be blocked by phlorizin pretreatment. Compound 5c passed the blood-brain barrier to some extent.

CONCLUSION

The data indicate that 2'-(18)F-fluoroethyl-beta-D-glucoside (5a) is a specific tracer of Na(+)-dependent glucose transport that may be used to visualize this transport activity in the S3 segments of renal proximal tubules.

Evaluation of 18F-FA-4 and 11C-pipzA-4 as radioligands for the in vivo evaluation of the high-affinity choline uptake system

4,4'-Bis-1-hydroxy-2-(4-methylpiperidin-1-yl)ethyl-biphenyl (A-4), a tertiary amine analog of hemicholinium-3 (HC-3), is an inhibitor of the sodium-dependent high-affinity choline uptake (HACU) system. We have evaluated 4-[1-hydroxy-2-(4-(18)F-fluoromethylpiperidin-1-yl)ethyl]-4'-[1-hydroxy-2-(4-methylpiperidin-1-yl)ethyl]biphenyl ((18)F-FA-4) and 4-[1-hydroxy-2-(4-(11)C-methylpiperazin-1-yl)ethyl]-4'-[1-hydroxy-2-(4-methylpiperidin-1-yl)ethyl]biphenyl ((11)C-pipzA-4), an (18)F- and a (11)C-labeled derivative of A-4 as potential in vivo tracers for the HACU system.

METHODS

The biodistribution of both compounds was determined in mice, and the intracerebral distribution was visualized by ex vivo and in vitro autoradiography. The in vitro affinity of the compounds was determined by a displacement study with (3)H-HC-3 on mice brain slices.

RESULTS

In mice, both tracers show a high and persistent brain uptake. In vitro autoradiography shows binding to the striatum, whereas ex vivo autoradiography shows homogeneous binding throughout the brain. FA-4 and pipzA-4 inhibited the (3)H-HC-3 binding with a 50% inhibitory concentration of 57 nmol/L and 320 nmol/L, respectively.

CONCLUSION

The evaluated compounds have affinity for HACU and show high uptake in the brain. In vitro binding of the compounds to the striatum cannot be inhibited by the presence of HC-3, whereas binding of HC-3 was inhibited by the presence of both FA-4 and pipzA-4, suggesting allosteric binding.