Quality Control of 225Ac and associated Radiopharmaceuticals: 221Fr to be or not to be?

Background: Actinium-225 radiopharmaceuticals have drawn great interest in cancer therapy due to their tumor-specific delivery of cytotoxic alpha-particles. Detection and quality control (QC) are critical for these potent agents. There is currently no consensus for best practice of ²²⁵ Ac QC. Detection of ²²⁵ Ac (T _1/2 = 9.92 days) is challenging; however, the gamma-emitting progenies ²²¹ Fr (T _1/2 = 4.8 min, 218 keV) and ²¹³ Bi (T _1/2 = 45 min, 440 keV) facilitate the indirect measurement of ²²⁵ Ac. Using several instruments, we compared multiple analytical methods for ²²⁵ Ac limit of quantification and the radiopharmaceutical radiochemical purity (RCP%). The RCP% of ²²⁵ Ac-radiopharmaceutical was evaluated at both ²²¹ Fr and ²¹³ Bi secular equilibrium. RCP% was measured using two TLC plate readers: a gas-filled proportional counter, under mixed emission and alpha specific parameters; and a plastic silicon photomultiplier detector and compared. Chromatographic strips were also analyzed using LS, HPGe, and NaI(Tl) gamma well counting. We correlated these RCP% to HPLC results measured at both equilibria utilizing multiple energy windows. Finally, examining accuracy and precision for each instrument, free ²²⁵ Ac spiking assays were conducted on the radiopharmaceutical and measured. Results: The TLC plate-reader using the plastic silicon detector resulted in significant RCP% differences between 30 min and >5 hr readings, whereas the gas-filled proportional counter showed no-significant differences between alpha-specific (30 min) and the mixed-isotope setting (>5 hr). HPGe-TLC demonstrated RCP% equivalence at ²²¹ Fr and ²¹³ Bi equilibrium. NaI(Tl) and LS measurements significantly underestimated the RCP% at 30 min. Finally, gamma well counting of HPLC fractions resulted in 3-5 % RCP% underestimation at ²²¹ Fr equilibrium. Conclusion: Five instruments have been tested for their accuracy, sensitivity, linearity and specificity response to ²²⁵ Ac quantification using ²²¹ Fr and ²¹³ Bi measurements. ²²¹ Fr secular equilibrium was deemed acceptable for TLC RCP% analyses using HPGe and gas-filled proportional counter under adequate parameters. LS, gamma well counting and TLC-scanner with plastic silicon detector required a reading at ²¹³ Bi equilibrium for accurate RCP% characterization. Low content radioimpurity detection was most accurately measured using TLC rather than gamma well counting of HPLC fraction collection due to gamma counting limitations.