Synthesis of tritium-labelled Nτ-methylhistamine for the improvement of extraction efficiency of Nτ-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-[H-3]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 [H-3]methyl iodide. The products, N(alpha)t-butoxycarbonyl-N-tau-[H-3]methylhistamine (III) and N(alpha)t-butoxycarbonyl-N-pi-[H-3]methylhistamine (IV), were then hydrolysed with iodotrimethylsilane under mild and short reaction conditions. Facile purification with Sep-Pak(TM) silica cartridges gave the combined two isomers of N-tau-[H-3]methylhistamine and N-pi-[H-3]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 [H-3]methylhistamine from normal human plasma. (C) 1997 Elsevier Science Ltd. All rights reserved.