Development of a non-radiometric method for measuring the arterial input function of a C-11-labeled PET radiotracer

Positron emission tomography (PET) uses radiotracers to quantify important biochemical parameters in human subjects. A radiotracer arterial input function (AIF) is often essential for converting brain PET data into robust output measures. For radiotracers labeled with carbon-11 (t(1/2)=20.4 min), AIF is routinely determined with radio-HPLC of blood sampled frequently during the PET experiment. There has been no alternative to this logistically demanding method, neither for regular use nor validation. A C-11-labeled tracer is always accompanied by a large excess of non-radioactive tracer known as carrier. In principle, AIF might be obtained by measuring the molar activity (A(m); ratio of radioactivity to total mass; Bq/mol) of a radiotracer dose and the time-course of carrier concentration in plasma after radiotracer injection. Here, we implement this principle in a new method for determining AIF, as shown by using [C-11]PBR28 as a representative tracer. The method uses liquid chromatography-tandem mass spectrometry for measuring radiotracer A(m) and then the carrier in plasma sampled regularly over the course of a PET experiment. A(m) and AIF were determined radiometrically for comparison. The new non-radiometric method is not constrained by the short half-life of carbon-11 and is an attractive alternative to conventional AIF measurement

Image-Derived Input Function for Human Brain Using High Resolution PET Imaging with [11C](R)-rolipram and [11C]PBR28

The aim of this study was to test seven previously published image-input methods in state-of-the-art high resolution PET brain images. Images were obtained with a High Resolution Research Tomograph plus a resolution-recovery reconstruction algorithm using two different radioligands with different radiometabolite fractions. Three of the methods required arterial blood samples to scale the image-input, and four were blood-free methods. values was quantified using a scoring system. Using the image input methods that gave the most accurate results with Logan analysis, we also performed kinetic modelling with a two-tissue compartment model.)-rolipram, which has a lower metabolite fraction. Compartment modeling gave less reliable results, especially for the estimation of individual rate constants.C]PBR28), the more difficult it is to obtain a reliable image-derived input function; and 4) in association with image inputs, graphical analyses should be preferred over compartmental modelling

The effect of pH on the fatty acid composition of Penicillium chrysogenum

Age-related changes in the pH of the medium, fatty acid composition and biosynthesis of fatty acids in various subcellular fractions have been studied in submerged cultures of P. chrysogenum harvested at pH's 7.2, 4.8, 4.4 and 3.8. 14C - labeled lauric and myristic acids were each incubated with cell fractions prepared by sonification and fractional centrifugation. Products were analyzed by thin layer chromatography, gas liquid scintillation spectrometry. Conversion of labeled precursors into longer chain saturated and unsaturated fatty acids was employed as a measure of the activity of the various cell fractions.It was found that between pH 4.8 and 4.4 of the age of the culture a period of maximum fatty acid synthesis occured at the time when glucose was being taken up to a greatest extent from the medium and when cells start rapidly increasing in growth. This correlated to the increased rate of de novo synthesis in the soluble cytoplasmic fraction.The fermentation products, formic, acetic, proprionic, butyric and gluconic acid, produced at different time points of the age of the culture, accounted for the drop in the pH of the medium.Thesis (M.S.

EFFECT OF N-METHYLGLUCAMINE SALICYLATE ON THE GASTROINTESTINAL ABSORBTION OF ALPHA-METHYLDOPA IN RATS

N-Methylglucamine, 1-deoxy-1-(methylamine)-D-glucitol, is a solubilizing and stabilizing agent in pharmaceutical preparations. Through the formation of N-methylglucamine salts, several drugs, which were otherwise not water soluble, become soluble and readily absorbed from the gastrointestinal tract. Pharmacokinetic studies in man and animals have shown that N-methylglucamine salicylate has acquired therapeutic blood salicylate levels with considerable less gastrointestinal damage than acetylsalicylate. The compound was suggested for use in conditions requiring systemic salicylate therapy. (alpha)-Methyldopa, 3-(3,4-dihydroxyphenyl)-2-methyl-L-alanine, is a highly effective antihypertensive agent. Pharmacologic studies in man and rats have shown that about 15-50% of the orally administered dose was absorbed from the gastrointestinal tract. For effectiveness of the drug and without causing any adverse side effects, patients that are being treated with (alpha)-methyldopa usually undergo individual adjustment of dosage. N-Methylglucamine salicylate is completely ionized when in aqueous media. If this drug were administered with a compound such as (alpha)-methyldopa, the possibility of N-methylglucamine influencing the absorption of (alpha)-methyldopa is probable. This research involved the in vivo study of the blood and plasma levels of (alpha)-methyldopa when (alpha)-methyldopa was administered orally with and without N-methylglucamine salicylate. Preliminary experiments were conducted utilizing (\u273)H-(alpha)-methyldopa. The radiochemical purity was determined using thin layer chromatography in conjunction with liquid scintillation techniques. (\u273)H-(alpha)-methyldopa was shown to be at least 96% radiochemically pure. (\u273)H-(alpha)-methyldopa was administered orally, to fasted rats, with and without N-methylglucamine. Periodical blood withdrawal from cannulated rats, made it feasible to monitor the blood radioactivity levels. Blood levels of radioactivity in the rats showed that they differed, indicating that there was interference between N-methylglucamine and (alpha)-methyldopa. However variability within treatment groups was high, probably due to variability in the rate of metabolism of (alpha)-methyldopa. Consequently, the measurement of radioactivity in the blood was not considered a good indication of (alpha)-methyldopa absorption. Fluorometric assays, specific for (alpha)-methyldopa, were done on plasma samples obtained from three groups of rats. Plasma samples were collected from rats by decapitation and exsanguination. Each of the three groups of rats received orally either (alpha)-methyldopa, (alpha)-methyldopa with N-methyl-glucamine salicylate simultaneously, or (alpha)-methyldopa 2 hours after N-methylglucamine salicylate. Results showed that N-methylglucamine salicylate decreased the gastrointestinal absorption of (alpha)-methyldopa when the two were given simultaneously, but there was no interference when the two doses of (alpha)-methyldopa and N-methylglucamine salicylate were staggered

Plasma radiometabolite correction in dynamic PET studies:Insights on the available modeling approaches

Full kinetic modeling of dynamic PET images requires the measurement of radioligand concentrations in the arterial plasma. The unchanged parent radioligand must, however, be separated from its radiometabolites by chromatographic methods. Thus, only few samples can usually be analyzed and the resulting measurements are often noisy. Therefore, the measurements must be fitted with a mathematical model. This work presents a comprehensive analysis of the different models proposed in the literature to describe the plasma parent fraction (PPf) and of the alternative approaches for radiometabolite correction. Finally, we used a dataset of [(11)C]PBR28 brain PET data as a case study to guide the reader through the PPf model selection process

Comparison of two PET radioligands, [11C]FPEB and [11C]SP203, for quantification of metabotropic glutamate receptor 5 in human brain

Of the two (18)F-labeled PET ligands currently available to image metabotropic glutamate receptor 5 (mGluR5), [(18)F]FPEB is reportedly superior because [(18)F]SP203 undergoes glutathionlyation, generating [(18)F]-fluoride ion that accumulates in brain and skull. To allow multiple PET studies on the same day with lower radiation exposure, we prepared [(11)C]FPEB and [(11)C]SP203 from [(11)C]hydrogen cyanide and compared their abilities to accurately quantify mGluR5 in human brain, especially as regards radiometabolite accumulation. Genomic plot was used to estimate the ratio of specific-to-nondisplaceable uptake (BPND) without using a receptor blocking drug. Both tracers quantified mGluR5; however [(11)C]SP203, like [(18)F]SP203, had radiometabolite accumulation in brain, as evidenced by increased distribution volume (VT) over the scan period. Absolute VT values were ∼30% lower for (11)C-labeled compared with (18)F-labeled radioligands, likely caused by the lower specific activities (and high receptor occupancies) of the (11)C radioligands. The genomic plot indicated ∼60% specific binding in cerebellum, which makes it inappropriate as a reference region. Whole-body scans performed in healthy subjects demonstrated a low radiation burden typical for (11)C-ligands. Thus, the evidence suggests that [(11)C]FPEB is superior to [(11)C]SP203. If prepared in higher specific activity, [(11)C]FPEB would presumably be as effective as [(18)F]FPEB for quantifying mGluR5 in human brain

Improved models for plasma radiometabolite correction and their impact on kinetic quantification in PET studies

The quantification of dynamic positron emission tomography studies performed with arterial sampling usually requires correcting the input function for the presence of radiometabolites by using a model of the plasma parent fraction (PPf). Here, we show how to include the duration of radioligand injection in the PPf model formulations to achieve a more physiologic description of the plasma measurements. This formulation (here called convoluted model) was tested on simulated data and on three datasets with different parent kinetics: [11C]NOP-1A, [11C]MePPEP, and [11C](R)-rolipram. Results showed that convoluted PPf models better described the fraction of unchanged parent in the plasma compared with standard models for all three datasets (weighted residuals sum of squares up to 25% lower). When considering the effect on tissue quantification, the overall impact on the total volume of distribution (VT) was low. However, the impact was significant and radioligand-dependent on the binding potential (BP) and the microparameters (K1, k2, k3, and k4). Simulated data confirmed that quantification is sensitive to different degrees to PPf model misspecification. Including the injection duration allows obtaining a more accurate correction of the input function for the presence of radiometabolites and this yields a more reliable quantification of the tissue parameters

Development of <i>N</i>‑Methyl-(2-arylquinolin-4-yl)oxypropanamides as Leads to PET Radioligands for Translocator Protein (18 kDa)

Translocator protein (18 kDa), known as TSPO, is a recognized biomarker of neuroinflammation. Radioligands with PET accurately quantify TSPO in neuroinflammatory conditions. However, the existence of three human TSPO genotypes that show differential affinity to almost all useful TSPO PET radioligands hampers such studies. There is an unmet need for genotype-insensitive, high-affinity, and moderately lipophilic TSPO ligands that may serve as leads for PET radioligand development. To address this need, we varied the known high-affinity TSPO ligand (<i>l</i>)-<i>N</i>,<i>N</i>-diethyl-2-methyl-3-(2-phenylquinolin-4-yl)­propanamide in its aryl scaffold, side chain tether, and pendant substituted amido group while retaining an <i>N</i>-methyl group as a site for labeling with carbon-11. From this effort, oxygen-tethered <i>N</i>-methyl-aryloxypropanamides emerged as new high-affinity TSPO ligands with attenuated lipophilicity, including one example with attractive properties for PET radioligand development, namely <i>N</i>-methyl-<i>N</i>-phenyl-2-{[2-(pyridin-2-yl)­quinolin-4-yl]­oxy}­propanamide (<b>22a</b>; rat <i>K</i>_i = 0.10 nM; human TSPO genotypes <i>K</i>_i = 1.4 nM; clogD = 4.18)