Thallium (201Tl) Chloride

Thallium-201 (t_1/2 = 73 h) emits around 37 Auger and other secondary electrons per decay and is highly radiotoxic when internalized into cancer cells. However, the lack of effective chelators hinders its application in molecular radiotherapy. This study evaluates Prussian blue nanoparticles, coated with citric acid (²⁰¹Tl-caPBNPs) or chitosan (²⁰¹Tl-chPBNPs), as a ²⁰¹Tl delivery vehicle compared with unbound ²⁰¹Tl⁺. Cellular uptake and efflux kinetics and radiotoxicity using clonogenic and γH2AX DNA damage assays were evaluated in vitro for both nanoparticle types. Subcellular localization was also assessed using electron microscopy with energy-dispersive X-ray spectroscopy. Biodistribution of ²⁰¹Tl-chPBNPs was evaluated in vivo in mice bearing subcutaneous A549 tumor xenografts, using single photon computed tomography imaging and ex vivo tissue counting. Compared with unbound ²⁰¹Tl⁺, ²⁰¹Tl-chPBNPs showed higher cellular uptake, while ²⁰¹Tl-caPBNP uptake was reduced. Both showed delayed efflux of ²⁰¹Tl from cancer cells. PBNPs prelocalized within cells enhanced the capture and retention of ²⁰¹Tl⁺ ions. Both types of PBNPs accumulated in cytoplasmic vesicular compartments and were not visible in the nuclei. Furthermore, ²⁰¹Tl-radiolabeled chPBNPs but not ²⁰¹Tl-caPBNPs showed significantly greater radiotoxicity than unbound ²⁰¹Tl⁺ per Becquerel of radiotoxicity provided in media, resulting from their higher uptake and delayed efflux. However, when corrected for the greater activity accumulated in cells and delayed efflux, the radiotoxicity of ²⁰¹Tl-chPBNPs was lower than that of unbound ²⁰¹Tl⁺, possibly due to differences in subcellular localization. These findings highlight the potential of chPBNPs for enhancing the uptake and retention of ²⁰¹Tl in cancer cells and development of targeted radionuclide therapy.

The lung-to-heart ratio (L/H ratio) in myocardial perfusion scintigraphy (MPS) is a useful marker that complements the sensitivity of ischemia detection. However, it requires planar imaging acquired following a separate protocol in addition to single-photon emission computed tomography (SPECT). We developed a novel method for constructing virtual planar image (VPI) from SPECT data.

Myocardial phantoms using Tl-201 were built with different amounts of radioactivity in the lungs. SPECT data and conventional planar images of these phantoms were collected with an Anger-type gamma camera. VPIs were constructed by adding all coronal images reconstructed from SPECT data. The clinical utility of VPIs obtained from 52 patients who underwent MPS with Tc-99m sestamibi was evaluated.

The radioactivity linearity of VPIs was satisfactory, with a correlation coefficient of r ≥ .99 between the measured amounts of radioactivity and image counts. The L/H ratios obtained from VPI analysis were strongly correlated with those of conventional planar images with a correlation coefficient of r ≥ .99 in the phantom study and r = .929 in clinical application.

The accuracy of VPI-based L/H ratio analysis was comparable to that of conventional planar image-based analysis. VPIs could be used as an alternative method of obtaining planar images in clinical settings.