The impact of age on radium-223 distribution and an evaluation of molecular imaging surrogates

Abstract

Introduction: Radium-223 dichloride is the first alpha-particle emitting therapeutic agent approved by FDA and EMA for bone metastatic castration-resistant prostate cancer. We studied its age-dependent biodistribution in mice, and compared it with [^99mTc]Tc-MDP and [¹⁸F]NaF aiming to identify a potential imaging surrogate to predict [²²³Ra]RaCl₂ whole-body localization.

Methods: Male C57Bl/6 mice dosed with [²²³Ra]RaCl₂ were sacrificed at different time points to explore [²²³Ra]RaCl₂ whole-body distribution. In another experiment, mice at different ages were dosed with [²²³Ra]RaCl₂ to evaluate the aging impact. Finally, [^99mTc]Tc-MDP and [¹⁸F]NaF were administered to mice, and we compared their biodistributions with [²²³Ra]RaCl₂. Detailed micro-localization of each tracer was visualized using autoradiography and histochemical staining.

Results: [²²³Ra]RaCl₂ uptake in bone was rapid and stable. We observed persistent localization at bone epiphyses, as well as the red pulp of the spleen, while its uptake in most soft tissues cleared within 24 h. [²²³Ra]RaCl₂ distribution in soft tissues is similar in all age groups tested, while bone activity significantly decreased with aging. Although the diagnostic tracers cleared much faster from soft tissues than the therapeutic radionuclide, [^99mTc]Tc-MDP and [¹⁸F]NaF both co-localized with [²²³Ra]RaCl₂ in the skeletal compartment.

Conclusions: Radium-223 localization to the bone is dependent on age-varying factors, which implies that radium-223 dosimetry should take patient age into account. [^99mTc]Tc-MDP shows a different biodistribution from [²²³Ra]RaCl₂, both in soft tissues and in bone. [¹⁸F]NaF presents a high similarity with [²²³Ra]RaCl₂ in skeletal uptake, which validates the potential of [¹⁸F]NaF as an imaging surrogate to predict radium-223 radiotherapeutic distribution in bone.