Gamma counting protocols for the accurate quantification of 225Ac and 213Bi without the need for a secular equilibrium between parent and gamma-emitting daughter

Abstract

Background: Quantification of actinium-225 through gamma counter measurements, when there is no secular equilibrium between actinium-225 and its gamma emitting daughters bismuth-213 and/or francium-221, can provide valuable information regarding the possible relocation of recoiled daughters such that related radiotoxicity effects can be evaluated. This study proposes a multiple time-point protocol using the bismuth-213 photopeak with measurements before secular equilibrium between actinium-225 and bismuth-213, and a single time-point protocol using both the francium-221 and bismuth-213 photopeak while assuming secular equilibrium between actinium-225 and francium-221 but not between bismuth-213 and actinium-225.

Results: Good agreement (i.e. 3% accuracy) was obtained when relying on a multiple time-points measurement of bismuth-213 to quantify both actinium-225 and excess of bismuth-213. Following scatter correction, actinium-225 can be accurately quantified using the francium-221 in a single time-point measurement within 3% of accuracy. The analysis performed on the stability data of [²²⁵Ac]Ac-DEPA and [²²⁵Ac]Ac-DOTA complexes, before secular equilibrium between bismuth-213 and actinium-225 was formed, revealed considerable amounts of unbound bismuth-213 (i.e. more than 90%) after 24 h of the radiolabeling most likely due to the recoiled daughter effect.

Conclusion: Both protocols were able to accurately estimate ²²⁵Ac-activities provided the francium-221 energy window was corrected for the down scatter of the higher-energy gamma-emissions by bismuth-213. This could prove beneficial to study the recoiled daughter effect and redistribution of free bismuth-213 by monitoring the accumulation or clearance of bismuth-213 in different tissues during biodistribution studies or in patient samples during clinical studies. On the other hand, the single gamma counter measurement protocol, although required a 30 min waiting time, is more time and cost efficient and therefore more appropriate for standardized quality control procedures of ²²⁵Ac-labeled radiopharmaceuticals.

Keywords: Actinium-225; Bismuth-213; Francium-221; Gamma counter; Radiopharmaceutical quality control; Recoiled daughter effect.

Fig. 1

GC photon energy spectrum measured for ²²⁵Ac, in SEq with both ²²¹Fr and ²¹³Bi, and for pure ²¹³Bi

Fig. 2

Schematic representation of the migration of radiolabeled compounds in the iTLC strip. Upon exposure of the strip to the mobile phase, unbound radionuclides will remain in the spotting site, whereas bound radionuclides will move with the solvent front to the upper part of the iTLC strip

Fig. 3

GC response for ²²⁵Ac in SEq with ²²¹Fr and ²¹³Bi, measured with both EWs of ²²¹Fr and ²¹³Bi. Error bars represent the standard deviation provided by the GC software

Fig. 4

Count rate of the down scatter of gamma emissions by ²¹³Bi in the ${\text{EW}}_{{221}_{\text{Fr}}}$ ($CP{M}_{{213}_{\text{Bi}}\to {\text{EW}}_{{221}_{\text{Fr}}}}$) for different count rates of ²¹³Bi measured with the ${\text{EW}}_{{213}_{\text{Bi}}}$ ($CP{M}_{{\text{213}}_{\text{Bi}}}$)

Fig. 5

²¹³Bi activities of sample 3 (see Table 1) containing ²¹³Bi in SEq with ²²⁵Ac and an added amount of ²¹³Bi. GC measurements were performed at different time-points before and after SEq between ²¹³Bi and ²²⁵Ac using the ${\text{EW}}_{{213}_{\text{Bi}}}$. Error bars represent the standard deviation provided by the GC software

Fig. 6

A ²²⁵Ac activities measured with ${\text{EW}}_{{221}_{\text{Fr}}}$ with and without scatter correction are plotted as function of measurement time for the sample 3 (see Table 1) containing ²²⁵Ac and additional ²¹³Bi activity. Error bars represent the standard deviation provided by the GC software. B Relative bias (percentage relative difference) of ²²⁵Ac activity estimated using ${\text{EW}}_{{221}_{\text{Fr}}}$ as function of the added ²¹³Bi activity (compared to SEq between ²²⁵Ac and ²¹³Bi) when no correction for the additional down scatter of ²¹³Bi gamma-emissions in the ${\text{EW}}_{{221}_{\text{Fr}}}$ is applied

Fig. 7

Multiple time-point GC measurements were performed of the upper part of iTLC strip using ${\text{EW}}_{{213}_{\text{Bi}}}$ before SEq was reached between ²¹³Bi and ²²⁵Ac (< 5 h), to determine the amount of bound ²¹³Bi after 24 h of incubation in human serum of [²²⁵Ac]Ac-DEPA (A) and [²²⁵Ac]Ac-DOTA (B). The solid line represents the total amount of ²²⁵Ac (from GC of both parts of the iTLC strips) and the dotted line the amount of bound ²²⁵Ac. Error bars represent the standard deviation provided by the GC software