A mouse model for calculating cross-organ beta doses from yttrium-90-labeled immunoconjugates

Internal absorbed dose calculation in body organs due to injection of Rhenium-188 labeled to Mu-9 antibody

The use of radiopharmaceuticals has gained a special place in the diagnosis and treatment of cancers and evaluation of the function of different organs of the body. In this study, the absorbed dose distribution of organs after injection of 188Re-Mu-9 has been investigated using MIRD method and MCNP-4C simulation code. The 188Re-Mu-9 labeled was injected the mouse body and the amount of 188Re-labeled accumulation was evaluated after 1, 4 and 2 4 h. Having a map of the distribution of radiopharmaceutical activity in the animal body, it is possible to convert it into a human model to obtain the internal dose received by 188Re-Mu-9 injection using the MIRD calculation method and the MCNP simulation code. According to the results of the study, the animal/human model can be acceptable method for dose estimation of antibody-based radiopharmaceuticals.

Skeletal dosimetry in a microCT image-based rat model for external photon irradiation

Human skeletal dosimetry has experienced great developments in radiation protection in recent years by using the heterogeneous skeletal model. While for the rats experimentally used in radiation medicine, the investigation on skeletal dosimetry were mainly based on the homogeneous skeletal model, leading to inaccurate assessments of dose to radiosensitive tissues of red bone marrow (RBM) and bone surface. The purpose of this study is to develop a rat model with heterogeneous skeletal system and to investigate the dose difference in bone tissues for external photon irradiation. The high resolution of microCT images of a rat weighing 335 g were segmented into bone cortical, bone trabecular, bone marrow as well as other organs to construct the rat model. The absorbed dose to bone cortical, bone trabecular and bone marrow were calculated respectively by using Monte Carlo simulation for 22 external monoenergetic photon beams between 10 keV and 10 MeV under four different irradiation geometries conditions (left lateral [LL], right lateral [RL], dorsal-ventral [DV], ventral-dorsal [VD]). The calculated absorbed dose data were expressed as dose conversion coefficients and presented in this article, and the effect of irradiation conditions, photon energies and bone tissues density on the skeletal dose was discussed. The results showed that the dose conversion coefficients varying the photon energy for bone cortical, bone trabecular and bone marrow exhibit different trends and have the same sensitivity to irradiation conditions. The dose difference in bone tissues indicated that bone cortical and bone trabecular have significant attenuation effect on the energy deposition in bone marrow and bone surface for photon energies below 0.2 MeV. The set of dose conversion coefficients in this work can be used to determine the absorbed dose to skeletal system for external photon irradiation and to supplement the rat skeletal dosimetry.

Effects of low-dose oxygen ions on cardiac function and structure in female C57BL/6J mice

PURPOSE

Astronauts in space vehicles beyond low-Earth orbit will be exposed to high charge and energy (HZE) ions, and there is concern about potential adverse effects on the cardiovascular system. Thus far, most animal studies that assess cardiac effects of HZE particles have included only males. This study assessed the effects of oxygen ions (¹⁶O) as a representative ion of the intravehicular radiation environment on the heart of female mice.

MATERIALS AND METHODS

Female C57BL/6 J mice at 6 months of age were exposed to ¹⁶O (600 MeV/n) at 0.25-0.26 Gy/min to a total dose of 0, 0.1, or 0.25 Gy. Cardiac function and abdominal aorta blood velocity were measured with ultrasonography at 3, 5, 7, and 9 months after irradiation. At 2 weeks, 3 months, and 9 months, cardiac tissue was collected to assess collagen deposition and markers of immune cells.

RESULTS

Ultrasonography revealed increased left ventricle mass, diastolic volume and diameter but there was no change in the abdominal aorta. There was no indication of cardiac fibrosis however, a 75 kDa peptide of left ventricular collagen type III and α-smooth muscle cell actin were increased suggesting some remodeling had occurred. Left ventricular protein levels of the T-cell marker CD2 was significantly increased at all time points, while the neutrophil marker myeloperoxidase was decreased at 2 weeks and 9 months.

CONCLUSIONS

These results taken together suggest ¹⁶O ion exposure did not result in cardiac fibrosis or cardiac dysfunction in female mice. However, it does appear mild cardiac remodeling occurs in response to HZE radiation.

Biodistribution of extracellular vesicles following administration into animals: A systematic review

In recent years, attention has turned to examining the biodistribution of EVs in recipient animals to bridge between knowledge of EV function in vitro and in vivo. We undertook a systematic review of the literature to summarize the biodistribution of EVs following administration into animals. There were time-dependent changes in the biodistribution of small-EVs which were most abundant in the liver. Detection peaked in the liver and kidney in the first hour after administration, while distribution to the lungs and spleen peaked between 2-12 h. Large-EVs were most abundant in the lungs with localization peaking in the first hour following administration and decreased between 2-12 h. In contrast, large-EV localization to the liver increased as the levels in the lungs decreased. There was moderate to low localization of large-EVs to the kidneys while localization to the spleen was typically low. Regardless of the origin or size of the EVs or the recipient species into which the EVs were administered, the biodistribution of the EVs was largely to the liver, lungs, kidneys, and spleen. There was extreme variability in the methodology between studies and we recommend that guidelines should be developed to promote standardization where possible of future EV biodistribution studies.

Quantification of Macrophage-Driven Inflammation During Myocardial Infarction with 18F-LW223, a Novel TSPO Radiotracer with Binding Independent of the rs6971 Human Polymorphism

Myocardial infarction (MI) is one of the leading causes of death worldwide, and inflammation is central to tissue response and patient outcomes. The 18-kDa translocator protein (TSPO) has been used in PET as an inflammatory biomarker. The aims of this study were to screen novel, fluorinated, TSPO radiotracers for susceptibility to the rs6971 genetic polymorphism using in vitro competition binding assays in human brain and heart; assess whether the in vivo characteristics of our lead radiotracer, 18F-LW223, are suitable for clinical translation; and validate whether 18F-LW223 can detect macrophage-driven inflammation in a rat MI model. Methods: Fifty-one human brain and 29 human heart tissue samples were screened for the rs6971 polymorphism. Competition binding assays were conducted with 3H-PK11195 and the following ligands: PK11195, PBR28, and our novel compounds (AB5186 and LW223). Naïve rats and mice were used for in vivo PET kinetic studies, radiometabolite studies, and dosimetry experiments. Rats underwent permanent coronary artery ligation and were scanned using PET/CT with an invasive input function at 7 d after MI. For quantification of PET signal in the hypoperfused myocardium, K1 (rate constant for transfer from arterial plasma to tissues) was used as a surrogate marker of perfusion to correct the binding potential for impaired radiotracer transfer from plasma to tissue (BPTC). Results: LW223 binding to TSPO was not susceptible to the rs6971 genetic polymorphism in human brain and heart samples. In rodents, 18F-LW223 displayed a specific uptake consistent with TSPO expression, a slow metabolism in blood (69% of parent at 120 min), a high plasma free fraction of 38.5%, and a suitable dosimetry profile (effective dose of 20.5-24.5 μSv/MBq). 18F-LW223 BPTC was significantly higher in the MI cohort within the infarct territory of the anterior wall relative to the anterior wall of naïve animals (32.7 ± 5.0 vs. 10.0 ± 2.4 cm3/mL/min, P ≤ 0.001). Ex vivo immunofluorescent staining for TSPO and CD68 (macrophage marker) resulted in the same pattern seen with in vivo BPTC analysis. Conclusion:18F-LW223 is not susceptible to the rs6971 genetic polymorphism in in vitro assays, has favorable in vivo characteristics, and is able to accurately map macrophage-driven inflammation after MI.

Therapy of Myeloid Leukemia using Novel Bispecific Fusion Proteins Targeting CD45 and 90Y-DOTA

Pretargeted radioimmunotherapy (PRIT) has been investigated as a multi-step approach to decrease relapse and toxicity for high-risk acute myeloid leukemia (AML). Relevant factors including endogenous biotin and immunogenicity, however, have limited the use of PRIT with an anti-CD45 antibody streptavidin conjugate and radiolabeled DOTA-biotin. To overcome these limitations we designed anti-murine and anti-human CD45 bispecific antibody constructs using 30F11 and BC8 antibodies, respectively, combined with an anti-yttrium (Y)-DOTA single-chain variable fragment (C825) to capture a radiolabeled ligand. The bispecific construct targeting human CD45 (BC8-Fc-C825) had high uptake in leukemia HEL xenografts [7.8 ± 0.02% percent injected dose/gram of tissue (% ID/g)]. Therapy studies showed that 70% of mice with HEL human xenografts treated with BC8-Fc-C825 followed by 44.4 MBq (1,200 μCi) of ⁹⁰Y-DOTA-biotin survived at least 170 days after therapy, while all nontreated controls required euthanasia because of tumor progression by day 32. High uptake at sites of leukemia (spleen and bone marrow) was also seen with 30F11-IgG1-C825 in a syngeneic disseminated SJL murine leukemia model (spleen, 9.0 ± 1.5% ID/g and bone marrow, 8.1 ± 1.2% ID/g), with minimal uptake in all other normal organs (<0.5% ID/g) at 24 hours after ⁹⁰Y-DOTA injections. SJL leukemia mice treated with the bispecific 30F11-IgG1-C825 and 29.6 MBq (800 μCi) of ⁹⁰Y-DOTA-biotin had a survival advantage compared with untreated leukemic mice (median, 43 vs. 30 days, respectively; P < 0.0001). These data suggest bispecific antibody-mediated PRIT may be highly effective for leukemia therapy and translation to human studies.

Preclinical Voxel-Based Dosimetry in Theranostics: a Review

Due to the increasing use of preclinical targeted radionuclide therapy (TRT) studies for the development of novel theranostic agents, several studies have been performed to accurately estimate absorbed doses to mice at the voxel level using reference mouse phantoms and Monte Carlo (MC) simulations. Accurate dosimetry is important in preclinical theranostics to interpret radiobiological dose-response relationships and to translate results for clinical use. Direct MC (DMC) simulation is believed to produce more realistic voxel-level dose distribution with high precision because tissue heterogeneities and nonuniform source distributions in patients or animals are considered. Although MC simulation is considered to be an accurate method for voxel-based absorbed dose calculations, it is time-consuming, computationally demanding, and often impractical in daily practice. In this review, we focus on the current status of voxel-based dosimetry methods applied in preclinical theranostics and discuss the need for accurate and fast voxel-based dosimetry methods for pretherapy absorbed dose calculations to optimize the dose computation time in preclinical TRT.

18F-FIMP: a LAT1-specific PET probe for discrimination between tumor tissue and inflammation

Positron emission tomography (PET) imaging can assist in the early-phase diagnostic and therapeutic evaluation of tumors. Here, we report the radiosynthesis, small animal PET imaging, and biological evaluation of a L-type amino acid transporter 1 (LAT1)-specific PET probe, ¹⁸F-FIMP. This probe demonstrates increased tumor specificity, compared to existing tumor-specific PET probes (¹⁸F-FET, ¹¹C-MET, and ¹⁸F-FDG). Evaluation of probes by in vivo PET imaging, ¹⁸F-FIMP showed intense accumulation in LAT1-positive tumor tissues, but not in inflamed lesions, whereas intense accumulation of ¹⁸F-FDG was observed in both tumor tissues and in inflamed lesions. Metabolite analysis showed that ¹⁸F-FIMP was stable in liver microsomes, and mice tissues (plasma, urine, liver, pancreas, and tumor). Investigation of the protein incorporation of ¹⁸F-FIMP showed that it was not incorporated into protein. Furthermore, the expected mean absorbed dose of ¹⁸F-FIMP in humans was comparable or slightly higher than that of ¹⁸F-FDG and indicated that ¹⁸F-FIMP may be a safe PET probe for use in humans. ¹⁸F-FIMP may provide improved specificity for tumor diagnosis, compared to ¹⁸F-FDG, ¹⁸F-FET, and ¹¹C-MET. This probe may be suitable for PET imaging for glioblastoma and the early-phase monitoring of cancer therapy outcomes.

Optimization of a table-top x-ray fluorescence computed tomography (XFCT) system

Pencil beam x-ray fluorescence computed tomography (XFCT) has typically used a single spectrometer and prohibitively long scan times. However, detecting backscattered fluorescent x-rays from gold nanoparticles (AuNPs) using multiple spectrometers greatly reduces image noise and scan time. The arrangement of eight spectrometers for combined K-shell and L-shell XFCT was investigated along with a variety of conditions. A 2.5 cm-diameter cylindrical water phantom containing 4 mm-diameter vials with 0.1%-2% AuNP concentrations by weight was modeled by TOPAS, a GEANT4-based Monte Carlo software. The phantom was irradiated to 30 mGy by a 0.5 mm Pb-filtered 120 kVp and 1 mm Al-filtered 30 kVp 1 mm² x-ray pencil beam to yield respective Au K-shell and L-shell fluorescent x-rays, with 50 0.5 mm translation and 2-degree rotation steps. Eight CdTe and silicon drift detector (SDD) spectrometers were placed 2.25 cm away from the isocentre. The respective energy resolution was applied to the detected energy spectra and the spectra were corrected for detector response before extracting the fluorescence signal. Three CdTe and SDD spectrometer configurations (isotropic/backscattered grid/backscattered row arrangements), two CdTe crystal sizes (9 mm²/25 mm²), two scanning techniques (moving/stationary spectrometers) and five vial-edge depths (0-4 mm) were considered in optimizing the contrast-to-noise ratio (CNR) for each XFCT image reconstructed with a maximum-likelihood expectation maximization (MLEM) algorithm. The isotropic spectrometer arrangement had AuNP detection sensitivities of 0.106% for K-shell and 0.132% for L-shell XFCT at 4 mm depth. Comparatively, the backscattered grid arrangement had the best AuNP sensitivity of 0.055% and 0.095%. The highest K-shell (0.044%) and L-shell (0.004%) AuNP sensitivities were found for vials at 0 mm depth. Using stationary spectrometers or the 9 mm² CdTe crystal compromised the CNR. For the best-case arrangement, L-shell XFCT is superior at vial-edge depths less than 3.0 mm. This work demonstrated the importance of spectrometer arrangement and vial depth for improving AuNP sensitivity and will guide the design for our table-top XFCT system.

Sheet beam x-ray fluorescence computed tomography (XFCT) imaging of gold nanoparticles

PURPOSE

X-ray fluorescence computed tomography (XFCT) experiments have typically used pencil beams for data acquisition, which yielded good quality images of gold nanoparticles (AuNP) but prolonged the imaging time. Here we propose three novel collimator geometries for use with faster sheet beam XFCT data acquisition. The feasibility of a multipinhole, parallel, and converging collimator was investigated in a Monte Carlo study.

METHODS

A cylindrical water phantom with 2 cm in diameter and 3 cm in height containing 0.5-2 mm diameter vials with 0.4%-1.6% AuNP concentrations was modelled by FLUKA. A 15 and 81 keV monoenergetic x-ray sheet beam of 0.4 mm in width was used to image the phantom with L-shell and K-shell XFCT, respectively, with a dose of 30 mGy. The collimator thickness for L-shell and K-shell data acquisition was 3.3 and 5.1 mm, respectively. The XFCT images resulting from three collimator geometries were generated using the maximum likelihood expectation maximization (MLEM) iterative reconstruction method. With a resolution of 0.4 mm they were corrected for x-ray attenuation. The sheet beam XFCT images were compared against pencil beam geometry images that were generated using 55 translations. To assess image quality, the contrast-to-noise ratio (CNR) was evaluated for each vial. The Rose criterion was used to determine the lowest AuNP concentration detectable for each image.

RESULTS

Among the three collimator geometry types, the sheet beam L-shell and K-shell parallel collimator XFCT images yielded AuNP sensitivity limits at 0.09% and 0.08%, respectively, for a 2 mm diameter vial. The AuNP sensitivity limits of the pencil beam XFCT images were 0.07% and 0.01% for L-shell and K-shell XFCT, respectively. The L-shell parallel collimator AuNP imaging sensitivity approached that of the pencil beam geometry with a 55-fold reduction in imaging time. The AuNP sensitivity limits for the 1 mm diameter vial for the L-shell and K-shell parallel collimator XFCT images were 0.19% and 0.16%, respectively, and those of the pencil beam XFCT images were 0.08% and 0.01% for L-shell and K-shell XFCT, respectively. The remaining two collimator geometries resulted in a lower CNR and poorer image quality. For a 2 mm diameter vial, the AuNP sensitivity limits for the L-shell and K-shell multipinhole collimator XFCT images were 0.23% and 0.52%, respectively, while for the L-shell and K-shell converging collimator XFCT images the AuNP sensitivity limits were 0.38% and 0.13%, respectively.

CONCLUSION

This work demonstrates the feasibility of sheet beam L-shell XFCT imaging for small animal studies using parallel-oriented lead collimators which can detect AuNP concentrations approaching the level of pencil beam images with reduced imaging time.

Murine-specific Internal Dosimetry for Preclinical Investigations of Imaging and Therapeutic Agents

There is a growing need to estimate the absorbed dose to small animals from preclinical investigations involving diagnostic and therapeutic radiopharmaceuticals. This paper introduces a Monte Carlo-based dosimetry platform called RAPID, which is capable of calculating murine-specific three-dimensional (3D) dose distributions. A comparison is performed between absorbed doses calculated with RAPID and absorbed doses calculated in a commonly used reference mouse phantom called MOBY. Four test mice containing different xenografts underwent serial PET/CT imaging using a novel diagnostic therapy (theranostic) agent NM404, which can be labeled with I for imaging or I for therapy. Using the PET/CT data, 3D dose distributions from I-NM404 were calculated in the mice using RAPID. Mean organ doses in these four test mice were compared to mean organ doses derived by using two previously published I S-values datasets in MOBY. In addition, mean tumor doses calculated in RAPID were compared to mean organ doses derived from unit density spheres. Large differences were identified between mean organ doses calculated in the test mice using RAPID and those derived in the MOBY phantom. Mean absorbed dose percent errors in organs ranged between 0.3% and 333%. Overall, mass scaling improved agreement between MOBY phantom calculations and RAPID, where percent errors were all less than 26%, with the exception of the lung in which percent errors reached values of 48%. Percent errors in mean tumor doses in the test mice and unit density spheres were less pronounced but still ranged between 8% and 23%. This work demonstrates the limitations of using pre-computed S-values in computational phantoms to predict organ doses in small animals from theranostic procedures. RAPID can generate accurate 3D dose distributions in small animals and in turn offer much greater insight on the ability of a given theranostic agent to image and treat diseases.

CD38-bispecific antibody pretargeted radioimmunotherapy for multiple myeloma and other B-cell malignancies

Pretargeted radioimmunotherapy (PRIT) has demonstrated remarkable efficacy targeting tumor antigens, but immunogenicity and endogenous biotin blocking may limit clinical translation. We describe a new PRIT approach for the treatment of multiple myeloma (MM) and other B-cell malignancies, for which we developed an anti-CD38-bispecific fusion protein that eliminates endogenous biotin interference and immunogenic elements. In murine xenograft models of MM and non-Hodgkin lymphoma (NHL), the CD38-bispecific construct demonstrated excellent blood clearance and tumor targeting. Dosimetry calculations showed a tumor-absorbed dose of 43.8 Gy per millicurie injected dose of ⁹⁰Y, with tumor-to-normal organ dose ratios of 7:1 for liver and 15:1 for lung and kidney. In therapy studies, CD38-bispecific PRIT resulted in 100% complete remissions by day 12 in MM and NHL xenograft models, ultimately curing 80% of mice at optimal doses. In direct comparisons, efficacy of the CD38 bispecific proved equal or superior to streptavidin (SA)-biotin-based CD38-SA PRIT. Each approach cured at least 75% of mice at the highest radiation dose tested (1200 µCi), whereas at 600- and 1000-µCi doses, the bispecific outperformed the SA approach, curing 35% more mice overall (P < .004). The high efficacy of bispecific PRIT, combined with its reduced risk of immunogenicity and endogenous biotin interference, make the CD38 bispecific an attractive candidate for clinical translation. Critically, CD38 PRIT may benefit patients with unresponsive, high-risk disease because refractory disease typically retains radiation sensitivity. We posit that PRIT might not only prolong survival, but possibly cure MM and treatment-refractory NHL patients.

Scattered Dose Calculations and Measurements in a Life-Like Mouse Phantom

Anatomically accurate phantoms are useful tools for radiation dosimetry studies. In this work, we demonstrate the construction of a new generation of life-like mouse phantoms in which the methods have been generalized to be applicable to the fabrication of any small animal. The mouse phantoms, with built-in density inhomogeneity, exhibit different scattering behavior dependent on where the radiation is delivered. Computer models of the mouse phantoms and a small animal irradiation platform were devised in Monte Carlo N-Particle code (MCNP). A baseline test replicating the irradiation system in a computational model shows minimal differences from experimental results from 50 Gy down to 0.1 Gy. We observe excellent agreement between scattered dose measurements and simulation results from X-ray irradiations focused at either the lung or the abdomen within our phantoms. This study demonstrates the utility of our mouse phantoms as measurement tools with the goal of using our phantoms to verify complex computational models.

Development of computational small animal models and their applications in preclinical imaging and therapy research

The development of multimodality preclinical imaging techniques and the rapid growth of realistic computer simulation tools have promoted the construction and application of computational laboratory animal models in preclinical research. Since the early 1990s, over 120 realistic computational animal models have been reported in the literature and used as surrogates to characterize the anatomy of actual animals for the simulation of preclinical studies involving the use of bioluminescence tomography, fluorescence molecular tomography, positron emission tomography, single-photon emission computed tomography, microcomputed tomography, magnetic resonance imaging, and optical imaging. Other applications include electromagnetic field simulation, ionizing and nonionizing radiation dosimetry, and the development and evaluation of new methodologies for multimodality image coregistration, segmentation, and reconstruction of small animal images. This paper provides a comprehensive review of the history and fundamental technologies used for the development of computational small animal models with a particular focus on their application in preclinical imaging as well as nonionizing and ionizing radiation dosimetry calculations. An overview of the overall process involved in the design of these models, including the fundamental elements used for the construction of different types of computational models, the identification of original anatomical data, the simulation tools used for solving various computational problems, and the applications of computational animal models in preclinical research. The authors also analyze the characteristics of categories of computational models (stylized, voxel-based, and boundary representation) and discuss the technical challenges faced at the present time as well as research needs in the future.

Comparative Analysis of Bispecific Antibody and Streptavidin-Targeted Radioimmunotherapy for B-cell Cancers

Streptavidin (SA)-biotin pretargeted radioimmunotherapy (PRIT) that targets CD20 in non-Hodgkin lymphoma (NHL) exhibits remarkable efficacy in model systems, but SA immunogenicity and interference by endogenous biotin may complicate clinical translation of this approach. In this study, we engineered a bispecific fusion protein (FP) that evades the limitations imposed by this system. Briefly, one arm of the FP was an anti-human CD20 antibody (2H7), with the other arm of the FP an anti-chelated radiometal trap for a radiolabeled ligand (yttrium[Y]-DOTA) captured by a very high-affinity anti-Y-DOTA scFv antibody (C825). Head-to-head biodistribution experiments comparing SA-biotin and bispecific FP (2H7-Fc-C825) PRIT in murine subjects bearing human lymphoma xenografts demonstrated nearly identical tumor targeting by each modality at 24 hours. However, residual radioactivity in the blood and normal organs was consistently higher following administration of 1F5-SA compared with 2H7-Fc-C825. Consequently, tumor-to-normal tissue ratios of distribution were superior for 2H7-Fc-C825 (P < 0.0001). Therapy studies in subjects bearing either Ramos or Granta subcutaneous lymphomas demonstrated that 2H7-Fc-C825 PRIT is highly effective and significantly less myelosuppressive than 1F5-SA (P < 0.0001). All animals receiving optimal doses of 2H7-Fc-C825 followed by ⁹⁰Y-DOTA were cured by 150 days, whereas the growth of tumors in control animals progressed rapidly with complete morbidity by 25 days. In addition to demonstrating reduced risk of immunogenicity and an absence of endogenous biotin interference, our findings offer a preclinical proof of concept for the preferred use of bispecific PRIT in future clinical trials, due to a slightly superior biodistribution profile, less myelosuppression, and superior efficacy. Cancer Res; 76(22); 6669-79. ©2016 AACR.

Data on biodistribution and radiation absorbed dose profile of a novel (64)Cu-labeled high affinity cell-specific peptide for positron emission tomography imaging of tumor vasculature

New peptide-based diagnostic and therapeutic approaches hold promise for highly selective targeting of cancer leading to more precise and effective diagnostic and therapeutic modalities. An important feature of these approaches is to reach the tumor tissue while limiting or minimizing the dose to normal organs. In this context, efforts to design and engineer materials with optimal in vivo targeting and clearance properties are important. This Data In Brief article reports on biodistribution and radiation absorbed dose profile of a novel high affinity radiopeptide specific for bone marrow-derived tumor vasculature. Background information on the design, preparation, and in vivo characterization of this peptide-based targeted radiodiagnostic is described in the article "Synthesis and comparative evaluation of novel 64Cu-labeled high affinity cell-specific peptides for positron emission tomography of tumor vasculature" (Merrill et al., 2016) [1]. Here we report biodistribution measurements in mice and calculate the radiation absorbed doses to normal organs using a modified Medical Internal Radiation Dosimetry (MIRD) methodology that accounts for physical and geometric factors and cross-organ beta doses.

Determining organ dose conversion coefficients for external neutron irradiation by using a voxel mouse model

A set of fluence-to-dose conversion coefficients has been calculated for neutrons with energies <20 MeV using a developed voxel mouse model and Monte Carlo N-particle code (MCNP), for the purpose of neutron radiation effect evaluation. The calculation used 37 monodirectional monoenergetic neutron beams in the energy range 10(-9) MeV to 20 MeV, under five different source irradiation configurations: left lateral, right lateral, dorsal-ventral, ventral-dorsal, and isotropic. Neutron fluence-to-dose conversion coefficients for selected organs of the body were presented in the paper, and the effect of irradiation geometry conditions, neutron energy and the organ location on the organ dose was discussed. The results indicated that neutron dose conversion coefficients clearly show sensitivity to irradiation geometry at neutron energy below 1 MeV.

90Y-Labeled Anti-ROBO1 Monoclonal Antibody Exhibits Antitumor Activity against Small Cell Lung Cancer Xenografts

INTRODUCTION

ROBO1 is a membrane protein that contributes to tumor metastasis and angiogenesis. We previously reported that 90Y-labeled anti-ROBO1 monoclonal antibody (90Y-anti-ROBO1 IgG) showed an antitumor effect against ROBO1-positive tumors. In this study, we performed a biodistribution study and radioimmunotherapy (RIT) against ROBO1-positive small cell lung cancer (SCLC) models.

METHODS

For the biodistribution study, 111In-labeled anti-ROBO1 monoclonal antibody (111In-anti-ROBO1 IgG) was injected into ROBO1-positive SCLC xenograft mice via the tail vein. To evaluate antitumor effects, an RIT study was performed, and SCLC xenograft mice were treated with 90Y-anti-ROBO1 IgG. Tumor volume and body weight were periodically measured throughout the experiments. The tumors and organs of mice were then collected, and a pathological analysis was carried out.

RESULTS

As a result of the biodistribution study, we observed tumor uptake of 111In-anti-ROBO1 IgG. The liver, kidney, spleen, and lung showed comparably high accumulation of 111In-labeled anti-ROBO1. In the RIT study, 90Y-anti-ROBO1 IgG significantly reduced tumor volume compared with baseline. Pathological analyses of tumors revealed coagulation necrosis and fatal degeneration of tumor cells, significant reduction in the number of Ki-67-positive cells, and an increase in the number of apoptotic cells. A transient reduction of hematopoietic cells was observed in the spleen, sternum, and femur.

CONCLUSIONS

These results suggest that RIT with 90Y-anti-ROBO1 IgG is a promising treatment for ROBO1-positive SCLC.

Construction of mouse phantoms from segmented CT scan data for radiation dosimetry studies

We present the complete construction methodology for an anatomically accurate mouse phantom made using materials which mimic the characteristics of tissue, lung, and bone for radiation dosimetry studies. Phantoms were constructed using 2 mm thick slices of tissue equivalent material which was precision machined to clear regions for insertion of lung and bone equivalent material where appropriate. Images obtained using a 3D computed tomography (CT) scan clearly indicate regions of tissue, lung, and bone that match their position within the original mouse CT scan. Additionally, radiographic films are used with the phantom to demonstrate dose mapping capabilities. The construction methodology presented here can be quickly and easily adapted to create a phantom of any specific small animal given a segmented CT scan of the animal. These physical phantoms are a useful tool to examine individual organ dose and dosimetry within mouse systems that are complicated by density inhomogeneity due to bone and lung regions.

Comparative efficacy of 177Lu and 90Y for anti-CD20 pretargeted radioimmunotherapy in murine lymphoma xenograft models

PURPOSE

Pretargeted radioimmunotherapy (PRIT) is a multi-step method of selectively delivering high doses of radiotherapy to tumor cells while minimizing exposure to surrounding tissues. Yttrium-90 (90Y) and lutetium-177 (177Lu) are two of the most promising beta-particle emitting radionuclides used for radioimmunotherapy, which despite having similar chemistries differ distinctly in terms of radiophysical features. These differences may have important consequences for the absorbed dose to tumors and normal organs. Whereas 90Y has been successfully applied in a number of preclinical and clinical radioimmunotherapy settings, there have been few published pretargeting studies with 177Lu. We therefore compared the therapeutic potential of targeting either 90Y or 177Lu to human B-cell lymphoma xenografts in mice.

METHODS

Parallel experiments evaluating the biodistribution, imaging, dosimetry, therapeutic efficacy, and toxicity were performed in female athymic nude mice bearing either Ramos (Burkitt lymphoma) or Granta (mantle cell lymphoma) xenografts, utilizing an anti-CD20 antibody-streptavidin conjugate (1F5-SA) and an 90Y- or 177Lu-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-biotin second step reagent.

RESULTS

The two radionuclides displayed comparable biodistributions in tumors and normal organs; however, the absorbed radiation dose delivered to tumor was more than twice as high for 90Y (1.3 Gy/MBq) as for 177Lu (0.6 Gy/MBq). More importantly, therapy with 90Y-DOTA-biotin was dramatically more effective than with 177Lu-DOTA-biotin, with 100% of Ramos xenograft-bearing mice cured with 37 MBq 90Y, whereas 0% were cured using identical amounts of 177Lu-DOTA-biotin. Similar results were observed in mice bearing Granta xenografts, with 80% of the mice cured with 90Y-PRIT and 0% cured with 177Lu-PRIT. Toxicities were comparable with both isotopes.

CONCLUSION

90Y was therapeutically superior to 177Lu for streptavidin-biotin PRIT approaches in these human lymphoma xenograft models.

Anti-CD45 radioimmunotherapy with 90Y but not 177Lu is effective treatment in a syngeneic murine leukemia model

Radioimmunotherapy (RIT) for treatment of hematologic malignancies has primarily employed monoclonal antibodies (Ab) labeled with 131I or 90Y which have limitations, and alternative radionuclides are needed to facilitate wider adoption of RIT. We therefore compared the relative therapeutic efficacy and toxicity of anti-CD45 RIT employing 90Y and 177Lu in a syngeneic, disseminated murine myeloid leukemia (B6SJLF1/J) model. Biodistribution studies showed that both 90Y- and 177Lu-anti-murine CD45 Ab conjugates (DOTA-30F11) targeted hematologic tissues, as at 24 hours 48.8 ± 21.2 and 156 ± 14.6% injected dose per gram of tissue (% ID/g) of 90Y-DOTA-30F11 and 54.2 ± 9.5 and 199 ± 11.7% ID/g of 177Lu-DOTA-30F11 accumulated in bone marrow (BM) and spleen, respectively. However, 90Y-DOTA-30F11 RIT demonstrated a dose-dependent survival benefit: 60% of mice treated with 300 µCi 90Y-DOTA-30F11 lived over 180 days after therapy, and mice treated with 100 µCi 90Y-DOTA-30F11 had a median survival 66 days. 90Y-anti-CD45 RIT was associated with transient, mild myelotoxicity without hepatic or renal toxicity. Conversely, 177Lu- anti-CD45 RIT yielded no long-term survivors. Thus, 90Y was more effective than 177Lu for anti-CD45 RIT of AML in this murine leukemia model.

In vivo localization of ⁹⁰Y and ¹⁷⁷Lu radioimmunoconjugates using Cerenkov luminescence imaging in a disseminated murine leukemia model

Cerenkov radiation generated by positron-emitting radionuclides can be exploited for a molecular imaging technique known as Cerenkov luminescence imaging (CLI). Data have been limited, however, on the use of medium- to high-energy β-emitting radionuclides of interest for cancer imaging and treatment. We assessed the use of CLI as an adjunct to determine localization of radioimmunoconjugates to hematolymphoid tissues. Radiolabeled (177)Lu- or (90)Y-anti-CD45 antibody (Ab; DOTA-30F11) was administered by tail vein injection to athymic mice bearing disseminated murine myeloid leukemia, with CLI images acquired at times afterward. Gamma counting of individual organs showed preferential uptake in CD45(+) tissues with significant retention of radiolabeled Ab in sites of leukemia (spleen and bone marrow). This result was confirmed in CLI images with 1.35 × 10(5) ± 2.2 × 10(4) p/s/cm(2)/sr and 3.45 × 10(3) ± 7.0 × 10(2) p/s/cm(2)/sr for (90)Y-DOTA-30F11 and (177)Lu-DOTA-30F11, respectively, compared with undetectable signal for both radionuclides using the nonbinding control Ab. Results showed that CLI allows for in vivo visualization of localized β-emissions. Pixel intensity variability resulted from differences in absorbed doses of the associated energies of the β-emitting radionuclide. Overall, our findings offer a preclinical proof of concept for the use of CLI techniques in tandem with currently available clinical diagnostic tools.

Assessment of S values in stylized and voxel-based rat models for positron-emitting radionuclides

PURPOSE

Positron emission tomography (PET) is a powerful tool in small animal research, enabling noninvasive quantitative imaging of biochemical processes in living subjects. However, the dosimetric characteristics of small animal PET imaging are usually overlooked, although the radiation dose may be significant. The variations of anatomical characteristics between the various computational models may result in differences in the dosimetric outcome.

METHODS

We used five different anatomical rat models (two stylized and three voxel based) to compare calculated absorbed fractions and S values for eight positron-emitting radionuclides (C-11, N-13, O-15, F-18, Cu-64, Ga-68, Y-86, and I-124) commonly used to label various probes for small animal PET imaging. The MCNPX radiation transport code was used for radiation dose calculations.

RESULTS

For most source/target organ pairs, O-15 and Ga-68 produce the highest self-absorbed S values because of the high-energy and high-frequency of positron emissions, while Y-86 produces the highest cross-absorbed S values because of the high energy and high frequency of γ-rays emission. Anatomical models produced from different rat strains or modeling techniques exhibit different organ masses, volumes, and thus give rise to different S values and absorbed dose. The variations of absorbed fractions between models of the same type are less than those between models with different types. The calculated S values depend strongly on organ mass, and as such, different models produce similar S values for organs of comparable masses. In most source organs presenting with high cumulated activity, the absorbed dose is less affected by model difference compared with other organs.

CONCLUSIONS

The produced S values for common positron-emitting radionuclides can be exploited in the assessment of radiation dose to rats from different radiotracers used in small animal PET experiments. This work contributes to a better understanding of the influence of different computational models on small animal dosimetry.

166Ho and 90Y labeled 6D2 monoclonal antibody for targeted radiotherapy of melanoma: comparison with 188Re radiolabel

INTRODUCTION

An approach to radioimmunotherapy (RIT) of metastatic melanoma is the targeting of melanin pigment with monoclonal antibodies (mAbs) to melanin radiolabeled with therapeutic radionuclides. The proof of principle experiments were performed using a melanin-binding antibody 6D2 of IgM isotype radiolabeled with a β emitter (188)Re and demonstrated the inhibition of tumor growth. In this study we investigated the efficacy of 6D2 antibody radiolabeled with two other longer lived β emitters (90)Y and (166)Ho in treatment of experimental melanoma, with the objective to find a possible correlation between the efficacy and half-life of the radioisotopes which possess high energy β (E(max)>1.5 MeV) emission properties.

METHODS

6D2 was radiolabeled with longer lived β emitters (90)Y and (166)Ho in treatment of experimental melanoma in A2058 melanoma tumor-bearing nude mice. The immunoreactivity of the radiolabeled 6D2 mAb, its in vitro binding to the MNT1 human melanoma cells, the biodistribution and therapy in A2058 human melanoma bearing nude mice as well as dosimetry calculations were performed.

RESULTS

When labeled with the longer lived (90)Y radionuclide, the 6D2 mAb did not produce any therapeutic effect in tumor bearing mice while the reduction of the tumor growth by (166)Ho-6D2 was very similar to the previously reported therapy results for (188)Re-6D2. In addition, (166)Ho-labeled mAb produced the therapeutic effect on the tumor without any toxic effects while the administration of the (90)Y-labeled radioconjugate was toxic to mice with no appreciable anti-tumor effect.

CONCLUSIONS

(166)Ho-labeled mAb to melanin produced some therapeutic effect on the tumor without any toxic effects while the administration of the (90)Y-labeled radioconjugate was toxic to mice with no appreciable anti-tumor effect. We concluded that the serum half-life of the 6D2 carrier antibody matched well the physical half-life of (166)Ho to deliver the tumoricidal absorbed dose to the tumor. Further investigation of this radionuclide for RIT of melanoma is warranted.

A preclinical model of CD38-pretargeted radioimmunotherapy for plasma cell malignancies

The vast majority of patients with plasma cell neoplasms die of progressive disease despite high response rates to novel agents. Malignant plasma cells are very radiosensitive, but the potential role of radioimmunotherapy (RIT) in the management of plasmacytomas and multiple myeloma has undergone only limited evaluation. Furthermore, CD38 has not been explored as a RIT target despite its uniform high expression on malignant plasma cells. In this report, both conventional RIT (directly radiolabeled antibody) and streptavidin-biotin pretargeted RIT (PRIT) directed against the CD38 antigen were assessed as approaches to deliver radiation doses sufficient for multiple myeloma cell eradication. PRIT demonstrated biodistributions that were markedly superior to conventional RIT. Tumor-to-blood ratios as high as 638:1 were seen 24 hours after PRIT, whereas ratios never exceeded 1:1 with conventional RIT. (90)Yttrium absorbed dose estimates demonstrated excellent target-to-normal organ ratios (6:1 for the kidney, lung, liver; 10:1 for the whole body). Objective remissions were observed within 7 days in 100% of the mice treated with doses ranging from 800 to 1,200 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin, including 100% complete remissions (no detectable tumor in treated mice compared with tumors that were 2,982% ± 2,834% of initial tumor volume in control animals) by day 23. Furthermore, 100% of animals bearing NCI-H929 multiple myeloma tumor xenografts treated with 800 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin achieved long-term myeloma-free survival (>70 days) compared with none (0%) of the control animals.

The synthesis and biodistribution of [(11)C]metformin as a PET probe to study hepatobiliary transport mediated by the multi-drug and toxin extrusion transporter 1 (MATE1) in vivo

In order to develop a new positron emission tomography (PET) probe to study hepatobiliary transport mediated by the multi-drug and toxin extrusion transporter 1 (MATE1), (11)C-labelled metformin was synthesized and then evaluated as a PET probe. [(11)C]Metformin ([(11)C]4) was synthesized in three steps, from [(11)C]methyl iodide. Evaluation by small animal PET of [(11)C]4 showed that there was increased concentrations of [(11)C]4 in the livers of mice pre-treated with pyrimethamine, a potential inhibitor of MATEs, inhibiting the hepatobiliary excretion of metformin. Radiometabolite analysis showed that [(11)C]4 was not degraded in vivo during the PET scan. Biodistribution studies were undertaken and the organ distributions were extrapolated into a standard human model. In conclusion, [(11)C]4 may be useful as a PET probe to non-invasively study the in vivo function of hepatobiliary transport and drug-drug interactions, mediated by MATE1 in future clinical investigations.

Effect of emaciation and obesity on small-animal internal radiation dosimetry for positron-emitting radionuclides

PURPOSE

Rats are widely used in biomedical research involving molecular imaging and therefore the radiation dose to animals has become a concern. The weight of laboratory animals might change through emaciation or obesity as a result of their use in various research experiments including those investigating different diet types. In this work, we evaluated the effects of changes in body weight induced by emaciation and obesity on the internal radiation dose from common positron-emitting radionuclides.

METHODS

A systematic literature review was performed to determine normal anatomical parameters for adult rats and evaluate how organs change with variations in total body weight. The ROBY rat anatomical model was then modified to produce a normal adult rat, and mildly, moderately and severely emaciated and obese rats. Monte Carlo simulations were performed using MCNPX to estimate absorbed fractions, specific absorbed fractions (SAFs) and S-values for these models using different positron-emitting radionuclides. The results obtained for the different models were compared to corresponding estimates from the normal rat model.

RESULTS

The SAFs and S-values for most source-target pairs between the various anatomical models were not significantly different, except where the intestine and the total body were considered as source regions. For the intestine, irradiating other organs in the obese model, the SAFs in organs in the anterior region of the splanchnocoele (e.g. kidney, liver and stomach) increased slightly, whereas the SAFs in organs in the posterior region of the splanchnocoele (e.g. bladder and testes) decreased owing to the increase in the distance separating the intestine and posterior abdominal organs because of the rat epididymal fat pad. For the total body, irradiating other organs, the SAFs and S-values were inversely related to body weight.

CONCLUSION

The effect of obesity on internal radiation dose is insignificant in most conditions for common positron-emitting radionuclides. Emaciation increases the cross-absorbed dose to organs from surrounding tissues, which might be a notable issue in laboratory animal internal dosimetry.

Anti-CD45 radioimmunotherapy using (211)At with bone marrow transplantation prolongs survival in a disseminated murine leukemia model

Despite aggressive chemotherapy combined with hematopoietic stem cell transplantation (HSCT), many patients with acute myeloid leukemia (AML) relapse. Radioimmunotherapy (RIT) using monoclonal antibodies labeled with β-emitting radionuclides has been explored to reduce relapse. β emitters are limited by lower energies and nonspecific cytotoxicity from longer path lengths compared with α emitters such as (211)At, which has a higher energy profile and shorter path length. We evaluated the efficacy and toxicity of anti-CD45 RIT using (211)At in a disseminated murine AML model. Biodistribution studies in leukemic SJL/J mice showed excellent localization of (211)At-anti-murine CD45 mAb (30F11) to marrow and spleen within 24 hours (18% and 79% injected dose per gram of tissue [ID/g], respectively), with lower kidney and lung uptake (8.4% and 14% ID/g, respectively). In syngeneic HSCT studies, (211)At-B10-30F11 RIT improved the median survival of leukemic mice in a dose-dependent fashion (123, 101, 61, and 37 days given 24, 20, 12, and 0 µCi, respectively). This approach had minimal toxicity with nadir white blood cell counts >2.7 K/µL 2 weeks after HSCT and recovery by 4 weeks. These data suggest that (211)At-anti-CD45 RIT in conjunction with HSCT may be a promising therapeutic option for AML.

Evaluation of absorbed doses in voxel-based and simplified models for small animals

Internal dosimetry in non-human biota is desirable from the viewpoint of radiation protection of the environment. The International Commission on Radiological Protection (ICRP) proposed Reference Animals and Plants using simplified models, such as ellipsoids and spheres and calculated absorbed fractions (AFs) for whole bodies. In this study, photon and electron AFs in whole bodies of voxel-based rat and frog models have been calculated and compared with AFs in the reference models. It was found that the voxel-based and the reference frog (or rat) models can be consistent for the whole-body AFs within a discrepancy of 25%, as the source was uniformly distributed in the whole body. The specific absorbed fractions (SAFs) and S values were also evaluated in whole bodies and all organs of the voxel-based frog and rat models as the source was distributed in the whole body or skeleton. The results demonstrated that the whole-body SAFs reflect SAFs of all individual organs as the source was uniformly distributed per mass within the whole body by about 30% uncertainties with exceptions for body contour (up to -40%) for both electrons and photons due to enhanced radiation leakages, and for the skeleton for photons only (up to +185%) due to differences in the mass attenuation coefficients. For nuclides such as (90)Y and (90)Sr, which were concentrated in the skeleton, there were large differences between S values in the whole body and those in individual organs, however the whole-body S values for the reference models with the whole body as the source were remarkably similar to those for the voxel-based models with the skeleton as the source, within about 4 and 0.3%, respectively. It can be stated that whole-body SAFs or S values in simplified models without internal organs are not sufficient for accurate internal dosimetry because they do not reflect SAFs or S values of all individual organs as the source was not distributed uniformly in whole body. Thus, voxel-based models would be good candidates for dosimetry in non-human biota if further accuracy in environmental dosimetry is desired.

Organ dose conversion coefficients based on a voxel mouse model and MCNP code for external photon irradiation

A set of conversion coefficients from kerma free-in-air to the organ absorbed dose for external photon beams from 10 keV to 10 MeV are presented based on a newly developed voxel mouse model, for the purpose of radiation effect evaluation. The voxel mouse model was developed from colour images of successive cryosections of a normal nude male mouse, in which 14 organs or tissues were segmented manually and filled with different colours, while each colour was tagged by a specific ID number for implementation of mouse model in Monte Carlo N-particle code (MCNP). Monte Carlo simulation with MCNP was carried out to obtain organ dose conversion coefficients for 22 external monoenergetic photon beams between 10 keV and 10 MeV under five different irradiation geometries conditions (left lateral, right lateral, dorsal-ventral, ventral-dorsal, and isotropic). Organ dose conversion coefficients were presented in tables and compared with the published data based on a rat model to investigate the effect of body size and weight on the organ dose. The calculated and comparison results show that the organ dose conversion coefficients varying the photon energy exhibits similar trend for most organs except for the bone and skin, and the organ dose is sensitive to body size and weight at a photon energy approximately <0.1 MeV.

Comparison of pretargeted and conventional CC49 radioimmunotherapy using 149Pm, 166Ho, and 177Lu

The therapeutic efficacies of radiolabeled biotin, pretargeted by monoclonal antibody (mAb)-streptavidin fusion protein CC49 scFvSA, were compared to those of radiolabeled mAb CC49, using the three radiolanthanides in an animal model of human colon cancer. The purpose of the present study was to compare antibody pretargeting to conventional radioimmunotherapy using (149)Pm, (166)Ho, or (177)Lu. Nude mice bearing LS174T colon tumors were injected sequentially with CC49 scFvSA, the blood clearing agent biotin-GalNAc(16), and (149)Pm-, (166)Ho-, or (177)Lu-DOTA-biotin. Tumor-bearing mice were alternatively administered (149)Pm-, (166)Ho-, or (177)Lu-MeO-DOTA-CC49. Therapy with pretargeted (149)Pm-,(166)Ho-, and (177)Lu-DOTA-biotin increased the median time of progression to a 1 g tumor to 50, 41, and 50 days post-treatment, respectively. Therapy with (149)Pm-,(166)Ho-, and (177)Lu-MeO-DOTA-CC49 increased the median time to progression to 53, 24, and 67 days post-treatment, respectively. In contrast, saline controls showed a median time to progression of 13 days postinjection. Treatment with pretargeted (149)Pm-, (166)Ho-, and (177)Lu-biotin or (149)Pm-, (166)Ho-, and (177)Lu-CC49 increased tumor doubling time to 18-36 days, compared to 3 days for saline controls. Among treated mice, 23% survived >84 days post-therapy, and 11% survived 6 months, but controls survived <29 days. Long-term survivors showed tumor growth inhibition or partial regression, extensive necrosis in residual masses, and no evidence of nontarget tissue toxicity at necropsy. Both pretargeted and conventional RIT demonstrated considerable efficacy in an extremely aggressive animal model of cancer. Our results identified (177)Lu as an optimal radiolanthanide for future evaluation of these agents in toxicity and multiple-dose therapy studies.

Monte Carlo calculations of absorbed doses in tumours using a modified MOBY mouse phantom for pre-clinical dosimetry studies

BACKGROUND

Clinical treatment with radionuclides is usually preceded by biokinetic and dosimetry studies in small animals. Evaluation of the therapeutic efficacy is essential and must rely on accurate dosimetry, which in turn must be based on a realistic geometrical model that properly describes the transport of radiation. It is also important to include the source distribution in the dosimetry calculations. Tumours are often implanted subcutaneously in animals, constituting an important additional source of radiation that often is not considered in the dosimetry models. The aims of this study were to calculate S values of the mouse, and determine the absorbed dose contribution to and from subcutaneous tumours inoculated at four different locations.

METHODS

The Moby computer program generates a three dimensional (3D) voxel-based phantom. Tumours were modelled as half-spheres on the body surface, and the radius was varied to study different tumour masses. The phantoms were used as input for Monte Carlo simulations of absorbed fractions and S factors with the radiation transport code MCNPX 2.6f. Calculations were performed for monoenergetic photons and electrons, and the radionuclides (125)I, (131)I, (111)In, (177)Lu and (90)Y.

RESULTS

Electron energy and tumour size are important for both self- and cross-doses. If the activity is non-uniformly distributed within the body, the position of the tumour must be considered in order to calculate the tumour absorbed dose accurately. If the uptake in the tumour is high compared with that in adjacent organs the absorbed dose contribution to organs from the tumour cannot be neglected.

CONCLUSIONS

In order to perform accurate tumour dosimetry in mouse models it is necessary to take the additional contribution from the activity distribution within the body of the mouse into account. This may be of significance in the interpretation of radiobiological tumour response in pre-clinical studies.

Anti-CD45 pretargeted radioimmunotherapy using bismuth-213: high rates of complete remission and long-term survival in a mouse myeloid leukemia xenograft model

Pretargeted radioimmunotherapy (PRIT) using an anti-CD45 antibody (Ab)-streptavidin (SA) conjugate and DOTA-biotin labeled with β-emitting radionuclides has been explored as a strategy to decrease relapse and toxicity. α-emitting radionuclides exhibit high cytotoxicity coupled with a short path length, potentially increasing the therapeutic index and making them an attractive alternative to β-emitting radionuclides for patients with acute myeloid leukemia. Accordingly, we have used (213)Bi in mice with human leukemia xenografts. Results demonstrated excellent localization of (213)Bi-DOTA-biotin to tumors with minimal uptake into normal organs. After 10 minutes, 4.5% ± 1.1% of the injected dose of (213)Bi was delivered per gram of tumor. α-imaging demonstrated uniform radionuclide distribution within tumor tissue 45 minutes after (213)Bi-DOTA-biotin injection. Radiation absorbed doses were similar to those observed using a β-emitting radionuclide ((90)Y) in the same model. We conducted therapy experiments in a xenograft model using a single-dose of (213)Bi-DOTA-biotin given 24 hours after anti-CD45 Ab-SA conjugate. Among mice treated with anti-CD45 Ab-SA conjugate followed by 800 μCi of (213)Bi- or (90)Y-DOTA-biotin, 80% and 20%, respectively, survived leukemia-free for more than 100 days with minimal toxicity. These data suggest that anti-CD45 PRIT using an α-emitting radionuclide may be highly effective and minimally toxic for treatment of acute myeloid leukemia.

Preclinical animal research on therapy dosimetry with dual isotopes

Preclinical research into radionuclide therapies based on radiation dosimetry will enable the use of any LET-equivalent radionuclide. Radiation dose and dose rate have significant influence on dose effects in the tumour depending on its radiation sensitivity, possibilities for repair of sublethal damage, and repopulation during or after the therapy. Models for radiation response of preclinical tumour models after peptide receptor radionuclide therapy based on the linear quadratic model are presented. The accuracy of the radiation dose is very important for observation of dose-effects. Uncertainties in the radiation dose estimation arise from incomplete assay of the kinetics, low accuracy in volume measurements and absorbed dose S-values for stylized models instead of the actual animal geometry. Normal dose uncertainties in the order of 20% might easily make the difference between seeing a dose-effect or missing it altogether. This is true for the theoretical case of a homogeneous tumour type behaving in vivo in the same way as its cells do in vitro. Heterogeneity of tumours induces variations in clonogenic cell density, radiation sensitivity, repopulation capacity and repair kinetics. The influence of these aspects are analysed within the linear quadratic model for tumour response to radionuclide therapy. Preclinical tumour models tend to be less heterogenic than the clinical conditions they should represent. The results of various preclinical radionuclide therapy experiments for peptide receptor radionuclide therapy are compared to the outcome of theoretical models and the influence of increased heterogeneity is analysed when the results of preclinical research is transferred to the clinic. When the radiation dose and radiobiology of the tumour response is known well enough it may be possible to leave the current phenomenological approach in preclinical radionuclide therapy and start basing these experiments on radiation dose. Then the use of a gamma ray-emitting radionuclides for a chemically comparable beta-particle-emitting paired isotope for therapy evaluation would be feasible.

Influence of voxel size on specific absorbed fractions and S-values in a mouse voxel phantom

Photon and electron specific absorbed fractions (SAFs) and S-values have been evaluated using mouse voxel phantoms. In voxel phantoms, it is important to choose the voxel size carefully since it affects the accuracy of results. In this study, two mouse voxel phantoms were constructed, with cubic voxels, one with 0.1-mm sides and the other with 0.4-mm sides. The sources were considered to be distributed uniformly in the main organs and the radiation transport was simulated using the Monte Carlo code EGS4. It was found that the effect of voxel size on SAFs for self-irradiation was not high (<10 %) for electrons and photons. However, it was appreciable for cross-irradiation especially for electrons. The effect of voxel size was investigated on S-values for some beta emitters such as (131)I, (153)Sm, (188)Re and (90)Y.

Radiation dose estimation using preclinical imaging with 124I-metaiodobenzylguanidine (MIBG) PET

PURPOSE

A pretherapy 124I-metaiodobenzylguanidine (MIBG) positron emission tomography (PET)/computed tomography (CT) provides a potential method to estimate radiation dose to normal organs, as well as tumors prior to 131I-MIBG treatment of neuroblastoma or pheochromocytoma. The aim of this work was to estimate human-equivalent internal radiation dose of 124I-MIBG using PET/CT data in a murine xenograft model.

METHODS

Athymic mice subcutaneously implanted with NB1691 cells that express high levels of human norepinephrine transporter (n = 4) were imaged using small animal microPET/CT over 96 h (approximate imaging time points: 0.5, 2, 24, 52, and 96 h) after intravenous administration of 3.07-4.84 MBq of 124I-MIBG via tail vein. The tumors did not accumulate 124I-MIBG to a detectable level. All four animals were considered as control and organ radiation dosimetry was performed. Volumes of interest were drawn on the coregistered CT images for thyroid, heart, lung, liver, kidney, and bladder, and transferred to PET images to obtain pharmacokinetic data. Based on tabulated organ mass distributions for both mice and adult male human, preclinical pharmacokinetic data were extrapolated to their human-equivalent values. Radiation dose estimations for different age groups were performed using the OLINDA/EXM software with modified tissue weighting factors in the recent International Commission on Radiological Protection (ICRP) Publication 103.

RESULTS

The mean effective dose from 124I-MIBG using weighting factors from ICRP 103 to the adult male was estimated at 0.25 mSv/MBq. In different age groups, effective doses using values from ICRP 103 were estimated as follows: Adult female: 0.34, 15-yr-old: 0.39 mSv/MBq, 10-yr-old: 0.58 mSv/MBq, 5-yr-old: 1.03 mSv/MBq, 1-yr-old: 1.92 mSv/MBq, and newborn: 3.75 mSv/ MBq. For comparison, the reported effective dose equivalent of 124I-NaI for adult male (25% thyroid uptake, MIRD Dose Estimate Report No. 5) was 6.5 mSv/MBq.

CONCLUSIONS

The authors estimated human-equivalent internal radiation dose of 124I-MIBG using preclinical imaging data. As a reference, the effective dose estimation showed that 124I-MIBG would deliver less radiation dose than 124I-NaI, a radiotracer already being used in patients with thyroid cancer.

RADAR realistic animal model series for dose assessment

Rodent species are widely used in the testing and approval of new radiopharmaceuticals, necessitating murine phantom models. As more therapy applications are being tested in animal models, calculating accurate dose estimates for the animals themselves becomes important to explain and control potential radiation toxicity or treatment efficacy. Historically, stylized and mathematically based models have been used for establishing doses to small animals. Recently, a series of anatomically realistic human phantoms was developed using body models based on nonuniform rational B-spline. Realistic digital mouse whole-body (MOBY) and rat whole-body (ROBY) phantoms were developed on the basis of the same NURBS technology and were used in this study to facilitate dose calculations in various species of rodents.

METHODS

Voxel-based versions of scaled MOBY and ROBY models were used with the Vanderbilt multinode computing network (Advanced Computing Center for Research and Education), using geometry and tracking radiation transport codes to calculate specific absorbed fractions (SAFs) with internal photon and electron sources. Photon and electron SAFs were then calculated for relevant organs in all models.

RESULTS

The SAF results were compared with values from similar studies found in reference literature. Also, the SAFs were used with standardized decay data to develop dose factors to be used in radiation dose calculations. Representative plots were made of photon electron SAFs, evaluating the traditional assumption that all electron energy is absorbed in the source organs.

CONCLUSION

The organ masses in the MOBY and ROBY models are in reasonable agreement with models presented by other investigators noting that considerable variation can occur between reported masses. Results consistent with those found by other investigators show that absorbed fractions for electrons for organ self-irradiation were significantly less than 1.0 at energies above 0.5 MeV, as expected for many of these small-sized organs, and measurable cross-irradiation was observed for many organ pairs for high-energy electrons (as would be emitted by nuclides such as (32)P, (90)Y, or (188)Re).

An image-based rat model for Monte Carlo organ dose calculations

An anatomically realistic rat model was developed from color images of successive cryosections of a mature Sprague-Dawley rat. Images were obtained, by digital scanning, of 9475 slices with thickness of 0.02 mm. A total of 13 major organs and tissues were selected, and models of these organs and tissues constructed from the images were used for calculations of absorbed dose from external photon sources. A detailed set of conversion coefficients from kerma free-in-air to organ absorbed dose have been calculated for external monoenergetic photon beams with energies ranging from 10 keV to 10 MeV under five idealized irradiation conditions (left lateral, right lateral, dorsal-ventral, ventral-dorsal, and isotropic) using the Monte Carlo code MCNPX. Dose results are presented in form of tables as supplemental data for practical use and comparison. The influence of anatomical characteristics, including organ volume, shape, location, and orientation, on dose distributions were evaluated. It would also be possible to make internal dose assessments using the computational rat model.

203Pb-labeled alpha-melanocyte-stimulating hormone peptide as an imaging probe for melanoma detection

Peptide-targeted alpha-therapy with 7.4 MBq of (212)Pb-[1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid]-ReO-[Cys(3,4,10),d-Phe(7),Arg(11)]alpha-MSH(3-13) ((212)Pb-DOTA-Re(Arg(11))CCMSH) cured 45% of B16/F1 murine melanoma-bearing C57 mice in a 120-d study, highlighting its melanoma treatment potential. However, there is a need to develop an imaging surrogate for patient-specific dosimetry and to monitor the tumor response to (212)Pb-DOTA-Re(Arg(11))CCMSH therapy. The purpose of this study was to evaluate the potential of (203)Pb-DOTA-Re(Arg(11))CCMSH as a matched-pair SPECT agent for (212)Pb-DOTA-Re(Arg(11))CCMSH.

METHODS

DOTA-Re(Arg(11))CCMSH was labeled with (203)Pb in 0.5 M NH(4)OAc buffer at pH 5.4. The internalization and efflux of (203)Pb-DOTA-Re(Arg(11))CCMSH were determined in B16/F1 melanoma cells. The pharmacokinetics of (203)Pb-DOTA-Re(Arg(11))CCMSH was examined in B16/F1 melanoma-bearing C57 mice. A micro-SPECT/CT study was performed with (203)Pb-DOTA-Re(Arg(11))CCMSH in a B16/F1 melanoma-bearing C57 mouse at 2 h after injection.

RESULTS

(203)Pb-DOTA-Re(Arg(11))CCMSH was easily prepared in NH(4)OAc buffer and completely separated from the excess nonradiolabeled peptide by reversed-phase high-performance liquid chromatography (RP-HPLC). (203)Pb-DOTA-Re(Arg(11))CCMSH displayed fast internalization and extended retention in B16/F1 cells. Approximately 73% of (203)Pb-DOTA-Re(Arg(11))CCMSH activity internalized after a 20-min incubation at 25 degrees C. After incubation of the cells in culture medium for 20 min, 78% of internalized activity remained in the cells. (203)Pb-DOTA-Re(Arg(11))CCMSH exhibited a biodistribution pattern similar to that of (212)Pb-DOTA-Re(Arg(11))CCMSH in B16/F1 melanoma-bearing mice. (203)Pb-DOTA-Re(Arg(11))CCMSH exhibited a peak tumor uptake of 12.00+/-3.20 percentage injected dose per gram (%ID/g) at 1 h after injection. The tumor uptake gradually decreased to 3.43+/-1.12 %ID/g at 48 h after injection. (203)Pb-DOTA-Re(Arg(11))CCMSH exhibited a peak tumor-to-kidney uptake ratio of 1.53 at 2 h after injection. The absorbed doses to the tumor and kidneys were 4.32 and 4.35 Gy, respectively, per 37 MBq. Whole-body clearance of (203)Pb-DOTA-Re(Arg(11))CCMSH was fast, with approximately 89% of the injected activity cleared through the urinary system by 2 h after injection. (203)Pb showed 1.6-mm SPECT resolution, which was comparable to (99m)Tc. Melanoma lesions were visualized through SPECT/CT images of (203)Pb-DOTA-Re(Arg(11))CCMSH at 2 h after injection.

CONCLUSION

(203)Pb-DOTA-Re(Arg(11))CCMSH exhibited favorable pharmacokinetic and tumor imaging properties, highlighting its potential as a matched-pair SPECT agent for (212)Pb-DOTA-Re(Arg(11))CCMSH melanoma treatment.

Mouse S-Factors Based on Monte Carlo Simulations in the Anatomical Realistic Moby Phantom for Internal Dosimetry

INTRODUCTION

Biokinetic and dosimetry studies in small animals often precede clinical radionuclide therapies. As in human studies, a reliable evaluation of therapeutic efficacy is essential and must be based on accurate dosimetry, which must be based on a realistic dosimetry model. The aim of this study was to evaluate the differences in the results when using a more anatomic realistic mouse phantom, as compared to previously mathematically described phantoms, based mainly on ellipsoids and cylinders. The difference in results from the two Monte Carlo codes, EGS4 and MCNPX 2.6a, was also evaluated.

METHODS

An anatomical correct mouse phantom (Moby) was developed by Segars et al. for the evaluation and optimization of the in vivo imaging of mice. The Moby phantom is based on surfaces, which allows for an easy and flexible definition of organ sizes. It includes respiratory movements and a beating heart. It also allows for a redefinition of the location of several internal organs. The execution of the Moby program generates a three-dimensional voxel-based phantom of a specified size, which was modified and used as input for Monte Carlo simulations of absorbed fractions and S-factors. The radiation transport was simulated both with the EGS4 system and the MCNPX 2.6a code. Calculations were done for the radionuclides (18)F, (124)I, (131)I, (111)In, (177)Lu, and (90)Y. S-factors were calculated using in-house-developed IDL programs and compared with results from previously published models.

RESULTS

The comparison of S-factors obtained by the Moby model and mathematical phantoms showed that these, in many cases, were within the same range, whereas for some organs, they were underestimated in the mathematical phantoms. The results were closer to the more anatomically realistic phantom than to the mathematical phantoms, with some exceptions. When investing differences between MCNPX 2.6a and EGS4 using the Moby phantom, results indicated some differences in absorbed fractions for electrons. This reason may be owing to differences in the codes regarding the theory for which electron transport are simulated.

CONCLUSIONS

It is possible to calculate S-factors that are specific for small animals, such as mice. The Moby phantom is useful as a dosimetry model because it is anatomically realistic, but still very flexible, with 35 accurately segmented regions.

A voxel-based mouse for internal dose calculations using Monte Carlo simulations (MCNP)

Murine models are useful for targeted radiotherapy pre-clinical experiments. These models can help to assess the potential interest of new radiopharmaceuticals. In this study, we developed a voxel-based mouse for dosimetric estimates. A female nude mouse (30 g) was frozen and cut into slices. High-resolution digital photographs were taken directly on the frozen block after each section. Images were segmented manually. Monoenergetic photon or electron sources were simulated using the MCNP4c2 Monte Carlo code for each source organ, in order to give tables of S-factors (in Gy Bq-1 s-1) for all target organs. Results obtained from monoenergetic particles were then used to generate S-factors for several radionuclides of potential interest in targeted radiotherapy. Thirteen source and 25 target regions were considered in this study. For each source region, 16 photon and 16 electron energies were simulated. Absorbed fractions, specific absorbed fractions and S-factors were calculated for 16 radionuclides of interest for targeted radiotherapy. The results obtained generally agree well with data published previously. For electron energies ranging from 0.1 to 2.5 MeV, the self-absorbed fraction varies from 0.98 to 0.376 for the liver, and from 0.89 to 0.04 for the thyroid. Electrons cannot be considered as 'non-penetrating' radiation for energies above 0.5 MeV for mouse organs. This observation can be generalized to radionuclides: for example, the beta self-absorbed fraction for the thyroid was 0.616 for I-131; absorbed fractions for Y-90 for left kidney-to-left kidney and for left kidney-to-spleen were 0.486 and 0.058, respectively. Our voxel-based mouse allowed us to generate a dosimetric database for use in preclinical targeted radiotherapy experiments.

Radiolabeled Melanin-Binding Peptides Are Safe and Effective in Treatment of Human Pigmented Melanoma in a Mouse Model of Disease

The incidence of melanoma is rising, and therapeutic options for metastatic melanoma are limited. We report the results of experimental melanoma therapy with 188-Rhenium-labeled melanin-binding decapeptide ((188)RE-HYNIC-4B4) and a comprehensive safety evaluation of this treatment. (188)RE-HYNIC- 4B4 bound only to nonviable eumelanotic MNT1 and pheomelanotic SK-28-MEL human melanoma cells in vitro, as determined by immunofluorescence, which is consistent with the inaccessibility of intracellular melanin in live cells, and suggests specificity for tumors with a significant amount of extracellular melanin. Administration of 1 mCi (188)RE-HYNIC-4B4 to MNT1 tumor-bearing mice significantly slowed tumor growth, with the therapeutic effect being a result of specific binding to tumor melanin, as irrelevant (188)RE-labeled decapeptide did not produce therapeutic gain. Repeated doses of (188)RE-HYNIC-4B4 had a more profound effect on tumor growth than a single dose. Treatment of tumors with 0.3-0.4 cm diameter was more effective than of larger ones (0.5-0.7 cm). There was no difference in uptake of (188)REHYNIC- 4B4 in melanized tissues of black C57BL6 mice and no histologically apparent damage to these tissues in comparison with white BALB/C mice. Treatment of C57BL6 mice with (188)RE-HYNIC-4B4 did not change their behavior, as established by SHIRPA protocol, and did not cause damage to neurons and glial cells. These results indicate that radiolabeled melanin-binding peptides are efficient and safe in treatment of melanoma and could be potentially useful against this tumor.