Radiopharmaceutical Quality Control Considerations for Accelerator-Produced Actinium Therapies

Background: Alpha-particle-emitting radiotherapies are of great interest for the treatment of disseminated cancer. Actinium-225 (²²⁵Ac) produces four α-particles through its decay and is among the most attractive radionuclides for use in targeted radiotherapy applications. However, supply issues for this isotope have limited availability and increased cost for research and translation. Efforts have focused on accelerator-based methods that produce ²²⁵Ac in addition to long-lived ²²⁷Ac. Objective: The authors investigated the impact of ²²⁵Ac/²²⁷Ac material in the radiolabeling and radiopharmaceutical quality control evaluation of a DOTA chelate-conjugated peptide under good manufacturing practices. The authors use an automated module under identical conditions with either generator or accelerator-produced actinium radiolabeling. Methods: The authors have performed characterization of the radiolabeled products, including thin-layer chromatography, high-pressure liquid chromatography, gamma counting, and high-energy resolution gamma spectroscopy. Results: Peptide was radiolabeled and assessed at >95% radiochemical purity with high yields for generator produced ²²⁵Ac. The radiolabeling results produced material with subtle but detectable differences when using ²²⁵Ac/²²⁷Ac. Gamma spectroscopy was able to identify peptide initially labeled with ²²⁷Th, and at 100 d for quantification of ²²⁵Ac-bearing peptide. Conclusion: Peptides produced using ²²⁵Ac/²²⁷Ac material may be suitable for translation, but raise new issues that include processing times, logistics, and contaminant detection.

Automated radiochemical synthesis and biodistribution of [¹¹C]l-α-acetylmethadol ([¹¹C]LAAM)

Long-acting opioid agonists methadone and l-α-acetylmethadol (LAAM) prevent withdrawal in opioid-dependent persons. Attempts to synthesize [(11)C]-methadone for PET evaluation of brain disposition were unsuccessful. Owing, however, to structural and pharmacologic similarities, we aimed to develop [(11)C]LAAM as a PET ligand to probe the brain exposure of long-lasting opioids in humans. This manuscript describes [(11)C]LAAM synthesis and its biodistribution in mice. The radiochemical synthetic strategy afforded high radiochemical yield, purity and specific activity, thereby making the synthesis adaptable to automated modules.

The three-photon yield from e+annihilation in various fluids

Positronium in the triplet state decays by the emission of three photons and it has been proposed that their simultaneous detection can be used for medical imaging. The three-photon yield has been observed to be enhanced in low O(2) levels in some fluids but has never been measured in biologically relevant liquids. In this study, the delayed three-photon decay yield, at both high and low O(2) levels, has been extracted by fitting the time dependence of the two-photon yield to a set of coupled differential equations. The differential equations, in a simple yet seemingly satisfactory fashion, account for the e(+) capture to form positronium, its decay and the interconversion of the two spin configurations. Our results indicate that the delayed three-photon fraction is 0.25% in water (or blood-like) samples and exhibits no (or exceedingly small) dependence on the dissolved oxygen content. If one assumes that the direct component contributes a fraction expected by annihilation on free electrons (1/372), then the total three-photon fraction is 0.52% in the samples of biological relevance.