AN AUTOMATED LIQUID-CHROMATOGRAPHIC PLASMA ANALYSIS OF AMINO-ACIDS USED IN COMBINATION WITH POSITRON EMISSION TOMOGRAPHY (PET) FOR DETERMINATION OF IN-VIVO PLASMA KINETICS

Quantification of physiological processes measured with positron emission tomography (PET) requires an ''input'' function which can be the concentration of administered radio-tracer in plasma. Radioactive nuclides used in PET have short half lives (2-20 min) and a limited time is available for the PET investigation including analysis of the composition of the radioactive signal in plasma. Therefore, an automated method for the analysis and separation of beta-[C-11]-L-5-hydroxy-tryptophan ([C-11]-L-5-HTP), beta-[C-11]-L-3,4-dihydroxy-phenylalanine ([C-11]-L-DOPA) and L-[methyl-C-11]-methionine and their respective metabolites in plasma was developed. A size exclusion column was used for isolation of the low molecular weight fraction. In the case of [C-11]-L-5-HTP and [C-11]-L-DOPA, the low molecular weight fraction was injected onto a liquid chromatographic system for separation of radioactive tracer from in vivo formed radio-labelled metabolites. The elution volume from the size exclusion column was 7.0, 5.0 and 3.5 ml for [C-11]-L-5-HTP, [C-11]-L-DOPA and L-[methyl-C-11]-methionine, respectively. An interaction with the column matrix and the solutes was observed for both [C-11]-L-5-HTP and [C-11]-L-DOPA. The yield in the isolation step was >98%. Separation of [C-11]-L-5-HTP and [C-11]-L-DOPA from their respective metabolites was performed with high-performance liquid chromatography with automated collection of fractions of the eluate corresponding to those of administered tracer and metabolites. The fractions were measured for radioactivity in a well counter. Inter- and intraday variation in retention were less than 3% RSD. Fortyfive minutes after injection of [C-11]-L-5-HTP or [C-11]-L DOPA the radioactivity in the [C-11]-L-5-HTP or [C-11]-L-DOPA fraction was 58.5 +/- 6.4 (RSD, n = 4) and 31.0 +/- 2.7 (RSD, n = 3), respectively. High and low molecular weight fractions from plasma after injection of L-[methyl-C-11]-methionine were collected and measured for radioactivity. An incorporation of [C-11]-L-methionine in plasma proteins, i.e, the high molecular weight fraction, was observed but with large inter-individual differences.