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 emiss ion tomography (PET) requires an ''input'' function which can be the c oncentration of administered radio-tracer in plasma. Radioactive nucli des used in PET have short half lives (2-20 min) and a limited time is available for the PET investigation including analysis of the composi tion of the radioactive signal in plasma. Therefore, an automated meth od for the analysis and separation of beta-[C-11]-L-5-hydroxy-tryptoph an ([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 metabolit es in plasma was developed. A size exclusion column was used for isola tion of the low molecular weight fraction. In the case of [C-11]-L-5-H TP and [C-11]-L-DOPA, the low molecular weight fraction was injected o nto a liquid chromatographic system for separation of radioactive trac er 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 in teraction 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 respe ctive metabolites was performed with high-performance liquid chromatog raphy with automated collection of fractions of the eluate correspondi ng to those of administered tracer and metabolites. The fractions were measured for radioactivity in a well counter. Inter- and intraday var iation in retention were less than 3% RSD. Fortyfive minutes after inj ection 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) an d 31.0 +/- 2.7 (RSD, n = 3), respectively. High and low molecular weig ht 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 f raction, was observed but with large inter-individual differences.