Novel direct alpha spectroscopy technique for ²²⁵Ac radiopharmaceutical detection in cancer cells

<i>Objective.</i>Targeted Alpha Therapy (TAT) is a promising approach for treating metastatic cancers, utilizing alpha-emitting radionuclides conjugated to tumor-targeting molecules. Actinium-225 (²²⁵Ac) has emerged as a clinically relevant candidate due to its decay chain, which produces four successive alpha emissions, effectively damaging cancer cells. However, the nuclear recoil effect can lead to off-target redistribution of decay daughters, complicating dosimetry and increasing potential toxicity. This study aims to address these challenges by developing a direct alpha spectroscopy method for<i>in vitro</i>investigations of²²⁵Ac radiopharmaceuticals.<i>Approach.</i>We developed the Bio-Sample Alpha Detector (BAD), a silicon-based detector designed to operate under ambient conditions, enabling direct alpha spectroscopy of cell samples. AR42J rat pancreatic tumor cells, which express somatostatin receptor 2 (SSTR2), were incubated with [²²⁵Ac]Ac-crown-TATE, [²²⁵Ac]Ac-PSMA-617, and [²²⁵Ac]Ac³⁺. The BAD setup allowed radiolabeled cell samples to be positioned within 100<i>µ</i>m of the detector for alpha spectra acquisition with statistical uncertainties of less than 1% in count rates. Geant4 Monte Carlo simulations were employed to validate the experimental results.<i>Main results.</i>Distinct spectral differences between radiolabeled cells and reference samples confirmed the uptake of [²²⁵Ac]Ac-crown-TATE by AR42J cells. Detection of²¹³Po, a decay daughter of²²⁵Ac, indicated partial retention and release of decay products from cells, providing insight into intracellular retention and radionuclide redistribution. Geant4 simulations confirmed the alignment of experimental data with theoretical predictions.<i>Significance.</i>This study introduces a novel method for directly measuring the behavior of²²⁵Ac and its decay daughters in biological samples using alpha spectroscopy. The BAD setup provides a valuable tool for investigating radionuclide retention, redistribution, and microdosimetry in radiopharmaceutical research.