Characterization of uptake of 3-[131I]iodo-α-methyl-L-tyrosine in human monocyte-macrophages

Radiometal-theranostics: the first 20 years*

This review describes the basic principles of radiometal-theranostics and its dawn based on the development of the positron-emitting 86Y and 86Y-labeled radiopharmaceuticals to quantify biodistribution and dosimetry of 90Y-labeled analogue therapeutics. The nuclear and inorganic development of 86Y (including nuclear and cross section data, irradiation, radiochemical separation and recovery) led to preclinical and clinical evaluation of 86Y-labeled citrate and EDTMP complexes and yielded organ radiation doses in terms of mGy/MBq 90Y. The approach was extended to [86/90Y]Y-DOTA-TOC, yielding again yielded organ radiation doses in terms of mGy/MBq 90Y. The review further discusses the consequences of this early development in terms of further radiometals that were used (68Ga, 177Lu etc.), more chelators that were developed, new biological targets that were addressed (SSTR, PSMA, FAP, etc.) and subsequent generations of radiometal-theranostics that resulted out of that.

Effect of α-Methyl versus α-Hydrogen Substitution on Brain Availability and Tumor Imaging Properties of Heptanoic [F-18]Fluoroalkyl Amino Acids for Positron Emission Tomography (PET)

Two [(18)F]fluoroalkyl substituted amino acids differing only by the presence or absence of a methyl group on the α-carbon, (S)-2-amino-7-[(18)F]fluoro-2-methylheptanoic acid ((S)-[(18)F]FAMHep, (S)-[(18)F]14) and (S)-2-amino-7-[(18)F]fluoroheptanoic acid ((S)-[(18)F]FAHep, (S)-[(18)F]15), were developed for brain tumor imaging and compared to the well-established system L amino acid tracer, O-(2-[(18)F]fluoroethyl)-l-tyrosine ([(18)F]FET), in the delayed brain tumor (DBT) mouse model of high-grade glioma. Cell uptake, biodistribution, and PET/CT imaging studies showed differences in amino acid transport of these tracer by DBT cells. Recognition of (S)-[(18)F]15 but not (S)-[(18)F]14 by system L amino acid transporters led to approximately 8-10-fold higher uptake of the α-hydrogen substituted analogue (S)-[(18)F]15 in normal brain. (S)-[(18)F]15 had imaging properties similar to those of (S)-[(18)F]FET in the DBT tumor model while (S)-[(18)F]14 afforded higher tumor to brain ratios due to much lower uptake by normal brain. These results have important implications for the future development of α-alkyl and α,α-dialkyl substituted amino acids for brain tumor imaging.

Synthesis, uptake mechanism characterization and biological evaluation of (18)F labeled fluoroalkyl phenylalanine analogs as potential PET imaging agents

INTRODUCTION

Amino acids based tracers represent a promising class of tumor metabolic imaging agents with successful clinical applications. Two new phenylalanine derivatives, p-(2-[(18)F]fluoroethyl)-L-phenylalanine (FEP, [(18)F]2) and p-(3-[(18)F]fluoropropyl)-L-phenylalanine (FPP, [(18)F]3) were synthesized and evaluated in comparison to clinically utilized O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET, [(18)F]1).

METHODS

FEP ([(18)F]2) and FPP ([(18)F]3) were successfully synthesized by a rapid and efficient two-step nucleophilic fluorination of tosylate precursors and deprotection reaction. In vitro cell uptake studies were carried out in 9L glioma cells. In vivo studies, 9L tumor xenografts were implanted in Fisher 344 rats.

RESULTS

FEP ([(18)F]2) and FPP ([(18)F]3) could be efficiently labeled within 90 min with good enantiomeric purity (>95%), good yield (11-37%) and high specific activity (21-69 GBq/μmol). Cell uptake studies showed FEP had higher uptake than FPP as well as reference ligand FET ([(18)F]1). Uptake mechanism studies suggested that FEP is a selective substrate for system L and prefers its subtype LAT1. In vivo biodistribution studies demonstrated FEP had specific accumulation in tumor cells and tumor to background ratio reached 1.45 at 60 min. Small animal positron emission tomography (PET) imaging studies showed FEP was comparable to FET for imaging rats bearing 9L tumor model. FEP had high uptake in 9L tumor compared to surrounding tissue and was quickly excreted through urinary tract.

CONCLUSION

Biological evaluations indicate that FEP ([(18)F]2) is a potential useful tracer for tumor imaging with PET.

Uptake of 3-[125I]iodo-alpha-methyl-L-tyrosine into colon cancer DLD-1 cells: characterization and inhibitory effect of natural amino acids and amino acid-like drugs

INTRODUCTION

We examined 3-[(123)I]iodo-alpha-methyl-L-tyrosine ([(123)I]IMT) uptake and inhibition by amino acids and amino acid-like drugs in the human DLD-1 colon cancer cell line, to discuss correlation between the inhibition effect and structure.

METHODS

Expression of relevant neutral amino acid transporters was examined by real-time PCR with DLD-1 cells. The time course of [(125)I]IMT uptake, contributions of transport systems, concentration dependence and inhibition effects by amino acids and amino acid-like drugs (1 mM) on [(125)I]IMT uptake were examined.

RESULTS

Expression of system L (4F2hc, LAT1 and LAT2), system A (ATA1, ATA2) and system ASC (ASCT1) was strongly detected; system L (LAT3, LAT4) and MCT8 were weakly detected; and B(0)AT was not detected. [(125)I]IMT uptake in DLD-1 cells involved Na(+)-independent system L primarily and Na(+)-dependent system(s). Uptake of [(125)I]IMT in Na(+)-free buffer followed Michaelis-Menten kinetics, with a K(m) of 78 microM and V(max) of 333 pmol/10(6) cells per minute. Neutral D- and L-amino acids with branched or aromatic large side chains inhibited [(125)I]IMT uptake. Tyrosine analogues, tryptophan analogues, L-phenylalanine and p-halogeno-L-phenylalanines, and gamma amino acids [including 3,4-dihydroxy-L-phenylalanine (L-DOPA), DL-threo-beta-(3,4-dihydroxyphenyl)serine (DOPS), 4-[bis(2-chloroethyl)amino]-L-phenylalanine and 1-(aminomethyl)-cyclohexaneacetic acid] strongly inhibited [(125)I]IMT uptake, but L-tyrosine methyl ester and R(+)/S(-)-baclofen weakly inhibited uptake. The substrates of system ASC and A did not inhibit [(125)I]IMT uptake except L-serine and D/L-cysteine.

CONCLUSIONS

[(125)I]IMT uptake in DLD-1 cells involves mostly LAT1 and its substrates' (including amino acid-like drugs derived from tyrosine, tryptophan and phenylalanine) affinity to transport via LAT1. Whether transport of gamma amino acid analogues is involved in LAT1 depends on the structure of the group corresponding to the amino acid residue. Beta-hydroxylation may confer reduction of transport affinity of tyrosine analogues via LAT1.

Stimulation of 125I-3-iodo-alpha-methyl-L-tyrosine uptake in Chinese hamster ovary (CHO-K1) cells by tyrosine esters

INTRODUCTION

Transport of the amino acid analog (123)I-3-iodo-alpha-methyl-L-tyrosine, which is used in clinical SPECT imaging, occurs mainly via L-type amino acid transporter type 1 (LAT1; an amino acid exchanger). As LAT1 is highly expressed in actively proliferating tumors, we made a preliminary investigation of the effects of amino acid esters on enhancement of (125)I-3-iodo-alpha-methyl-L-tyrosine (IMT) uptake via LAT1 in Chinese hamster ovary (CHO-K1) cells.

METHODS

Because the sequence of the CHO-K1 LAT1 gene is not available, we confirmed LAT1 expression through IMT (18.5 kBq) uptake mechanisms using specific inhibitors. L-Gly, L-Ser, L-Leu, L-Phe, L-Met, L-Tyr, D-Tyr, L-Val and L-Lys ethyl/methyl esters were tested in combination with IMT. Time-course studies over a 3-h period were conducted, and the concentration dependence of L-Tyr ethyl and methyl esters (0.001 to 10 mM) in combination with IMT was also examined. For a proof of de-esterification of L- and D-Tyr ethyl and methyl esters in the cells (by enzymatic attack or other cause), the concentration of L- and D-Tyr was analyzed by high-performance liquid chromatography of the esters in phosphate buffer (pH 7.4) and cell homogenates at 37 degrees C or under ice-cold conditions.

RESULTS

Inhibition tests suggested that LAT1 is involved in IMT uptake by CHO-K1 cells. Co-administration of 1 mM of l-Tyr ethyl or methyl ester with IMT produced the greatest enhancement. The de-esterification reaction was stereo selective and temperature dependent in the homogenate. De-esterification kinetics were very fast in the homogenate and very slow in the phosphate buffer.

CONCLUSIONS

The L-Tyr ethyl or methyl esters were the most effective enhancers of IMT uptake into CHO-K1 cells and acted by trans-stimulation of the amino acid exchange function of LAT1. This result suggests that de-esterification in the cells may be caused by enzymatic attack. We will use IMT and L-Tyr ethyl or methyl esters to examine LAT1 function in tumor cells or tissues in vivo.

In vitro characterization of the thyroidal uptake of O-(2-[18F]fluoroethyl)-l-tyrosine

OBJECTIVES

Positron emission tomography (PET) using O-(2-[(18)F]fluoroethyl)-l-tyrosine (FET) has been successfully employed in the diagnostic workup of brain tumors. Knowledge on the mechanisms of the uptake of radiolabeled amino acids into thyroidal tissues and well-differentiated thyroid carcinomas is limited. We therefore studied several factors potentially governing the uptake of FET in the rat thyroid cell line FRTL-5 in comparison with thyroid tumor cell lines of human origin.

METHODS

FET uptake was determined in thyroid-stimulating hormone (TSH)-stimulated and TSH-deprived FRTL-5 cells, as well as in the cell lines U-138 MG (human glioblastoma), Onco DG-1 (human papillary thyroid carcinoma) and ML-1 (human follicular thyroid carcinoma). The TSH responsiveness of cells was measured by the incubation of TSH-treated and untreated control cells with 2-[(18)F]fluoro-2-deoxyglucose (FDG). All cellular tracer uptake values were related to total protein mass and expressed as percentage per milligram. For countertransport studies, FRTL-5 cells were exposed to 10-300 microM tyrosine methyl ester. TSH-stimulated and TSH-deprived FRTL-5 cells were incubated with 100 kBq/ml FET for 20 min. 2-Aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH), alpha-(methylamino)-isobutyric acid, L-serine and tryptophan were used as competitive inhibitors of FET uptake. All inhibition experiments were repeated with the human thyroid carcinoma cell lines to obtain comparative FET uptake values.

RESULTS

The FET uptake was 155+/-30%/mg in FRTL-5 cells (n=6), 108+/-14%/mg in U-138 MG cells (n=6), 194+/-60%/mg in ML-1 cells (n=9) and 64+/-23%/mg in Onco DG-1 cells (n=6) under identical incubation conditions. Preloading with tyrosine methyl ester increased cellular FET uptake dose dependently in FRTL-5 cells (165+/-25%, n=6). While TSH increased the uptake of FDG in FRTL-5 cells by sixfold, there was no TSH effect on FET accumulation. FET uptake by TSH-treated FRTL-5 cells was sodium independent and significantly inhibited by BCH (91.4+/-3.0%, n=9), tryptophan (94.8+/-1.6%, n=8) and serine (83.2+/-10.8%, n=12). TSH-starved FRTL-5 cells had a sodium-dependent component with a similar inhibition pattern. Onco DG-1 mainly confirmed the inhibition pattern of FET uptake in FRTL-5 cells, reflecting System-L-mediated FET uptake that was blocked by BCH and serine (72-85%, n=9). ML-1 cells revealed a pronounced sodium-dependent FET uptake that was inhibited by tryptophan (70+/-10%, n=9, P<.05) in the presence and in the absence of sodium, suggesting a contribution of alternative amino acid carriers.

CONCLUSION

FET uptake by FRTL-5 cells is not TSH dependent. FET uptake by FRTL-5 cells seems to be mainly mediated by a carrier exhibiting the characteristics of the System L amino acid transporter. FET uptake in thyroid cells and thyroid carcinoma cells was in the same range as that in a glioblastoma cell line. This encourages further research efforts towards the clinical evaluation of FET for the diagnostic workup of well-differentiated thyroid carcinomas.