The aim of this study was to develop a positron emission tomography (PET) tracer based on the dual P-glycoprotein (P-gp) breast cancer resistance protein (BCRP) inhibitor tariquidar (1) to study the interaction of 1 with P-gp and BCRP in the blood-brain barrier (BBB) in vivo. Odesmethyl-1 was synthesized and reacted with [ 11 C]methyl triflate to afford [ 11 C]-1. Small-animal PET imaging of [ 11 C]-1 was performed in naïve rats, before and after administration of unlabeled 1 (15 mg/kg, n=3) or the dual P-gp/BCRP inhibitor elacridar (5 mg/kg, n=2), as well as in wildtype, Mdr1a/b (−/−) , Bcrp1 (−/−) and Mdr1a/b (−/−) Bcrp1 (−/−) mice (n=3). In vitro autoradiography was performed with [ 11 C]-1 using brain sections of all 4 mouse types, with and without coincubation with unlabeled 1 or elacridar (1 μM). In PET experiments in rats, administration of unlabeled 1 or elacridar increased brain activity uptake by a factor of 3-4, whereas blood activity levels remained unchanged. In Mdr1a/b (−/−) , Bcrp1 (−/−) and Mdr1a/b (−/−) Bcrp1 (−/−) mice, brain-toblood ratios of activity at 25 min after tracer injection were 3.4, 1.8 and 14.5 times higher, respectively, as compared to wild-type animals. Autoradiography showed approximately 50% less [ 11 C]-1 binding in transporter knockout mice compared to wild-type mice and significant displacement by unlabeled elacridar in wild-type and Mdr1a/b (−/−) mouse brains. Our data suggest that [ 11 C]-1 interacts specifically with P-gp and BCRP in the BBB. However, further investigations are needed to assess if [ 11 C]-1 behaves in vivo as a transported or a non-transported inhibitor.

With the aim to develop a positron emission tomography (PET) tracer to assess the distribution of P-glycoprotein (P-gp) at the blood-brain barrier (BBB) in vivo, the potent third-generation P-gp inhibitor elacridar (1) was labeled with 11 C by reaction of O-desmethyl 1 with [ 11 C]-methyl triflate. In vitro autoradiography and small-animal PET imaging of [ 11 C]-1 was performed in rats (n=3), before and after administration of unlabeled 1, as well as in wild-type, Mdr1a/b (−/−) and Bcrp1 (−/−) mice (n=3). In PET experiments in rats, administration of unlabeled 1 increased brain activity uptake 5.4-fold, whereas blood activity levels remained unchanged. In Mdr1a/b (−/−) mice, brain activity uptake was 2.5-fold higher compared to wild-type animals, whereas in Bcrp1 (−/−) mice brain activity uptake was only 1.3-fold higher. In vitro autoradiography showed that 63% of [ 11 C]-1 binding was displaceable by an excess of unlabeled 1. As the signal obtained with [ 11 C]-1 appeared to be specific for P-gp at the BBB, its utility for the visualization of cerebral P-gp merits further investigation.

A new goup of acridone derivatives, obtained by reaction of acridone-4-carboxylic acid derivatives with aromatic amines, was tested to determine the inhibitory properties toward the NS3 helicase of hepatitis C virus (HCV). Six compounds inhibited the NS3 helicase at low concentrations (IC(50) from 1.5 to 20 microM). The acridone derivatives probably act via intercalation into double-stranded nucleic acids with a strong specificity for double-stranded RNA, although an interaction with the enzyme cannot be excluded. Testing in the subgenomic HCV replicon system revealed that compounds 10 and 13 are efficient RNA replication inhibitors, with EC(50) of 3.5 and 1 microM and therapeutic indexes of >28 and 20, respectively. Compound 16, with EC(50) < 1 microM and TI > 1000, is extremely specific and practically noncytotoxic at the concentrations tested, proving that the acridone derivatives may be regarded as potential antiviral agents. Although the mechanism of action of 16 in the replicon system remains unclear, it is the key lead compound for further development of anti-HCV drugs.

Aim of this study was to label the potent dual P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) inhibitor elacridar (1) with 18 F to provide a positron emission tomography (PET) radiotracer to visualize Pgp and BCRP. A series of new 1-and 2-halogen-and nitro-substituted derivatives of 1 (4a-e) was synthesized as precursor molecules and reference compounds for radiolabelling and shown to display comparable in vitro potency to 1 in increasing rhodamine 123 accumulation in a cell line overexpressing human Pgp (MDCKII-MDR1). 1-[ 18 F]fluoroelacridar ([ 18 F]4b) was synthesized in a decay-corrected radiochemical yield of 1.7±0.9% by a 1-step nocarrier added nucleophilic aromatic 18 F-substitution of 1-nitro precursor 4c. Small-animal PET imaging of [ 18 F]4b was performed in naïve rats, before and after administration of unlabelled 1 (5 mg/kg, n=3), as well as in wild-type and Mdr1a/b (−/−) Bcrp1 (−/−) mice (n=3). In PET experiments in rats, administration of unlabelled 1 increased brain activity uptake by a factor of 9.5 (p=0.0002, 2-tailed Student's t-test), whereas blood activity levels remained unchanged. In Mdr1a/ b (−/−) Bcrp1 (−/−) mice, the mean brain-to-blood ratio of activity at 60 min after tracer injection was 7.6 times higher as compared to wild-type animals (p=0.0002). HPLC analysis of rat brain tissue extracts collected at 40 min after injection of [ 18 F]4b revealed that 93±7% of total radioactivity in brain was in the form of unchanged [ 18 F]4b. In conclusion, the in vivo behavior of [ 18 F]4b was found to be similar to previously described

Our data suggest that [(11)C]-1 preferably interacts with P-gp rather than BCRP at the murine BBB which questions its reported in vitro BCRP selectivity. Consequently, [(11)C]-1 appears to be unsuitable as a PET tracer to map cerebral BCRP expression.

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