SYNTHESIS OF RADIOACTIVE C-11 MOLECULES FOR MEDICAL-RESEARCH ON A MICROSCALE - A THEORETICAL AND PRACTICAL APPROACH

How the specific activity of the labeling agent .**GRAPHIC**. affects the synthesis yield of the radiopharmaceutical .**GRAPHIC**. in the following reaction is discussed: V1 .**GRAPHIC**. + V2 A2 .dblarw. V''1 .**GRAPHIC**. + V''2 .**GRAPHIC**. A specific activity increase can improve, reduce, or have no effect on the yield according to the respective values of V1, V2, V''1, V''2. Different ways to shift an equilibrium unfavorable to labeling such as reaction temperature, concentrations, addition of starting products and removal of final products are discussed. Reaction kinetics are important since the half-life of 11C is 20 mn, but this parameter is a complex function of concentrations which can only be determined by experiment. In special cases the percentage of radioactivity incorporated in a given time is independent of specific activity. The effect of impurities, which can be present in concentrations of the same order of magnitude as those of the labeling agent is also discussed. Each theoretical development is illustrated by concrete examples studied in the laboratory. Theoretically it should not be impossible to synthesize strictly carrier-free 11C radiopharmaceuticals in certain favorable cases (if it were known how to prepare such carrier-free labeling agents).