Synthesis of radioactive ¹¹C molecules for medical research on a microscale : A theoretical and practical approach

Abstract

Abstract It is often necessary to obtain certain 11 C radiopharmaceuticals with a high specific activity (> 1000 Ci/mmole), especially when these are intended for medical purposes and are either toxic or used for “in vivo” specific receptor binding studies. The amounts of labelling agent must be very small (concentrations often below to 10 −3 M) and some synthesis are difficult or impossible under these conditions. This article shows how the specific activitry of the labelling agent A affects the synthesis yield of the radiopharmaceutical A ′ in the reaction: A specific activity increase can improve, reduce or have no effect on the yield according to the respective valuse of V 1 , V 2 , V ′ 1 , V ′ 2 . Different ways to shift an epuilibrium unfavourabel to labelling are also given : reaction temperature, concentrations, addition of starting products, removal of final products. The reaction kinetics are important since the half‐life of carbon 11 is 20 mn, but this parameter is a complex function of concentrations which can only be detemined by experiment. It seems however that in special cases the percentage of radioactivity incorporated in a given times is imdependent of specific activity. The effect of impurities, which can be present in concentration of the same order of magnitude as those of the labelling agent is also discussed. Each thereticl development is illustrated by concrete examples studied in the laboratory. The conclusion is that theoretically it should not be impossible to synthesize strictly carrier‐free 11 C radiopharmaceuticals in certain favourable cases (if it were known how to prepare such carrier‐free labelling agents).