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Several radiolabeled folic acid conjugates have been developed for targeted imaging and therapy. However, the therapeutic concept with radiolabeled folate conjugates has not yet been applied to clinical applications owing to the high renal absorbed dose. The effectiveness of targeted radionuclide therapy (TRT) depends primarily on the absorbed dose rate and on the total absorbed dose delivered to the tumor and to normal tissue. Owing to various limitations associated with organ level dosimetry, voxelbased dosimetry has become essential for the assessment of a more accurate absorbed dose during TRT. In this study, we synthesized iron oxide nanoparticle (IONP)-conjugated radiolabeled folate ( 177 Lu-IONP-Folate) and performed voxel-based dosimetry using SPECT/CT images of normal mice through direct Geant4 application for emission tomography (GATE) Monte Carlo (MC) simulation. We also prepared 177 Lu-Folate and 177 Lu-IONPs for the comparison of absorbed doses with that of 177 Lu-IONP-Folate. In addition, we calculated the mean absorbed dose at the organ-level using the medical internal radiation dose (MIRD) schema. The radioactivities of all three radiotracers were mainly accumulated in the liver and kidneys immediately after injection. For the kidneys, the voxel-based absorbed doses obtained with 177 Lu-IONP-Folate, 177 Lu-Folate, and 177 Lu-IONPs were 1.01 ± 0.17, 2.46 ± 0.50, and 0.52 ± 0.08 Gy/MBq, respectively. The renal absorbed dose decreased significantly (∼half) when 177 Lu-IONP-Folate was used compared with when the 177 Lu-Folate only was used. The mean absorbed dose values obtained at organ-level using the MIRD schema were comparable to voxel-based absorbed doses estimated with GATE MC. The voxel-based absorbed dose values obtained in this study of individualized activity show that the renal absorbed dose could be reduced to almost half with 177 Lu-IONP-Folate. Therefore, 177 Lu-IONP-Folate could be clinically applicable in the TRT of folate receptor-positive cancers in a personalized manner when using the voxel-based dosimetry method.

Technetium-99m-labeled human serum albumin (99mTc-HSA) has been utilized as a blood pool imaging agent in the clinic for several decades. However, 99mTc-HSA has a short circulation time, which is a critical shortcoming for a blood pool imaging agent. Herein, we developed a novel 99mTc-labeled HSA with a long circulation time using click chemistry and a chelator, 2,2′-dipicolylamine (DPA), (99mTc-DPA-HSA). Specifically, we examined the feasibility of copper-free strain-promoted alkyne-azide cycloaddition (SPAAC) for the incorporation of HSA to the [99mTc (CO)3(H2O)3]+ system by adopting a chelate-then-click approach. In this strategy, a potent chelate system, azide-functionalized DPA, was first complexed with [99mTc (CO)3(H2O)3]+, followed by the SPAAC click reaction with azadibenzocyclooctyne-functionalized HSA (ADIBO–HSA) under biocompatible conditions. Radiolabeling efficiency of azide-functionalized DPA (99mTc-DPA) was >98%. Click conjugation efficiency of 99mTc-DPA with ADIBO–HSA was between 76 and 99% depending on the number of ADIBO moieties attached to HSA. In whole-body in vivo single photon emission computed tomography images, the blood pool uptakes of 99mTc-DPA-HSA were significantly enhanced compared to those of 99mTc-HSA at 10 min, 2, and 6 h after the injection (P < 0.001, 0.025, and 0.003, respectively). Furthermore, the blood activities of 99mTc-DPA-HSA were 8 times higher at 30 min and 10 times higher at 3 h after the injection compared to those of conventional 99mTc-HSA in ex vivo biodistribution experiment. The results exhibit the potential of 99mTc-DPA-HSA as a blood pool imaging agent and further illustrate the promise of the pre-labeling SPAAC approach for conjugation of heat-sensitive biological targeting vectors with [99mTc (CO)3(H2O)3]+.

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