Biodistribution of free Francium-221 and Bismuth-213 in Tumour-bearing SCID mice after successful development of Actinium-225/Francium-221 radionuclide generator Set-up.

Abstract

Despite the clinical evidence of actinium-225 (²²⁵Ac)-based targeted alpha therapies (TαT) efficacy, optimized treatment regimens are needed to improve overall clinical response rates and decrease toxicities. The nuclear recoil effect of ²²⁵Ac and its resulting daughter nuclides have been hypothesized to contribute to non-targeted damage. However, a lack of generator concepts for radionuclidically pure francium-221 (²²¹Fr), involvement of strong acids for elution, and its short half-life (4.8 min), has limited in vivo studies. Here, we report on a successful application of an ²²⁵Ac/²²¹Fr generator concept and the in vivo distribution of ²²¹Fr and bismuth-213 (²¹³Bi). The immobilization of ²²⁵Ac and elution of ²²¹Fr was performed on a LN2 resin column. The biodistribution of ²²¹Fr and ²¹³Bi was investigated in male SCID mice with LNCaP tumors at 5 and 15 min p.i. Our results indicate that LN2 resin is a highly efficient resin for selective separation of ²²⁵Ac and ²²¹Fr. The use of 0.1 M NaOAc enabled continuous elution at a constant pH. The biodistribution study revealed a fast distribution of ²²¹Fr and ²¹³Bi already 5 min p.i. We observed a strong accumulation of ²²¹Fr to the kidneys, salivary glands and small intestine. In ²¹³Bi-injected mice, the highest accumulation was in kidney and liver. We present an unprecedented concept utilizing LN2 resin in ²²⁵Ac/²²¹Fr generator applications. Successfully eluted and injected ²²¹Fr fractions showed strong accumulation of ²²¹Fr and ²¹³Bi in key organs. Our data provide preliminary evidence of the potential contribution of recoiled progeny radionuclides to side-effects in non-targeted organs.