An alternative and expedient synthesis of radioiodinated 4-iodophenylalanine

Objectives Radioiodinated 4-iodophenylalanine is an amino acid analog that may be used to target LAT1 transporter upregulation in breast cancer. An expedient and high yield synthesis could facilitate further development for both novel imaging and therapy applications.

Methods A novel tin precursor, (S)-tert-butyl 2-(tert-butoxycarbonylamino)-3-(4-(tributylstannyl)phenyl)propanoate, where both amino and acid functions were protected with acid-labile moieties was synthesized from the known, corresponding iodo derivative. This tin precursor was converted to radioiodinated 4-iodophenylalanine (4-[*I]I-Phe) by a two-step or a single step synthesis. The suitability of 4-[¹²⁵I]I-Phe for breast cancer targeting was evaluated by determining its uptake in MCF-7 breast carcinoma cells.

Results Using the two-step process, synthesis of 4-[¹²⁵I]I-Phe provided radiochemical yields of 91.6 ± 2.7% and 83.7 ± 1.7% (n = 5), for the radioiodination and subsequent deprotection steps, respectively; an average radiochemical yield of 94.8 ± 3.4% (n = 5) was obtained using the one-step synthesis. After incubation with MCF-7 breast carcinoma cells for 60 min, uptake was 49.0 ± 0.7% of the input dose; in comparison, the uptake of [¹⁴C]phenylalanine under the same conditions was 55.9 ± 0.5%. Furthermore, the uptake of both tracers was inhibited in similar fashion in a concentration-dependent manner by both unlabeled phenylalanine and 4-iodophenylalanine. With [¹⁴C]phenylalaine as the tracer, IC₅₀ values of 1.45 mM and 2.50 mM were obtained for Phe and I-Phe, respectively; IC₅₀ values for [¹²⁵I]I-Phe inhibition were 1.3 and 1.0 mM.

Conclusions An improved and convenient method for the synthesis of no-carrier-added 4-[*I]phenylalanine was developed and the radiotracer prepared by this route demonstrated an amino acid transporter-mediated uptake in MCF-7 breast cancer cells in vitro that was comparable to that of [¹⁴C]phenylalanine.

Research Support BC073244 US DOD Breast Cancer Research (BBC), P30 CA14236-34 Duke Pilot Project Grant (BBC and GV), NIH R21EB00875 (GV