The five “W”s and “How” of Targeted Alpha Therapy: Why? Who? What? Where? When? and How?

Theranostic Imaging Surrogates for Targeted Alpha Therapy: Progress in Production, Purification, and Applications

This article highlights recent developments of SPECT and PET diagnostic imaging surrogates for targeted alpha particle therapy (TAT) radiopharmaceuticals. It outlines the rationale for using imaging surrogates to improve diagnostic-scan accuracy and facilitate research, and the properties an imaging-surrogate candidate should possess. It evaluates the strengths and limitations of each potential imaging surrogate. Thirteen surrogates for TAT are explored: 133La, 132La, 134Ce/134La, and 226Ac for 225Ac TAT; 203Pb for 212Pb TAT; 131Ba for 223Ra and 224Ra TAT; 123I, 124I, 131I and 209At for 211At TAT; 134Ce/134La for 227Th TAT; and 155Tb and 152Tb for 149Tb TAT.

Challenges and opportunities in developing Actinium-225 radiopharmaceuticals

Actinium-225 (225Ac) has emerged as a promising therapeutic radioisotope for targeted alpha therapy. It emits net four alpha particles during its decay to stable daughter bismuth-209, rightly called an in-vivo nano-generator. Compared to the worldwide demand of 225Ac, the amount produced via depleted thorium-229 sources is minimal, making it an expensive radionuclide. However, many research groups are working on optimizing the parameters for the production of 225Ac via different routes, including cyclotrons, reactors and high-energy linear accelerators. The present review article focuses on the various aspects associated with the development of 225Ac radiopharmaceuticals. It includes the challenges and opportunities associated with the production methods, labeling chemistry, in-vivo kinetics and dosimetry of 225Ac radiopharmaceuticals. A brief description is also given about the 225Ac radiopharmaceuticals at preclinical stages, clinical trials and used routinely.

ED-B-Containing Isoform of Fibronectin in Tumor Microenvironment of Thymomas: A Target for a Theragnostic Approach

Simple Summary

The extra-domain B fibronectin (ED-B FN) is highly expressed in thymic epithelial tumors (TETs), as demonstrated by in vivo targeting using 131I-labeled L19 small immunoprotein (131I-L19-SIP) and immunohistochemistry with a predominant expression by stromal cells of a thymoma microenvironment rather than epithelial cells. Such high expression derived from the induction of stromal cells shifts FN production to the ED-B subtype. Our results suggest that Radretumab radioimmunotherapy (R-RIT) inefficacy is not related to low TET ED-B expression but to multifactorial aspects including patients’ inherent characteristics, the pattern expression of the target, the biological characteristics of the tumor, and the format of the target agent, which contribute to the resistance of tumor cells to treatment.

Abstract

Aim: to exploit tissue-specific interactions among thymic epithelial tumor (TETs) cells and extra-domain B fibronectin (ED-B FN). Material and methods: The stromal pattern of ED-B FN expression was investigated through tumor specimen collection and molecular profiling in 11 patients with recurrent TETs enrolled in prospective theragnostic phase I/II trials with Radretumab, an ED-B FN specific recombinant human antibody. Radretumab radioimmunotherapy (R-RIT) was offered to patients who exhibited the target expression. Experiments included immunochemical analysis (ICH), cell cultures, immunophenotypic analysis, Western blot, slot-blot assay, and quantitative RT-PCR of two primary thymoma cultures we obtained from patients’ samples and in the Ty82 cell line. Results: The in vivo scintigraphic demonstration of ED-B FN expression resulted in R-RIT eligibility in 8/11 patients, of which seven were treated. The best observed response was disease stabilization (n = 5/7) with a duration of 4.3 months (range 3–5 months). IHC data confirmed high ED-B FN expression in the peripherical microenvironment rather than in the center of the tumor, which was more abundant in B3 thymomas. Further, there was a predominant expression of ED-B FN by the stromal cells of the thymoma microenvironment rather than the epithelial cells. Conclusions: Our data support the hypothesis that thymomas induce stromal cells to shift FN production to the ED-B subtype, likely representing a favorable hallmark for tumor progression and metastasis. Collectively, results derived from clinical experience and molecular insights of the in vitro experiments suggested that R-RIT inefficacy is unlikely related to low target expression in TET, being the mechanism of R-RIT resistance eventually related to patients’ susceptibility (i.e., inherent characteristics), the pattern expression of the target (i.e., at periphery), the biological characteristics of the tumor (i.e., aggressive and resistant phenotypes), and/or to format of the target agent (i.e., 131I-L19-SIP).

Dosimetry in targeted alpha therapy. A systematic review: current findings and what is needed

Objective. A systematic review of dosimetry in Targeted Alpha Therapy (TAT) has been performed, identifying the common issues. Approach. The systematic review was performed in accordance with the PRISMA guidelines, and the literature was searched using the Scopus and PubMed databases. Main results. From the systematic review, three key points should be considered when performing dosimetry in TAT. (1) Biodistribution/Biokinetics: the accuracy of the biodistribution data is a limit to accurate dosimetry in TAT. The biodistribution of alpha-emitting radionuclides throughout the body is difficult to image directly, with surrogate radionuclide imaging, blood/faecal sampling, and animal studies able to provide information. (2) Daughter radionuclides: the decay energy of the alpha-emissions is sufficient to break the bond to the targeting vector, resulting in a release of free daughter radionuclides in the body. Accounting for daughter radionuclide migration is essential. (3) Small-scale dosimetry and microdosimetry: due to the short path length and heterogeneous distribution of alpha-emitters at the target site, small-scale/microdosimetry are important to account for the non-uniform dose distribution in a target region, organ or cell and for assessing the biological effect of alpha-particle radiation. Significance. TAT is a form of cancer treatment capable of delivering a highly localised dose to the tumour environment while sparing the surrounding healthy tissue. Dosimetry is an important part of treatment planning and follow up. Being able to accurately predict the radiation dose to the target region and healthy organs could guide the optimal prescribed activity. Detailed dosimetry models accounting for the three points mentioned above will help give confidence in and guide the clinical application of alpha-emitting radionuclides in targeted cancer therapy.

Dendrimers based cancer nanotheranostics: An overview

Cancer is a known deadliest disease that requires a judicious diagnostic, targeting, and treatment strategy for an early prognosis and selective therapy. The major pitfalls of the conventional approach are non-specificity in targeting, failure to precisely monitor therapy outcome, and cancer progression leading to malignancies. The unique physicochemical properties offered by nanotechnology derived nanocarriers have the potential to radically change the landscape of cancer diagnosis and therapeutic management. An integrative approach of utilizing both diagnostic and therapeutic functionality using a nanocarrier is termed as nanotheranostic. The nanotheranostics platform is designed in such a way that overcomes various biological barriers, efficiently targets the payload to the desired locus, and simultaneously supports planning, monitoring, and verification of treatment delivery to demonstrate an enhanced therapeutic efficacy. Thus, a nanotheranostic platform could potentially assist in drug targeting, image-guided focal therapy, drug release and distribution monitoring, predictionof treatment response, and patient stratification. A class of highly branched nanocarriers known as dendrimers is recognized as an advanced nanotheranostic platform that has the potential to revolutionize the oncology arena by its unique and exciting features. A dendrimer is a well-defined three-dimensional globular chemical architecture with a high level of monodispersity, amenability of precise size control, and surface functionalization. All the dendrimer properties exhibit a reproducible pharmacokinetic behavior that could ensure the desired biodistribution and efficacy. Dendrimers are thus being exploited as a nanotheranostic platform embodying a diverse class of therapeutic, imaging, and targeting moieties for cancer diagnosis and treatment.

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.