High-Yielding Automated Convergent Synthesis of No-Carrier-Added [¹¹C-Carbonyl]-Labeled Amino Acids Using the Strecker Reaction

Abstract

A new variant of the Strecker synthesis using no-carrier-added [¹¹C]cyanide for the synthesis of radiolabeled amino acids is described. The protocol is fully automated using a radiochemistry synthesis module and applied to the production of a number of new PET radiotracers. [¹¹C-Carbonyl]sarcosine, [¹¹C-carbonyl]methionine, [¹¹C-carbonyl]-N-phenylglycine, and [¹¹C-carbonyl]glycine are all synthesized in moderate to good radiochemical yields. The synthesis of [¹¹C-carbonyl]sarcosine has been validated for production of doses for clinical use, and preliminary evaluation of the new radiotracer in PC3 tumor-bearing mice is also reported.

Key words

PET imaging - PET radiochemistry - carbon-11

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• 12 Radiochemical conversion was determined by HPLC; %RCC = (area of product radioactivity peak / total area of all radioactive peaks) × 100.
• 13 Automated Synthesis of [¹¹C-Carbonyl]sarcosine 2 Methylamine hydrochloride (0.5 mg in 50 μL H₂O) and 37% formaldehyde (0.56 μL in 50 μL H₂O) were added to the reaction vessel of a TRACERLab FXm synthesis module. [¹¹C]NaCN (ca. 900 mCi) was delivered to the reaction vessel from the PETTrace cyclotron, and the reaction was stirred for 5 min at r.t. 10 M NaOH (250 μL) was then added to the reaction vessel and was heated to 60 °C for 5 min to perform the hydrolysis. The crude reaction mixture was cooled, diluted with HPLC mobile phase (0.6 mL), and purified by semipreparative HPLC (column: Phenomenex Luna NH₂ 250 × 10 mm; mobile phase: 10 mM NaH₂PO4 in 60% MeCN, pH 5.6; flow rate: 4 mL/min; t _R = 10 min). The fraction corresponding to [¹¹C-carbonyl]sarcosine was collected into 60 mL of Milli-Q water containing 1 M NaOH (0.2 mL), and the resulting solution was passed through a SAX cartridge to trap the radiotracer. The cartridge was washed with sterile water for injection, USP (SWI) and then [¹¹C-carbonyl]sarcosine and eluted with 2 M NaCl (0.5 mL) into a product collection vial precharged with sterile water (4.3 mL) and sodium phosphates, USP (0.2 mL). The SAX cartridge was rinsed with additional SWI (5 mL) to give a final dose of 10 mL. This dose was passed through a 0.22 μm sterile filter into a sterile dose vial and submitted for QC testing (see Supporting Information for details of QC methods). The yield of [¹¹C-carbonyl]sarcosine 2 was 30±12 mCi (n = 3), corresponding to 1% isolated and formulated end-of-synthesis radiochemical yield (RCY) based on [¹¹C]CO₂, or 3% based on [¹¹C]HCN). The product was also obtained in high radiochemical purity (RCP ≥90%) and specific activity (SA ≥1500 Ci/mmol using the 0.3 μg/mL HPLC limit-of-detection for sarcosine).
• 14 General Procedure (Table 3, Entries 1–4) The carbonyl compound (1 equiv) and amine (1–3 equiv) in water and/or EtOH were added to the reaction vessel of a TRACERLab FXm synthesis module. [¹¹C]NaCN (ca. 100–800 mCi) in H₂O (200–250 μL) was added, and reaction was stirred for 5–7 min at r.t. to 100 °C. After this time, 10 M NaOH (250–350 μL) was added, and the reaction mixture was heated at 60–100 °C for 5–7 min. The reaction was cooled, diluted, and purified by semipreparative HPLC to yield [¹¹C-carbonyl] methionine 4 (RCY = 5%; RCP >90%; SA = 1256 Ci/mmol), [¹¹C-carbonyl]glycine 5 (RCY = 14%; RCP >95%; SA > 1500 Ci/mmol), or [¹¹C-carbonyl]-N-phenylglycine 6 (RCY = 2%; RCP >99%; SA = 15453 Ci/mmol).
• 15 Baeza A, Nájera C, Sansano JM. Synthesis 2007; 1230