HIGH-SPEED LIQUID-CHROMATOGRAPHY OF PHENYLETHANOLAMINES FOR THE KINETIC-ANALYSIS OF [C-11] META-HYDROXYEPHEDRINE AND METABOLITES IN PLASMA

A method is developed and described for analysis of [C-11]-meta-hydrox yephedrine, [C-11]MHED, a tracer of cardiac function, and its metaboli tes in plasma samples. The method combines on-column solid-phase extra ction and separation on a single weak cation-exchange column. Phenylet hanolamines were used to develop the separation method that concentrat es the analytes on-column from physiological saline and then elutes th em by changing to an acidic mobile phase. Hydrophobic interactions det ermine the selectivity, and elution order is the same as for reversed- phase liquid chromatography on a C,, stationary phase. The mechanism o f separation is mixed mode, with ion-exchange coupled with a reversed- phase liquid chromatography mechanism. Each sample analysis requires o nly 10 min and does not require deproteinization or the use of organic solvents. In human samples, a single plasma metabolite of [C-11]MHED along with the parent compound were observed using this method. The me thod was sufficiently rapid so that in 70 min seven samples were assay ed, providing a well-defined time course for MHED and its metabolites in blood. The metabolite concentration increased with time to approxim ate to 85% of the plasma activity 50 min after administration. The res ults with the developed method are comparable to those described for r eversed-phase separations, with the advantage that our method does not require deproteinization, reducing sample analysis time by a factor o f two.