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 Nτ-methylhistamine, a principal metabolite of histamine, during extraction and purification from human plasma and urine samples, Nτ-[3H]methylhistamine was prepared in two steps from Nαt-butoxycarbonylhistamine (II). In the first step, compound II was deprotonated with NaH in an aprotic solvent and treated with [3H]methyl iodide. The products, Nαt-butoxycarbonyl-Nτ-[3H]methylhistamine (III) and Nαt-butoxycarbonyl-Nπ-[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 Nτ[3H]methylhistamine and Nτ[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.