Synthesis of tritium-labelled N tau-methylhistamine for the improvement of extraction efficiency of N tau-methylhistamine from biological fluids.

In order to trace the loss of N tau-methylhistamine, a principal metabolite of histamine, during extraction and purification from human plasma and urine samples, N tau-[3H]methylhistamine was prepared in two steps from N alpha t-butoxycarbonylhistamine (II). In the first step, compound II was deprotonated with NaH in an aprotic solvent and treated with [3H]methyl iodide. The products, N alpha t-butoxycarbonyl-N tau-[3H]methylhistamine (III) and N alpha t-butoxycarbonyl-N pi-[3H]methylhistamine (IV), were then hydrolysed with iodotrimethylsilane under mild and short reaction conditions. Facile purification with Sep-Pak silica cartridges gave the combined two isomers of N tau-[3H]methylhistamine and N pi-[3H]methylhistamine in 10.7% radiochemical yield with a radiochemical purity of > 94% and a ratio of approximately 2:1. Improvements in the extraction of methylhistamine using chromatography on Sep-Pak silica cartridges led to an overall recovery of 82.5 +/- 0.3% (n = 3) based upon total [3H]methylhistamine from normal human plasma.