Biomedical subjects

N Satyamurthy

Publications and source records attributed to N Satyamurthy.

At least 37 records · Page 2Linked to original sources

PubMed1999

Imaging adenoviral-directed reporter gene expression in living animals with positron emission tomography.

We are developing quantitative assays to repeatedly and noninvasively image expression of reporter genes in living animals, using positron emission tomography (PET). We synthesized positron-emitting 8-[18F]fluoroganciclovir (FGCV) and demonstrated that this compound is a substrate for the herpes simplex virus 1 thymidine kinase enzyme (HSV1-TK). Using positron-emitting FGCV as a PET reporter probe, we imaged adenovirus-directed hepatic expression of the HSV1-tk reporter gene in living mice. There is a significant positive correlation between the percent injected dose of FGCV retained per gram of liver and the levels of hepatic HSV1-tk reporter gene expression (r2 > 0.80). Over a similar range of HSV1-tk expression in vivo, the percent injected dose retained per gram of liver was 0-23% for ganciclovir and 0-3% for FGCV. Repeated, noninvasive, and quantitative imaging of PET reporter gene expression should be a valuable tool for studies of human gene therapy, of organ/cell transplantation, and of both environmental and behavioral modulation of gene expression in transgenic mice.

Animals↗

PubMed1999

Synthesis of stereo (R and S) and geometric (E and Z) isomers of [18F]fluoro-beta-fluoromethylene-m-tyrosine derivatives: in vivo probes of central dopaminergic function.

Fluorination of pure R and S enantiomers of (E)-beta-fluoromethylene-m-tyrosine [(E)-FMMT] and its racemic geometric isomer, (Z)-beta-fluoromethylene-m-tyrosine [(Z)-FMMT] with [18F]acetyl hypofluorite ([18F]AcOF) gave a mixture of aromatic ring fluorinated products and a pair of diastereomeric products of addition across the exocyclic double bond. Semipreparative high performance liquid chromatography (HPLC) enabled a complete separation and isolation of these products, namely, 6-[18F]fluoro, 2-[18F]fluoro, and 2,6-[18F]difluoro (E)-FMMT and (Z)-FMMT derivatives. No attempt was made to isolate the individual components of the addition product. Pure racemic 4-[18F]fluoro-(E)-beta-fluoromethylene-m-tyrosine was also synthesized from a substituted (E)-FMMT precursor involving a fluorodestannylation reaction with [18F]F2. The availability of stereo (R and S) isomers of 6-[18F]fluoro and 2-[18F]fluoro (E)-FMMT and those of the racemic (Z)-FMMT along with 4-[18F]fluoro-(E)-beta-fluoromethylene-m-tyrosine would now enable a systematic investigation of the central monoamine oxidase/aromatic amino acid decarboxylase enzyme system with positron emission tomography.

Brain↗

PubMed1999

[18F]Fluoro-beta-fluoromethylene-m-tyrosine derivatives show stereo, geometrical, and regio specificities as in vivo central dopaminergic probes in monkeys.

Stereo (D and L), geometrical (E and Z), and regiospecific (2-, 4-, and 6-[18F]fluoro) analogs of beta-fluoromethylene-m-tyrosine (FMMT) have been investigated in adult vervet monkeys (Cercopithecus aethiops sabaeus, n = 12) in vivo with positron emission tomography (PET). Brain transport through the blood-brain barrier and central aromatic amino acid decarboxylase (AAAD)-mediated decarboxylation rates were established. Results show strict structural dependency of the kinetic behavior of radiofluorinated FMMT analogs, with the E-isomer exhibiting a higher specificity over the (Z) geometrical counterpart for central dopaminergic structures. The 6-[18F]fluoro substituted L-(E)-FMMT was also favored over the 2- and 4-[18F]fluorosubstituted isomers in terms of their ability to localize in the same brain areas. The role of PET in drug development is also exemplified in this work.

Animals↗

PubMed1999

Synthesis of 3beta-(4-[18F]fluoromethylphenyl)- and 3beta-(2-[18F] fluoromethylphenyl)tropane-2beta-carboxylic acid methyl esters: new ligands for mapping brain dopamine transporter with positron emission tomography.

The synthesis of two new dopamine transporter ligands, 3beta-(4-fluoromethylphenyl)tropane-2beta-carboxylic acid methyl ester and 3beta-(2-fluoromethylphenyl)tropane-2beta-carboxylic acid methyl ester, and their spectral characterization are described. The precursors for these ligands were prepared by TiCl4 catalyzed chloromethylation of 3beta-phenyltropane-2beta-carboxylic acid methyl ester followed by separation of the isomeric product mixture of 2- and 4-chloromethylphenyltropane derivatives. Reaction of the chloromethyl analogs with no-carrier-added [18F]fluoride ion followed by high performance liquid chromatography purification provided the corresponding [18F]fluoromethyltropanes, in good radiochemical yields, useful for imaging the brain dopamine transporter system in vivo with positron emission tomography.

Brain↗

PubMed1999

2Beta-carbomethoxy-3beta-(4- and 2-[18F]fluoromethylphenyl)tropanes: specific probes for in vivo quantification of central dopamine transporter sites.

Dopamine reuptake transporter binding kinetics of 2beta-carbomethoxy-3beta-(4-[18F]fluoromethylphenyl)tropane (p-FWIN) and 2beta-carbomethoxy-3beta-(2-[18F]fluoromethylphenyl)tropane (o-FWIN) were determined in vervet monkeys using positron emission tomography (PET). Ligand localization was rapid and specific to the striatum with kinetic estimates comparable with those of 11C-labeled WIN 35,428 (CWIN). Binding was more specific with p-FWIN than with CWIN or o-FWIN. The relatively longer half-life of the 18F radiolabel enabled longer acquisition times with p-FWIN, resulting in less variability in the kinetic estimates.

Animals↗

PubMed1999

Electronic Generators for the Production of Positron-Emitter Labeled Radiopharmaceuticals. Where Would PET Be Without Them?

The emergence of clinical positron emission tomography (PET) has enabled a routine and noninvasive assessment of cancer, neurological disorders, and coronary artery disease in humans. Vital to a sustained growth and widespread utilization of this novel methodology, in a clinical environment, is the ready and convenient availability of positron-emitter labeled radiopharmaceuticals. This requirement is aptly met by integrated radiopharmaceutical production systems or electronic radiopharmaceutical generators that comprise of: (1) a low energy, self-shielded negative ion cyclotron; (2) small volume targets for the production of positron emitting precursors; and (3) unit operations based automated synthesizers all under the full control of a personal computer (PC) and entirely operated by a technician. Efforts by both academia and industry during the past 20 years have now led to the inception of such integrated systems that are efficient and highly suitable for the production of multiple doses of numerous radiotracers in clinical settings. The theme of this review is the evolution, over the years, of integrated automated synthesis units from the standpoint of the cyclotrons, targetry, and automated synthesizers. PET radiopharmacies are now making a reality the availability of positron-emitter labeled radiopharmaceuticals for clinical use and are positioning themselves to extend the availability of new PET probes to research environments stimulated by the micro-PET technology. Stand alone automatic synthesis modules also offer a unique avenue to PET radiopharmaceuticals for research. The new Food and Drug Administration (FDA) regulatory environment mandated by the Food and Drug Administration Modernization Act (FDAMA) in 1997, combined with these developments will permit the accessibility of PET radiopharmaceuticals at low cost for a variety of clinical and research applications.

Journal Article↗

PubMed1999

Repetitive, non-invasive imaging of the dopamine D2 receptor as a reporter gene in living animals.

Reporter genes (e.g. beta-galactosidase, chloramphenicol-acetyltransferase, green fluorescent protein, luciferase) play critical roles in investigating mechanisms of gene expression in transgenic animals and in developing gene delivery systems for gene therapy. However, measuring expression of these reporter genes requires biopsy or death. We now report a procedure to image reporter gene expression repetitively and non-invasively in living animals with positron emission tomography (PET), using the dopamine type 2 receptor (D2R) as a reporter gene and 3-(2'-[18F]fluoroethyl)spiperone (FESP) as a reporter probe. We use a viral delivery system to demonstrate the ability of this PET reporter gene/PET reporter probe system to image reporter gene expression following somatic gene transfer. In mice injected intravenously with replication-deficient adenovirus carrying a D2R reporter gene, PET in vivo measures of hepatic [18F] retention are proportional to in vitro measures of hepatic FESP retention, D2R ligand binding and D2R mRNA. We use tumor-forming cells carrying a stably transfected D2R gene to demonstrate imaging of this PET reporter gene/PET reporter probe system in 'tissues'. Tumors expressing the transfected D2R reporter gene retain substantially more FESP than control tumors. The D2R/FESP reporter gene/reporter probe system should be a valuable technique to monitor, in vivo, expression from both gene therapy vectors and transgenes.

Adenoviridae↗

PubMed1998

Distribution volume of radiolabeled large neutral amino acids in brain tissue.

Variations in the cerebellum to plasma ratio at late times in 6-[18F]fluoro-L-DOPA studies are shown to be consistent with competitive binding of large neutral amino acids for a common transporter in the blood-brain barrier and the stability of brain tissue large neutral amino acid level in the presence of plasma level changes. The distribution volume of an inert large neutral amino acid can be estimated from plasma and tissue large neutral amino acid levels and apparent half-saturation concentrations (Km) of the transporter in the blood-brain barrier. Stability of brain large neutral amino acid levels is supported by literature findings and can be explained by high saturation of the large neutral amino acid transporter at physiologic conditions.

Amino Acids↗

PubMed1998

Imaging of adenoviral-directed herpes simplex virus type 1 thymidine kinase reporter gene expression in mice with radiolabeled ganciclovir.

UNLABELLED: We are developing procedures to repeatedly and noninvasively image the expression of transplanted reporter genes in living animals and in patients, using PET. We have investigated the use of the Herpes Simplex Virus type 1 thymidine kinase gene (HSV1-tk) as a reporter gene and [8-14C]-ganciclovir as a reporter probe. HSV1-tk, when expressed, leads to phosphorylation of [8-14C]-ganciclovir. As a result, specific accumulation of phosphorylated [8-14C]-ganciclovir should occur almost exclusively in tissues expressing the HSV1-tk gene. METHODS: An adenoviral vector was constructed carrying the HSV1-tk gene along with a control vector. C6 rat glioma cells were infected with either viral vector and uptake of [8-3H]-ganciclovir was determined. In addition, 12 mice were injected with varying levels of either viral vector. Adenovirus administration in mice leads primarily to liver infection. Forty-eight hours later the mice were injected with [8-14C]-ganciclovir, and 1 hr later the mice were sacrificed and biodistribution studies performed. Digital whole-body autoradiography also was performed on separate animals. HSV1-tk expression was assayed, using both normalized HSV1-tk mRNA levels and relative HSV1-TK enzyme levels, in both the cell culture and murine studies. RESULTS: Cell culture, murine tissue biodistribution and murine in vivo digital whole-body autoradiography all demonstrate the feasibility of HSV1-tk as a reporter gene and [8-14C]-ganciclovir as an imaging reporter probe. A good correlation (r2 = 0.86) between the [8-14C]-ganciclovir percent injected dose per gram tissue from HSV1-tk positive tissues and HSV1-TK enzyme levels in vivo was found. An initial study in mice with [8-18F]-fluoroganciclovir and microPET imaging supports further investigation of [8-18F]-fluoroganciclovir as a PET reporter probe for imaging HSV1-tk gene expression. CONCLUSION: These results demonstrate the feasibility of using [8-14C]-ganciclovir as a reporter probe for the HSV1-tk reporter gene, using an in vivo adenoviral mediated gene delivery system in a murine model. The results form the foundation for further investigation of [8-18F]-fluoroganciclovir for noninvasive and repeated imaging of gene expression with PET.

Adenoviridae↗

PubMed1996

Production of [18F]F2 using the 16O(3He,p)18F reaction.

For the first time, the 16O(3He,p)18F reaction has been used for the production of 18F electrophilic reagents. The predominant reactive chemical specie recovered in the single-step postirradiation gases was [18F]F2. Decay curve and Ge(Li) analyses established the main radionuclidic impurities in this nuclear reaction as 11C and 15O. Recovery of the 18F activity was exceptionally high, approaching 90% using an electroformed nickel target body. Consistent 18F recoveries for this nuclear reaction (approximately 70%) were realized with a novel aluminum target body. The utility of the [18F]F2 recovered from this oxygen gas target system in radiopharmaceutical preparations is demonstrated by the successful synthesis of 6-[18F]fluoro-L-dopa (11.0 mCi at EOB) via a fluorodemercuration method.

Fluorine Radioisotopes↗

PubMed1996

Proton irradiation of [18O]O2: production of [18F]F2 and [18F]F2 + [18F] OF2.

The production of 18F electrophilic reagents via the 18O(p,n)18F reaction has been investigated in small-volume target bodies made of aluminum, copper, gold-plated copper and nickel, having straight or conical bore shapes. Three irradiation protocols-single-step, two-step and modified two-step-were used for the recovery of the 18F activity. The single-step irradiation protocol was tested in all the target bodies. Based on the single-step performance, aluminum targets were utilized extensively in the investigation of the two-step and modified two-step irradiation protocols. With an 11-MeV cyclotron and using the two-step irradiation protocol, > 1Ci [18F]F2 was recovered reproducibly from an aluminum target body. Probable radical mechanisms for the formation of OF2 and FONO2 (fluorine nitrate) in the single-step and modified two-step targets are proposed based on the amount of ozone generated and the nitrogen impurity present in the target gases, respectively.

Aluminum↗

PubMed1996

Metals suitable for fluorine gas target bodies: first use of aluminum for the production of [18F]F2.

A comprehensive evaluation of different metals (aluminum, silver, copper, nickel, and gold-plated copper) was undertaken for the fabrication of target bodies with straight and conical bore shapes for the production of [18F]F2 via the 20Ne(d,alpha)18F nuclear reaction. Of these metals, aluminum, silver and copper have never been used for the production of [18F]F2. All these target bodies were easily passivated using a mild beam-induced plasma technique in the presence of 1% F2 in neon or argon. The recovery of 18F activity was higher with electroformed nickel and silver bodies, probably due to favorable thermal conductivities. Aluminum proved to be a useful material for fluorine gas targets. The consistent recovery of 18F activities, ease and low cost of manufacturing and low nuclear activation properties all make aluminum an ideal choice for fluorine gas targetry. To our knowledge, this investigation is the first to highlight the use of aluminum as a target body material for the routine production of [18F]F2. A reasonable mechanism based on the Langmuir-Rideal surface atom recombination is also proposed for the behavior of [18F]F2 recovery from a nickel target body.

Aluminum↗

PubMed1996

Identification and quantitation of gaseous compounds of fluorine generated in [18F]F2 target systems.

The first direct evidence for the chemical identity of the electrophilic fluorinating agents generated in 20Ne(d, alpha)18F (single-step), 18O(p,n)18F (single-step and two-step) and 16O(3He2+, p)18F (single-step) gas target systems, utilizing aluminum, silver, copper, nickel and gold plated copper target bodies, has been established with multinuclear NMR and mass spectral techniques. The major components of the reactive fraction from these targets were also quantitated using 19F NMR. Fluorine-19 NMR data of the reactive fraction of all proton and 3He2+ irradiated oxygen gas target systems showed the presence of oxygen difluoride in various proportions. Samples from the single-step method contained up to 20% OF2 while those from the two-step process had 0-5%. Fluorine nitrate (FONO2) was observed only as a minor component (0-3%) in the reactive fraction. The presence of OF2 and FONO2 was further confirmed by 17O and 15N NMR, respectively, using [17O]O2 and [15N]N2 spiked oxygen gas targets. The NMR results were supported by mass spectral data collected with a residual gas analyzer (RGA). Both 19F NMR and mass spectroscopy showed CF4 as the only major inert component in the single-step oxygen target products. As expected, the 19F NMR and mass spectral data showed that the reactive fraction of the neon gas target constituted only F2 and the inert fraction comprised of CF4 and NF3.

Fluorine Compounds↗

PubMed1996

Chemical reactivity of the 18F electrophilic reagents from the 18O(p,n) 18F gas target systems.

A comprehensive evaluation of the reactivity of 18F electrophilic fluorinating agents derived from the 18O(p,n)18F reaction conducted in target bodies made of aluminum, silver, copper, nickel, and gold-plated copper is reported. Two representative electrophilic reactions, namely addition across a double bond and substitution on an aromatic ring (fluorodemercuration and fluorodestannylation), were tested with the 18F activity generated in the [18O]O2 target systems. Identical reactions were also conducted with analogous nonradioactive fluorinating agents as control experiments. The products of all these reactions were analyzed by NMR spectroscopy. Results of these experiments clearly showed that 18F activities recovered from aluminum, silver, and copper target bodies were better suited for radiochemical syntheses, irrespective of the irradiation protocols employed. The 18F activity from a gold-plated target was suitable only for addition reactions. Further, the fluorodemercuration reaction essentially failed with the single-step irradiation technique conducted in nickel and gold-plated targets. In contrast, the fluorodestannylation reaction was quite successful with the 18F reagents recovered from all the target bodies and irradiation protocols.

Aluminum↗

PubMed1996

Radiofluorinated L-m-tyrosines: new in-vivo probes for central dopamine biochemistry.

In this work, we introduce 6-[18F]fluoro-L-m-tyrosine (6-FMT) and compare its in-vivo kinetic and bio-chemical behaviors in monkeys and rodents with those of 4-FMT and 6-[18F]fluoro-L-3, 4-dihydroxyphenylalanine (DOPA) (FDOPA). These radiofluorinated m-tyrosine presynaptic dopaminergic probes, resistant to peripheral 3-O-methylation, offer a nonpharmacological alternative to the use of catechol-O-methyltransferase inhibitors. Like FDOPA, 4-FMT and 6-FMT are analogs that essentially follow the L-DOPA pathway of central metabolism. After i.v. administration in nonhuman primates and rodents, these new radiofluorinated m-tyrosine analogs accumulate selectively in striatal structures and allow for the detection of additional innervation sites (e.g., brain stem) rich in aromatic amino acid decarboxylase. Bio-chemical analyses in rodents and monkeys revealed the specificity of their central and peripheral metabolism. Molecular and enzymatic mechanisms involved in their retention in central brain structures are consistent with involvement of dopaminergic neurons. The high signal-to-noise ratios observed make these radiofluorinated m-tyrosine analogs outstanding candidates for probing the integrity of central dopaminergic mechanisms in humans.

Animals↗

PubMed1993

Synthesis of 6-[18F] and 4-[18F]fluoro-L-m-tyrosines via regioselective radiofluorodestannylation.

The regioselective radiofluorodestannylation of 6-trimethylstannyl-L-m-tyrosine derivative 6 with [18F]F2 and [18F]acetyl hypofluorite afforded, after acid hydrolysis, 6-[18F]fluoro-L-m-tyrosine (8a) in radiochemical yields of 23 and 17%, respectively. Similarly, 4-[18F]fluoro-L-m-tyrosine (13a) was synthesized in 11% radiochemical yield from the corresponding 4-trimethylstannyl-L-m-tyrosine derivative 11 using [18F]F2. The structural analyses of precursors (6,11), intermediates, and the final products (after 18F decay), were carried out by 1H, 13C, 19F, 119Sn-NMR and high resolution mass spectroscopy.

Fluorine Radioisotopes↗

PubMed1992

Detection and quantitation of 17F formation during the cyclotron production of 13N.

The positron emitter 17F has been identified for the first time as a radioactive contaminant formed during 13N production. It was determined that 17F made up 18% of the short-lived radioactivity previously thought to be exclusively 15O. Mathematical deconvolution of decay curve data was employed as a complement to radioanalytical techniques. The nuclidic composition of the effluents from the proton bombardment of (a) water, (b) dilute ethanol and (c) 13C powder/water slurry was determined and compared with previous literature values.

Fluorine Radioisotopes↗

PubMed1992

Regioselective radiofluorodestannylation with [18F]F2 and [18F]CH3COOF: a high yield synthesis of 6-[18F]Fluoro-L-dopa.

A protected 6-trimethylstannyl dopa derivative 6 has been synthesized for the first time as a precursor for the preparation of 6-[18F]fluoro-L-dopa. The tin derivative 6 readily reacted with electrophilic radiofluorinating agents such as [18F]F2 and [18F]AcOF. The [18F]fluoro intermediate 7 was easily hydrolyzed with HBr and the product 6-[18F]fluoro-L-dopa was isolated after HPLC purification in a maximum radiochemical yield of 25%, ready for human use. The various intermediates, the stannyl precursor 6 and the final product (after 18F decay) were all fully characterized by 1H, 13C, 19F and 119Sn NMR as well as high resolution mass spectroscopy.

Dihydroxyphenylalanine↗