The purpose of this project was to synthesize a-methyl-L-tryptophan labeled with carbon- 11 (T1/2 = 20.4 min), a cyclotron produced, positron-emitting radionuclide, which will permit imaging the brain utilizing positron emission tomography (PET). Drs. Paul Andreason and Susan Shoaf are interested in using PET to study the use of alpha- [11C-methyl]-L-tryptophan as a tracer of brain serotonin synthesis. The rate of accumulation of radioactivity of this tracer as measured by PET is assumed to be indicative of the rate of serotonin synthesis and may be correlated with increased aggressive/impulsive behavior of patients. In order to use this tracer for serotonin synthesis, it is first necessary to establish the pharmacokinetic parameters of the tracer. Also, in order to relate the accumulation of the tracer to the rate of serotonin synthesis, the differences in the enzyme kinetics for the tracer and tryptophan must be determined. Since alpha-methyl- L-tryptophan is not commercially available, I developed a HPLC procedure for the resolution of the racemate into its enantiometers using a cyclodextrin column. The method provided the L-form in ammonium acetate solution, but enzyme kinetics requires the L-form free of ammonium acetate. This was achieved by concentrating the solution from 1 ml to 50 microl in a Speed- Vac at 60. The concentrate was injected onto a Waters C-18 Nova-pak column (15 x 4 mm) using 8:92 MeOH:H2O at 1.0 mL/min. The eluate was concentrated in a Speed- Vac at 60 until the final concentration was 4 mg/mL in H2O. The MeOH came off quickly and completely. During the course of this work, a chiral precursor to alpha-methyl-L- tryptophan became commercially available. Literature procedures were reported utilizing this material for the synthesis of [11C] and [14C]- labeled alpha-methyl-L-tryptophan. Although the methylation reaction using [11C]CH3I are similar, the hydrolysis step leading to the labeled alpha-methyl-L-tryptophan are different Using an adaptation of these precedures, we have synthesized alpha-[11C-methyl]-L-tryptophan in about 20% yield.
