Development of Radiolabeled Probes with Improved Imaging Contrast by Releasing Urinary Excretable Radiolabeled Compounds from Thermosensitive Liposomes in the Blood

In this study, thermosensitive liposomes (TSLs) encapsulating urinary excretable radiolabeled compounds were developed. We considered that the release of the radiolabeled compounds from the TSLs in the blood by heating the blood in peripheral tissues can achieve rapid clearance of radioactivity, resulting in improved imaging contrast. To demonstrate the hypothesis, classical TSLs mainly composed of 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine with a phase transition temperature of 41 °C were used. The optimal composition of TSLs was determined by an in vitro release test using [¹¹¹In]In-diethylenetriaminepentaacetic acid (DTPA)-encapsulated liposomes, which showed that the cholesterol content drastically changed the release characteristics of classical TSLs. In the biodistribution experiments, [¹¹¹In]In-DTPA was significantly released from the TSLs in the blood when the tails of mice were heated at 43 °C. The tumor-to-blood ratio of the heated group was three times higher than that of the nonheated group, and accumulation in normal tissues of the heated group was lower than that of the nonheated group. These results demonstrate the usefulness of the method using TSLs to encapsulate urinary excretable radiolabeled compounds for improving imaging contrast.