Development of a transmission alpha particle dosimetry technique using A549 cells and a Ra-223 source for targeted alpha therapy

Alpha-Emitter Radium-223 Induces STING-Dependent Pyroptosis to Trigger Robust Antitumor Immunity

Radium-223 (223 Ra) is the first-in-class alpha-emitter to mediate tumor eradication, which is commonly thought to kill tumor cells by directly cleaving double-strand DNA. However, the immunogenic characteristics and cell death modalities triggered by 223 Ra remain unclear. Here, it is reported that the 223 Ra irradiation induces the pro-inflammatory damage-associated molecular patterns including calreticulin, HMGB1, and HSP70, hallmarks of tumor immunogenicity. Moreover, therapeutic 223 Ra retards tumor progression by triggering pyroptosis, an immunogenic cell death. Mechanically, 223 Ra-induced DNA damage leads to the activation of stimulator of interferon genes (STING)-mediated DNA sensing pathway, which is critical for NLRP3 inflammasome-dependent pyroptosis and subsequent DCs maturation as well as T cell activation. These findings establish an essential role of STING in mediating alpha-emitter 223 Ra-induced antitumor immunity, which provides the basis for the development of novel cancer therapeutic strategies and combinatory therapy.

α-Particle-induced DNA damage tracks in peripheral blood mononuclear cells of [223Ra]RaCl2-treated prostate cancer patients

PURPOSE

One therapy option for prostate cancer patients with bone metastases is the use of [²²³Ra]RaCl₂. The α-emitter ²²³Ra creates DNA damage tracks along α-particle trajectories (α-tracks) in exposed cells that can be revealed by immunofluorescent staining of γ-H2AX+53BP1 DNA double-strand break markers. We investigated the time- and absorbed dose-dependency of the number of α-tracks in peripheral blood mononuclear cells (PBMCs) of patients undergoing their first therapy with [²²³Ra]RaCl₂.

METHODS

Multiple blood samples from nine prostate cancer patients were collected before and after administration of [²²³Ra]RaCl₂, up to 4 weeks after treatment. γ-H2AX- and 53BP1-positive α-tracks were microscopically quantified in isolated and immuno-stained PBMCs.

RESULTS

The absorbed doses to the blood were less than 6 mGy up to 4 h after administration and maximally 16 mGy in total. Up to 4 h after administration, the α-track frequency was significantly increased relative to baseline and correlated with the absorbed dose to the blood in the dose range < 3 mGy. In most of the late samples (24 h - 4 weeks after administration), the α-track frequency remained elevated.

CONCLUSION

The γ-H2AX+53BP1 assay is a potent method for detection of α-particle-induced DNA damages during treatment with or after accidental incorporation of radionuclides even at low absorbed doses. It may serve as a biomarker discriminating α- from β-emitters based on damage geometry.

Targeted Alpha Therapy: Progress in Radionuclide Production, Radiochemistry, and Applications

This review outlines the accomplishments and potential developments of targeted alpha (α) particle therapy (TAT). It discusses the therapeutic advantages of the short and highly ionizing path of α-particle emissions; the ability of TAT to complement and provide superior efficacy over existing forms of radiotherapy; the physical decay properties and radiochemistry of common α-emitters, including ²²⁵Ac, ²¹³Bi, ²²⁴Ra, ²¹²Pb, ²²⁷Th, ²²³Ra, ²¹¹At, and ¹⁴⁹Tb; the production techniques and proper handling of α-emitters in a radiopharmacy; recent preclinical developments; ongoing and completed clinical trials; and an outlook on the future of TAT.

Construction and Preclinical Evaluation of 211At Labeled Anti-mesothelin Antibodies as Potential Targeted Alpha Therapy Drugs

Targeted alpha therapy (TAT) is a promising tumor therapy that can specifically transport α particle to the vicinity of tumor cells while the normal cells are only slightly irradiated. Mesothelin is a highly promising molecular signature for many types of solid tumors including malignant mesothelioma, pancreatic cancer, ovarian cancer and lung adenocarcinoma etc., while the expression in normal human tissues are limited, thus making mesothelin a promising antigen for TAT. Previously we developed a theoretical model that could predict and optimize in vitro screening of potential TAT drugs. The aim of the study is construction and preclinical evaluation of 211At labeled anti-mesothelin antibodies as potential TAT drugs. Mesothelin expression of two tumor cell lines were confirmed by flow cytometry, and their radiosensitivities were also evaluated. We used two kinds of anti-mesothelin antibodies, ET210-6 and ET210-28, to construct TAT drugs. Then, radiochemical purity, stability in vitro, affinity of the conjugates and mesothelin expression level were assessed. The specific killing of mesothelin-positive cancer cells treated by 211At-ET210-28 and 211At-ET210-6 were studied via Cell Counting Kit-8 assay and colony formation assay. 211At-ET210-28 and 211At-ET210-6 revealed excellent affinity and stability in both phosphate buffer saline and fetal bovine serum environment. Radiolabeled antibody conjugates bound specifically to mesothelin-positive cells in vitro. Both 211At-ET210-28 and 211At-ET210-6 could specifically kill mesothelin-positive cells with negligible damages to mesothelin-negative cells. Our findings provide initial proof-of-concept for the potential use of 211At labeled ET210-28/ET210-6 anti-mesothelin antibody in specific killings of mesothelin-positive tumor cells.

Selectivity Conversion of Protease Inhibitory Antibodies

Solid tumors are inherently difficult to treat because of large regions of hypoxia and are often chemotherapy- or radiotherapy-resistant. It seems that cancer stem cells reside in hypoxic and adjacent necrotic tumor areas. Therefore, new treatments that are highly selective for tumors and can eradicate cells in both hypoxic and necrotic tumor regions are desirable. Antibody α-radioconjugates couple an α-emitting radionuclide with the specificity of a tumor-targeting monoclonal antibody. The large mass and energy of α-particles result in radiation dose delivery within a smaller area independent of oxygen concentration, thus matching key criteria for killing hypoxic tumor cells. With advances in radionuclide production and chelation chemistry, α-radioconjugate therapy is regaining interest as a cancer therapy. Here, we will review current literature examining radioconjugate therapy specifically targeting necrotic and hypoxic tumor cells and outline how α-radioconjugate therapy could be used to treat tumor regions harboring more resistant cancer cell types.

Statement of Significance

Tumor-targeting antibodies are excellent vehicles for the delivery of toxic payloads directly to the tumor site. Tumor hypoxia and necrosis promote treatment recurrence, resistance, and metastasis. Targeting these areas with antibody α-radioconjugates would aid in overcoming treatment resistance.

Selectivity Conversion of Protease Inhibitory Antibodies

Background: Proteases are one of the largest pharmaceutical targets for drug developments. Their dysregulations result in a wide variety of diseases. Because proteolytic networks usually consist of protease family members that share high structural and catalytic homology, distinguishing them using small molecule inhibitors is often challenging. To achieve specific inhibition, this study described a novel approach for the generation of protease inhibitory antibodies. As a proof of concept, we aimed to convert a matrix metalloproteinase (MMP)-14 specific inhibitor to MMP-9 specific inhibitory antibodies with high selectivity. Methods: An error-prone single-chain Fv (scFv) library of an MMP-14 inhibitor 3A2 was generated for yeast surface display. A dual-color competitive FACS was developed for selection on MMP-9 catalytic domain (cdMMP-9) and counter-selection on cdMMP-14 simultaneously, which were fused/conjugated with different fluorophores. Isolated MMP-9 inhibitory scFvs were biochemically characterized by inhibition assays on MMP-2/-9/-12/-14, proteolytic stability tests, inhibition mode determination, competitive ELISA with TIMP-2 (a native inhibitor of MMPs), and paratope mutagenesis assays. Results: We converted an MMP-14 specific inhibitor 3A2 into a panel of MMP-9 specific inhibitory antibodies with dramatic selectivity shifts of 690-4,500 folds. Isolated scFvs inhibited cdMMP-9 at nM potency with high selectivity over MMP-2/-12/-14 and exhibited decent proteolytic stability. Biochemical characterizations revealed that these scFvs were competitive inhibitors binding to cdMMP-9 near its reaction cleft via their CDR-H3s. Conclusions: This study developed a novel approach able to convert the selectivity of inhibitory antibodies among closely related protease family members. This methodology can be directly applied for mAbs inhibiting many proteases of biomedical importance.

Radiation Imagers for Quantitative, Single-particle Digital Autoradiography of Alpha- and Beta-particle Emitters

Promising therapies are being developed or are in early-stage clinical trials that employ the use of alpha- and beta-emitting radionuclides to cure hematologic malignancies. However, these targeted radionuclide therapies have not yet met their expected potential for cancer treatment. A primary reason is lack of biodistribution, dosimetry, and dose-response information at cellular levels, which are directly related to optimal targeting, achieving a requisite therapeutic dose, and assessing the safety profile in normal organs and tissues. The current set of imaging tools, such as film autoradiography, scintigraphy, and SPECT/CT, available to researchers and clinicians do not allow the effective assessment of radiation absorbed dose distributions at cellular levels because resolutions are poor, measurement and analytical times are long, and the spatial resolutions are low-generally resulting in poor signal-to-noise ratios. Recently, new radiation digital autoradiography imaging tools have been developed that promise to address these challenges. They include scintillation-, gaseous-, and semiconductor-based radiation-detection technologies that localize the emission location of charged particles on an event-by-event basis at resolutions up to 20 µm FWHM for alpha and beta emitters. These imaging systems allow radionuclide activity concentrations to be quantified to unprecedented levels (mBq/µg) and provide real-time imaging and simultaneous imaging capabilities of both high- and low-activity samples without dynamic range limitations that plague traditional autoradiography. Additionally, large-area imagers are available (>20 × 20 cm²) to accommodate high-throughput imaging studies. This article reviews the various detector classes and their associated performance trade-offs to provide researchers with an overview of the current technologies available for selecting an optimal detector configuration to meet imaging requirement needs.

Progress in Targeted Alpha-Particle Therapy. What We Learned about Recoils Release from In Vivo Generators

This review summarizes recent progress and developments as well as the most important pitfalls in targeted alpha-particle therapy, covering single alpha-particle emitters as well as in vivo alpha-particle generators. It discusses the production of radionuclides like ²¹¹At, ²²³Ra, ²²⁵Ac/²¹³Bi, labelling and delivery employing various targeting vectors (small molecules, chelators for alpha-emitting nuclides and their biomolecular targets as well as nanocarriers), general radiopharmaceutical issues, preclinical studies, and clinical trials including the possibilities of therapy prognosis and follow-up imaging. Special attention is given to the nuclear recoil effect and its impacts on the possible use of alpha emitters for cancer treatment, proper dose estimation, and labelling chemistry. The most recent and important achievements in the development of alpha emitters carrying vectors for preclinical and clinical use are highlighted along with an outlook for future developments.

DNA damage in leukocytes after internal ex-vivo irradiation of blood with the α-emitter Ra-223

Irradiation with high linear energy transfer α-emitters, like the clinically used Ra-223 dichloride, severely damages cells and induces complex DNA damage including closely spaced double-strand breaks (DSBs). As the hematopoietic system is an organ-at-risk for the treatment, knowledge about Ra-223-induced DNA damage in blood leukocytes is highly desirable. Therefore, 36 blood samples from six healthy volunteers were exposed ex-vivo (in solution) to different concentrations of Ra-223. Absorbed doses to the blood were calculated assuming local energy deposition of all α- and β-particles of the decay, ranging from 0 to 142 mGy. γ-H2AX + 53BP1 co-staining and analysis was performed in leukocytes isolated from the irradiated blood samples. For DNA damage quantification, leukocyte samples were screened for occurrence of α-induced DNA damage tracks and small γ-H2AX + 53BP1 DSB foci. This revealed a linear relationship between the frequency of α-induced γ-H2AX damage tracks and the absorbed dose to the blood, while the frequency of small γ-H2AX + 53BP1 DSB foci indicative of β-irradiation was similar to baseline values, being in agreement with a negligible β-contribution (3.7%) to the total absorbed dose to the blood. Our calibration curve will contribute to the biodosimetry of Ra-223-treated patients and early after incorporation of α-emitters.