Astatine-211 imaging by a Compton camera for targeted radiotherapy

Hybrid PET/Compton-camera imaging: an imager for the next generation

Compton cameras can offer advantages over gamma cameras for some applications, since they are well suited for multitracer imaging and for imaging high-energy radiotracers, such as those employed in radionuclide therapy. While in conventional clinical settings state-of-the-art Compton cameras cannot compete with well-established methods such as PET and SPECT, there are specific scenarios in which they can constitute an advantageous alternative. The combination of PET and Compton imaging can benefit from the improved resolution and sensitivity of current PET technology and, at the same time, overcome PET limitations in the use of multiple radiotracers. Such a system can provide simultaneous assessment of different radiotracers under identical conditions and reduce errors associated with physical factors that can change between acquisitions. Advances are being made both in instrumentation developments combining PET and Compton cameras for multimodal or three-gamma imaging systems, and in image reconstruction, addressing the challenges imposed by the combination of the two modalities or the new techniques. This review article summarizes the advances made in Compton cameras for medical imaging and their combination with PET.

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

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

Targeted Alpha Therapy of Glioma Using 211At-Labeled Heterodimeric Peptide Targeting Both VEGFR and Integrins

Targeted radionuclide therapy based on α-emitters plays an increasingly important role in cancer treatment. In this study, we proposed to apply a heterodimeric peptide (iRGD-C6-lys-C6-^DA7R) targeting both VEGFR and integrins as a new vector for ²¹¹At radiolabeling to obtain high-performance radiopharmaceuticals with potential in targeted alpha therapy (TAT). An astatinated peptide, iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R, was prepared with a radiochemical yield of ∼45% and high radiochemical purity of >95% via an electrophilic radioastatodestannylation reaction. iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R showed good stability in vitro and high binding ability to U87MG (glioma) cells. Systematic in vitro antitumor investigations involving cytotoxicity, apoptosis, distribution of the cell cycle, and reactive oxygen species (ROS) clearly demonstrated that ²¹¹At-labeled heterodimeric peptides could significantly inhibit cell viability, induce cell apoptosis, arrest the cell cycle in G2/M phase, and increase intracellular ROS levels in a dose-dependent manner. Biodistribution revealed that iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R had rapid tumor accumulation and fast normal tissue/organ clearance, which was mainly excreted through the kidneys. Moreover, in vivo therapeutic evaluation indicated that iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R was able to obviously inhibit tumor growth and prolong the survival of mice bearing glioma xenografts without notable toxicity to normal organs. All these results suggest that TAT mediated by iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R can provide an effective and promising strategy for the treatment of glioma and some other tumors.

Compton imaging for medical applications

Compton imaging exploits inelastic scattering, known as Compton scattering, using a Compton camera consisting of a scatterer detector in the front layer and an absorber detector in the back layer. This method was developed for astronomy, and in recent years, research and development for environmental and medical applications has been actively conducted. Compton imaging can discriminate gamma rays over a wide energy range from several hundred keV to several MeV. Therefore, it is expected to be applied to the simultaneous imaging of multiple nuclides in nuclear medicine and prompt gamma ray imaging for range verification in particle therapy. In addition, multiple gamma coincidence imaging is expected to be realized, which allows the source position to be determined from a single coincidence event using nuclides that emit multiple gamma rays simultaneously, such as nuclides that emit a single gamma ray simultaneously with positron decay. This review introduces various efforts toward the practical application of Compton imaging in the medical field, including in vivo studies, and discusses its prospects.

Medical application of particle and heavy ion transport code system PHITS

The Particle and Heavy Ion Transport code System (PHITS) is a general-purpose Monte Carlo simulation code that has been applied in various areas of medical physics. These include application in different types of radiotherapy, shielding calculations, application to radiation biology, and research and development of medical tools. In this article, the useful features of PHITS are explained by referring to actual examples of various medical applications.

Absorbed dose simulation of meta-211At-astato-benzylguanidine using pharmacokinetics of 131I-MIBG and a novel dose conversion method, RAP

OBJECTIVE

We aimed to estimate in vivo 211At-labeled meta-benzylguanidine (211At-MABG) absorbed doses by the two dose conversion methods, using 131I-MIBG biodistribution data from a previously reported neuroblastoma xenograft model. In addition, we examined the effects of different cell lines and time limitations using data from two other works.

METHODS

We used the framework of the Monte Carlo method to create 3200 virtual experimental data sets of activity concentrations (kBq/g) to get the statistical information. Time activity concentration curves were produced using the fitting method of a genetic algorithm. The basic method was that absorbed doses of 211At-MABG were calculated based on the medical internal radiation dose formalism with the conversion of the physical half-life time of 131I to that of 211At. We have further improved the basic method; that is, a novel dose conversion method, RAP (Ratio of Pharmacokinetics), using percent injected dose/g.

RESULTS

Virtual experiments showed that 211At-MABG and 131I-MIBG had similar properties of initial activity concentrations and biological components, but the basic method did not simulate the 211At-MABG dose. Simulated 211At-MABG doses from 131I-MIBG using the RAP method were in agreement with those from 211At-MABG, so that their boxes overlapped in the box plots. The RAP method showed applicability to the different cell lines, but it was difficult to predict long-term doses from short-term experimental data.

CONCLUSIONS

The present RAP dose conversion method could estimate 211At-MABG absorbed doses from the pharmacokinetics of 131I-MIBG with some limitations. The RAP method would be applicable to a large number of subjects for targeted nuclide therapy.

Anti-tumor effects and potential therapeutic response biomarkers in α-emitting meta-211At-astato-benzylguanidine therapy for malignant pheochromocytoma explored by RNA-sequencing

Targeted α-particle therapy is a promising option for patients with malignant pheochromocytoma. Recent observations regarding meta-²¹¹At-astato-benzylguanidine (²¹¹At-MABG) in a pheochromocytoma mouse model showed a strong anti-tumor effect, though the molecular mechanism remains elusive. Here, we present the first comprehensive RNA-sequencing (RNA-seq) data for pheochromocytoma cells based on in vitro²¹¹At-MABG administration experiments. Key genes and pathways in the tumor α-particle radiation response are also examined to obtain potential response biomarkers.

Methods: We evaluated genome-wide transcriptional alterations in the rat pheochromocytoma cell line PC12 at 3, 6, and 12 h after ²¹¹At-MABG treatment; a control experiment using ⁶⁰Co γ-ray irradiation was carried out to highlight ²¹¹At-MABG-specific gene expression. For comparisons, 10% and 80% iso-survival doses (0.8 and 0.1 kBq/mL for ²¹¹At-MABG and 10 and 1 Gy for ⁶⁰Co γ-rays) were used.

Results: Enrichment analysis of differentially expressed genes (DEGs) and analysis of the gene expression profiles of cell cycle checkpoints revealed similar modes of cell death via the p53-p21 signaling pathway after ²¹¹At-MABG treatment and γ-ray irradiation. The top list of ranked DEGs demonstrated the expression of key genes on the decrease in the survival following ²¹¹At-MABG exposure, and four potential genes (Mien1, Otub1, Vdac1 and Vegfa genes) of ²¹¹At-MABG therapy. Western blot analysis indicated increased expression of TSPO in ²¹¹At-MABG-treated cells, suggesting its potential as a PET imaging probe.

Conclusion: Comprehensive RNA-seq revealed contrasting cellular responses to γ-ray and α-particle therapy, leading to the identification of four potential candidate genes that may serve as molecular imaging and ²¹¹At-MABG therapy targets.