Alpha and Beta Radiation for Theragnostics

Targeted radionuclide therapy (TRT) has significantly evolved from its beginnings with iodine-131 to employing carrier molecules with beta emitting isotopes like lutetium-177. With the success of Lu-177-DOTATATE for neuroendocrine tumors and Lu-177-PSMA-617 for prostate cancer, several other beta emitting radioisotopes, such as Cu-67 and Tb-161, are being explored for TRT. The field has also expanded into targeted alpha therapy (TAT) with agents like radium-223 for bone metastases in prostate cancer, and several other alpha emitter radioisotopes with carrier molecules, such as Ac-225, and Pb-212 under clinical trials. Despite these advancements, the scope of TRT in treating diverse solid tumors and integration with other therapies like immunotherapy remains under investigation. The success of antibody-drug conjugates further complements treatments with TRT, though challenges in treatment optimization continue.

Therapeutic efficacy of an alpha-particle emitter labeled anti-GD2 humanized antibody against osteosarcoma-a proof of concept study

PURPOSE

Current treatments for osteosarcoma (OS) have a poor prognosis, particularly for patients with metastasis and recurrence, underscoring an urgent need for new targeted therapies to improve survival. Targeted alpha-particle therapy selectively delivers cytotoxic payloads to tumors with radiolabeled molecules that recognize tumor-associated antigens. We have recently demonstrated the potential of an FDA approved, humanized anti-GD2 antibody, hu3F8, as a targeted delivery vector for radiopharmaceutical imaging of OS. The current study aims to advance this system for alpha-particle therapy of OS.

METHODS

The hu3F8 antibody was radiolabeled with actinium-225, and the safety and therapeutic efficacy of the [225Ac]Ac-DOTA-hu3F8 were evaluated in both orthotopic murine xenografts of OS and spontaneously occurring OS in canines.

RESULTS

Significant antitumor activity was proven in both cases, leading to improved overall survival. In the murine xenograft's case, tumor growth was delayed by 16-18 days compared to the untreated cohort as demonstrated by bioluminescence imaging. The results were further validated with magnetic resonance imaging at 33 days after treatment, and microcomputed tomography and planar microradiography post-mortem. Histological evaluations revealed radiation-induced renal toxicity, manifested as epithelial cell karyomegaly and suggestive polyploidy in the kidneys, suggesting rapid recovery of renal function after radiation damage. Treatment of the two canine patients delayed the progression of metastatic spread, with an overall survival time of 211 and 437 days and survival beyond documented metastasis of 111 and 84 days, respectively.

CONCLUSION

This study highlights the potential of hu3F8-based alpha-particle therapy as a promising treatment strategy for OS.

Girth-based administered activity for pediatric 99m Tc-DMSA SPECT

BACKGROUND

Pediatric molecular imaging requires a balance between administering an activity that will yield sufficient diagnostic image quality while maintaining patient radiation exposure at acceptable levels. In current clinical practice, this balance is arrived at by the current North American Consensus Guidelines in which patient weight is used to recommend the administered activity (AA).

PURPOSE

We have previously demonstrated that girth (waist circumference at the level of the kidneys) is better at equalizing image quality than patient weight for pediatric Tc-99m DMSA renal function imaging. However, the correlation between image quality (IQ), AA, and patient girth has not been rigorously and systematically developed. In this work, we generate a series of curves showing the tradeoff between AA and IQ as a function of patient girth, providing the data for standards bodies to develop the next generation of dosing guideline for pediatric DMSA SPECT.

METHODS

An anthropomorphic phantom series that included variations in age (5, 10, and 15 years), gender (M, F), local body morphometry (5, 10, 50, 90, and 95th girth percentiles), and kidney size (±15% standard size), was used to generate realistic SPECT projections. A fixed and clinically challenging defect-to-organ volume percentage (0.49% of renal cortex value) was used to model a focal defect with zero uptake (i.e., full local loss of renal function). Task-based IQ assessment methods were used to rigorously measure IQ in terms of renal perfusion defect detectability. This assessment was performed at multiple count levels (corresponding to various AAs) for groups of patients that had similar girths and defect sizes. Receiver-operating characteristics (ROC) analysis was applied; the area under the ROC curve (AUC) was used as a figure-of-merit for task performance. Curves showing the tradeoff between AUC and AA were generated for these groups of phantoms.

RESULTS

Overall, the girth-based dosing method suggested different amounts of AA compared to weight-based dosing for the phantoms that had a relatively large body weight but a small girth or phantoms with relatively small bodyweight but large girth. Reductions of AA to 62.9% compared to weight-based dosing guidelines can potentially be realized while maintaining a baseline (AUC = 0.80) IQ for certain 15-year-olds who have a relatively small girth and large defect size. Note that the task-based IQ results are heavily dependent on the simulated defect size for the defect detection task and the appropriate AUC value must be decided by the physicians for this diagnostic task. These results are based purely on simulation and are subject to future clinical validation.

CONCLUSIONS

The study provides simulation-based IQ-AA data for a girth-based dosing method for pediatric renal SPECT, suggesting that patient waist circumference at the level of kidneys should be considered in selecting the AA needed to achieve an acceptable IQ. This data may be useful for standards bodies to develop girth-based dosing guidelines.

Early Normal Tissue Effects and Bone Marrow Relative Biological Effectiveness for an Actinium 225-Labeled HER2/neu-Targeting Antibody

PURPOSE

In this study we determined the dose-independent relative biological effectiveness (RBE2) of bone marrow for an anti-HER2/neu antibody labeled with the alpha-particle emitter actinium 225 (225Ac). Hematologic toxicity is often a consequence of radiopharmaceutical therapy (RPT) administration, and dosimetric guidance to the bone marrow is required to limit toxicity.

METHODS AND MATERIALS

Female neu/N transgenic mice (MMTV-neu) were intravenously injected with 0 to 16.65 kBq of the alpha-particle emitter labeled antibody, 225Ac-DOTA-7.16.4, and euthanized at 1 to 9 days after treatment. Complete blood counts were performed. Femurs and tibias were collected, and bone marrow was isolated from 1 femur and tibia and counted for radioactivity. Contralateral intact femurs were fixed, decalcified, and assessed by histology. Marrow cellularity was the biologic endpoint selected for RBE2 determination. For the reference radiation, both femurs of the mice were photon irradiated with 0 to 5 Gy using a small animal radiation research platform.

RESULTS

Response as measured by cellularity for the alpha-particle emitter RPT (αRPT) RPT and the external beam radiation therapy were linear and linear quadratic, respectively, as a function of absorbed dose. The resulting dose-independent RBE2 for bone marrow was 6.

CONCLUSIONS

As αRPT gains prominence, preclinical studies evaluating RBE in vivo will be important in relating to human experience with beta-particle emitter RPT. Such normal tissue RBE evaluations will help mitigate unexpected toxicity in αRPT.

Gel-forming therapeutic peptide exhibits sustained delivery and efficacy in a mouse model of triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a particularly aggressive and invasive subtype of breast cancer that represents a major cause of death of women worldwide. Here we describe the efficacy of an integrin-binding antiangiogenic peptide in a variety of delivery methods and dosing conditions. This peptide, AXT201, demonstrated consistent anti-tumor efficacy when administered intraperitoneally, subcutaneously, and intratumorally, and retained this activity even when dosing frequency was reduced to once every two weeks. Finally, in vivo imaging and biodistribution studies of AXT201 showed a long-term persistence of at least 10 days at the site of injection and a stable detectable signal in the blood over 48 h, indicating a sustained release profile. Taken together, these findings indicate AXT201 exhibits favorable pharmacokinetic properties for a 20-mer peptide.

Large-scale in vitro microdosimetry via live cell microscopy imaging: implications for radiosensitivity and RBE evaluations in alpha-emitter radiopharmaceutical therapy

BACKGROUND

Alpha-emitter radiopharmaceutical therapy (αRPT) has shown promising outcomes in metastatic disease. However, the short range of the alpha particles necessitates dosimetry on a near-cellular spatial scale. Current knowledge on cellular dosimetry is primarily based on in vitro experiments using cell monolayers. The goal of such experiments is to establish cell sensitivity to absorbed dose (AD). However, AD cannot be measured directly and needs to be modeled. Current models, often idealize cells as spheroids in a regular grid (geometric model), simplify binding kinetics and ignore the stochastic nature of radioactive decay. It is unclear what the impact of such simplifications is, but oversimplification results in inaccurate and non-generalizable results, which hampers the rigorous study of the underlying radiobiology.

METHODS

We systematically mapped out 3D cell geometries, clustering behavior, agent binding, internalization, and subcellular trafficking kinetics for a large cohort of live cells under representative experimental conditions using confocal microscopy. This allowed for realistic Monte Carlo-based (micro)dosimetry. Experimentally established surviving fractions of the HER2 + breast cancer cell line treated with a ²¹²Pb-labelled anti-HER2 conjugate or external beam radiotherapy, anchored a rigorous statistical approach to cell sensitivity and relative biological effectiveness (RBE) estimation. All outcomes were compared to a reference geometric model, which allowed us to determine which aspects are crucial model components for the proper study of the underlying radiobiology.

RESULTS

In total, 567 cells were measured up to 26 h post-incubation. Realistic cell clustering had a large (2x), and cell geometry a small (16.4% difference) impact on AD, compared to the geometric model. Microdosimetry revealed that more than half of the cells do not receive any dose for most of the tested conditions, greatly impacting cell sensitivity estimates. Including these stochastic effects in the model, resulted in significantly more accurate predictions of surviving fraction and RBE (permutation test; p < .01).

CONCLUSIONS

This comprehensive integration of the biological and physical aspects resulted in a more accurate method of cell survival modelling in αRPT experiments. Specifically, including realistic stochastic radiation effects and cell clustering behavior is crucial to obtaining generalizable radiobiological parameters.

A persistent belief in radiopharmaceutical therapy

Imaging and dosimetry physics are essential to the long-term success of radiopharmaceutical therapy (RPT), a cancer treatment modality that can deliver potent cytotoxic radiation to disseminated cancer cells. This is a review of my personal journey in this field.

Bone Marrow Relative Biological Effectiveness for a 212Pb-labeled Anti-HER2/neu Antibody

PURPOSE

We have determined the in vivo relative biological effectiveness (RBE) of an alpha-particle-emitting radiopharmaceutical therapeutic agent (²¹²Pb-labeled anti-HER2/neu antibody) for the bone marrow, a potentially dose-limiting normal tissue.

METHODS AND MATERIALS

The RBE was measured in mice using femur marrow cellularity as the biological endpoint. External beam radiation therapy (EBRT), delivered by a small-animal radiation research platform was used as the reference radiation. Alpha-particle emissions were delivered by ²¹²Bi after the decay of its parent nuclide ²¹²Pb, which was conjugated onto an anti-HER2/neu antibody. The alpha-particle absorbed dose to the marrow after an intravenous administration (tail vein) of 122.1 to 921.3 kBq ²¹²Pb-TCMC-7.16.4 was calculated. The mice were sacrificed at 0 to 7 days after treatment and the radioactivity from the femur bone marrow was measured. Changes in marrow cellularity were assessed by histopathology.

RESULTS

The dose response for EBRT and ²¹²Pb-anti-HER2/neu antibody were linear-quadratic and linear, respectively. On transforming the EBRT dose-response relationship into a linear relationship using the equivalent dose in 2-Gy fractions of external beam radiation formalism, we obtained an RBE (denoted RBE2) of 6.4, which is independent of cellularity and absorbed dose.

CONCLUSIONS

Because hematologic toxicity is dose limiting in almost all antibody-based RPT, in vivo measurements of RBE are important in helping identify an initial administered activity in phase 1 escalation trials. Applying the RBE2 and assuming typical antibody clearance kinetics (biological half-life of 48 hours), using a modified blood-based dosimetry method, an average administered activity of approximately 185.5 MBq (5.0 mCi) per patient could be administered before hematologic toxicity is anticipated.

Cure of Micrometastatic B-Cell Lymphoma in a SCID Mouse Model Using 213Bi-Anti-CD20 Monoclonal Antibody

We studied the feasibility of using the α-emitting ²¹³Bi-anti-CD20 therapy with direct bioluminescent tracking of micrometastatic human B-cell lymphoma in a SCID mouse model. Methods: A highly lethal SCID mouse model of minimal-tumor-burden disseminated non-Hodgkin lymphoma (NHL) was established using human Raji lymphoma cells transfected to express the luciferase reporter. In vitro and in vivo radioimmunotherapy experiments were conducted. Single- and multiple-dose regimens were explored, and results with ²¹³Bi-rituximab were compared with various controls, including no treatment, free ²¹³Bi radiometal, unlabeled rituximab, and ²¹³Bi-labeled anti-HER2/neu (non-CD20-specific antibody). ²¹³Bi-rituximab was also compared in vivo with the low-energy β-emitter ¹³¹I-tositumomab and the high-energy β-emitter ⁹⁰Y-rituximab. Results: In vitro studies showed dose-dependent target-specific killing of lymphoma cells with ²¹³Bi-rituximab. Multiple in vivo studies showed significant and specific tumor growth delays with ²¹³Bi-rituximab versus free ²¹³Bi, ²¹³Bi-labeled control antibody, or unlabeled rituximab. Redosing of ²¹³Bi-rituximab was more effective than single dosing. With a single dose of therapy given 4 d after intravenous tumor inoculation, disease in all untreated controls, and in all mice in the 925-kBq ⁹⁰Y-rituximab group, progressed. With 3,700 kBq of ²¹³Bi-rituximab, 75% of the mice survived and all but 1 survivor was cured. With 2,035 kBq of ¹³¹I-tositumomab, 75% of the mice were tumor-free by bioluminescent imaging and 62.5% survived. Conclusion: Cure of micrometastatic NHL is achieved in most animals treated 4 d after intravenous tumor inoculation using either ²¹³Bi-rituximab or ¹³¹I-tositumomab, in contrast to the lack of cures with unlabeled rituximab or ⁹⁰Y-rituximab or if there was a high tumor burden before radioimmunotherapy. α-emitter-labeled anti-CD20 antibodies are promising therapeutics for NHL, although a longer-lived α-emitter may be of greater efficacy.

Anti-GD2 antibody for radiopharmaceutical imaging of osteosarcoma

PURPOSE

Osteosarcoma (OS) is the most frequently diagnosed bone cancer in children with little improvement in overall survival in the past decades. The high surface expression of disialoganglioside GD2 on OS tumors and restricted expression in normal tissues makes it an ideal target for anti-OS radiopharmaceuticals. Since human and canine OS share many biological and molecular features, spontaneously occurring OS in canines has been an ideal model for testing new imaging and treatment modalities for human translation. In this study, we evaluated a humanized anti-GD2 antibody, hu3F8, as a potential delivery vector for targeted radiopharmaceutical imaging of human and canine OS.

METHODS

The cross-reactivity of hu3F8 with human and canine OS cells and tumors was examined by immunohistochemistry and flow cytometry. The hu3F8 was radiolabeled with indium-111, and the biodistribution of [¹¹¹In]In-hu3F8 was assessed in tumor xenograft-bearing mice. The targeting ability of [¹¹¹In]In-hu3F8 to metastatic OS was tested in spontaneous OS canines.

RESULTS

The hu3F8 cross reacts with human and canine OS cells and canine OS tumors with high binding affinity. Biodistribution studies revealed selective uptake of [¹¹¹In]In-hu3F8 in tumor tissue. SPECT/CT imaging of spontaneous OS canines demonstrated avid uptake of [¹¹¹In]In-hu3F8 in all metastatic lesions. Immunohistochemistry confirmed the extensive binding of radiolabeled hu3F8 within both osseous and soft lesions.

CONCLUSION

This study demonstrates the feasibility of targeting GD2 on OS cells and spontaneous OS canine tumors using hu3F8-based radiopharmaceutical imaging. Its ability to deliver an imaging payload in a targeted manner supports the utility of hu3F8 for precision imaging of OS and potential future use in radiopharmaceutical therapy.

Improved accuracy of S-value-based dosimetry: a guide to transition from Cristy-Eckerman to ICRP adult phantoms

Background

In 2016, the International Commission on Radiological Protection (ICRP) published the results of Monte Carlo simulations performed using updated and anatomically realistic voxelized phantoms. The resulting specific absorbed fractions are based on more realistic human anatomy than those computed in the stylized, geometrical Cristy–Eckerman (CE) phantom. Despite this development, the ICRP-absorbed fractions have not been widely adopted for radiopharmaceutical dosimetry. To help make the transition, we have established a correspondence between source and target tissues defined in the CE phantom and those defined in the ICRP phantoms.

Results

The ICRP phantom has 79 source regions and 43 target regions in comparison with the 23 source and 18 target tissue regions defined in the CE phantom. The ICRP phantom provides tissue regions with greater anatomical detail. Some of this additional detail is focused on radiation protection and dosimetry of inhaled/ingested radioactivity. Some, but not all, of this detail is useful and appropriate for radiopharmaceutical therapy. We have established the correspondence between CE and ICRP phantom source and target regions and attempted to highlight the ICRP source tissues relevant to radiopharmaceutical therapy (RPT). This paper provides tables and figures highlighting the correspondences established.

Conclusion

The results provide assistance in transitioning from CE-stylized phantoms to the anatomically accurate voxelized ICRP phantoms. It provides specific guidance for porting the total absorbed activity for regions as defined in the CE phantom to regions within the ICRP phantoms.

Combination of Carriers with Complementary Intratumoral Microdistributions of Delivered -Particles May Realize the Promise for 225Ac in Large, Solid Tumors

α-particle radiotherapy has already been shown to be impervious to most resistance mechanisms. However, in established (i.e., large, vascularized) soft-tissue lesions, the diffusion-limited penetration depths of radiolabeled antibodies or nanocarriers (≤50-80 μm) combined with the short range of α-particles (4-5 cell diameters) may result in only partial tumor irradiation, potentially limiting treatment efficacy. To address this challenge, we combined carriers with complementary intratumoral microdistributions of the delivered α-particles. We used the α-particle generator ²²⁵Ac, and we combined a tumor-responsive liposome (which, on tumor uptake, releases into the interstitium a highly diffusing form of its radioactive payload [²²⁵Ac-DOTA], potentially penetrating the deeper parts of tumors where antibodies do not reach) with a separately administered, less-penetrating radiolabeled antibody (irradiating the tumor perivascular regions where liposome contents clear too quickly). Methods: In a murine model with orthotopic human epidermal growth factor receptor 2-positive BT474 breast cancer xenografts, the biodistributions of each carrier were evaluated, and the control of tumor growth was monitored after administration of the same total radioactivity of ²²⁵Ac delivered by the ²²⁵Ac-DOTA-encapsulating liposomes, by the ²²⁵Ac-DOTA-SCN--labeled trastuzumab, and by both carriers at equally split radioactivities. Results: Tumor growth was significantly more inhibited when the same total injected radioactivity was divided between the 2 separate carriers than when delivered by either of the carriers alone. The combined carriers enabled more uniform intratumoral microdistributions of α-particles, at a tumor dose that was lower than the dose delivered by the antibody alone. Conclusion: This strategy demonstrates that more uniform microdistributions of the delivered α-particles within established solid tumors improve efficacy even at lower tumor doses. Augmentation of antibody-targeted α-particle therapies with tumor-responsive liposomes may address partial tumor irradiation, improving therapeutic effects.

Methods of improving brain dose estimates for internally deposited radionuclides. J Radiol Prot 2022;42:033001. [PMID: 35785774 DOI: 10.1088/1361-6498/ac7e02]

The US National Council on Radiation Protection and Measurements (NCRP) convened Scientific Committee 6-12 (SC 6-12) to examine methods for improving dose estimates for brain tissue for internally deposited radionuclides, with emphasis on alpha emitters. This Memorandum summarises the main findings of SC 6-12 described in the recently published NCRP Commentary No. 31, 'Development of Kinetic and Anatomical Models for Brain Dosimetry for Internally Deposited Radionuclides'. The Commentary examines the extent to which dose estimates for the brain could be improved through increased realism in the biokinetic and dosimetric models currently used in radiation protection and epidemiology. A limitation of most of the current element-specific systemic biokinetic models is the absence of brain as an explicitly identified source region with its unique rate(s) of exchange of the element with blood. The brain is usually included in a large source region calledOtherthat contains all tissues not considered major repositories for the element. In effect, all tissues inOtherare assigned a common set of exchange rates with blood. A limitation of current dosimetric models for internal emitters is that activity in the brain is treated as a well-mixed pool, although more sophisticated models allowing consideration of different activity concentrations in different regions of the brain have been proposed. Case studies for 18 internal emitters indicate that brain dose estimates using current dosimetric models may change substantially (by a factor of 5 or more), or may change only modestly, by addition of a sub-model of the brain in the biokinetic model, with transfer rates based on results of published biokinetic studies and autopsy data for the element of interest. As a starting place for improving brain dose estimates, development of biokinetic models with explicit sub-models of the brain (when sufficient biokinetic data are available) is underway for radionuclides frequently encountered in radiation epidemiology. A longer-term goal is development of coordinated biokinetic and dosimetric models that address the distribution of major radioelements among radiosensitive brain tissues.

Abstract 3313: Single-cell level absorbed dose assessment and radiosensitivity modeling for alpha-emitter radiopharmaceutical therapy

The radiobiological response to alpha-emitter radiopharmaceutical therapy (αRPT) is often studied in cell monolayers. In these experiments, the goal is to establish the relationship between absorbed dose and cell survival probability. However, absorbed dose is not readily known and needs to be estimated. Current models to estimate this commonly idealize cells as biologically inert spheres (geometric model). This results in inaccurate and non-generalizable results, which could hamper the rigorous study of the underlying radiobiology. The purpose of this study was to estimate the variability in absorbed dose on a single-cell level by combining 3D measurements of cell geometries, as well as the dynamics of carrier molecule binding and trafficking in individual cells with full physics simulations of the alpha emissions. This comprehensive integration of the biological and physical aspects allows for a more accurate way to model cell survival in these αRPT experiments. Live cells were imaged on a confocal microscope. Experimental conditions of previous cell survival experiments with 212Pb on NT2.5 HER2+ breast cancer cells were replicated. A relevant antibody was tagged with AF488. 3D time lapses of membrane binding kinetics and internalization were recorded. All cells were segmented into nuclei, membrane and cytosol compartments using a purpose-build algorithm. Pharmacokinetic models were fit to the temporal antibody signals, which enabled validation with experimental binding assays. Monte Carlo simulations using the exact antibody locations in each time frame allowed for the precise estimation of dose rate over time. Single-cell absorbed dose estimates were used to model previously obtained cell survival curves. Over 300 cells were measured between 0 and 26 hours post incubation. A large range in absorbed doses was observed (coefficient of variation 0.74). The median contribution of membrane-bound activity to absorbed dose was in agreement with geometric models (error less than 6%). However the contribution of antibody internalization and perinuclear trafficking to absorbed dose varied widely between cells and over time. Cell clustering contributed 46% of the total dose, and was 6x higher than in the geometric model. Applying this to previous cell survival data yielded an estimated radiosensitivity kappa of 7.1 (geometric model: 2.8). Cell clustering has a larger, and cell geometry has a smaller impact than is assumed in current models. Perinuclear trafficking of internalized Ab positively impacts cell nucleus absorbed dose, which is typically ignored. Dose variability should be included in radiosensitivity modeling. More accurate absorbed dose estimations which result from a better understanding of the different contributing factors, will improve generalizability of the radiobiological models established in these cell monolayer experiments to more complex models.

Citation Format: Remco Bastiaannet, Ioanna Liatsou, Robert Hobbs, George Sgouros. Single-cell level absorbed dose assessment and radiosensitivity modeling for alpha-emitter radiopharmaceutical therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3313.

Abstract 213: Assessing the role of X-ray radiation in combination with a DNA-PK inhibitor on cellular activity and adhesion using an in vitro HER2-positive breast cancer model

Introduction: Breast cancer (BC) is the most common cancer for women regardless of ethnicity, and the leading cause of cancer-related deaths in women. Radiation therapy (XRT) plays a vital role in multidisciplinary treatment approaches. Although XRT has been found to reduce breast cancer recurrence and mortality rate, like all other therapeutics, XRT contains some risks. Radiation can increase the mortality rate due to the risk of ischemic heart diseases. As part of an overall effort to understand the interaction between different types of radiations (e.g., radiopharmaceutical therapy (RPT) with β or α-particle emitters) and DNA double-strand break repair (DSB) repair pathways, we observed that impairment of DNA DSB by DNA-PKcs inhibitor (NU7441) fragmented HER2-positive BC spheroids in the presence of XRT. This could be possibly due to the modulation of cellular adhesion protein mediated by DNA-PK inhibition. In this study, we aim to assess the spheroids fragmentations and also measure the cell viability and levels of a cell adhesion protein, β1-integrin in irradiated HER2-positive monolayer cells, to get a mechanistic view for XRT and NU7441 combinatorial action so as to compare with β- or α-emitter RPT.

Methods: In brief, NT2.5 BC spheroids were formed in agarose-coated 96-well plates. For both spheroids and monolayer cells, half of the media was replaced with fresh media containing 1 nM-5μM of NU7441 every other day. A colorimetric cell viability kit was used to measure cell viability and western blot analysis was performed to measure the β1-integrin level, which was normalized against β-actin.

Results: Microscopic analysis suggested that XRT increased spheroid fragmentations in the presence of NU7441 compared to the XRT alone. Interaction between XRT and NU7441 (6.68% of the total variation, P&lt;0.0001) was found for the cell viability data, where XRT (74.07% of the total variation, P&lt;0.0001) and NU7441 (5.07% of the total variation, P&lt;0.0001) both regulated cell viability on their own. Western blot data suggest that β1-integrin level was reduced for 8 Gy XRT and 5 μM NU7441 combination compared to the no XRT and vehicle combination. Moreover, 2-way ANOVA analysis revealed that XRT plays a role to regulate the β1-integrin level in NT2.5 monolayer cells (57.61% of the total variation, P&lt;0.0001).

Conclusion: XRT increased BC spheroids fragmentations when DNA damage was inhibited by a small molecule inhibitor. XRT and NU7441 both regulated NT2.5 cell viability with an interaction between them. Furthermore, XRT reduced β1-integrin level in monolayer NT2.5 cells. The XRT and NU7441 combination may induce fragmentations by decreasing cell viability and adhesion proteins. Future studies assessing the mechanisms of XRT and DNA PK inhibitor in BC spheroids and in vivo are warranted to confirm these results.

Citation Format: Mahmud Hasan, Ioanna Liatsou, George Sgouros. Assessing the role of X-ray radiation in combination with a DNA-PK inhibitor on cellular activity and adhesion using an in vitro HER2-positive breast cancer model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 213.

212Pb-conjugated anti-rat HER2/neu antibody against a neu-N derived murine mammary carcinoma cell line: cell kill and RBE in vitro

PURPOSE

In the current work, the RBE of a ²¹²Pb-conjugated anti-HER2/neu antibody construct has been evaluated, in vitro, by colony formation assay. The RBE was estimated by comparing two absorbed dose-survival curves: the first obtained from the conjugated ²¹²Pb experiments (test radiation), the second obtained by parallel experiments of single bolus irradiation of external beam (reference radiation).

MATERIALS AND METHODS

Mammary carcinoma NT2.5 cells were treated with (0-3.70) kBq/ml of radiolabeled antibody. Nonspecific binding was assessed with addition of excess amount of unlabeled antibody. The colony formation curves were converted from activity concentration to cell nucleus absorbed dose by simulating the decay and transport of all daughter and secondary particles of ²¹²Pb, using the Monte Carlo code GEANT 4.

RESULTS

The radiolabeled antibody yielded an RBE of 8.3 at 37% survival and a survival independent RBE (i.e. RBE2) of 9.9. Unbound/untargeted ²¹²Pb-labeled antibody, as obtained in blocking experiments yielded minimal alpha-particle radiation to cells. Conclusions: These results further highlight the importance of specific targeting toward achieving tumor cell kill and low toxicity to normal tissue.

Dosimetry for Radiopharmaceutical Therapy: Current Practices and Commercial Resources

With the ongoing dramatic growth of radiopharmaceutical therapy, research and development in internal radiation dosimetry continue to advance both at academic medical centers and in industry. The basic paradigm for patient-specific dosimetry includes administration of a pretreatment tracer activity of the therapeutic radiopharmaceutical; measurement of its time-dependent biodistribution; definition of the pertinent anatomy; integration of the measured time-activity data to derive source-region time-integrated activities; calculation of the tumor, organ-at-risk, and/or whole-body absorbed doses; and prescription of the therapeutic administered activity. This paper provides an overview of the state of the art of patient-specific dosimetry for radiopharmaceutical therapy, including current methods and commercially available software and other resources.

Abstract P169: Dynamic cell-level modeling of antibody binding and internalization for radiosensitivity assessements in alpha-emitter radiopharmaceutical therapy

The radiobiological response to alpha-emitter radiopharmaceutical therapy (αRPT) is often studied in cell monolayers. The geometric models which are currently used to estimate the relationship between isotope activity concentration and cell survival probability typically idealize cells as spheres without considering cellular biological processes. This results in inaccurate and non-generalizable results, which could hamper the rigorous study of the underlying radiobiology. The purpose of this study was to create accurate absorbed dose models by combining Monte Carlo simulations with 3D measurements of cell clustering and geometries, as well as dynamic carrier molecule binding and trafficking in individual cells of cell monolayers. This allows for a more accurate way to model cell survival in these αRPT experiments. Experimental conditions of previous cell survival experiments with 212Pb on NT2.5 HER2+ breast cancer cells were replicated. Live cells were imaged on a confocal microscope. Nuclei were stained with Hoechst and the media was stained with labelled dextran, creating a negative template of the cells. A relevant antibody (Ab) was tagged with AF488. 3D time lapses of membrane binding kinetics and internalization were recorded. Photobleaching was modelled and corrected for. All cells were segmented into nuclei, membrane and cytosol compartments using a purpose-build algorithm. The temporal antibody signals were used to fit pharmacokinetic models, which enabled interpolation and validation with experimental binding assays. The segmentations were used in a Monte Carlo code. S-values for every compartment and time frame were calculated using the Ab distribution directly, capturing the effect of Ab trafficking. Absorbed doses were calculated for each cell and were used to model previously obtained cell survival curves. Statistics were calculated for &gt;100 cells. We observed a large range in absorbed doses (coefficient of variation 0.74). Absorbed doses to the nucleus per unit decay on the membrane, which are mainly determined by cellular geometries, agreed with the geometric model (error &lt;6%). S-values for intercellular decays increased &gt;50% over time, which corresponds to perinuclear trafficking of Abs. The dose contribution of neighboring cells was high (46% of total dose; 6x geometric model), highlighting the importance of cell clustering. Applying this to previous cell survival data yielded an estimated radiosensitivity kappa of 7.1 (geometric model: 2.8). Cell clustering has a larger, and cell geometry has a smaller impact than is assumed in current models. Perinuclear trafficking of internalized Ab positively impacts cell nucleus absorbed dose, which is typically ignored. Dose variability should be included in radiosensitivity modeling. We intend to use such rigorous and highly detailed, cell-level analyses to arrive at simplifications that are generalizable and whose accuracy is better understood. For example, based on our findings a better accounting of cell clustering would substantially improve geometric model calculations.

Citation Format: Remco Bastiaannet, Ioanna Liatsou, Robert Hobbs, George Sgouros. Dynamic cell-level modeling of antibody binding and internalization for radiosensitivity assessements in alpha-emitter radiopharmaceutical therapy [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P169.

Tumor Response to Radiopharmaceutical Therapies: The Knowns and the Unknowns

Radiopharmaceutical therapy (RPT) is defined as the delivery of radioactive atoms to tumor-associated targets. In RPT, imaging is built into the mode of treatment since the radionuclides used in RPT often emit photons or can be imaged using a surrogate. Such imaging may be used to estimate tumor-absorbed dose. We examine and try to elucidate those factors that impact the absorbed dose-versus-response relationship for RPT agents. These include the role of inflammation- or immune-mediated effects, the significance of theranostic imaging, radiobiology, differences in dosimetry methods, pharmacokinetic differences across patients, and the impact of tumor hypoxia on response to RPT.

Imaging and dosimetry for alpha-particle emitter radiopharmaceutical therapy: improving radiopharmaceutical therapy by looking into the black box

Radiopharmaceutical therapy using α-particle emitting radionuclides (αRPT) is a novel treatment modality that delivers highly potent alpha-particles to cancer cells or their environment. We review the advantages and challenges of imaging and dosimetry in implementing αRPT for cancer patients.

Real-time quantitation of thyroidal radioiodine uptake in thyroid disease with monitoring by a collar detection device

Radioactive iodine (RAI) is safe and effective in most patients with hyperthyroidism but not all individuals are cured by the first dose, and most develop post-RAI hypothyroidism. Postoperative RAI therapy for remnant ablation is successful in 80–90% of thyroid cancer patients and sometimes induces remission of nonresectable cervical and/or distant metastatic disease but the effective tumor dose is usually not precisely known and must be moderated to avoid short- and long-term adverse effects on other tissues. The Collar Therapy Indicator (COTI) is a radiation detection device embedded in a cloth collar secured around the patient’s neck and connected to a recording and data transmission box. In previously published experience, the data can be collected at multiple time points, reflecting local cervical RAI exposure and correlating well with conventional methods. We evaluated the real-time uptake of RAI in patients with hyperthyroid Graves’ disease and thyroid cancer. We performed a pilot feasibility prospective study. Data were analyzed using R^© (version 4.0.3, The R Foundation for Statistical Computing, 2020), and Python (version 3.6, Matplotlib version 3.0.3). The COTI was able to provide a quantitative temporal pattern of uptake within the thyroid in persons with Graves’ disease and lateralized the remnant tissue in persons with thyroid cancer. The study has demonstrated that the portable collar radiation detection device outside of a healthcare facility is accurate and feasible for use after administration of RAI for diagnostic studies and therapy to provide a complete collection of fractional target radioactivity data compared to that traditionally acquired with clinic-based measurements at one or two time-points.

Clinical Trials Registration NCT03517579, DOR 5/7/2018.

Dosimetric impact of Ac-227 in accelerator-produced Ac-225 for alpha-emitter radiopharmaceutical therapy of patients with hematological malignancies: a pharmacokinetic modeling analysis

Background

Actinium-225 is an alpha-particle emitter under investigation for use in radiopharmaceutical therapy. To address limited supply, accelerator-produced ²²⁵Ac has been recently made available. Accelerator-produced ²²⁵Ac via ²³²Th irradiation (denoted ^225/7Ac) contains a low percentage (0.1–0.3%) of ²²⁷Ac (21.77-year half-life) activity at end of bombardment. Using pharmacokinetic modeling, we have examined the dosimetric impact of ²²⁷Ac on the use of accelerator-produced ²²⁵Ac for radiopharmaceutical therapy. We examine the contribution of ²²⁷Ac and its daughters to tissue absorbed doses. The dosimetric analysis was performed for antibody-conjugated ^225/7Ac administered intravenously to treat patients with hematological cancers. Published pharmacokinetic models are used to obtain the distribution of ^225/7Ac-labeled antibody and also the distribution of either free or antibody-conjugated ²²⁷Th.

Results

Based on our modeling, the tissue specific absorbed dose from ²²⁷Ac would be negligible in the context of therapy, less than 0.02 mGy/MBq for the top 6 highest absorbed tissues and less than 0.007 mGy/MBq for all other tissues. Compared to that from ²²⁵Ac, the absorbed dose from ²²⁷Ac makes up a very small component (less than 0.04%) of the total absorbed dose delivered to the 6 highest dose tissues: red marrow, spleen, endosteal cells, liver, lungs and kidneys when accelerator produced ^225/7Ac-conjugated anti-CD33 antibody is used to treat leukemia patients. For all tissues, the dominant contributor to the absorbed dose arising from the ²²⁷Ac is ²²⁷Th, the first daughter of ²²⁷Ac which has the potential to deliver absorbed dose both while it is antibody-bound and while it is free. CONCLUSIONS: These results suggest that the absorbed dose arising from ²²⁷Ac to normal organs would be negligible for an ^225/7Ac-labeled antibody that targets hematological cancer.

Renal 99mTc-DMSA pharmacokinetics in pediatric patients

Abstract

^99mTc-DMSA is one of the most commonly used pediatric nuclear medicine imaging agents. Nevertheless, there are no pharmacokinetic (PK) models for ^99mTc-DMSA in children, and currently available pediatric dose estimates for ^99mTc-DMSA use pediatric S values with PK data derived from adults. Furthermore, the adult PK data were collected in the mid-70’s using quantification techniques and instrumentation available at the time. Using pediatric imaging data for DMSA, we have obtained kinetic parameters for DMSA that differ from those applicable to adults.

Methods

We obtained patient data from a retrospective re-evaluation of clinically collected pediatric SPECT images of ^99mTc-DMSA in 54 pediatric patients from Boston’s Children Hospital (BCH), ranging in age from 1 to 16 years old. These were supplemented by prospective data from twenty-three pediatric patients (age range: 4 months to 6 years old).

Results

In pediatric patients, the plateau phase in fractional kidney uptake occurs at a fractional uptake value closer to 0.3 than the value of 0.5 reported by the International Commission on Radiological Protection (ICRP) for adult patients. This leads to a 27% lower time-integrated activity coefficient in pediatric patients than in adults. Over the age range examined, no age dependency in uptake fraction at the clinical imaging time was observed. Female pediatric patients had a 17% higher fractional kidney uptake at the clinical imaging time than males (P < 0.001).

Conclusions

Pediatric ^99mTc-DMSA kinetics differ from those reported for adults and should be considered in pediatric patient dosimetry. Alternatively, the differences obtained in this study could reflect improved quantification methods and the need to re-examine DMSA kinetics in adults.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-021-00401-7.

Abstract 1395: Humanized GD2 antibody for targeted radiopharmaceutical therapy of human and canine osteosarcoma

Osteosarcoma (OS) is the most frequently diagnosed bone tumor in children in the United States. The prognosis for metastatic or recurrent OS has remained poor (5-year survival&lt;30%) with no new effective therapies developed during the past 30 years. The high expression of tumor antigen, ganglioside GD2, on a variety of tumors, including OS, with its restricted expression on normal tissue makes GD2 an ideal target for anti-OS radiopharmaceutical therapy. Since human and canine OS shares many biological and molecular features and the prevalence of OS in dogs is 27 times higher than that in humans, spontaneously occurring OS in dogs has been shown to be an ideal model for testing new treatments for human translation. In this study, we evaluated a humanized GD2 antibody, hu3F8, that was developed for neuroblastoma therapy, as a potential delivery vector for targeted radiopharmaceutical therapy of human and canine OS.The cross immunoreactivity of hu3F8 with canine OS cells (OSCA78) and tissue, and human OS cells was confirmed by immunohistochemistry staining and flow cytometry. The binding affinity of hu3F8 to GD2 was assessed in vitro in OSCA78 and IMR32 (a human neuroblastoma cell line known expressing GD2) cell lines using 111In-DTPA-hu3F8. The dissociation constant Kd was 7.4 ± 1.0 nM for OSCA78, and 6.2 ± 1.9 nM for IMR32. Biodistribution study was performed in Nu/Nu mice bearing either OSCA78 tumor or IMR32 tumor. At 24 h after 111In-DTPA-hu3F8 injection, the highest uptake was observed in the tumor, followed by the blood, spleen, lung, and kidneys. The mean tumor uptake was 12.0% ID/g for OSCA78 tumors and 15.0% ID/g for IMR32 tumors, with a tumor-to-muscle ratio of 10.6 and 21.1, and a tumor-to-blood ratio of 1.1 and 2.4, for OSCA78 and IMR32 tumors, respectively. The 72 h biodistribution study revealed the highest uptake of 111In-DTPA-hu3F8 in both OSCA78 (28.0% ID/g) and IMR32 (51.6% ID/g ) tumors, with a tumor-to-muscle ratio of 93.3 and 206.6, and a tumor-to-blood ratio of 6.7 and 8.4, for OSCA78 tumors and IMR32 tumors, respectively. The improved uptake of 111In-DTPA-hu3F8 in tumors at 72 h was indicative of selective binding of 111In-DTPA-hu3F8 to GD2 expressing tumors. SPECT imaging showed that both OSCA78 and IMR32 tumors with 111In-DTPA-hu3F8 had superior contrast to the background, while 111In-DTPA-Rituximab (an irrelevant antibody) injected OSCA78-bearing mouse only showed moderate contrast to the background in the kidney.The cross immunoreactivity and high binding affinity of hu3F8 to canine OS cells/tissue and its ability to deliver an imaging payload (111In) suggest that conjugating hu3F8 with a radionuclide, such as alpha-emitter, 225Ac, may provide a potent radiopharmaceutical therapy for human and canine OS.

Citation Format: Yingli Fu, Jing Yu, Ioanna Liatsou, Anders Josefsson, Yong Du, Jeffrey Bryan, Dara L. Kraitchman, George Sgouros. Humanized GD2 antibody for targeted radiopharmaceutical therapy of human and canine osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1395.

Preclinical Evaluation of 213Bi- and 225Ac-Labeled Low-Molecular-Weight Compounds for Radiopharmaceutical Therapy of Prostate Cancer

Prostate-specific membrane antigen (PSMA)-targeted radiopharmaceutical therapy is a new option for patients with advanced prostate cancer refractory to other treatments. Previously, we synthesized a β-particle-emitting low-molecular-weight compound, ¹⁷⁷Lu-L1 which demonstrated reduced off-target effects in a xenograft model of prostate cancer. Here, we leveraged that scaffold to synthesize α-particle-emitting analogs of L1, ²¹³Bi-L1 and ²²⁵Ac-L1, to evaluate their safety and cell kill effect in PSMA-positive (+) xenograft models. Methods: The radiochemical synthesis, cell uptake, cell kill, and biodistribution of ²¹³Bi-L1 and ²²⁵Ac-L1 were evaluated. The efficacy of ²²⁵Ac-L1 was determined in human PSMA+ subcutaneous and micrometastatic models. Subacute toxicity at 8 wk and chronic toxicity at 1 y after administration were evaluated for ²²⁵Ac-L1. The absorbed radiation dose of ²²⁵Ac-L1 was determined using the biodistribution data and α-camera imaging. Results: ²¹³Bi- and ²²⁵Ac-L1 demonstrated specific cell uptake and cell kill in PSMA+ cells. The biodistribution of ²¹³Bi-L1 and ²²⁵Ac-L1 revealed specific uptake of radioactivity within PSMA+ lesions. Treatment studies of ²²⁵Ac-L1 demonstrated activity-dependent, specific inhibition of tumor growth in the PSMA+ flank tumor model. ²²⁵Ac-L1 also showed an increased survival benefit in the micrometastatic model compared with ¹⁷⁷Lu-L1. Activity-escalated acute and chronic toxicity studies of ²²⁵Ac-L1 revealed off-target radiotoxicity, mainly in kidneys and liver. The estimated maximum tolerated activity was about 1 MBq/kg. α-Camera imaging of ²²⁵Ac-L1 revealed high renal cortical accumulation at 2 h followed by fast clearance at 24 h. Conclusion: ²²⁵Ac-L1 demonstrated activity-dependent efficacy with minimal treatment-related organ radiotoxicity. ²²⁵Ac-L1 is a promising therapeutic for further clinical evaluation.

Transport-driven engineering of liposomes for delivery of α-particle radiotherapy to solid tumors: effect on inhibition of tumor progression and onset delay of spontaneous metastases

PURPOSE

Highly cytotoxic α-particle radiotherapy delivered by tumor-selective nanocarriers is evaluated on metastatic Triple Negative Breast Cancer (TNBC). On vascularized tumors, the limited penetration of nanocarriers (<50-80 μm) combined with the short range of α-particles (40-100 μm) may, however, result in only partial tumor irradiation, compromising efficacy. Utilizing the α-particle emitter Actinium-225 (²²⁵Ac), we studied how the therapeutic potential of a general delivery strategy using nanometer-sized engineered liposomes was affected by two key transport-driven properties: (1) the release from liposomes, when in the tumor interstitium, of the highly diffusing ²²⁵Ac-DOTA that improves the uniformity of tumor irradiation by α-particles and (2) the adhesion of liposomes on the tumors' ECM that increases liposomes' time-integrated concentrations within tumors and, therefore, the tumor-delivered radioactivities.

METHODS

On an orthotopic MDA-MB-231 TNBC murine model forming spontaneous metastases, we evaluated the maximum tolerated dose (MTD), biodistributions, and control of tumor growth and/or spreading after administration of ²²⁵Ac-DOTA-encapsulating liposomes, with different combinations of the two transport-driven properties.

RESULTS

At 83% of MTD, ²²⁵Ac-DOTA-encapsulating liposomes with both properties (1) eliminated formation of spontaneous metastases and (2) best inhibited the progression of orthotopic xenografts, compared to liposomes lacking one or both properties. These findings were primarily affected by the extent of uniformity of the intratumoral microdistributions of ²²⁵Ac followed by the overall tumor uptake of radioactivity. At the MTD, long-term toxicities were not detected 9.5 months post administration.

CONCLUSION

Our findings demonstrate the potential of a general, transport-driven strategy enabling more uniform and prolonged solid tumor irradiation by α-particles without cell-specific targeting.

An improved 211At-labeled agent for PSMA-targeted alpha therapy

α-Particle emitters targeting the prostate-specific membrane antigen (PSMA) proved effective in treating patients with prostate cancer who were unresponsive to the corresponding β-particle therapy. Astatine-211 is an α-emitter that may engender less toxicity than other α-emitting agents. We synthesized a new ²¹¹At-labeled radiotracer targeting PSMA that resulted from the search for a pharmacokinetically optimized agent. Methods: A small series of ¹²⁵I-labeled compounds were synthesized from their tin precursors to evaluate the effect of location of radiohalogen within the molecule and the presence of lutetium in the chelate on biodistribution. On that basis, ²¹¹At-VK-02-90-Lu was selected and evaluated in cell uptake and internalization studies, biodistribution and PSMA+ PC3 PIP tumor growth control in experimental flank and metastatic (PC3-ML-Luc) models. A long-term (13-month) toxicity study was performed for ²¹¹At-VK-02-90-Lu, including tissue chemistries and histopathology. Results: The radiochemical yield of ²¹¹At-VK-02-90-Lu was 17.8 ± 8.2%. Lead compound ²¹¹At-VK-02-90-Lu demonstrated total uptake within PSMA+ PC3 PIP cells of 13.4 ± 0.5% of the input dose after 4 h of incubation with little uptake in control cells. In SCID mice, ²¹¹At-VK-02-90-Lu provided 30.6 ± 4.8 percentage of injected dose per gram (%ID/g) of uptake in PSMA+ PC3 PIP tumor at 1 h post-injection that decreased to 9.46 ± 0.96 %ID/g by 24 h. Tumor-to-salivary gland and tumor-to-kidney ratios were 129 ± 99 at 4 h and 130 ± 113 at 24 h, respectively. De-astatination was not significant (stomach 0.34 ± 0.20%ID/g at 4 h). Dose-dependent survival was demonstrated at higher doses (>1.48 MBq) in both flank and metastatic models. There was little off-target toxicity as demonstrated by hematopoietic stability, unchanged tissue chemistries, weight gain rather than loss throughout treatment, and favorable histopathology. Conclusion: Compound ²¹¹At-VK-02-90-Lu or close analogs may provide limited and acceptable toxicity while retaining efficacy in management of prostate cancer.

Process validation, current good manufacturing practice production, dosimetry, and toxicity studies of the carbonic anhydrase IX imaging agent [111 In]In-XYIMSR-01 for phase I regulatory approval

[¹¹¹ In]In-XYIMSR-01 is a promising single-photon emission computed tomography (SPECT) imaging agent for identification of tumors that overexpress carbonic anhydrase IX. To translate [¹¹¹ In]In-XYIMSR-01 to phase I trials, we performed animal toxicity and dosimetry studies, determined the maximum dose for human use, and completed the chemistry, manufacturing, and controls component of a standard regulatory application. The production process, quality control testing, stability studies, and specifications for sterile drug product release were based on United States Pharmacopeia chapters <823> and <825>, FDA 21 CFR Part 212. Toxicity was evaluated by using nonradioactive [^113/115 In]In-XYIMSR-01 according to 21 CFR Part 58 guidelines. Organ Level INternal Dose Assessment/EXponential Modeling (OLINDA/EXM) was used to calculate the maximum single dose for human studies. Three process validation runs at starting radioactivities of ~800 MBq were completed with a minimum concentration of 407 MBq/ml and radiochemical purity of ≥99% at the end of synthesis. A single intravenous dose of 55 μg/ml of [^113/115 In]In-XYIMSR-01 was well tolerated in male and female Sprague-Dawley rats. The calculated maximum single dose for human injection from dosimetry studies was 390.35 MBq of [¹¹¹ In]In-XYIMSR-01. We have completed toxicity and dosimetry studies as well as validated a manufacturing process to test [¹¹¹ In]In-XYIMSR-01 in a phase I clinical trial.

Overcoming Barriers to Radiopharmaceutical Therapy (RPT): An Overview From the NRG-NCI Working Group on Dosimetry of Radiopharmaceutical Therapy

Radiopharmaceutical therapy (RPT) continues to demonstrate tremendous potential in improving the therapeutic gains in radiation therapy by specifically delivering radiation to tumors that can be well assessed in terms of dosimetry and imaging. Dosimetry in external beam radiation therapy is standard practice. This is not the case, however, in RPT. This NRG (acronym formed from the first letter of the 3 original groups: National Surgical Adjuvant Breast and Bowel Project, the Radiation Therapy Oncology Group, and the Gynecologic Oncology Group)-National Cancer Institute Working Group review describes some of the challenges to improving RPT. The main priorities for advancing the field include (1) developing and adopting best practice guidelines for incorporating patient-specific dosimetry for RPT that can be used at both large clinics with substantial resources and more modest clinics that have limited resources, (2) establishing and improving strategies for introducing new radiopharmaceuticals for clinical investigation, (3) developing approaches to address the radiophobia that is associated with the administration of radioactivity for cancer therapy, and (4) solving the financial and logistical issues of expertise and training in the developing field of RPT.

DeepAMO: a multi-slice, multi-view anthropomorphic model observer for visual detection tasks performed on volume images

Purpose: We propose a deep learning-based anthropomorphic model observer (DeepAMO) for image quality evaluation of multi-orientation, multi-slice image sets with respect to a clinically realistic 3D defect detection task. Approach: The DeepAMO is developed based on a hypothetical model of the decision process of a human reader performing a detection task using a 3D volume. The DeepAMO is comprised of three sequential stages: defect segmentation, defect confirmation (DC), and rating value inference. The input to the DeepAMO is a composite image, typical of that used to view 3D volumes in clinical practice. The output is a rating value designed to reproduce a human observer's defect detection performance. In stages 2 and 3, we propose: (1) a projection-based DC block that confirms defect presence in two 2D orthogonal orientations and (2) a calibration method that "learns" the mapping from the features of stage 2 to the distribution of observer ratings from the human observer rating data (thus modeling inter- or intraobserver variability) using a mixture density network. We implemented and evaluated the DeepAMO in the context of   Tc 99 m -DMSA SPECT imaging. A human observer study was conducted, with two medical imaging physics graduate students serving as observers. A 5 × 2 -fold cross-validation experiment was conducted to test the statistical equivalence in defect detection performance between the DeepAMO and the human observer. We also compared the performance of the DeepAMO to an unoptimized implementation of a scanning linear discriminant observer (SLDO). Results: The results show that the DeepAMO's and human observer's performances on unseen images were statistically equivalent with a margin of difference ( Δ AUC ) of 0.0426 at p < 0.05 , using 288 training images. A limited implementation of an SLDO had a substantially higher AUC (0.99) compared to the DeepAMO and human observer. Conclusion: The results show that the DeepAMO has the potential to reproduce the absolute performance, and not just the relative ranking of human observers on a clinically realistic defect detection task, and that building conceptual components of the human reading process into deep learning-based models can allow training of these models in settings where limited training images are available.

Dosimetric considerations of 99mTc-MDP uptake within the epiphyseal plates of the long bones of pediatric patients

Skeletal scintigraphy is most performed in pediatric patients using the radiopharmaceutical ^99mTc labelled methylene diphosphonate (^99mTc-MDP). Reference biokinetic models for ^99mTc-MDP indicate 50% of the administered activity is uniformly localized to the interior bone surfaces (trabecular and cortical regions), yet imaging data clearly show some preferential uptake to the epiphyseal growth plates of the long bones. To explore the dosimetric consequences of these regional activity concentrations, we have modified mesh-type computational phantoms of the International Commission on Radiological Protection (ICRP) reference pediatric series to explicitly include geometric models of the epiphyseal growth plates (2 mm in thickness) within the left/right, distal/proximal ends of the humeri, radii, ulnae, femora, tibia, and fibulae. Bone mineral activity from the ICRP Publication 128 biokinetic model for ^99mTc-MDP (ICRP 2015) was then partitioned to the growth plates at values of 0.5%, 4.4%, 8.3%, 12.2%, 16.1%, and 20%. Radiation transport simulations were performed to compute ^99mTc S-values and organ dose coefficients to the soft tissues and to bone site-specific regions of spongiosa. As the percentage of bone activity assigned to the growth plates was increased (from 0.5% to 20%), absorbed doses to the soft tissue organs, active bone marrow, bone endosteum (BE), as well as the detriment-weighted dose, were shown to decrease from their nominal values (no substantial growth plate activity), while epiphyseal plate self-doses increased. In the 15 year old male phantom, moving from 0.5% to 20% relative bone activity within the epiphyseal plates resulted in a 15% reduction in active marrow (AM) and BE dose, a 10% reduction in mean soft tissue and detriment-weighted dose, and a 6.3-fold increase in epiphyseal plate self-dose. In the newborn female phantom, we observed a 18% decrease in AM and BE dose, a 10% decrease in mean soft tissue dose, a 15% decrease in detriment-weighted dose, and 12.8-fold increase in epiphyseal plate self-dose. Increases (to 3 mm) and decreases (to 1 mm) in the assumed growth plate thickness of our models were shown to impact only the growth plate self-dose. Future work in differential quantification of ^99mTc-MDP activity-growth plates versus other bone surfaces-is required to provide clinically realistic data on activity partitioning as a function of patient age, and perhaps skeletal site. The phantom series presented here may be used to develop more optimized age-related guidance on ^99mTc-MDP administered activities to children.

Specific absorbed fractions and radionuclide S-values for tumors of varying size and composition

Accurate estimates of tumor absorbed dose are essential for the evaluation of treatment efficacy in radiopharmaceutical cancer therapy. Although tumor dosimetry via the MIRD schema has been previously investigated, prior studies have been limited to the consideration of soft-tissue tumors. In the present study, specific absorbed fractions (SAFs) for monoenergetic photons, electrons, and alpha particles in tumors of varying compositions were computed using Monte Carlo simulations in MCNPX after which self-irradiation S-values for 22 radionuclides (along with 14 additional alpha-emitter progeny) were generated for tumors of both varying size and tissue composition. The tumors were modeled as spheres with radii ranging from 0.10 cm to 6.0 cm and with compositions varying from 100% soft tissue (ST) to 100% mineral bone (MB). The energies of the photons and electrons were varied on a logarithm energy grid from 10 keV to 10 MeV. The energies of alpha particles were varied along a linear energy grid from 0.5 MeV to 12 MeV. In all cases, a homogenous activity distribution was assumed throughout the tumor volume. Furthermore, to assess the effect of tumor shape, several ellipsoidal tumors of different compositions were modeled and absorbed fractions were computed for monoenergetic electrons and photons. S-values were then generated using detailed decay data from the 2008 MIRD Monograph on Radionuclide Data and Decay Schemes. Our study results demonstrate that a soft-tissue model yields relative errors of 25% and 71% in the absorbed fraction assigned to uniform sources of 1.5 MeV electrons and 100 keV photons, respectively, localized within a 1 cm diameter tumor of MB. The data further show that absorbed fractions for moderate ellipsoids can be well approximated by a spherical shape of equal mass within a relative error of < 8%. S-values for 22 radionuclides (and their daughter progeny) were computed with results demonstrating how relative errors in SAFs could propagate to relative errors in tumor dose estimates as high as 86%. A comprehensive data set of radionuclide S-values by tumor size and tissue composition is provided for application of the MIRD schema for tumor dosimetry in radiopharmaceutical therapy.

Body morphometry appropriate computational phantoms for dose and risk optimization in pediatric renal imaging with Tc-99m DMSA and Tc-99m MAG3

Current guidelines for administered activity (AA) in pediatric nuclear medicine imaging studies are based on a 2016 harmonization of the 2010 North American Consensus guidelines and the 2007 European Association of Nuclear Medicine pediatric dosage card. These guidelines assign AA scaled to patient body mass, with further constraints on maximum and minimum values of radiopharmaceutical activity. These guidelines, however, are not formulated based upon a rigor-ous evaluation of diagnostic image quality. In a recent study of the renal cortex imaging agent ^99mTc-DMSA (Li Y et al 2019), body mass-based dosing guidelines were shown to not give the same level of image quality for patients of differing body mass. Their data suggest that patient girth at the level of the kidneys may be a better morphometric parameter to consider when selecting AA for renal nuclear medicine imaging. The objective of the present work was thus to develop a dedicated series of computational phantoms to support image quality and organ dose studies in pediatric renal imaging using ^99mTc-DMSA or ^99mTc-MAG3. The final library consists of 50 male and female phantoms of ages 0 to 15 years, with percentile variations (5th to 95th) in waist circumference (WC) at each age. For each phantom, nominal values of kidney volume, length, and depth were incorporated into the phantom design. Organ absorbed doses, detriment-weighted doses, and stochastic risks were assessed using ICRP reference biokinetic models for both agents. In Monte Carlo radiation transport simulations, organ doses for these agents yielded detriment-weighted dose coefficients (mSv/MBq) that were in general larger than current ICRP values of the effective dose coefficients (age and WC-averaged ratios of e_DW/e were 1.40 for the male phantoms and 1.49 for the female phantoms). Values of risk index (ratio of radiation-induced to natural background cancer incidence risk x 100) varied between 0.062 (newborns) to 0.108 (15-year-olds) for ^99mTc-DMSA and between 0.026 (newborns) to 0.122 (15-year-olds) for ^99mTc-MAG3. Using tallies of photon exit fluence as a rough surrogate for uniform image quality, our study demonstrated that through body region-of-interest optimization of AA, there is the potential for further dose and risk reductions of between factors of 1.5 to 3.0 beyond simple weight-based dosing guidance.

Response of breast cancer carcinoma spheroids to combination therapy with radiation and DNA-PK inhibitor: growth arrest without a change in α/β ratio

PURPOSE

Agents that increase tumor radiosensitivity are of interest in improving outcomes in radiotherapy (XRT). DNA-PK inhibitors radiosensitize and alter cell adhesion proteins. We investigated combination radiation and a DNA-PK inhibitor in monolayers vs spheroids.

MATERIALS AND METHODS

Using HER2 positive mammary carcinoma cells, we investigated the impact of NU7441, a DNA-PK inhibitor, on irradiated monolayer and spheroid cultures. Colony formation assays were performed with monolayer culture cells and spheroids after irradiation with/without NU7441 (5 μM).

RESULTS

In monolayer culture cells, α/β increased from 3.0 ± 0.2 Gy (XRT alone) to 6.9 ± 0.2 Gy (XRT+NU7441). Corresponding α/β values for cells obtained by disaggregating treated spheroids were 3.6 ± 0.7 Gy (XRT alone) and 3.5 ± 0.2 Gy (XRT+NU7441). However, spheroid survival was highly sensitive to NU7441 incubation. After 4 Gy XRT alone 75% of the irradiated spheroids remained intact; when NU7441 treatment was involved, 13% remained intact. No spheroids survived to 3 weeks at 6 Gy or more. The discrepancy between the minimal change in α/β from cells derived from spheroids and the spheroid growth response was not related to poor penetration of NU7441.

CONCLUSIONS

DNA-PK inhibitor NU7441 radiosensitized monolayer cells but not cells obtained from spheroids. NU7441 and radiation increased spheroid fragmentation.

Author Correction: Radiopharmaceutical therapy in cancer: clinical advances and challenges

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Phase 2 Study of Iodine-131 Tositumomab Plus Chemotherapy in Patients With Previously Untreated Mantle-Cell Lymphoma

BACKGROUND

Mantle-cell lymphoma (MCL) is sensitive to radiotherapy, and the CD20 antigen is relatively highly expressed in MCL. Therefore, radioimmunotherapy using radiolabeled anti-CD20 monoclonal antibodies has the potential to treat MCL. The objective of this study was to investigate the efficacy, pharmacokinetics, and safety of tositumomab (TST) and iodine-131 tositumomab (I-131 TST) followed by 6 cycles of cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) in patients with previously untreated MCL (ClinicalTrials.govNCT00022945).

PATIENTS AND METHODS

In this phase 2 open-label study, patients received dosimetric (day 0: 450 mg TST, then 35 mg I-131 TST [5 mCi]) and therapeutic (between days 7 and 14: 450 mg TST, then an individualized dose of I-131 TST [65-75 cGy]) TST/I-131 TST, with CHOP treatment commencing approximately 13 weeks after the therapeutic dose. The primary end point was the MCL response rate to treatment; secondary end points included confirmed complete response rate and total body residence time.

RESULTS

Twenty-six patients were enrolled, and 25 were included in the intent-to-treat population. The overall unconfirmed response rate was 84%, and the confirmed complete response rate was 44%. The median progression free-survival was 27.6 months. The median total body residence time was 94.5 hours. No new or unexpected safety signals were identified.

CONCLUSION

Patients with previously untreated MCL who received radioimmunotherapy with TST/I-131 TST followed by CHOP had a high response rate and a long duration of response, indicating that radioimmunotherapy is a therapeutic option in this patient population.

Abstract 5350: RBE, in vivo, for a 212Pb-conjugated anti-HER2/neu antibody

The relative biological effectiveness is defined as the absorbed dose ratio (Dref(x)/Dtest(x)) of a reference radiation, Dref, to a test radiation, Dtest, that leads to a particular biological end-point, x. The majority of reported RBE values are obtained from in vitro irradiation of cells in monolayer culture wherein clonogenic survival is the end-point. As alpha-particle emitter radiopharmaceutical therapy (RPT) becomes a viable cancer therapy modality, RBE determinations, in vivo, are essential to assessing the potential efficacy and toxicity of alpha-emitter RPT. The aim of the study was to obtain the relative biological effectiveness, RBE, for alpha-particle emissions delivered by Bi-212, following the decay of Pb-212, conjugated onto an anti-HER2/neu antibody. Photon irradiation (200 kVp, 13 mA, 4 Gy/min, 10 × 10 mm2 collimator), delivered by a small animal radiation research platform (SARRP) was used as the reference radiation to irradiate the femurs of 6-8 weeks old female, neu/N transgenic mice (MMTV-neu). The biological endpoint was the reduction in hematopoietic cells in the region of the femur used for the RBE calculation. Alpha-particle emissions were delivered to this region by administering 325 kBq (8.8 μCi) of 212Pb-TCMC-7.16.4 intravenously (tail vein). Mice were sacrificed (n=3) at 1-7 days post-212Pb-TCMC-7.16.4 antibody administration. The long bones (femurs) were harvested, fixed with formalin and counted for radioactivity. The samples were also examined for histopathologic changes to assess the marrow cellularity. Clinical chemistry and hematological analysis were also performed on blood collected by cardiac puncture. The nadir in blood counts was observed 5 days after radiolabeled antibody injection. An absorbed dose to the marrow from the 212Bi alpha-particle emissions of 1.6 Gy yielded a marrow cellularity reduction of 50 %. The absorbed dose from XRT to achieve the same reduction in marrow cellularity was 5.7 Gy. This gives an RBE of 3.6 for a 212Pb-anti-HER2/neu antibody. Since hematologic toxicity is dose-limiting in almost all antibody-based RPT, in vivo measurements of RBE are critical to avoiding it. 2 Gy to the red marrow of low LET radiation is a threshold between low platelet toxicity grades (1 and 2) vs high (3 and 4) grade toxicity. These results suggest that the toxicity threshold for a 212Pb-antibody would be less than 1 Gy.

Citation Format: Ioanna Liatsou, Anders Josefsson, Angel Cortez, Robert F. Hobbs, Cory Brayton, Julien Torgue, George Sgouros. RBE, in vivo, for a 212Pb-conjugated anti-HER2/neu antibody [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5350.

Abstract 6281: Differential response of irradiated breast carcinoma cells to DNA-PK inhibition: Spheroid response is governed by cell adhesion effects rather than changes in radiosensitivity

Agents that increase tumor radiosensitivity are of interest in improving outcomes in radiotherapy (XRT). DNA-PKcs inhibitors lead to radiosensitization and also to alterations in cell adhesion proteins. Using a HER2 positive mammary carcinoma cell line, NT2.5, we investigated the impact of Nu7441, a potent and selective DNA-PKcs inhibitor together with external beam irradiation in both 2D monolayer and 3D spheroid cell culture systems. Colony formation assays were performed for cells irradiated as monolayers and also for cells disaggregated from spheroids. Cells grown in monolayer culture and as spheroids were incubated with Nu7441 (5 µM) for 24 h after different doses of external beam irradiation. In monolayer culture cells, α/β increased from 3.0 ± 0.2 Gy (XRT alone) to 6.9 ± 0.2 Gy when XRT was followed by Nu7441 incubation. Corresponding α/β values for cells obtained by disaggregating treated spheroids were 3.6 ± 0.7 Gy (XRT alone) and 3.5 ± 0.2 Gy (XRT+Nu7441). In contrast to the minimal change in α/β, spheroids survival was highly sensitive to Nu7441 incubation. 21 days after 0, 2, 4, 6, 8, or 10 Gy XRT alone 100, 83, 75, 63, 56, and 31%, respectively, of the irradiated spheroids remained intact; when Nu7441 treatment was involved, 56% remained intact at 2 Gy and 13% at 4 Gy, no spheroids survived to 3 weeks at 6 Gy or more. We also found a progressive increase in fragmentation for spheroids exposed only to Nu7441 (0 Gy): 3/24, 9/24 and 17/24 fragmented after 14, 21 and 28 days of incubation, respectively. The discrepancy between the minimal change in α/β from cells derived from spheroids and the spheroid growth response was not related to poor penetration of Nu7441 since the colony formation assay results for different sizes of spheroids (180 to 400 µm) treated with 4 Gy with or without Nu7441were not significantly different. The results suggest that spheroid response to DNA-PKcs inhibitors is driven by their known inhibition of cell-cell adhesion pathways and potentially less so by radiosensitization.

Citation Format: Jing Yu, Ryan Lu, Jessie Nedrow, George Sgouros. Differential response of irradiated breast carcinoma cells to DNA-PK inhibition: Spheroid response is governed by cell adhesion effects rather than changes in radiosensitivity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6281.

Current Status of Radiopharmaceutical Therapy

In radiopharmaceutical therapy (RPT), a radionuclide is systemically or locally delivered with the goal of targeting and delivering radiation to cancer cells while minimizing radiation exposure to untargeted cells. Examples of current RPTs include thyroid ablation with the administration of ¹³¹I, treatment of liver cancer with ⁹⁰Y microspheres, the treatment of bony metastases with ²²³Ra, and the treatment of neuroendocrine tumors with ¹⁷⁷Lu-DOTATATE. New RPTs are being developed where radionuclides are incorporated into systemic targeted therapies. To assure that RPT is appropriately implemented, advances in targeting need to be matched with advances in quantitative imaging and dosimetry methods. Currently, radiopharmaceutical therapy is administered by intravenous or locoregional injection, and the treatment planning has typically been implemented like chemotherapy, where the activity administered is either fixed or based on a patient's body weight or body surface area. RPT pharmacokinetics are measurable by quantitative imaging and are known to vary across patients, both in tumors and normal tissues. Therefore, fixed or weight-based activity prescriptions are not currently optimized to deliver a cytotoxic dose to targets while remaining within the tolerance dose of organs at risk. Methods that provide dose estimates to individual patients rather than to reference geometries are needed to assess and adjust the injected RPT dose. Accurate doses to targets and organs at risk will benefit the individual patients and decrease uncertainties in clinical trials. Imaging can be used to measure activity distribution in vivo, and this information can be used to determine patient-specific treatment plans where the dose to the targets and organs at risk can be calculated. The development and adoption of imaging-based dosimetry methods is particularly beneficial in early clinical trials. In this work we discuss dosimetric accuracy needs in modern radiation oncology, uncertainties in the dosimetry in RPT, and best approaches for imaging and dosimetry of internal radionuclide therapy.

α-Particle-Emitter Radiopharmaceutical Therapy: Resistance Is Futile

Alpha-emitter radiopharmaceutical therapy (α-RPT) is a treatment modality that is impervious to conventional cellular resistance mechanisms because of the unique properties of the α-particle. Radiobiological studies of α-particle emitters have been few as they require detailed consideration of both biology and physics. Clinical studies of this radiation delivery modality have shown highly promising results in cancers that are resistant to other treatments. The work by Yard and colleagues published in this issue introduces an innovative approach to radiobiological investigations of α-RPT and highlights the specific physics considerations required to properly investigate this multidisciplinary treatment modality.See related article by Yard et al., p. 5640.

SAT-417 Personalized Treatment Planning for Radioiodine Therapy of Graves’ Disease;The Collar Therapy Indicator(CoTI)

Introduction Since its introduction 80 years ago, the therapeutic I-131 dosage has usually been tailored to individual patient requirements based on the uptake of a tracer radio-iodine(RAI) dose. Estimated exposure has typically been extrapolated from the results of activity measurements at one or two time points, e.g., at 4 and 24 hours. We now know that treatment of hyperthyroid Graves disease with these methods lead to a 13–25% rate of failure to cure hyperthyroidism and a 46–80% rate of long-term hypothyroidism in cured patients. There is a need for a much more personalized approach to RAI dosing based on individual RAI tissue uptake, kinetics. This can be achieved only after including multiple data points during the evaluation of tissue uptake. The Collar Therapy Indicator (CoTI), a device placed in cloth collar around the neck resembling a turtle neck sweater collar with a connecting wire and recording box, has been shown in small feasibility studies to provide data regarding radioiodine exposure that correlates with conventional methods of measuring I-123 and I-131 uptakes after diagnostic dose administration and/or therapy for thyroid disorders Methods; We hypothesized that the device’s continuous measurement capability will permit more accurate estimates of radiation exposure to thyroid tissue than conventionally employed methods assessing fractional uptake at one or a few time points. It may also provide information about the extent of variability in the absorbed radiation dose among patients with hyperthyroidism. We performed a feasibility study in a patient with graves’ disease to see the difference between tradition methods of I-123 uptake and the CoTI; (1) We compared the conventional quantitative uptake-derived thyroid time activity curve (TAC) as well as the Area Under the Curve (AUC)(based on percent uptake at 6 hour and 24 hour time points) to that obtained using the CoTI.(2) We evaluated the uptake and clearance kinetics of diagnostic I-123 administered.(3) We also evaluated patient experience in using the CoTI device with a survey instrument. Results; The CoTI plotted TAC and AUC offered a different approach from the conventional methods of calculation (6 hr and 24 hr % uptake) of I-123 TAC and AUC. The patient reported no difficulty in using the device and the device itself was not inconvenient. Conclusions; The calculation of % uptake as well as rate of uptake within the thyroid by CoTI might help us, in achieving a more personalized approach to I-131 RAI dose calculation for treatment of Graves’ disease. The preliminary research findings that we have generated will help us investigate different aspects of RAI uptake within the thyroid and will hopefully lead to solutions, for some of the common issues and problems arising out of random dosing of RAI.

Accuracy in dosimetry of diagnostic agents: impact of the number of source tissues used in whole organ S value-based calculations

BACKGROUND

Dosimetry for diagnostic agents is performed to assess the risk of radiation detriment (e.g., cancer) associated with the imaging agent and the risk is assessed by computing the effective dose coefficient, e. Stylized phantoms created by the MIRD Committee and updated by work performed by Cristy-Eckerman (CE) have been the standard in diagnostic dosimetry. Recently, the ICRP developed voxelized phantoms, which are described in ICRP Publication 110. These voxelized phantoms are more realistic and detailed in describing human anatomy compared with the CE stylized phantoms. Ideally, all tissues should be represented and their pharmacokinetics collected for an as accurate a dosimetric calculation as possible. As the number of source tissues included increases, the calculated e becomes more accurate. There is, however, a trade-off between the number of source tissues considered, and the time and effort required to measure the time-activity curve for each tissue needed for the calculations. In this study, we used a previously published 68Ga-DOTA-TATE data set to examine how the number of source tissues included for both the ICRP voxelized and CE stylized phantoms affected e.

RESULTS

Depending upon the number of source tissues included e varied between 14.0-23.5 μSv/MBq for the ICRP voxelized and 12.4-27.7 μSv/MBq for the CE stylized phantoms. Furthermore, stability in e, defined as a < 10% difference between e obtained using all source tissues compared to one using fewer source tissues, was obtained after including 5 (36%) of the 14 source tissues for the ICRP voxelized, and after including 3 (25%) of the 12 source tissues for the CE stylized phantoms. In addition, a 2-fold increase in e was obtained when all source tissues where included in the calculation compared to when the TIAC distribution was lumped into a single reminder-of-body source term.

CONCLUSIONS

This study shows the importance of including the larger tissues like the muscles and remainder-of-body in the dosimetric calculations. The range of e based on the included tissues were less for the ICRP voxelized phantoms using tissue weighting factors from ICRP Publication 103 compared to CE stylized phantoms using tissue weighting factors from ICRP Publication 60.

Dosimetry, Radiobiology and Synthetic Lethality: Radiopharmaceutical Therapy (RPT) With Alpha-Particle-Emitters

As a treatment modality that is fundamentally different from other therapies against cancer, radiopharmaceutical therapy with alpha-particle emitters has drawn the attention of the therapy community and also the biopharmaceutical industry. Alpha-particles cause a preponderance of complex DNA double-strand breaks (DSBs). This provides an opportunity to either enhance cell kill by using DNA DSB repair inhibitors or identify patients who are likely to be high responders to alpha-emitter RPT. The short-range and high potency of alpha-particles requires special dosimetry considerations. These are reviewed in light of recent updates to the phantoms and associated dosimetric quantities used for dosimetry calculations. A formalism for obtaining the necessary microscale pharmacokinetic information from patient nuclear medicine imaging is presented. Alpha-emitter based radiopharmaceutical therapy is an exciting cancer therapy modality that is being revisited. Further development of imaging and dosimetric methods specific to alpha-particle emitters, coupled with standardization of the methods and rigorous evidence that dosimetry applied to alphaRPT improves patient care are needed moving forward.

Radium-223 mechanism of action: implications for use in treatment combinations

The targeted alpha therapy radium-223 (223Ra) can prolong survival in men with castration-resistant prostate cancer (CRPC) who have symptomatic bone metastases and no known visceral metastases. Preclinical studies demonstrate that 223Ra preferentially incorporates into newly formed bone matrix within osteoblastic metastatic lesions. The emitted high-energy alpha particles induce DNA double-strand breaks that might be irreparable and lead to cell death in nearby exposed tumour cells, osteoblasts and osteoclasts. Consequently, tumour growth and abnormal bone formation are inhibited by these direct effects and by the disruption of positive-feedback loops between tumour cells and the bone microenvironment. 223Ra might also modulate immune responses within the bone. The clinical utility of 223Ra has encouraged the development of other anticancer targeted alpha therapies. A thorough understanding of the mechanism of action could inform the design of new combinatorial treatment strategies that might be more efficacious than monotherapy. On the basis of the current mechanistic knowledge and potential clinical benefits, combination therapies of 223Ra with microtubule-stabilizing cytotoxic drugs and agents targeting the androgen receptor axis, immune checkpoint receptors or DNA damage response proteins are being explored in patients with CRPC and metastatic bone disease.