Monte Carlo simulation study to explore optimum conditions for Astatine-211 SPECT.

211At is a promising nuclide for targeted radioisotope therapy. Direct imaging of this nuclide is important for in vivo evaluation of its distribution. We investigated suitable conditions for single-photon emission computed tomography (SPECT) imaging of 211At and assessed their feasibility using a homemade Monte Carlo simulation code, MCEP-SPECT. Radioactivity concentrations of 5, 10, or 20 kBq/mL were distributed in six spheres in a National Electrical Manufactures Association (NEMA) body phantom with a background of 1 kBq/mL. The energy window, projection number, and acquisition time were 71-88 keV, 60, and 60 s, respectively, per projection. A medium-energy collimator and three low-energy collimators were tested. SPECT images were reconstructed using the ordered subset expectation maximization (OSEM) method with attenuation correction (Chang method) and scatter correction (triple-energy-windows method). Image quality was evaluated using the contrast-to-noise ratio (CNR) for detectability and the contrast recovery coefficient (CRC) for quantitavity. The low-energy, high-sensitivity collimator exhibited the best detectability among the four types of collimators, with a maximum CNR value of 43. In contrast, the low-energy, high-resolution collimator exhibited excellent quantitavity, with a maximum CRC value of 102%. Scatter correction improved the image quality. In particular, the CRC value almost doubled after scatter correction. The detection of spheres smaller than 20 mm in diameter was difficult. In summary, low-energy collimators were suitable for the SPECT imaging of 211At. In addition, scatter correction was extremely effective in improving the image quality. The feasibility of 211At SPECT was demonstrated for lesions larger than 20 mm.

cGMP production of astatine-211-labeled anti-CD45 antibodies for use in allogeneic hematopoietic cell transplantation for treatment of advanced hematopoietic malignancies.

The objective of this study was to translate reaction conditions and quality control methods used for production of an astatine-211(211At)-labeled anti-CD45 monoclonal antibody (MAb) conjugate, 211At-BC8-B10, from the laboratory setting to cGMP production. Five separate materials were produced in the preparation of 211At-BC8-B10: (1) p-isothiocyanato-phenethyl-closo-decaborate(2-) (B10-NCS), (2) anti-CD45 MAb, BC8, (3) BC8-B10 MAb conjugate, (4) [211At]NaAt, and (5) 211At-BC8-B10. The 211At-labeling reagent, B10-NCS, was synthesized as previously reported. BC8 was produced, then conjugated with B10-NCS under cGMP conditions to form BC8-B10. [211At]NaAt was produced by α-irradiation of Bi targets, followed by isolation of the 211At using a "wet chemistry" method. The clinical product, 211At-BC8-B10, was prepared by reacting [211At]NaAt with BC8-B10 in NH4OAc buffer (pH 5.5) for 2 min at room temperature, followed by size-exclusion chromatography purification. Quality control tests conducted on the 211At-BC8-B10 included evaluations for purity and identity, as well as pyrogen and sterility tests. Stability of the 211At-BC8-B10 in 25 mg/mL sodium ascorbate solution was evaluated at 1, 2, 4, 6 and 21 h post isolation. For qualification, three consecutive 211At-BC8-B10 clinical preparations were successfully conducted in the cGMP suite, and an additional cGMP clinical preparation was carried out to validate each step required to deliver 211At-BC8-B10 to a patient. These cGMP preparations provided 0.80-1.28 Gbq (21.5-34.5 mCi) of 211At-BC8-B10 with radiochemical purity of >97%. The preparations were found to be sterile and have a pyrogen level <0.50 EU/mL. Cell binding was retained by the 211At-BC8-B10. 211At-BC8-B10 in ascorbic acid solution demonstrated a radiochemical stability of >95% for up to 21 h at room temperature. The experiments conducted have defined conditions for translation of 211At-BC8-B10 production from the laboratory to cGMP suite. This study has allowed the initiation of a phase I/II clinical trial using 211At-BC8-B10 (NCT03128034).

Astatine-211 imaging by a Compton camera for targeted radiotherapy.

Astatine-211 is a promising radionuclide for targeted radiotherapy. It is required to image the distribution of targeted radiotherapeutic agents in a patient's body for optimization of treatment strategies. We proposed to image 211At with high-energy photons to overcome some problems in conventional planar or single-photon emission computed tomography imaging. We performed an imaging experiment of a point-like 211At source using a Compton camera, and demonstrated the capability of imaging 211At with the high-energy photons for the first time.

Evaluation of 209At as a theranostic isotope for 209At-radiopharmaceutical development using high-energy SPECT.

The development of alpha-emitting radiopharmaceuticals using 211At requires quantitative determination of the time-dependent nature of the 211At biodistribution. However, imaging-based methods for acquiring this information with 211At have not found wide-spread use because of its low abundance of decay emissions suitable for external detection. In this publication we demonstrate the theranostic abilities of the 211At/209At isotope pair and present the first-ever 209At SPECT images. The VECTor microSPECT/PET/CT scanner was used to image 209At with a collimator suitable for the 511 keV annihilation photons of PET isotopes. Data from distinct photopeaks of the 209At energy spectrum (195 keV (22.6%), 239 keV (12.4 %), 545 keV (91.0 %), a combined 782/790 keV peak (147 %), and 209Po x-rays (139.0 %)) were independently evaluated for use in image reconstructions using Monte Carlo (GATE) simulations and phantom studies. 209At-imaging in vivo was demonstrated in a healthy mouse injected with 10 MBq of free [209At]astatide. Image-based measurements of 209At uptake in organs of interest-acquired in 5 min intervals-were compared to ex vivo gamma counter measurements of the same organs. Simulated and measured data indicated that-due to the large amount of scatter from high energy (>750 keV) gammas-reconstructed images using the x-ray peak outperformed those obtained from other peaks in terms of image uniformity and spatial resolution, determined to be <0.85 mm. 209At imaging using the x-ray peak revealed a biodistribution that matched the known distribution of free astatide, and in vivo image-based measurements of 209At uptake in organs of interest matched ex vivo measurements within 10%. We have acquired the first 209At SPECT images and demonstrated the ability of quantitative SPECT imaging with 209At to accurately determine astatine biodistributions with high spatial and temporal resolution.

Evaluating 99mTc Auger electrons for targeted tumor radiotherapy by computational methods.

PURPOSE: Technetium-99m (99mTc) has been widely used as an imaging agent but only recently has been considered for therapeutic applications. This study aims to analyze the potential use of 99mTc Auger electrons for targeted tumor radiotherapy by evaluating the DNA damage and its probability of correct repair and by studying the cellular kinetics, following 99mTc Auger electron irradiation in comparison to iodine-131 (131I) beta minus particles and astatine-211 (211At) alpha particle irradiation. METHODS: Computational models were used to estimate the yield of DNA damage (fast Monte Carlo damage algorithm), the probability of correct repair (Monte Carlo excision repair algorithm), and cell kinetic effects (virtual cell radiobiology algorithm) after irradiation with the selected particles. RESULTS: The results obtained with the algorithms used suggested that 99mTc CKMMX (all M-shell Coster-Kroning--CK--and super-CK transitions) electrons and Auger MXY (all M-shell Auger transitions) have a therapeutic potential comparable to high linear energy transfer 211At alpha particles and higher than 131I beta minus particles. All the other 99mTc electrons had a therapeutic potential similar to 131I beta minus particles. CONCLUSIONS: 99mTc CKMMX electrons and Auger MXY presented a higher probability to induce apoptosis than 131I beta minus particles and a probability similar to 211At alpha particles. Based on the results here, 99mTc CKMMX electrons and Auger MXY are useful electrons for targeted tumor radiotherapy.

Preparation and preclinical assessment of folate-conjugated, radiolabelled antibodies.

BACKGROUND: The folate receptor (FR) is frequently over-expressed on human cancer cells and may be a suitable target for radiopharmaceuticals. Because of FR expression in the kidneys, the rapidly renal clearing folate is not well suited as a carrier for therapeutic radionuclides. As an alternative, folate-immunoglobulin conjugates were studied as potential carriers for radionuclides. MATERIALS AND METHODS: Two types of conjugate were evaluated: (i) folate conjugated to osteosarcoma antigen directed murine monoclonal antibodies TP-1 and TP-3 or (ii) folate conjugated to non-specific polyclonal human IgG (HIg6). These constructs were labelled with 211At or 125I. RESULTS: The folate-HIg6-radionuclide conjugate showed high affinity to immobilized folate binding protein and also to folate receptor-expressing cells. Folate conjugates of TP-1 and TP-3 had a selective binding in vitro to antigen-expressing tumor cells and also to cells expressing FR only, thus the folate antibody constructs possessed dual affinity binding. Comparisons between folate-conjugated and non-folated antibody in Balb/C mice showed that the folate did not markedly change the properties of the radiolabelled antibody. CONCLUSION: It was demonstrated that folate-conjugated antibodies carrying therapeutic radionuclides have relevant properties for the targeting of tumor cells expressing FR.

Dosimetry calculations on a tissue level by using the MCNP4c2 Monte Carlo code.

OBJECTIVE: The aim of this study was to develop a MCNP4c2-code and to further refine the small-scale anatomy intestinal dosimetry model based on a EGS4-code developed by Jonsson et al.(1,2) METHOD: The small intestine was modeled as a hexagonal tube system and includes cross-dose contribution from activity in nearby intestine loops. The model includes villi (height, 500 microm), radiosensitive crypt cells (height, 150 microm), and an overlying mucus layer of thicknesses (5-200 microm). The developed intestinal model used in either of the two Monte Carlo codes make it possible to calculate S-values and subsequent mean absorbed dose to the radiation-sensitive crypt cells in the small intestinal wall by considering contributions from the self-dose and from the cross-dose from nearby intestinal loops. Results are given for monoenergetic electrons and photons and for full decay schemes of (99m)Tc, (111)In, (131)I, (67)Ga, (90)Y, and (211)At. RESULTS: Results show that the cross-dose from nearby intestinal loops is significant, and that the fraction of cumulated activity in the intestinal wall contents is important for accurate absorbed-dose estimation. CONCLUSION: It is evident from our study that previous Medical Internal Radiation Dose (MIRD) and International Conference on Radiological Protection (ICRP) models tend to overestimate the absorbed dose to the wall. Our work on the gastrointestinal tract model includes several noticeable refinements, as compared to the MIRD- and ICRP model, and the "onion shell" geometry can easily be transferred to similar geometrical dosimetry applications.

Microdosimetry of astatine-211 single-cell irradiation: role of daughter polonium-211 diffusion.

A microdosimetric analysis of previously published data on 211At-albumin, free 211At, and 211At-C215 irradiation of Colo-205 cells in a slowly rotating single-cell suspension is presented. A custom-built computer program based on the Monte Carlo method was used to simulate the irradiation and the energy deposition in individual cell nuclei. Separate simulations were made for the assumption that the 211Po atom stays in the position where it is created, and that it diffuses away. The mean event number at which 37% of all cells survived, n37, and the frequency mean specific energy per event, zF, were estimated. The Poisson distribution of events and simulated single and multievent distributions of specific energy were used to find the single-cell specific energy at which the probability of survival is reduced to 37%, z37. The calculated single-cell radiosensitivity values show that 211Po atoms, created on a cell surface by the decay of 211At atoms, will diffuse from the cell during its life-span. The increasing distance to the cell nucleus will drastically decrease the probability of the emitted alpha particle to hit the nucleus. This will result in fewer alpha-particle events in the cell nucleus. For dispersed cells, the diffusion of 211Po atoms will reduce the total dose from cell-bound 211At by a factor of 2.

Astatine-211-labeled antibodies for treatment of disseminated ovarian cancer: an overview of results in an ovarian tumor model.

PURPOSE: The aim of the study was to establish and refine a preclinical model to alpha-immunoradiotherapy of ovarian cancer. EXPERIMENTAL DESIGN: At-211 was produced by cyclotron irradiation of a bismuth-209 target and isolated using a novel dry distillation procedure. Monoclonal antibodies were radiohalogenated with the intermediate reagent N-succinimidyl 3-(trimethylstannyl)benzoate and characterized in terms of radiochemical yield and in vitro binding properties. In vitro OVCAR-3 cells were irradiated using an external Cobalt-60 beam, as reference, or At-211-albumin and labeled antibody. Growth assays were used to establish cell survival. A Monte Carlo program was developed to simulate the energy imparted and the track length distribution. Nude mice were used for studies of WBC depression, with various activities of Tc-99m antibodies, as reference, and At-211 antibodies. In efficacy studies, OVCAR-3 cells were inoculated i.p., and animals were treated 2 weeks later. The animals were either dissected 6 weeks later or followed-up for long-term survival. RESULTS: A rapid distillation procedure, as well as a rapid and high-yield, single-pot labeling procedure, was achieved. From growth inhibition data, the relative biological effectiveness of the alpha-emission for OVCAR-3 cells was estimated to be approximately 5, which is in the same range as found in vivo for hematological toxicity. At-211 MOv18 was found to effectively inhibit the development of tumors and ascites, also resulting in long-term survival without significant toxic effect. CONCLUSIONS: Use of the short-range, high-linear energy transfer alpha-emitter At-211 conjugated to a surface epitope-recognizing monoclonal antibody appears to be highly efficient without significant toxicity in a mouse peritoneal tumor model, urging a Phase I clinical trial.

Some microdosmetric data on Astatine-211.

Radionuclides which emit short range, high LET radiations such as alpha and Auger electrons have very promising applications in cancer therapy. Such radionuclides should eventually be incorporated into cell nuclei to achieve high radiotoxic effectiveness. This means that the dose distribution within the cell nucleus at microscopic levels is very important for comparison of the real differences between the radiotoxic effectiveness of different radionuclides. An experimental setup to determine real dose absorption on the microscopic scale is extremely difficult to design. For this reason, calculation procedures for microscopic dose absorption are of special interest for the diagnostic and therapeutic applications of radionuclides which emit short-range and high LET radiations. A specific calculation method for microscopic energy absorptions within the cell nucleus from Auger electrons of 125I was described earlier. In this study, the radiotoxic effectiveness of 211At and 125I has been compared using the data obtained by this calculation method. The data obtained show clearly that the radiotoxicity of the alpha and Auger emitter radionuclide 211At is comparable to that of 125I.

Radiation effects in spheroids of cells exposed to alpha emitters.

PURPOSE: To simulate spheroids of cells and use the spheroid models to establish methods of calculating cell survival following exposure to alpha irradiation from decays of 211At and 213Bi. METHODS: Using a Monte Carlo technique, two models of spheroids of cells were generated in which cells either did not or were allowed to overlap. At 40% packing of the volume it is shown that the two models are equivalent for longer-ranged alpha particles. The overlapping model was used to examine the effects of spheroid size and cell packing on cell survival. Cell survival was also calculated for different distributions of alpha decays such as uniform distribution within the spheroid, external to the spheroid and as a shell on the periphery of the spheroid. RESULTS: Three examples of the cell structure of the spheroids are shown. Detailed calculations show that cell survival decreases from 57% to 37% as the spheroid diameter increases from 75 microm to 225 microm for an average of one 211At decay per cell and 50% packing. Increasing the packing of the cells in the spheroid from 40% to 70% reduces survival from 46% to 26% for 200 microm diameter spheroids for one 211At or 213Bi decay per cell. The presence of small regions of unlabelled cells within the spheroids does not significantly change cell survival. CONCLUSIONS: Monte Carlo programmes generating spheroids of cells and their subsequent use to score cell survival for alpha irradiation should be useful in the design and interpretation of work on spheroids of cells in vitro and the application of such modelling to the study of very small tumours in vivo. Copies of the programmes are available from the author on request.

Theoretical treatment of human haemopoietic stem cell survival following irradiation by alpha particles.

PURPOSE: To calculate survival of human haemopoietic stem cells irradiated by alpha particles from 149Tb (initial energy 3.97 MeV) and 211At (average initial energy 6.87 MeV). METHODS: Following as closely as possible radiobiological data, Monte Carlo methods were used to calculate passages of alpha particles (originating in three geometries) through the nuclei of haemopoietic stem cells. Survival of stem cell populations was calculated from the probability of surviving each passage (a function of LET). RESULTS: For decays targeted to the surface of individual cells 37% survival was found at 1.3 passages per nucleus for 149Tb and 6.5 for 211At/Po decays. For decays distributed in a large volume the D0 doses were 0.81 and 0.87 for 149Tb and 211At respectively. When 36% of the marrow is occupied by fat cells alphas from 149Tb are more effective with a D0 of 0.68Gy compared to 0.82Gy for the 211At/Po combination. CONCLUSIONS: When the isotopes are targeted to the cell, in terms of passages, 149Tb is five times more effective at cell killing than 211At, which when expressed in terms of dose, increases to a factor of 9. When the isotopes are broadly distributed (such as in marrow or in vitro) the differences are considerably reduced.

A model of cell inactivation by alpha-particle internal emitters.

Tumor-associated antibodies labeled with 131I and 90Y have been used in the treatment of malignant disease with some success. The use of alpha-particle-emitting radionuclides as radiolabels offers potential advantages over beta-particle sources. The short range in tissue (< 100 microns) and the high linear energy transfer associated with alpha-particle emitters will result in a more concentrated deposition of energy at the site of radionuclide decay. Thus, if radiolabeled antibodies can be bound to malignant cells specifically, a high differential cell killing can be achieved between the malignant and the normal cells. However, the energy deposition pattern will be strongly dependent upon the configuration of alpha-particle sources relative to the cells, and will consequently impact upon the dose-response characteristics. The purpose of this paper is to study distributions of energy deposition from alpha-particle-emitting radioimmunoconjugates distributed uniformly and nonuniformly around cells through theoretical modeling. Energy deposition spectra for cell nuclei are calculated and used to estimate the survival fraction by a simple biological model. We show that survival curves resulting from nonuniform distributions of alpha-particle-emitting radiolabeled antibodies can depart significantly from the classical exponential survival model applied to external alpha-particle beams. The survival curves may have initial slopes much steeper than those produced by a uniform distribution of sources, and they may also depart from linearity. Furthermore, the results of the modelling indicate how survival curves are dependent on the cell and radiolabel spacing. The results from our model compare reasonably well with published experimental data and can be used to facilitate the design and interpretation of radiobiological experiments.

A microdosimetric model of astatine-211 labeled antibodies for radioimmunotherapy.

Astatine-211 is an alpha-emitter with a short half-life (7.2 hr). This paper discusses the potential of 211At targeted by antibodies for tumor therapy and the possible advantage of 211At over beta- and gamma-emitting radionuclides such as 131I currently employed in the field of radioimmunotherapy. Since the longest range alpha-particle from 211At is only 67 microns and the rate of energy loss is high (track averaged linear energy transfer LT approximately 120 keV/micron), a disintegration of 211At produces a large and extremely localized deposition of energy. A Monte-Carlo model has been developed for studying the stochastic fluctuation of alpha-particle hits and energy deposition in cell nuclei in an attempt to determine the efficacy of 211At-labeled antibodies for tumor cell inactivation. Calculations have been performed for 2 extreme conditions: (a) the case of 211At retained in the capillary, and (b) for a homogeneous distribution of 211At-labeled antibody in the tumor. The results of these two calculations represent the boundary conditions between which any real solution must lie. Finally, developments to the model to include antibody transport across the capillary membrane and through the tumor tissue are discussed.