Studies of the mechanism of the in-loop synthesis of radiopharmaceuticals.

A series of experiments were performed to better understand the mechanism of the In-loop [11C]CH3I-methylation. The timing of [11C]CH3I delivery is critical for the high yield of radiolabeling since in-loop radioactivity trapping is reversible. Trapped radioactivity escapes faster from a Tefzel loop compared to a PEEK- or stainless steel loop. Up to 50% of delivered radioactivity may be concentrated at the loop origin (representing 8.1% of the total loop volume). A five-fold reduction of the reaction solvent volume and/or precursor amount may lead to a decrease of the product radiochemical yield either by lowering the in-loop radioactivity trapping or by diminishing conversion of [11C]CH3I into the product.