Naengnoi Limpa-Amara

Melanin-Targeted Preclinical PET Imaging of Melanoma Metastasis

Dialkylamino-alkyl-benzamides possess an affinity for melanin, suggesting that labeling of such benzamides with 18F could potentially produce melanin-targeted PET probes able to identify melanotic melanoma metastases in vivo with high sensitivity and specificity. Methods: In this study, N-[2-(diethylamino)ethyl]-4-18F-fluorobenzamide (18F-FBZA) was synthesized via a 1-step conjugation reaction. The σ-receptor binding affinity of 19F-FBZA was determined along with the in vitro cellular uptake of radiofluorinated 18F-FBZA in B16F10 cells. In vivo distribution and small-animal PET studies were conducted on mice bearing B16F10 melanoma, A375M amelanotic melanoma, and U87MG tumors, and comparative studies were performed with 18F-FDG PET in the melanoma models. Results: In vitro, uptake of 18F-FBZA was significantly higher in B16F10 cells treated with l-tyrosine (P < 0.001). In vivo, 18F-FBZA displayed significant tumor uptake; at 2 h, 5.94 ± 1.83 percentage injected dose (%ID) per gram was observed in B16F10 tumors and only 0.75 ± 0.09 %ID/g and 0.56 ± 0.13 %ID/g was observed in amelanotic A375M and U87MG tumors, respectively. Lung uptake was significantly higher in murine lungs bearing melanotic B16F10 pulmonary metastases than in normal murine lungs (P < 0.01). Small-animal PET clearly identified melanotic lesions in both primary and pulmonary metastasis B16F10 tumor models. Coregistered micro-CT with small-animal PET along with biopsies further confirmed the presence of tumor lesions in the mouse lungs. Conclusion: 18F-FBZA specifically targets primary and metastatic melanotic melanoma lesions with high tumor uptake and may have translational potential.

Adsorption of metallic radionuclides on plastic phantom walls

Metal radionuclide solutions at neutral pH adhere to plastic containers. Adsorption of radionuclides on the walls of phantoms leads to a nonuniform activity distribution, which could adversely affect imaging studies, as well as phantom-based validations of absorbed dose calculations used in radioimmunotherapy, requiring accurate knowledge of the underlying activity distribution. In the work reported here, the authors determined the degree of metal chelation required to minimize metallic radionuclide oxide formation and adsorption on phantom walls in order to yield more reliable experimental data for validating image-based dosimetry. Using hollow spherical plastic phantoms, the authors evaluated three different radionuclides, I-131, In-111, and Y-90, in solutions containing three different concentrations of the chelator, ethylenediaminetetraacetate (EDTA). Adsorption to plastic walls was determined using microSPECT imaging and/or by counting aliquots of solutions. Reconstructed images and measurements of I-131 activity showed that it was uniformly distributed within all spheres; however, images of In-111 in 0.25-microM EDTA indicated that the activity concentration near the wall was much higher than that in the middle of the sphere. The decrease in activity concentration near the center of the spheres was approximately 47%. Y-90 in 0.25-microM EDTA behaved similarly; the activity concentration of Y-90 decreased by 46%. For an In-111 or Y-90 radioactivity concentration of 0.74 MBq/mL, a 2.5-microM EDTA solution was required to achieve a uniform distribution, suggesting that, under our experimental conditions, approximately 700 EDTA molecules were required for each radiometal atom to prevent precipitation and adsorption on poly(methylmethacrylate). For certain radiometals, e.g., In-111 or Y-90, adequate chelation is essential to achieve uniform activity concentration values and homogeneous distribution within the phantom compartments.

Tumor uptake of 99mTc-MIBI and 201T1 by a 9L gliosarcoma brain tumor model in rats

There have been several recent case reports of the accumulation of 99mTc-MIBI [hexakismethoxyisobutylisonitriletechnetium(I), Cardiolite, Sestamibi] in tumors, but no reports of the uptake of this radiopharmaceutical in an animal model. To address this question, the biodistributions of 99mTc-MIBI and 201Tl were compared in Fisher rats bearing 9L gliosarcomas. The results showed that, although the absolute uptake of the tracers by the tumor is relatively low (< 1% ID/g), the tumor-to-normal brain ratios are greater than 6:1 because of low uptake by normal brain. The tumor-to-normal brain ratio of 99mTc-MIBI exceeds that of other currently available 99mTc radiopharmaceuticals suggesting that 99mTc-MIBI may be of particular value in the clinical evaluation of brain tumors and that further investigation of this class of compounds as tumor-avid radiopharmaceuticals is necessary.

A new approach to labeling cells with technetium-99m, part I. Preparation of modified polylysine and in vitro cell labeling☆

A method for labeling cells with technetium-99m via hydrophilic, polycationic poly D-lysine modified by N-acetyl homocysteine has been developed. The modified polylysine (MPL) is labeled with 99mTc in > 95% yield and is stable for > 12 h. Maximum cell labeling is achieved by a 1-h incubation at room temperature with isolated leukocytes, granulocytes and peripheral blood mononuclear cells attaining 60-75% 99mTc incorporation, and red blood cells 35%. Ninety-two percent of the label is retained by leukocytes after a 1-h incubation at room temperature in 50% serum. The cell uptake of 99mTc-MPL is affected by the presence of negatively charged species in the medium; the inhibitory effects of 5% serum or serum albumin can be reversed by increasing the concentration of 99mTc-MPL, while those of heparin are not.

Validation of absorbed-dose calculations using Y-90 in different source and target geometries

Because of increasing interest in patient-image-based dosimetry, dosimetry validation in phantoms would be useful, particularly for modeling complex source-target geometries. We have devised a fluorescence technique to measure doses to liquid-filled compartments. Solutions containing 0.1 mM coumarin-3-carboxylic acid (CCA) with Y-90 (YA) and without Y-90 (NA) were prepared. We measured the self-absorbed dose in six spheres, three cylinders, and two shell-sphere geometries. Each shell-sphere geometry consisted of an inner sphere containing NA solution, surrounded by an outer shell containing YA solution. This enabled us to measure the self-dose to the source shell as well as the cross-dose to the target sphere. After a 24 or 48 hour exposure, the mean dose to each compartment was estimated by measurement of the induced fluorescence in the solutions. A calibration curve obtained using NA solution exposed in a Cs-137 irradiator (98.3 rads/min) for different times displayed a linear increase in fluorescence with increasing dose up to 1500 rads. The self-dose increased with sphere or cylinder size, and agreed within plusmn8.5% with values computed using a Y-90 dose point-kernel method. However, the dose delivered to the cold sphere compartment from the adjacent hot shell, which was a few percent of the self-dose to the hot compartment, showed a large discrepancy between measured and calculated values. CCA fluorescence dosimetry is a promising technique for verification of dose calculations

Assessment of the activity distribution of metal radionuclides in plastic phantoms using μSPECT imaging and gamma counting

At neutral pH, metal radionuclides, e.g., In-111 and Y-90, precipitate and adhere to container walls, yielding nonuniform activity distributions in compartments of plastic phantoms used for dosimetry validation. For various concentrations of a chelator, EDTA, we evaluated the uniformity of activity distributions in phantoms, as well as the accuracy of a fluorescent liquid dosimeter, coumarin-3-carboxylic acid (CCA). Solutions of In-111 and Y-90 with activity concentrations of 10-25 μCi/cc containing increasing EDTA concentrations (0-25 μM) were stored in polymethyl methacrylate (PMMA) phantoms for 24 h to accumulate dose. Activity in the solution was then measured in a gamma counter and/or by μSPECT imaging. Phantoms containing 0.1 mM CCA with EDTA were also irradiated either with a Cs-137 irradiator or with Y-90 to evaluate the effect of EDTA concentration on fluorescence intensity (FI). Activity measurements from images and gamma counting indicated that 40-50% of the activity of In-111 and Y-90 (25 μCi/cc) accumulated on phantom walls for EDTA concentration <2.5 μM, whereas no adherence was observed for EDTA concentration >2.5 μM. For a fixed absorbed dose, fluorescence intensity decreased by <15% when using 2.5 μM EDTA. For all EDTA concentrations, a linear dose-FI relationship was observed up to ~15Gy. For phantom dose measurements using metal radionuclides, solutions should contain ~1,000 EDTA molecules for each radiolabeled atom to obtain a uniform activity distribution.