Synthesis of L-2,4-diamino[4-11C]butyric acid and its use in some in vitro and in vivo tumour models

L-2,4-Diamino[4-11C]butyric acid (DAB) was synthesized by an enzyme catalysed carrier added (0.1 micromol KCN) reaction of hydrogen [11C]cyanide with O-acetyl-L-serine followed by reduction. L-[11C]DAB was obtained with a radiochemical purity higher than 96% and with a decay corrected radiochemical yield of 30-40% within a 32 min reaction time. The enantiomeric excess was 98%. The uptake of L-[11C]DAB was investigated in multicellular aggregates of six different cell lines and animal tumour models. L-[11C]DAB is potentially useful for the assessment of pharmacokinetics of L-DAB in vivo for part of its evaluation as an antitumoural agent, although its use for diagnostic purposes seems limited.

Supercritical fluid extraction of 11C-labeled metabolites in tissue using supercritical ammonia

Supercritical fluid extraction (SFE) of 11C-labeled tracer compounds and their metabolites from biological tissue was performed using supercritical ammonia in an attempt to develop a rapid extraction procedure that allowed subsequent analysis of the labeled metabolites. Metabolites were extracted from kidneys and brain in rats given in vivo injections of the radiotracers O-[2-11C]acetyl-L-carnitine and N-[11C]methylpiperidyl benzilate, respectively. Only a minimal sample pretreatment of the tissue was necessary, i.e., cutting into 10-20 pieces and mixing with the drying agent Hydromatrix, before it was loaded into the extraction vessel. Extraction efficiency was measured for SFE at temperatures over the range of 70-150 degrees C and a pressure of 400 bar. For O-[2-11C]acetyl-L-carnitine, 66% of the radioactivity was trapped in the collected fractions and 12% remained in the extraction vessel. For the more lipophilic N-[11C]methylpiperidyl benzilate, 93% of the activity was collected and less than 1% remained in the extraction vessel. Labeled metabolites were analyzed by LC and also, in the case, of O-[2-11C]acetyl-L-carnitine by LC/MS. The complete extraction procedure, from removal of the biological tissue until an extract was ready for analysis, was 25 min, corresponding to about one half-life of the radionuclide 11C.

Determination of flumazenil in human plasma by liquid chromatography-electrospray ionisation tandem mass spectrometry

A liquid chromatography-electrospray ionisation-tandem mass spectrometry (LC-ESI-MS/MS) method was developed to determine unlabelled flumazenil (Ro 15-1788) in human plasma in [11C]flumazenil positron emission tomography (PET) studies. N-Methyl tri-deuterated flumazenil was used as an internal standard. The analyte and internal standard were extracted from plasma samples using solid-phase extraction, with a recovery of 78%. This was determined through the convenience of radioactivity measurements of 11C-labelled flumazenil. The evaporated and reconstituted eluate was analysed by LC-ESI-MS/MS. The calibration curve was linear over the tested concentration range of 0.05-0.5 nM (15-150 pg/ml) with a correlation coefficient, R2, of 0.998+/-0.001. A high precision was achieved, with mean intra-assay and inter-assay relative standard deviations of at most 6 and 7%, respectively. The accuracy of the method ranged from 95 to 104%. As a proof of concept, the validated method was applied in the determination of flumazenil in plasma from two healthy volunteers participating in a PET study with three repeated investigations. A bolus-infusion protocol was used to achieve a constant concentration level of flumazenil. The average plasma concentrations ranged from 0.11 and 0.19 nM and all measurements were within the calibration standard range. The flumazenil concentrations were relatively constant within each scan and the average intra-scan precision was 15%.

Catechol-O-methyltransferase inhibition increases the uptake of 11C-3-(3,4-dihydroxyphenyl)-L-alanine in the rat pancreas

BACKGROUND

The objective of the present investigation was to evaluate the uptake and metabolism of 3-(3,4-dihydroxyphenyl-L-alanine) (L-DOPA) in the rat pancreas.

METHODS

The procedure included intravenous injection of the positron-emitting radiotracer L-[beta-11C] DOPA (DOP) into unanaesthetized male Sprague-Dawley rats and evaluation of uptake of radioactivity in organs in animals only given the tracer and in animals given therapeutic doses of three different catechol-O-methyltransferase (COMT) inhibitors, OR-486, OR-611, or Ro 41-0960. Selected pancreati were homogenized, and the chemical form bearing the radioactivity was analysed with high-performance liquid chromatography (HPLC).

RESULTS

The main finding was that the tracer uptake in the pancreas increased fourfold when the rats were pretreated with COMT inhibitors. Half maximum effect of OR-486 was found at a dose of 0.2 mg/kg. HPLC analysis showed that with COMT inhibitor, the radioactivity in the pancreas consisted of 90% DOPAC. When administering MAO-A and COMT inhibitor together, the pancreas radioactivity corresponded to dopamine. Also in the pig pancreas a significant increase of DOP was observed after COMT inhibition.

CONCLUSIONS

This study has shown a high turnover of L-DOPA in the rat pancreas, which can be modulated to give enhanced levels of DOPAC or dopamine by COMT and MAO inhibition.