2-Bromo-L-phenylalanine

This work describes the synthesis and the tumor affinity testing of no-carrier-added (n.c.a.) p-[(124)I]iodo-L-phenyalanine ([(124)I]IPA) and n.c.a. p-[(131)I]iodo-l-phenyalanine ([(131)I]IPA) as radiopharmaceuticals for imaging brain tumors with PET and for radionuclid-based therapy, respectively. Parameters for labeling were optimized with regard to the amount of precursor, temperature and time. Thereafter, n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA were investigated in rat F98 glioma and in primary human A1207 and HOM-T3868 glioblastoma cells in vitro, followed by an in vivo evaluation in CD1 nu/nu mice engrafted with human glioblastoma. No-carrier-added [(124)I]IPA and n.c.a. [(131)I]IPA were obtained in 90+/-6% radiochemical yield and >99% radiochemical purity by iododestannylation of N-Boc-4-(tri-n-butylstannyl)-L-phenylalanine methylester in the presence of chloramine-T, followed by hydrolysis of the protecting groups. The total synthesis time, including the HPLC separation and pharmacological formulation, was less than 60 min and compatible with a clinical routine production. Both amino acid tracers accumulated intensively in rat and in human glioma cells. The radioactivity incorporation in tumor cells following a 15-min incubation at 37 degrees C/pH 7.4 varied from 25% to 42% of the total loaded activity per 10(6) tumor cells (296-540 cpm/1000 cells). Inhibition experiments confirmed that n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA were taken up into tumor by the sodium-independent L- and ASC-type transporters. Biodistribution and whole-body imaging by a gamma-camera and a PET scanner demonstrated a high targeting level and a prolonged retention of n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA within the xenotransplanted human glioblastoma and a primarily renal excretion. However, an accurate delineation of the tumors in mice was not possible by our imaging systems. Radioactivity accumulation in the thyroid and in the stomach as a secondary indication of deiodination was less than 1% of the injected dose at 24h p.i., confirming the high in vivo stability of the radiopharmaceuticals. In conclusion, n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA are new promising radiopharmaceuticals, which can now be prepared in high radiochemical yields and high purity for widespread clinical applications. The specific and high-level targeting of n.c.a. [(124)I]IPA and n.c.a. [(131)I]IPA to glioma cells in vitro and to glioblastoma engrafts in vivo encourages further in vivo validations to ascertain their clinical potential as agent for imaging and quantitation of gliomas with PET, and for radionuclid-based therapy, respectively.

[125I]-2-iodo-L-phenylalanine, a new radioiodinated phenylalanine analog was evaluated as a potential specific tumor tracer for SPECT. The tracer is obtained with an overall radiochemical yield of at least 98%, a purity of > 99%, and a specific activity of 11 MBq/mmol in one pot Kit conditions using the Cu1+ assisted isotopic exchange. The tracer is evaluated in vitro using R1M rat rabdomyosarcoma cells in HEPES buffer with and without Na+ ions and in MEM buffer. The uptake of [125I]-2-iodo-L-phenylalanine follows a reversible pseudo-first-order reaction which is the same in presence and absence of Na+ ions, but the compound is not incorporated into the cell proteins. The reversible uptake is proven to occur with the same affinity as L-henylalanine by a saturable transport system which is competitively inhibited by BCH, an L transport type selective molecule. Trans-stimulation of the efflux by BCH and typical L transported amino acids shows that the transporter is of the antiport type and fulfils all the properties of the LAT1 heterodimer transport system. [125I]-2-iodo-L-phenylalanine is thus a phenylalanine analog that for the uptake uses for the major part the LAT1 transport system which is known to be over-expressed in tumor cells. This, together with the easy Kit preparation, makes [123I]-2-iodo-L-phenylalanine a promising tumor specific tracer for SPECT.

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.