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.

Uptake and kinetics of 5-aminolevulinic acid in oral squamous cell carcinoma.

Photodynamic diagnosis (PDD) is a form of cancer detection based on the administration of an exogenous photo-activated compound that accumulates in malignant cells, followed by appropriate photo irradiation. The authors describe a spectroscopic study of 5-aminolevulinic acid (5-ALA)-generated photosensitizer protoporphyrin IX (PpIX) fluorescence in human oral squamous cell carcinoma (OSCC) cell lines to validate its clinical use. 5-ALA-induced PpIX fluorescence intensity was measured in the presence and absence of deferoxamine mesylate (DFO). Two, one and two cell lines produced poorly, moderately and well differentiated carcinomas, respectively, on transplantation in scid mice. The fluorescence intensity was high in the poorly differentiated cell lines, followed by the moderately differentiated cell line; the intensity of the well differentiated cell lines was low and not significantly different from that of normal keratinocytes in the absence of DFO. It was elevated to the level of poorly differentiated cell lines following DFO treatment. This elevation was not observed in normal keratinocytes. The results indicate that DFO enhances the photodynamic sensitivity of 5-ALA in non-responsive carcinoma cells as a result of increased cellular accumulation by inhibiting haeme biosynthesis. This PDD system can be applied clinically to the detection of OSCC irrespective of the degree of differentiation.

The influx of neutral amino acids into the porcine brain during development: a positron emission tomography study.

Pigs of three different age groups (newborns, 1 week old, 6 weeks old) were used to study the transport of the large neutral amino acids 6-[18F]fluoro-L-DOPA ([18F]FDOPA) and 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]OMFD) across the blood-brain barrier (BBB) with positron emission tomography (PET). Compartmental modeling of PET data was used to calculate the blood-brain clearance (K1) and the rate constant for the brain-blood transfer (k2) of [18F]FDOPA and [18F]OMFD after i.v. injection. A 40-70% decrease of K1(OMFD), K1(FDOPA) and k2(OMFD) from newborns to juvenile pigs was found whereas k2(FDOPA) did not change. Generally, K1(OMFD) and k2(OMFD) are lower than K1(FDOPA) and k2(FDOPA) in all regions and age groups. The changes cannot be explained by differences in brain perfusion because the measured regional cerebral blood flow did not show major changes during the first 6 weeks after birth. In addition, alterations in plasma amino acids cannot account for the described transport changes. In newborn and juvenile pigs, HPLC measurements were performed. Despite significant changes of single amino acids (decrease: Met, Val, Leu; increase: Tyr), the sum of large neutral amino acids transported by LAT1 remained unchanged. Furthermore, treatment with a selective inhibitor of the LAT1 transporter (BCH) reduced the blood-brain transport of [18F]FDOPA and [18F]OMFD by 35% and 32%, respectively. Additional in-vitro studies using human LAT1 reveal a much lower affinity of FDOPA compared to OMFD or L-DOPA. The data indicate that the transport system(s) for neutral amino acids underlie(s) developmental changes after birth causing a decrease of the blood-brain barrier permeability for those amino acids during brain development. It is suggested that there is no tight coupling between brain amino acid supply and the demands of protein synthesis in the brain tissue.

Transport of amino acid esters and the amino-acid-based prodrug valganciclovir by the amino acid transporter ATB(0,+).

PURPOSE: The purpose of this study was to analyze the transport of amino acid esters and the amino-acid-based prodrug valganciclovir by the Na(+)/Cl(-)-coupled amino acid transporter ATB(0,+). METHODS: The interaction of amino acid esters and valganciclovir with the cloned rat ATB(0,+) was evaluated in a mammalian cell expression system and in the Xenopus oocyte expression system. RESULTS: In mammalian cells, expression of ATB(0,+) induced glycine uptake. This uptake was inhibited by valine and its methyl, butyl, and benzyl esters. The benzyl esters of other neutral amino acids were also effective inhibitors. Valganciclovir, the valyl ester of ganciclovir, was also found to inhibit ATB(0,+)-mediated glycine uptake competitively. Exposure of ATB(0,+)-expressing oocytes to glycine induced inward currents. Exposure to different valyl esters (methyl, butyl, and benzyl), benzyl esters of various neutral amino acids, and valganciclovir also induced inward currents in these oocytes. The current induced by valganciclovir was saturable with a K0.5 value of 3.1+/-0.7 mM and was obligatorily dependent on Na+ and Cl-. The Na+:Cl-:valganciclovir stoichiometry was 2 or 3:1:1. CONCLUSIONS: Amino acid esters and the amino-acid-based prodrug valganciclovir are transported by ATB(0,+). This shows that ATB(0,+) can serve as an effective delivery system for amino acid-based prodrugs.

Effect of liposomal delivery on in vitro antitumor activity of lipophilic conjugates of methotrexate with lipoamino acids.

Some selected lipophilic conjugates of the antifolate drug methotrexate (MTX) with lipoamino acids (LAA), previously described, were incorporated in liposomes with a different composition and charge (neutral, positive, or negative). The properties of the liposomal systems were determined. The inhibitory activity of the conjugates after incorporation in the vesicles was determined in a preliminary assessment against a human erythroleukemic cell line (K562 cells) and compared with the activity of the parent drug and of free conjugates. The influence of liposome surface charge and of the type of conjugate (i.e., in the carboxylic or ester form) on the biological effect is discussed.

Large neutral amino acids auto exchange when infused by microdialysis into the rat brain: implication for maple syrup urine disease and phenylketonuria.

Maple syrup urine disease (MSUD) and phenylketonuria (PKU) are associated with accumulation of large neutral amino acids (LNAA) in blood and tissues and a decrease of other LNAA not directly related to the enzyme defects. One characteristic shared by both the elevated and decreased amino acids is that all are substrates for transport via the large neutral amino acid transporter. In this study, the blood brain barrier was effectively bypassed using microdialysis to determine the immediate effect of infused phenylalanine, tyrosine, 2-amino-2-norborane-carboxylic acid (BCH), and leucine and alpha-ketoisocaproate on extracellular levels of LNAA. The concentration of non-infused LNAA increased in the interstitial fluid, presumably due to trans-stimulated exchange of these LNAA from intracellular pools as the infused LNAA entered the cells. Such trans-stimulated exchange can potentially deplete cells of multiple essential LNAA. It is proposed that brain cells in disorders such as MSUD and PKU may be subject to two mechanisms that limit the availability of a full complement of these amino acids: competition for transport of LNAAs at the blood brain barrier and trans-stimulated exchange out of neuronal cells for subsequent metabolism or sequestration in the periphery.