Radiotracers based on technetium-94m.

This review gives a survey on the use and applications of technetium-94m ((94m)Tc) as a non-conventional positron emission tomography (PET) radionuclide for molecular imaging. The first part of this review describes the production and processing of (94m)Tc. The second part covers basic concepts of technetium coordination chemistry with a special focus on the synthesis of (94m)Tc-labeled compounds for molecular imaging purposes. The review concludes with a summary and an outlook on the prospects of using (94m)Tc in the field of PET chemistry and molecular imaging.

Cyclotron production of (99m)Tc: experimental measurement of the (100)Mo(p,x)(99)Mo, (99m)Tc and (99g)Tc excitation functions from 8 to 18 MeV.

INTRODUCTION: The cyclotron-based (100)Mo(p,2n)(99m)Tc transformation has been proposed as a viable alternative to the reactor based (235)U(n,f)(99)Mo→(99m)Tc strategy for production of (99m)Tc. Despite efforts to theoretically model the amount of ground-state (99g)Tc present at end of bombardment for the (p,2n) reaction, experimental validation has yet to be performed. The co-production of (99g)Tc may have important implications in both the subsequent radiopharmaceutical chemistry and patient dosimetry upon injection. METHODS: To determine the extent of (99g)Tc co-production, we have experimentally measured the (100)Mo(p,x)(99)Mo, (99m)Tc, and (99g)Tc excitation functions in the 8-18 MeV range using a combination of natural abundance and 97.42% enriched (100)Mo foils along with γ-ray spectrometry and ICP-MS. Although the excitation functions for production of (99)Mo and (99m)Tc have been presented previously in the literature, to the best of our knowledge, this work presents the first experimental evaluation of the (100)Mo(p,2n)(99g)Tc excitation function. RESULTS: From the experimental cross-section measurements, the (99m)Tc production yields and (99m)Tc/(99m+g)Tc nuclei ratio were calculated for various thick target irradiation conditions. Results suggest that TBq quantities of (99m)Tc can be achieved with a (99m)Tc/(99m+g)Tc nuclei ratio that is on par with the current (99)Mo/(99m)Tc generator standard eluted at a 24-h frequency. CONCLUSION: These findings suggest that the cyclotron production of (99m)Tc may be a feasible alternative to the current reactor-based production strategy.

A new and simple calibration-independent method for measuring the beam energy of a cyclotron.

This work recommends a new and simple-to-perform method for measuring the beam energy of an accelerator. The proposed method requires the irradiation of two monitor foils interspaced by an energy degrader. The primary advantage of the proposed method, which makes this method unique from previous energy evaluation strategies that employ the use of monitor foils, is that this method is independent of the detector efficiency calibration. This method was evaluated by performing proton activation of (nat)Cu foils using both a cyclotron and a tandem Van de Graaff accelerator. The monitor foil activities were read using a dose calibrator set to an arbitrary calibration setting. Excellent agreement was noted between the nominal and measured proton energies.

Biodistribution and uptake of 3'-deoxy-3'-fluorothymidine in ENT1-knockout mice and in an ENT1-knockdown tumor model.

UNLABELLED: (18)F-3'-Deoxy-3'-fluorothymidine ((18)F-FLT) is a PET tracer that accumulates in proliferating tissues. The current study was undertaken to determine whether equilibrative nucleoside transporter 1 (ENT1) is important for (18)F-FLT uptake in normal tissues and tumors. METHODS: ENT1-knockout (ENT1(-/-)) mice were generated and compared with wild-type (ENT1(+/+)) mice using small-animal (18)F-FLT PET. In addition, ENT1(+/+) mice were also injected with the ENT1 inhibitor nitrobenzylmercaptopurine ribonucleoside phosphate (NBMPR-P) at 1 h before radiotracer injection, followed by (18)F-FLT small-animal PET. Tissues of interest were analyzed for thymidine kinase 1 and nucleoside transporters by immunoblotting and immunohistochemistry, respectively, and plasma thymidine levels were analyzed by liquid chromatography-mass spectrometry. Human lung carcinoma A549 cells were stably transfected with pSUPER-producing short-hairpin RNA against human ENT1 (hENT1) or a scrambled sequence with no homology to mammalian genes (A549-pSUPER-hENT1 and A549-pSUPER-SC, respectively). Cultured transfected cells were characterized for hENT1 transcript levels and (18)F-FLT uptake using real-time polymerase chain reaction and (3)H-FLT uptake assays, respectively. Transfected A549 cells were grown as xenograft tumors in NIH-III mice, which were analyzed by (18)F-FLT small-animal PET. RESULTS: Compared with noninjected ENT1(+/+) mice, ENT1(+/+) mice injected with NBMPR-P and ENT1(-/-) mice displayed a reduced percentage injected dose per gram (%ID/g) for (18)F-FLT in the blood (84 and 81%, respectively) and an increased %ID/g for (18)F-FLT in the spleen (188 and 469%, respectively) and bone marrow (266 and 453%, respectively). ENT1(-/-) mice displayed 1.65-fold greater plasma thymidine levels than did ENT1(+/+) mice. Spleen tissue from ENT1(+/+) and ENT1(-/-) mice displayed similar thymidine kinase 1 protein levels and significant concentrative nucleoside transporter 1 and 3 staining. Compared with A549-pSUPER-SC cells, A549-pSUPER-hENT1 cells displayed 0.45-fold hENT1 transcript levels and 0.68-fold (3)H-FLT uptake. Compared with A549-pSUPER-SC xenograft tumors, A549-pSUPER-hENT1 xenograft tumors displayed 0.76-fold %ID/g values (ex vivo gamma-counts) and 0.65-fold maximum standardized uptake values (PET image analysis) for (18)F-FLT uptake at 1 h after tracer injection. CONCLUSION: Loss of ENT1 activity significantly affected (18)F-FLT biodistribution in mice and (18)F-FLT uptake in xenograft tumors, suggesting that nucleoside transporters are important mediators of (18)F-FLT uptake in normal and transformed cells.

Iodine-124: a promising positron emitter for organic PET chemistry.

The use of radiopharmaceuticals for molecular imaging of biochemical and physiological processes in vivo has evolved into an important diagnostic tool in modern nuclear medicine and medical research. Positron emission tomography (PET) is currently the most sophisticated molecular imaging methodology, mainly due to the unrivalled high sensitivity which allows for the studying of biochemistry in vivo on the molecular level. The most frequently used radionuclides for PET have relatively short half-lives (e.g. 11C: 20.4 min; 18F: 109.8 min) which may limit both the synthesis procedures and the time frame of PET studies. Iodine-124 (124I, t1/2 = 4.2 d) is an alternative long-lived PET radionuclide attracting increasing interest for long term clinical and small animal PET studies. The present review gives a survey on the use of 124I as promising PET radionuclide for molecular imaging. The first part describes the production of 124I. The second part covers basic radiochemistry with 124I focused on the synthesis of 124I-labeled compounds for molecular imaging purposes. The review concludes with a summary and an outlook on the future prospective of using the long-lived positron emitter 124I in the field of organic PET chemistry and molecular imaging.

Enhancement of radiation cytotoxicity in breast-cancer cells by localized attachment of gold nanoparticles.

Gold nanoparticles (GNPs) and modified GNPs having two kinds of functional molecules, cysteamine (AET) and thioglucose (Glu), are synthesized. Cell uptake and radiation cytotoxicity enhancement in a breast-cancer cell line (MCF-7) versus a nonmalignant breast-cell line (MCF-10A) are studied. Transmission electron microscopy (TEM) results show that cancer cells take up functional Glu-GNPs significantly more than naked GNPs. The TEM results also indicate that AET-capped GNPs are mostly bound to the MCF-7 cell membrane, while Glu-GNPs enter the cells and are distributed in the cytoplasm. After MCF-7 cell uptake of Glu-GNPs, or binding of AET-GNPs, the in vitro cytotoxicity effects are observed at 24, 48, and 72 hours. The results show that these functional GNPs have little or no toxicity to these cells. To validate the enhanced killing effect on cancer cells, various forms of radiation are applied such as 200 kVp X-rays and gamma-rays, to the cells, both with and without functional GNPs. By comparison with irradiation alone, the results show that GNPs significantly enhance cancer killing.

Target specificity of 188Re-labeled B27.1 monoclonal antibodies to ovarian cancer cells in vivo.

BACKGROUND: Currently we are exploring a new multistep pretargeting approach involving administration of a bispecific antibody (B27.1 x P54) which has an anti-CA-125 (B27.1) and antibiotin (P54) paratope. It is followed by the administration of radiolabeled biotinylated liposomes to target the 188Re to the ovarian cancer cells. As a preliminary step to realize this goal, we determined the target specificity of the monoclonal antibodies (B27.1) to the ovarian cancer cells in vivo. MATERIALS AND METHODS: B27.1 monoclonal antibodies were photoreduced using UV light and incubated with reduced 188Re for 30 min at 25degreesC. 188Re-labeled B27.1 antibodies were purified using size exclusion chromatography. A comparative biodistribution of Re-B27.1 and 188Re was performed in nude mice xenografted with NIH:OVCAR-3 cells. RESULTS: While free rhenium distributed preferentially into thyroid and stomach with insignificant accumulation in the cancer cells, about 20% of the injected dose of 188Re-B27.1 was recovered in ascites cells with insignificant localization in other organs four hours after administration. CONCLUSION: The study validates the affinity of the B27.1 antibodies to the ovarian cancer cells in vivo.

A quantitative and comparative study of radionuclidic and chemical impurities in water samples irradiated in a niobium target with Havar vs. niobium-sputtered Havar as entrance foils.

Enriched and natural abundance water samples were irradiated in a niobium (Nb) chamber target with Havar and Nb-sputtered Havar foils. Irradiations were performed with 17.5MeV protons at currents from 35 to 100microA lasting for 1-2.5h. Radionuclidic and chemical (cationic) impurities were determined via gamma spectroscopy and ICP-MS, respectively. Anionic impurities were evaluated by ion chromatography. Impurities in water samples irradiated with the Havar-Nb foils were much lower than the samples irradiated with an unmodified Havar foil. No significant differences were observed in the impurity levels between samples of H(2)(18)O-enriched and natural abundance water. Radionuclidic impurities were observed to decrease after 3-4 irradiations on a fresh Havar entrance foil, and reached a constant value for subsequent irradiations with the same integrated current. For targets covered with Havar foil, radionuclidic impurities were found to be proportional to the beam-integrated current regardless of the beam power and, unexpectedly, dependant of the beam power when using a Havar-Nb foil.

Use of a simulated annealing algorithm to fit compartmental models with an application to fractal pharmacokinetics.

Increasingly, fractals are being incorporated into pharmacokinetic models to describe transport and chemical kinetic processes occurring in confined and heterogeneous spaces. However, fractal compartmental models lead to differential equations with power-law time-dependent kinetic rate coefficients that currently are not accommodated by common commercial software programs. This paper describes a parameter optimization method for fitting individual pharmacokinetic curves based on a simulated annealing (SA) algorithm, which always converged towards the global minimum and was independent of the initial parameter values and parameter bounds. In a comparison using a classical compartmental model, similar fits by the Gauss-Newton and Nelder-Mead simplex algorithms required stringent initial estimates and ranges for the model parameters. The SA algorithm is ideal for fitting a wide variety of pharmacokinetic models to clinical data, especially those for which there is weak prior knowledge of the parameter values, such as the fractal models.

Optimization of a two-step desolvation method for preparing gelatin nanoparticles and cell uptake studies in 143B osteosarcoma cancer cells.

PURPOSE: To establish a matrix of parameters to synthesize nanoparticles of different sizes and to investigate the cellular uptake of these nanoparticles by osteosarcoma cancer cells in order to investigate their potential as therapeutic drugdelivery carriers. METHODS: Gelatin A and B were used to synthesize nanoparticles by a two-step desolvation process. Different parameters were investigated, including temperature, pH, concentration of glutaraldehyde, type of desolvating agent and nature of gelatin. For cell uptake studies, Texas Red labeled nanoparticles were incubated with 143B osteosarcoma cells and then evaluated using confocal laser scanning microscopy (CLSM). RESULTS: The systematic investigation of the synthesis parameters showed that it is possible to prepare gelatin-based nanoparticles with different particle sizes and a narrow size distribution. Temperature and nature of the gelatin were the most important synthesis factors. Bioimaging using CLSM showed uptake of the nanoparticles by 143B osteosarcoma cancer cells. CONCLUSIONS: Osteosarcoma cancer cells take up gelatin nanoparticles. This might improve the clinical effectiveness of anti-cancer treatments if nanoparticles are used as a drug delivery system and has important implications for future cancer treatment strategies.

Preliminary results of nanopharmaceuticals used in the radioimmunotherapy of ovarian cancer.

PURPOSE: The treatment of late stage ovarian cancer presents an unmet clinical need for women around the world. A multistep radioimmunotherapeutic (RIT) approach, exploiting the combination of a bispecific monoclonal antibody (BsMAb) with 90Y labelled biotinylated long-circulating liposomes was tested as a potential adjuvant treatment for epithelial ovarian carcinomatosis in an attempt to meet this need. This approach was used to overcome some of the major obstacles associated with conventional strategies, in particular, to increase the amount of radioactivity delivered to the tumor site compared with conventional monoclonal antibody (MAb) radionuclide delivery. We hypothesize that sequential intraperitoneal administration of the targeting and therapeutic moieties provides the basis for an enhanced therapeutic ratio. METHODS: A BsMAb, with anti-CA 125 and anti-biotin epitopes was engineered for use with PEGylated liposomes coated with biotin to deliver the cytotoxic radionuclide 90Y to tumor sites. An in vivo therapy trial was used to test this RIT protocol with Balb/c nude mice (n=29) xenografted with the NIH:OVCAR-3 (CA 125+) human ovarian cancer cell line. RESULTS: A median tumor growth delay of 91 days for the combined treatment group versus 77.7 days for the control group was observed. CONCLUSION: An ongoing tumor growth delay/control study using this model has indicated an appreciable delay in progress of tumor and ascites development in treated vs. control populations.

Beta dose-rate distributions in microscopic spherical tumors for intraperitoneal radioimmunotherapy.

PURPOSE: This work was designed to calculate the radial beta dose-rate profiles through microscopic spherical tumors. Its application is in the treatment of micrometastases in the peritoneal cavity by the intraperitoneal administration of radiolabeled immunoliposomes. METHODS AND MATERIALS: Using previously published data for the dose-rate as a function of distance from a point source of activity, dose-rate profiles through five sizes of tumors (radii: 10 microm, 50 microm, 100 microm, 500 microm, 1 mm) for six different radionuclides ((188)Re, (186)Re, (32)P, (90)Y, (67)Cu, (131)I) were calculated. Dose-rate profiles were calculated for two source geometries: (1) a large bath of radioactivity in which the tumor is submerged, and (2) surface-bound radioactivity that results from tumor targeting. RESULTS: The bath geometry produced profiles that were uniform for sufficiently small tumors. For high-energy emitters (i.e., (90)Y and (188)Re), uniformity was maintained up to a tumor radius of 100 microm. For lower energy emitters (i.e., (67)Cu and (131)I) deviations from uniformity start to appear at a tumor radius of 50 microm. Surface-bound radioactivity produced a much greater range of dose-rates within tumors of all sizes. Lower energy emitters bound to the surface of tumors produce higher dose-rates for very small micrometastases compared with high-energy emitters. Upon consideration of the simultaneous contributions from both source geometries, we believe that liposome-mediated radioimmunotherapy would benefit from the inclusion of a high-energy beta emitter, possibly as a component of a cocktail of radionuclides. CONCLUSIONS: The calculated dose-rate profiles provide a tool for making tumor control probability estimations for micrometastases and for assessing the potential benefit offered by a targeted approach over a nontargeted approach. These calculations also suggest that the inclusion of a high-energy beta emitter is appropriate for this treatment modality.