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In Vivo Evaluation and Dosimetry of ¹²³I-2-Iodo-D-Phenylalanine, a New Potential Tumor-Specific Tracer for SPECT, in an R1M Rhabdomyosarcoma Athymic Mouse Model

¹ Laboratory for Radiopharmacy, Universiteit Gent, Gent, Belgium
² Laboratory for Molecular Imaging and Targeted Radiotherapy, University of Toronto, Toronto, Ontario, Canada
³ Department of Medicinal Physics and Radiation Protection, Universiteit Gent, Gent, Belgium
⁴ Laboratory for Medical Imaging and Physics, Vrije Universiteit Brussel, Brussels, Belgium
⁵ Division of Nuclear Medicine, Gent University Hospital, Gent, Belgium

Earlier reports described the preferential uptake of D-amino acids in tumor-bearing mice. Moreover, it was shown that in tumor cells in vitro the L-amino acid transporter system seemed to lack stereospecificity. Because of the successful results with ^123/125I-2-iodo-L-phenylalanine, ^123/125I-2-iodo-D-phenylalanine was developed, and its tumor-detecting characteristics were evaluated in vivo. Methods: ¹²³I labeling of 2-iodo-D-phenylalanine was performed with a kit formulation by use of Cu¹⁺-assisted nucleophilic exchange. ¹²³I-2-Iodo-D-phenylalanine was evaluated in R1M tumor–bearing athymic mice by dynamic planar imaging (DPI) and dissection. The in vivo stability of the tracer was tested by high-performance liquid chromatography. Tumor tracer retention and tracer contrast were evaluated as a function of time. Two-compartment blood modeling from DPI results and dosimetric calculations from biodistribution results were carried out. Moreover, ¹²⁵I-2-iodo-D-phenylalanine and ¹⁸F-FDG uptake in acute inflammation was investigated. Results: ¹²³I-2-Iodo-D-phenylalanine was metabolically stable. Fast, high, and specific tumor retention was observed. Two-compartment modeling confirmed the fast clearance of the tracer through the kidneys to the bladder, as observed by DPI and dissection. Moreover, compared with the L-isomer, ¹²³I-2-iodo-D-phenylalanine demonstrated faster clearance and faster uptake in the peripheral compartment. No accumulation in the abdomen or in the brain was noted. Dosimetry revealed that ¹²³I-2-iodo-D-phenylalanine demonstrated a low radiation burden comparable to those of ¹²³I-2-iodo-L-phenylalanine and ¹²³I-2-iodo-L-tyrosine. Although ¹²³I-2-iodo-D-phenylalanine showed a tumor retention of only 4%, the tumor contrast was increased up to 350% at 19 h after injection. Conclusion: ¹²³I-2-Iodo-D-phenylalanine is a promising tracer for diagnostic oncologic imaging because of its high, fast, and specific tumor uptake and fast clearance from blood.

Key Words: ¹²³I-2-iodo-D-phenylalanine • radiolabeled amino acid analog • tumor imaging • D-amino acid • biodistribution

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