Nuclear Medicine &a Biology, Vol. 23, 1005-1008, 1996 Copyright 0 1996 Elsevier Science Inc. ELSEVIER ISSN 0969-8051/96/$15.00 + 0.00 PII SO969-8051(96)00127~8 TECHNICAL NOTE A High-Yield and Simplified Procedure for the Synthesis of ~1, [’ ‘C]Methyl-L-Tryptophan Pulak K. Chakruborty , ‘Thomas J. Mangner, Diane C. Chugani, Otto Muzik and Harry 7’. Chuguni CHILDREN’S HOSPITAL OF MICHIGAN, DEPARTMENT OF PEDIATRIC IMAGING, WAYNE STATE UNIVERSITY SCHOOL OF MEDICINE, DETROIT, MI 48201, USA ABSTRACT. Alpha-[’ ‘Clmethyl-L-tryptophan (AMT) h as b een synthesized by stereoselective methylation with [l’C]methyl iodide of the lithium-enolate generated by treating dimethyl 2(S),3a(R),sa(S)-(+)- hexahydro-8(phenylsuIfonyl)pyrrolo[2,3-b]indole-l,2~d icarboxylate (2) with lithium diisopropyl amide (LDA) at -55”C, followed by ring opening using trifluoroacetic acid and alkaline hydrolysis of the protecting groups. The crude product was purified by a simple reverse-phase C-18 Sep-Pak procedure. The purified product was isolated with an average radiochemical yield of 53 * 12% (decay corrected) in 30-35 min from [“Clmethyl iodide. At end of synthesis (EOS), 138 f 35 mCi (n = 24) of product was collected with a specific activity of ca. l-l.3 Ci/pmol (EOS) (4-5 Ci/pmol @ EOB) starting from 1.5 Ci (EOB) of [“clco,. Copyright 0 1996 Elsevier Science Inc. NUCL MED BIOL 23;8:1005-1008, 1996. KEY WORDS. a-[“C]Methyl+tryptophan, Serotonin, Positron emission tomography INTRODUCTION The alteration of the level of brain serotonin (5-hydroxy tryptamine) and the rate of its in viwo synthesis from L-tryptophan have been implicated in a number of neurological diseases and disorders (3). cw-Methyl-L-tryptophan (AMT), radiolabeled at the c-u-methyl position with carbon-11 ([“CIAMT, l), has been ad- vanced as a tracer to study these processes via PET (4,5). As a result of our need for a PET tracer to study serotonergic involvement in a variety of neurological disorders, the synthesis of [“C]AMT was undertaken. The basic approach taken to the stereoselective synthesis of ra- diolabeled AMT was C-l 1 methylation of a cyclized fully protected tryptophan (7, 9). The initial step in this approach is the methyl- ation with [“Clmethyl iodide of the lithium enolate generated by treating dimethyl 2(S),3a(R),8a(S)-(+)-hexahydro-8(phenylsul fonyl)pyrrolo[2,3-b]indole-1,2-dicarboxylate (2) with lithium diiso- propyl amide (LDA) at -78”C, as described by Boume et al. (1) (Scheme 1). Hydrolysis of the [“Clmethyl adduct (3) has been described via two different procedures. Plenevaux et al. (7) hydro- lyzed the [“Clmethyl adduct in one step with 57% HI at 200°C for 5 min and isolated the final product by C-18 reverse-phase semi- prep HPLC. Venkatachalam et al. (9) adopted a two-step hydrolysis procedure, heating the [3H]- or [14C]-labeled adduct first with tri- fluoroacetic acid (210°C for 1 h) followed by strong base (8N NaOH at 210°C for 2 h or 5N KOH at 210°C for 15 min) in a sealed vessel with subsequent HPLC purification of the final prod- uct. Recently, the same group reported the synthesis of [“CIAMT Address reprint requests to: P&k K. Chakraborty, PhD, PET Center, Chil- dren’s Hospital of Michigan, 3901 Beaubien Blvd., Detroit, MI 48201. Part of this paper was reported first in abstract form: 11th Intematiowl Symposium on Radiophannueutical Chemistry, August 13-17, 1995, Vancou- ver, BC, Canada. J. Label. Compds. Radiopham~ (1995) 37, 619-621. Research was supported in part by a grant from Siemens Gammasonics, Inc. Received 9 March 1996. Accepted 20 May 1996. by applying this two-step hydrolysis procedure (TFA, 21O”C, 3 min and 10 N NaOH, 21O”C, 10 min) and subsequent purification of the final product by semi-prep HPLC (6). Another group has used essentially the same procedure to obtain [“CIAMT after a 70-min preparation time and in 40% (corrected) radiochemical yield from 1’ ‘Clmethyl iodide (8). Our goal in developing the synthesis of [“CIAMT for routine clinical use was to simplify the preparation by eliminating the need for HPLC isolation and purification of the final product, thus saving considerable time and eliminating potential pyrogenicity and prod- uct contamination problems associated with the multiple reuse of a semi-prep HPLC column. Removal of L-tryptophan, which is gen- erated from the hydrolysis of the excess unlabeled precursor, from the final product was unnecessary since the amount produced (l-2 mg total) is insignificant compared to normal blood levels (80 p,M or -16 mg/L of blood). Here we report the adaptation of a two-step hydrolysis procedure followed by a simple Sep-Pak purification of [“CIAMT. EXPERIMENTAL Materials and Methods Dimethyl 2(S),3a(R),8a(S)-(+)-hexahydro-8(phenylsulfonyl)pyr~ rolo[2,3-blindole-1,2-dicarboxylate (2), lithium diisopropylamide (LDA) (1.5M in cyclohexane), trifluoroacetic acid (spectrophoto- metric grade), and glacial acetic acid (reagent grade) were pur- chased from Aldrich Chemical Co. Sep-Pak (C-18) cartridges were obtained from Waters Associates. High-pressure liquid chromatog- raphy (HPLC) analyses were performed using a Waters HPLC sys- tem equipped with a Waters 486 UV detector and a sodium iodide scintillation detector (Ortec). Analytical HPLC was performed on an Ultremex-5 ODS-2 C-18 reverse-phase column (4,6 x 250 mm) (Phenomenex) using lo/90 acetonitrile/buffer (containing 0.7 mL of phosphoric acid and 1.4 mL of triethylamine per liter of water) as the eluent with a flow rate of 1.5 mL/min at 280 nm. With this