N-(4-[18F]fluorobenzyl)cholylglycine, a novel tracer for PET of enterohepatic circulation of bile acids: Radiosynthesis and proof-of-concept studies in rats.

INTRODUCTION: Enterohepatic circulation (EHC) of conjugated bile acids is an important physiological process crucial for regulation of intracellular concentrations of bile acids and their function as detergents and signal carriers. Only few bile acid-derived imaging agents have been synthesized and hitherto none have been evaluated for studies of EHC. We hypothesized that N-(4-[18F]fluorobenzyl)cholylglycine ([18F]FBCGly), a novel fluorine-18 labeled derivative of endogenous cholylglycine, would be a suitable tracer for PET of the EHC of conjugated bile acids, and we report here a radiosynthesis of [18F]FBCGly and a proof-of-concept study by PET/MR in rats. METHODS: A radiosynthesis of [18F]FBCGly was developed based on reductive alkylation of glycine with 4-[18F]fluorobenzaldehyde followed by coupling to cholic acid. [18F]FBCGly was investigated in vivo by dynamic PET/MR in anesthetized rats; untreated or treated with cholyltaurine or rifampicin. Possible in vivo metabolites of [18F]FBCGly were investigated by analysis of blood and bile samples, and the stability of [18F]FBCGly towards enzymatic de-conjugation by Cholylglycine Hydrolase was tested in vitro. RESULTS: [18F]FBCGly was produced with a radiochemical purity of 96% ±â€¯1% and a non-decay corrected radiochemical yield of 1.0% ±â€¯0.3% (mean ±â€¯SD; n = 12). PET/MR studies showed that i.v.-administrated [18F]FBCGly underwent EHC within 40-60 min with a rapid transhepatic transport from blood to bile. In untreated rats, the radioactivity concentration of [18F]FBCGly was approximately 15 times higher in bile than in liver tissue. Cholyltaurine and rifampicin inhibited the biliary secretion of [18F]FBCGly. No fluorine-18 metabolites of [18F]FBCGly were observed. CONCLUSION: We have developed a radiosynthesis of a novel fluorine-18 labeled bile acid derivative, [18F]FBCGly, and shown by PET/MR that [18F]FBCGly undergoes continuous EHC in rats without metabolizing. This novel tracer may prove useful in PET studies on the effect of drugs or diseases on the EHC of conjugated bile acids.

An efficient laboratory-scale preparative method for [1-(13)C]glycocholic acid.

A breath test using [1-(13)C]glycocholic acid as a substrate is a potential diagnostic method for small intestinal bacterial overgrowth syndrome. [1-(13)C]Glycocholic acid has been thus synthesized in an excellent yield from ethyl [1-(13)C]glycinate hydrochloride in a one-pot reaction. This method is suitable for the preparation of the labeled compound on a laboratory scale, which helps to perform extensive clinical studies of the breath test.

[Synthesis and properties of fluorescent labeled detergents analogues of glycocholic acid]. / Sintez i svoistva fluorestsentnomechenykh detergentov, analogov glikokholevoi kisloty.

For studying membrane processes with participation of detergents, fluorescent analogues of glycocholic acid containing p-hydroxybenzyl, 7-nitrobenz-2-oxa-1,3-diazol-4-yl, or fluorescein-5-thiocarbamoyl fluorophore in the glycyl moiety attached to glycocholic acid were synthesized. The fluorophores are in the probes near their carboxyl groups and, in membrane systems, should therefore be situated on the interface and be sensitive to phase transitions. The critical micelle concentrations were determined for the analogues and found to be close to those of cholate and glycocholate in the case of the first two compounds. We presume that the behavior of the probes in membrane systems will mimic the behavior of the bile acid salts.

Synthesis and characterization of sodium cis-dichlorochenodeoxycholylglycinato(O,N) platinum(II)--cytostatic activity.

With a view to using bile acids as shuttles for delivering platinum-related cytostatic drugs to liver tumors, a chenodeoxycholylglycinato(CDCG)-derivative of platinum(II) has been synthesized. The complex--named Bamet-M2--was chemically characterized by elemental analysis, FT-IR, NMR, FAB-MS, and UV spectroscopy. The results indicate the following composition: C26H42N2O5Cl2NaPt(II), the metal Pt(II) being bound to two Cl- and one bidentate CDCG moiety, i.e., Na[Pt CDCG(N,O) Cl2]. The compound is highly soluble (up to 20 mM) in water and (up to 35 mM) in ethanol, methanol and DMSO. Hydrolysis was investigated because this is assumed to be an important step in intracellular activation and interaction with DNA of this type of compounds. The reaction kinetics of this complex in aqueous solution show unusual behaviour; the substitution process with the displacement of two Cl- was almost instantaneous, and the resulting species were found to be very stable. Kinetic studies carried out in the presence of different NaCl concentrations (up to 500 mM) revealed similar fast and nonreversible aquations of Bamet-M2. This contrasts with the slow aquation of cisplatin in extracellular-line solutions (i.e., at high NaCl concentrations) as compared with fast hydrolysis in cells. This difference may partly account for the low cytostatic activity observed here for Bamet-M2 against several tumor cell-lines.

An improved synthesis of taurine- and glycine-conjugated bile acids.

A simple and efficient method for the synthesis of taurine- and glycine-conjugated bile acids is described. The condensation reaction was achieved by the simple mixing of unconjugated bile acid (1.0 eq.), taurine (2.0 eq.) (or glycinate ester), diethyl phosphorocyanidate (1.2 eq.) in the presence of triethylamine at room temperature for 30-60 min. Sample clean-up was effected by the use of a prepacked Sep-Pak C18 cartridge for reversed-phase solid extraction or by direct recrystallization, yielding the desired taurine and glycine conjugates in 89-93 and 92-96% isolated yields, respectively.

Synthesis and characterization of the new cytostatic complex cis-diammineplatinum(II)-chlorocholylglycinate.

Owing to the high efficiency of hepatocytes to take up bile acids, these endogenous compounds or their analogues can be considered as potential shuttles for delivering drugs to the liver. With the aim of using this strategy to target platinum(II)-related cytostatic drugs toward the hepatobiliary system, a cholylglycinate (CG) derivative of cis-diammineplatinum(II) has been synthesized by treatment of cis-diammineplatinum(II) dichloride with sodium cholylglycinate. The complex, named Bamet-R2, was characterized by spectroscopy and elemental analysis. Results obtained in these studies together with conductivity measurements, which pointed to nonelectrolyte behavior, allowed the structure of the complex to be identified as C26H48N3O6ClPt. The compound was found to be soluble (up to 3 mM) in water and was highly soluble (more than 10 mM) in ethanol, methanol, and dimethyl sulfoxide. Its stability in solution was monitored by HPLC analysis. In deionized water, the compound remains > 90% pure in solution for up to 7 days and > 80% for up to 28 days. However, in 150 mM NaCl it remains as > 90% pure compound in solution for only 1 day. By contrast with the parent compound CG, Bamet-R2 was found to significantly inhibit the growth of rat hepatocytes in primary culture and L1210 murine leukemia cells, although in a less marked way than that observed for cisplatin. The cytostatic effect of Bamet-R2 was particularly strong against human colon adenocarcinoma LS174T cells. The results point to the potential usefulness of Bamet-R2 in the antitumoral therapy of enterohepatic-derived neoplasias.

Synthesis and characterization of a new bile acid and platinum(II) complex with cytostatic activity.

With a view to using bile acids as shuttles for delivering platinum-related cytostatic drugs to the liver, a cholylglycine(CG)-derivative of platinum(II) has been synthesized. The complex, named Bamet-H2, was characterized by elemental analysis, FT-IR, NMR, FAB-MS, and UV spectroscopy. The results indicate the following composition: C52H84N2O12ClNa Pt(II). Conductivity data suggest that the complex behaves as a sodium salt (1:1) of a complex of Pt(II) bound to one cl-, one bidentate CG moiety, and another monodentate CG moiety, i.e., Na[Pt(CG-O,N)(CG-O)Cl]. The compound is highly soluble (up to 10 mM) in water, ethanol, methanol, DMF, and DMSO. Bamet-H2 was stable in solution (either water or 150 mM NaCl solution), as measured by HPLC, up to 24 h. At this time, more than 90% of the platinum present in water or saline solutions was found to be Bamet-H2. Cytostatic activity against L1210 murine leukemia cells was found. This characteristic was stronger against rat hepatocytes in primary culture. Isolated in situ rat livers were perfused for 40 min with a recirculating medium containing 1 microM Bamet-H2, CG, or cisplatin. Uptake and excretion into bile were much greater for Bamet-H2 than for cisplatin, but less than for CG. Liver content was higher for Bamet-H2 than for cisplatin or CG. The results point to the potential usefulness of Bamet-H2 in the antitumoral therapy of neoplasias derived from liver parenchymal cells.

The synthesis of glycine conjugated 3-oxo-bile acids.

The glycine conjugates of the 3-oxo-derivatives of lithocholic, deoxycholic, chenodeoxycholic and cholic acids have been synthesized from their respective parent bile acid using established procedures. The reaction sequence involved oxidation using either chromic acid or silver carbonate--Celite followed by a conjugation step employing the peptide coupling reagent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline. The final products were obtained in both high yield and purity for use as reference compounds in the analysis of conjugated bile acids in bile and faecal samples by high performance liquid chromatography. Analysis of the prepared 3-oxo-glycobile acids by fast atom bombardment negative ion mass spectrometry, infrared spectroscopy and nuclear magnetic resonance proton spectroscopy is also discussed along with the thin-layer chromatographic properties.

Hepatic handling of a gamma-emitting bile salt derivative, 123I-cholylglycylhistamine, in the dog.

The hepatic handling of the bile salt derivative 123I-cholylglycylhistamine has been compared with that of the physiologic bile salt taurocholate in boxer dogs equipped with a Thomas duodenal cannula. After intravenous injection of trace amounts, 123I-cholylglycylhistamine and 14C-taurocholate disappeared from plasma with nearly identical disappearance rate constants. Excellent scintiscans of the liver were obtained with the Anger camera after injection of 3 mCi of 123I-cholylglycylhistamine. Monitoring of radioactivity over a hepatic region of interest revealed rapid uptake of 123I-cholylglycylhistamine by the liver, which reached a maximum 5.7 +/- 0.4 min after injection. The biliary excretion rates of the compounds closely paralleled each other, reaching their maximum within 15 min after injection. Cumulative biliary excretion within 45 min was 58.5 +/- 2.9% and 61.4 +/- 5.0% of the dose for 123I-cholylglycylhistamine and 14C-taurocholate, respectively. Modification of the side chain and gamma-labelling of bile salts may provide scintigraphic agents for the study of the biliary system, which in the behaviour closely resemble the physiologic parent compounds.