Radiopharmaceutical chemistry of targeted radiotherapeutics, Part 3: alpha-particle-induced radiolytic effects on the chemical behavior of (211)At.
Two characteristics of alpha-particles that enhance their potential for targeted radiotherapy are their high energy and approximately cellular range. Unfortunately, these properties also can have negative consequences, confounding the production of clinically relevant levels of radiopharmaceutical because of radiolytic effects. The purpose of this study was to evaluate the effect of radiation dose on the astatine species present before initiation of a labeling reaction and the potential role of these molecules in the efficiency of N-succinimidyl 3-(211)At-astatobenzoate (SAB) synthesis. The ranges of radiation dose evaluated were selected to reflect those that might be encountered in SAB synthesis for the preparation of clinical doses of (211)At-labeled radiopharmaceuticals.
The distribution of astatine species present in methanol, and the yields for the synthesis of SAB from N-succinimidyl 3-(tri-n-butylstannyl)benzoate as a function of radiation dose, were determined by high-performance liquid chromatography. Radiation doses in the range of 500-12,000 Gy were evaluated using different (211)At time-activity combinations, and the effect of acetic acid, a normal component of astatodestannylation reactions, also was studied. Finally, the effect of the reducing agent sodium sulfite also was evaluated to characterize the nature of the species produced by radiolysis.
At radiation doses below 1,000 Gy, high-performance liquid chromatography analysis indicated that more than 90% of the (211)At was present in methanol as a single species, At(1), whereas at higher doses, a second peak, At(2), emerged. At(1) decreased and At(2) increased in a radiation dose-dependent fashion, with At(2) becoming the predominant species at about 3,000 Gy. At(2) was identified as a reduced form of astatine, presumably astatide, which could not be efficiently oxidized to a species suitable for electrophilic astatination. In methanol/acetic acid, more than 95% of the astatine was present as At(2) even at doses below 1,400 Gy.
The emergence of a reduced form of astatine, At(2), at higher radiation doses is consistent with the decline in SAB yields observed under these conditions. Alteration of the chemical form of the astatine by radiolysis could account for the declining yields noted in the preparation of clinical-level (211)At-labeled radiopharmaceuticals and when the labeling chemistry is initiated hours after (211)At production.
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