The slow metabolism of L-[2-18F]-fluorophenylalanine in rat

Quantitative evaluation of neutral amino acid transport in cerebral gliomas using positron emission tomography and fluorine-18 fluorophenylalanine

To elucidate the mechanism of large neutral amino acid (LNAA) transport in cerebral gliomas and to evaluate the clinical usefulness of positron emission tomography (PET) with fluorine-18 fluorophenylalanine (18F-Phe), we examined 18 patients with cerebral glioma using dynamic PET and 18F-Phe. By employing two-compartment model analysis, the influx rate K1, the efflux rate k2 and the distribution volume (Vd) of 18F-Phe were estimated in tumour tissue and contralateral normal grey matter. 18F-Phe showed increased accumulation in tumour tissue regardless of the grade of malignancy in all patients. The rate of uptake of 18F-Phe in high-grade glioma was significantly higher than in low-grade glioma (P <0.05). However, it was difficult to evaluate the tumour grade only from the 18F-Phe accumulation in individual cases. Values of K1 and Vd were significantly increased in the tumour tissue. The K1 value of the tumour tissue tended to decrease with increasing LNAA concentration in plasma. Therefore, influx of 18F-Phe into tumour tissue is mainly related to the carrier-mediated active transport. It is concluded that PET with 18F-Phe is of clinical value for tumour detection rather than assessment of tumour malignancy.

Does the anabolic metabolism of L-[2-18F]fluorophenylalanine and L-[2,6-3H]phenylalanine differ in the cerebrum and the cerebellum?

The anabolism of isotopically labeled amino acids was compared between the cerebrum and the cerebellum in conscious rat at three feeding conditions. After L-[2-18F]fluorophenylalanine and L-[2,6-3H]phenylalanine injections, the incorporation rate of both radioactivity into protein fraction showed no difference between the cerebrum and the cerebellum at normal condition, but the lipid fraction in the cerebellum was higher than that in the cerebrum in any conditions. These results show the usefulness of L-[2-18F]fluorophenylalanine as a positron emission tomography tracer and different anabolic rate of the amino acids to lipid between the cerebrum and cerebellum.

Accumulation of L-[2-(F-18)]fluorophenylalanine in peri-infarct area in a patient with acute cerebral infarction

We studied the brain uptake of amino acid in a patient with acute cerebral infarction with L-[2-(F-18)]fluorophenylalanine and positron emission tomography. The increased accumulation of the ligand was specifically found in the peri-infarct area where oxygen metabolism was still maintained but decreased later in the 72-day follow-up period. The kinetic analysis revealed that increased accumulation was not due to increased transport from the blood to the brain but delayed washout from the brain to the blood. Although the mechanism is still unknown, abnormally high accumulation of L-[F-18]fluorophenylalanine may predict delayed neuronal changes after ischemic insults of the brain.

L-[2-18F]fluorophenylalanine and L-[U-14C]phenylalanine: a comparative study of their transport to rat brain

The transport rates of L-[2-18F]fluorophenylalanine ([18F]Phe) and L-[U-14C]phenylalanine ([14C]Phe) across the blood-brain barrier were compared in saline, large neutral amino acids (NAA) and large basic amino acids (BAA) loaded rats. The fraction of transported tracer (FTT) was expressed as the ratio of brain radioactivity to integrated plasma radioactivity at 5 min after injection of the tracers. Cerebral FTTs of [18F]Phe and [14C]Phe in the control group were 0.042 and 0.045, respectively, and decreased to 0.016 and 0.019 in the NAA loaded group. The ratios of FTT of [18F]Phe and that of [14C]Phe were approx. 0.90, and did not differ among the three groups. These results indicate that the transport of [18F]Phe from the blood to the brain can be considered to be mediated by the NAA transport system and that its transport rate is almost the same as that of [14C]Phe.

Amino acids for the measurement of protein synthesis in vivo by PET

In principle, PET in combination with amino acids labelled with positron-emitting radionuclides and kinetic metabolic models, can quantify local protein synthesis rates in tissue in vivo. These PET measurements have clinical potential in, for example, oncology, neurology and psychiatry. An optimal positron-emitting amino acid for the measurement of PSR has a high protein incorporation, can easily be prepared by automated equipment and has minimal non-protein radioactive metabolites. Presently L-[methyl-11C]methionine, L-[1-11C]leucine, L-[1-11C]tyrosine, L-[1-11C]phenylalanine, L-[1-11C]methionine and L-[2-18F]fluorotyrosine are under evaluation in normal volunteers and/or in patients. Several other amino acids are suggested. No comparison of the clinical usefulness of the different amino acids in man is yet available. Because of the longer half life of 18F compared to 11C, [18F]fluoro amino acids may have advantages over [11C]amino acids for the investigation of tissue with relative slow protein synthesis, such as brain, and for application in institutions with an off site, but nearby cyclotron. The half life of [13N]amino acids is considered to be too short for flexible clinical application. As yet no metabolic compartmental model has been investigated for [13N]amino acids. For routine application reliable preparation of the radiopharmaceutical is essential. Of all the amino acids under evaluation, a reliable, high yield, easy to automate production procedure is available for L-[methyl-11C]methionine only. It is however unlikely that this tracer can accurately measure PSR because of its non-protein metabolism. For the other amino acids the main problems in production are associated with complex multistep syntheses and/or low radiochemical yields, complex purification methods and the need to isolate the L-enantiomer. The kinetic metabolic models under investigation, consist of 4 or 5 compartments depending on the necessity to compensate for labelled metabolites. The metabolic profile of the amino acids is mainly extracted from animal experiments. Because of the number and amount of labelled metabolites in plasma, [11C]carboxylic labelled amino acids are preferred to amino acids with carbon-11 in another position. As yet no recommendation can be given on the optimal labelled amino acid(s) for PSR measurement in vivo nor on the methods to prepare the amino acids reported for this purpose.

Synthesis and radiation dosimetry of 4-borono-2-[18F]fluoro-D,L-phenylalanine: a target compound for PET and boron neutron capture therapy

The 18F-labeling of 4-borono-D-L-phenylalanine (BPA), a potential target compound for cancer treatment with boron neutron capture therapy, is described. By direct fluorination of BPA with [18F]AcOF or [18F]F2 followed by HPLC separation, 4-borono-2-[18F]fluoro-D,L-phenylalanine was prepared with radiochemical yields of 25-35% and with a radiochemical purity of over 99%. The tissue distribution study showed that the compound has potential as a tracer for pancreas imaging with positron emission tomography. Radiation dosimetry is also described.