Synthesis and evaluation of (18)F-labeled choline analogs as oncologic PET tracers.

UNLABELLED: Elevated levels of choline (trimethyl-2-hydroxyethylammonium) and choline kinase (CK) activity in neoplasms have motivated the development of positron-labeled choline analogs for noninvasive detection of cancer using PET. The aim of this study was to further evaluate [(18)F]fluorocholine (fluoromethyl-dimethyl-2-hydroxyethylammonium [FCH]) as an oncologic probe in comparison with several other closely related molecules. METHODS: FCH, [(18)F]fluoromethyl-methylethyl-2-hydroxyethylammonium (FMEC), [(18)F]fluoroethyl-dimethyl-2-hydroxyethylammonium (FEC), and [(18)F]fluoropropyl-dimethyl-2-hydroxyethylammonium (FPC) were synthesized through [(18)F]fluoroalkylation reactions. In vitro phosphorylation rates of the (18)F-labeled choline analogs and [methyl-(14)C]choline (CH) were studied using yeast CK. Several choline radiotracers were also evaluated in cultured PC-3 human prostate cancer cells. Data on chemical stability, radiation dosimetry, and toxicity of FCH were obtained. PET studies with FCH were performed on a patient with prostate cancer and a patient with a brain tumor. RESULTS: FCH and FMEC revealed in vitro phosphorylation by CK that was similar to that of choline, whereas rates of phosphorylation of FEC and FPC were 30% (P < 0.01) and 60% (P < 0.01) lower, respectively. Accumulations of FCH, CH, and FPC in cultured PC-3 cancer cells were comparable, whereas uptake of FEC was approximately one fifth that of FCH. Dosimetry estimates using FCH biodistribution data in mice indicated that the kidneys are radiation-dose-critical organs for FCH. PET images of a patient with recurrent prostate cancer showed uptake of FCH in the prostatic bed and in metastases to lymph nodes. FCH PET showed uptake in malignancies in a patient with metastatic breast cancer. PET revealed FCH uptake in biopsy-proven recurrent brain tumor with little confounding uptake by normal brain tissues. CONCLUSION: The fluoromethyl choline analog FCH may serve as a probe of choline uptake and phosphorylation in cancer cells, whereas fluoroethyl (FEC) and fluoropropyl (FPC) analogs appear to have relatively poorer biologic compatibility. Preliminary PET studies on patients with prostate cancer and with breast cancer and brain tumor support further studies to evaluate the usefulness of FCH as an oncologic probe.

Synthesis and evaluation of 18F-labeled choline as an oncologic tracer for positron emission tomography: initial findings in prostate cancer.

The up-regulation of rates of choline uptake and phosphorylation in certain malignancies has motivated the development of positron-labeled choline analogues for noninvasive detection of cancer using positron emission tomography (PET). The choline analogue, no-carrier-added [18F]fluoromethyl-dimethyl-2-hydroxyethyl-ammonium (FCH), was synthesized through the intermediate [18F]fluorobromomethane. FCH was evaluated in relationship to 2-[18F]fluoro-2-deoxyglucose (FDG) as an oncological probe in cultured PC-3 human prostate cancer cells, a murine PC-3 human prostate cancer xenograft model, and in PET imaging studies of patients with prostate cancer. FCH was synthesized in 20-40% radiochemical yield and >98% radiochemical purity. Accumulation of FCH and FDG were comparable in cultured prostate cancer cells, whereas only FCH was inhibited (90%) by hemicholinium-3, a specific inhibitor of choline transport and phosphorylation. FCH showed similar biodistribution to [14C]choline in the tumor-bearing mouse, with prominent renal and hepatic uptake. Tumor uptake of FCH was similar to choline and FDG in the mouse model, although tumor:blood ratios were moderately higher for FCH. Initial PET imaging studies in prostate cancer patients showed high uptake of FCH in advanced prostate carcinoma and detection of osseous and soft tissue metastases. FCH uptake by tumors was markedly reduced in patients rescanned during androgen deprivation therapy. It is concluded that FCH closely mimics choline uptake by normal tissues and prostate cancer neoplasms. FCH is potentially useful as a PET tracer for detection and localization of prostate cancer and monitoring effects of therapy.

Total syntheses of (-)-haemanthidine, (+)-pretazettine, and (+)-tazettine.

[structures: see text] The total syntheses of the amaryllidaceae alkaloids haemanthidine, pretazettine, and tazettine as optically pure enantiomers are reported. Using D-mannose as the starting material, the critical relative stereochemical relationships are established with an intramolecular nitrone-alkene cycloaddition reaction. The synthetic route leads successively to (-)-haemanthidine and then to (+)-pretazettine and (+)-tazettine, taking advantage of the well-established complex relationships among these three alkaloids.

Characterization of recombinant human interleukin 4 receptor from CHO cells: role of N-linked oligosaccharides.

Interleukin 4 (IL-4) mediates its biological activities through interaction with its receptor on the cell surface. A recombinant extracellular domain of the alpha subunit of human interleukin 4 receptor was expressed in CHO cells and purified to homogeneity by a combination of ion exchange and immunoaffinity chromatography. Analysis of the purified protein by MALDI MS provided an average mass of 38,241 Da while microsequencing identified the site of the signal sequence processing to be Ser23-Gly24. The receptor was highly glycosylated, containing N-linked complex oligosaccharides with bi-, tri-, and tetraantennary structures. Five of the six potential glycosylation sites could be assigned to Asn residues 53, 98, 128, 134 and 176. N-deglycosylation increased aggregation and reduced solubility of the receptor but did not affect its IL-4 binding activity. These observations provide preliminary insights into the role of N-linked oligosaccharides in IL-4 receptor biosynthesis and function at the cell surface.

Structural features of 5,10-dideaza-5,6,7,8-tetrahydrofolate that determine inhibition of mammalian glycinamide ribonucleotide formyltransferase.

We have investigated the structural features of 5,10-dideaza-5,6,7,8-tetrahydrofolate (DDATHF) that determine the activity of this compound as an inhibitor of glycinamide ribonucleotide formyltransferase (GARFT) purified from mouse L1210 cells. 5-Deazatetrahydrofolate was as good an inhibitor of GARFT as DDATHF, indicating that isosteric replacement of nitrogen by carbon at the 5-position of tetrahydrofolate is sufficient for inhibition of GARFT. 5,10-Dideazafolic acid, 5,8,10-trideazatetrahydrofolate, and 2-desamino-5,10-dideazatetrahydrofolate were poor inhibitors of GARFT, indicating that a reduced pyridopyrimidine ring, N-8, and the 2-amino group of DDATHF, respectively, play an important role in the binding of tetrahydrofolate analogues to this enzyme. DDATHF analogues in which the phenyl ring was replaced either by a cyclohexyl ring or by methylene groups retained activity as inhibitors. 5,10-Dideazatetrahydrohomofolate was about 6 times more potent as an inhibitor of GARFT than DDATHF, but 5,10-dideazatetrahydronorfolate had about one-fifth of the activity of DDATHF. An analogue of DDATHF in which the glutamic acid side chain was replaced by aspartic acid (which was not a substrate for polyglutamation and was only weakly cytotoxic) was equiactive with DDATHF as an inhibitor of purified GARFT. Surprisingly, 5,10-dideazatetrahydropteroic acid was about as active as DDATHF as an inhibitor of GARFT, an indication that the glutamic acid in the side chain of DDATHF does not play a role in this ligand-enzyme interaction. The polyglutamate derivatives of DDATHF bound up to 100 times tighter to GARFT than DDATHF itself; longer chain polyglutamates conformed to Goldstein's zone B behavior under experimental conditions and were projected to be in zone C, i.e., stoichiometric inhibition, in vivo. We conclude that the presence of carbon at the 5-position of tetrahydrofolate analogues is sufficient for inhibition of GARFT, that N-8 and the 2-amino group are involved in binding of DDATHF to GARFT, probably through hydrogen bonds, and that the structures of the phenyl ring and amino acid side chain of DDATHF analogues are not primary determinants of GARFT inhibition by monoglutamate forms of these compounds. We also conclude that polyglutamation plays a major role in the potent cytotoxicity of DDATHF.

The 6S- and 6R-diastereomers of 5, 10-dideaza-5, 6, 7, 8-tetrahydrofolate are equiactive inhibitors of de novo purine synthesis.

The diasteromers of 5,10-dideaza-5,6,7,8-tetrahydrofolate (DDATHF) differing in chirality about carbon 6 were resolved and studied as inhibitors of folate-dependent processes in mouse leukemia cells. Both diastereomers of DDATHF were found to be potent inhibitors of leukemia cell growth due to effects on de novo purine synthesis. Cell growth inhibition by these compounds was prevented by 5-formyltetrahydrofolate in a dose-dependent manner. This indicated that the effects of the DDATHF diastereomers were due to inhibition of folate-dependent processes. Metabolite reversal experiments indicated that 5'-phosphoribosylglycinamide formyltransferase was the major site of action of these compounds in mouse cells. Another site in de novo purine synthesis was affected at higher concentrations of diastereomer B in L1210 cells. Low concentrations of both diastereomers were found to inhibit pure L1210 5'-phosphoribosylglycinamide formyltransferase competitively with the folate substrate. The two diastereomers were also efficient substrates for mouse liver folylpolyglutamate synthetase. We conclude that the 6R- and 6S-diastereomers of DDATHF are remarkably similar and equiactive antimetabolites inhibitory to de novo purine synthesis and that the biochemical processes involved in their cytotoxicity display little stereochemical specificity.