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Abstract Purpose Patients who develop metastatic melanoma have a very poor prognosis, and new treatments are needed to improve the response rates. Melanocortin-1 receptor (MC1R) is a promising target for radionuclide therapy of metastatic melanoma, and alpha-melanocyte stimulating hormone (α-MSH) peptide analogs show high affinities to MC1Rs. Because targeted alpha therapy (TAT) can be a desirable treatment for metastatic melanoma, this study aimed to develop an ²¹¹At-labeled α-MSH peptide analog for TAT of metastatic melanoma. Methods We designed an α-MSH analog labeled with ²¹¹At using a neopentyl glycol scaffold via a hydrophilic linker. Preliminary studies using ¹²⁵I-labeled α-MSH analogs were performed to identify suitable hydrophilic linkers. Then, [²¹¹At]NpG-GGN4c was prepared using a procedure similar to that of the ¹²⁵I-labeled counterpart, [¹²⁵I]NpG-GGN4b. The biodistribution profile of [²¹¹At]NpG-GGN4c in B16F10 tumor-bearing mice was compared with that of [¹²⁵I]NpG-GGN4b. B16F10 tumor-bearing mice were treated with a single dose of vehicle or [²¹¹At]NpG-GGN4c (1 or 0.4 MBq). Results The D-Glu-D-Arg linker was identified as the optimal hydrophilic linker because of its high affinity for MC1R and good biodistribution profile, especially with low accumulation in the liver and intestine. [²¹¹At]NpG-GGN4c showed tumor accumulation comparable to that of [¹²⁵I]NpG-GGN4b and maintained the tumor radioactivity retention from 1 to 3 h postinjection. [²¹¹At]NpG-GGN4c exhibited a dose-dependent inhibitory effect on B16F10 xenograft growth without apparent body weight loss. Conclusion [²¹¹At]NpG-GGN4c showed dose-dependent efficacy against B16F10 xenografts, suggesting that [²¹¹At]NpG-GGN4c is a promising TAT agent for treating metastatic melanoma.

Abstract In this study we developed a neopentyl ²¹¹At‐labeled activated ester that incorporates a triazole spacer and applied it to the synthesis of an ²¹¹At‐labeled cetuximab. The activated ester was synthesized via the nucleophilic ²¹¹At‐astatination of a neopentyl sulfonate carrying two long alkyl chains that serve as a lipid tag, which was followed by the hydrolysis of an acetal. Additionally, we developed a novel Resin‐Assisted Purification and Deprotection (RAPD) protocol involving a solid‐phase extraction of the protected ²¹¹At‐labeled compound from the mixture of the labeling reaction, hydrolysis of the acetal on the resin, and finally an elution of the ²¹¹At‐labeled activator from the resin. This method allows the synthesis of an ²¹¹At‐labeled activated ester with high purity through a simplified procedure that circumvents the need for HPLC purification. Using this ²¹¹At‐labeled activated ester, we efficiently synthesized ²¹¹At‐labeled cetuximab in 27±1 % radiochemical yield with 95 % radiochemical purity. This ²¹¹At‐activated ester demonstrated high reactivity, and enabled the completion of the reaction with the antibody within 10 min. In comparative biodistribution studies between ²¹¹At‐labeled cetuximab and the corresponding ¹²⁵I‐labeled cetuximab in normal mice, both the thyroid and stomach showed radioactivity levels that were less than 1.0 % of the injected dose.

Abstract Background Prostate cancer is a common cancer among men worldwide that has a very poor prognosis, especially when it progresses to metastatic castration-resistant prostate cancer (mCRPC). Therefore, novel therapeutic agents for mCRPC are urgently required. Because prostate-specific membrane antigen (PSMA) is overexpressed in mCRPC, targeted alpha therapy (TAT) for PSMA is a promising treatment for mCRPC. Astatine-211 (²¹¹At) is a versatile α-emitting radionuclide that can be produced using a cyclotron. Therefore, ²¹¹At-labeled PSMA compounds could be useful for TAT; however, ²¹¹At-labeled compounds are unstable against deastatination in vivo. In this study, to develop in vivo stable ²¹¹At-labeled PSMA derivatives, we designed and synthesized ²¹¹At-labeled PSMA derivatives using a neopentyl glycol (NpG) structure that can stably retain ²¹¹At in vivo. We also evaluated their biodistribution in normal and tumor-bearing mice. Results We designed and synthesized ²¹¹At-labeled PSMA derivatives containing two glutamic acid (Glu) linkers between the NpG structure and asymmetric urea (NpG-L-PSMA ((L-Glu)₂ linker used) and NpG-D-PSMA ((D-Glu)₂ linker used)). First, we evaluated the characteristics of ¹²⁵I-labeled NpG derivatives because ¹²⁵I was readily available. [¹²⁵I]I-NpG-L-PSMA and [¹²⁵I]I-NpG-D-PSMA showed low accumulation in the stomach and thyroid, indicating their high in vivo stability against deiodination. [¹²⁵I]I-NpG-L-PSMA was excreted in urine as hydrophilic radiometabolites in addition to the intact form. Meanwhile, [¹²⁵I]I-NpG-D-PSMA was excreted in urine in an intact form. In both cases, no radioactivity was observed in the free iodine fraction. [¹²⁵I]I-NpG-D-PSMA showed higher tumor accumulation than [¹²⁵I]I-NpG-L-PSMA. We then developed ²¹¹At-labeled PSMA using the NpG-D-PSMA structure. [²¹¹At]At-NpG-D-PSMA showed low accumulation in the stomach and thyroid in normal mice, indicating its high stability against deastatination in vivo. Moreover, [²¹¹At]At-NpG-D-PSMA showed high accumulation in tumor similar to that of [¹²⁵I]I-NpG-D-PSMA. Conclusions [²¹¹At]At-NpG-D-PSMA showed high in vivo stability against deastatination and high tumor accumulation. [²¹¹At]At-NpG-D-PSMA should be considered as a potential new TAT for mCRPC.

Radiolabeled antibodies are powerful tools for both imaging and therapy in the field of nuclear medicine. Radiolabeling methods that do not release radionuclides from parent antibodies are essential for radiolabeling antibodies, and practical radiolabeling protocols that provide high in vivo stability have been established for many radionuclides, with a few exceptions. However, several limitations remain, including undesirable side effects on the biodistribution profiles of antibodies. This review summarizes the numerous efforts made to tackle this problem and the recent advances, mainly in preclinical studies. These include pretargeting approaches, engineered antibody fragments and constructs, the secondary injection of clearing agents, and the insertion of metabolizable linkages. Finally, we discuss the potential of these approaches and their prospects for further clinical application.

Abstract Background L-type amino acid transporter 1 (LAT1) is overexpressed in various cancers; therefore, radiohalogen-labeled amino acid derivatives targeting LAT1 have emerged as promising candidates for cancer radiotheranostics. However, ²¹¹At-labeled amino acid derivatives exhibit instability against deastatination in vivo, making it challenging to use ²¹¹At for radiotherapy. In this study, radiohalogen-labeled amino acid derivatives with high dehalogenation stability were developed. Results We designed and synthesized new radiohalogen-labeled amino acid derivatives ([²¹¹At]At-NpGT, [¹²⁵I]I-NpGT, and [¹⁸F]F-NpGT) in which L-tyrosine was introduced into the neopentyl glycol (NpG) structure. The radiolabeled amino acid derivatives were recognized as substrates of LAT1 in the in vitro studies using C6 glioma cells. In a biodistribution study using C6 glioma-bearing mice, these agents exhibited high stability against in vivo dehalogenation and similar biodistributions. The similarity of [²¹¹At]At-NpGT and [¹⁸F]F-NpGT indicated that these pairs of radiolabeled compounds would be helpful in radiotheranostics. Moreover, [²¹¹At]At-NpGT exhibited a dose-dependent inhibitory effect on the growth of C6 glioma-bearing mice. Conclusions [²¹¹At]At-NpGT exhibited a dose-dependent inhibitory effect on the tumor growth of glioma-bearing mice, and its biodistribution was similar to that of other radiohalogen-labeled amino acid derivatives. These findings suggest that radiotheranostics using [¹⁸F]F-NpGT and [^123/131I]I-NpGT for diagnostic applications and [²¹¹At]At-NpGT and [¹³¹I]I-NpGT for therapeutic applications are promising.

Niosomes composed of non-ionic surfactants (NISs) are novel drug carriers that can deliver both hydrophilic and hydrophobic drugs similarly to liposomes, and have several advantages over liposomes, such as lower cost and higher availability owing to their relative abundance and variety. In this study, polyethylene glycol (PEG)-coated (PEGylated) Span 20 niosome was labeled with indium-111 using a remote-loading method, and its in vivo behavior was compared with that of the liposome. In addition, niosomes based on Span 20, 40, 60, and 80 were compared to evaluate the effects of differences in the lipophilic moiety of NIS. We succeeded in convenient labeling of PEGylated niosomes with high labeling efficiency (> 95%) and purity (> 95%). The stability of 111In-labeled niosomes in the serum was high enough to trace the in vivo behavior. Span 20 niosome exhibited significantly extended retention in the blood and high accumulation in the tumor at 48 h post-injection compared to the liposome. Niosomes composed of Span 80 with an unsaturated hydrocarbon chain showed decreased radioactivity in the blood and increased accumulation in the spleen compared with the other niosomes composed of Span 20, 40, and 60 with saturated hydrocarbon chain. Meanwhile, all studied niosomes were highly accu-mulated in the tumor. These results suggest that the PEGylated niosomes can be useful as tumor-targeting drug carriers.

Liposomes are highly biocompatible drug carriers in drug delivery systems (DDSs). Preferential accumulation of liposomes and acceleration of drug release at target tumor sites are essential for effective cancer therapy using liposomal formulations; however, conventional liposomes are unsuitable for on-demand drug release. We have previously reported that drug release can be accelerated via a bio-orthogonal inverse electron demand Diels-Alder (IEDDA) reaction between amphiphilic tetrazine (Tz)-containing liposomes and norbornene (NB) derivatives in vitro. In this study, we prepared HSTz-liposomes composed of hydrogenated soybean phosphatidylcholine (HSPC) and Tz compound (2-hexadecyl-N-(6-(6-(pyridin-2-yl)-1,2,4,5-tetrazin-3-yl)pyridin-3-yl)octadecanamide) with particle sizes of 60-80 nm and ζ-potentials of -5 to 0 mV. Similar to our previous report, the addition of 5-norbornene-2-carboxylic acid (NBCOOH) to HSTz-liposomes accelerated drug release from the liposomes in vitro. In the biodistribution study using colon26 tumor-bearing mice, the radiolabeled HSTz-liposomes were accumulated and retained in the tumor at 6-48 h post-injection, whereas the radioactivity in the blood almost disappeared at 48 h. Therefore, the timing of the injection of NBCOOH was selected to be 48 h after the injection of the HSTz-liposome to avoid the IEDDA reaction in the bloodstream. We investigated the in vivo drug release by evaluating the intratumoral localization of doxorubicin (DOX) encapsulated in HSTz-liposomes labeled with fluorescent lipids. In the tumors treated with HSTz-liposomes and NBCOOH, DOX was more widely dispersed in the tumor compared with fluorescent lipid, suggesting that the release of encapsulated drugs (DOX) from HSTz-liposomes was enhanced in the tumor tissue via the bio-orthogonal IEDDA reaction. Furthermore, the combination of DOX-encapsulated HSTz-liposomes with NBCOOH significantly suppressed tumor growth compared to conventional DOX-encapsulated liposomes. In conclusion, the bio-orthogonal IEDDA reactions in the liposomal membrane enabled the acceleration of drug release from HSTz-liposomes in vivo, suggesting a promising strategy for effective cancer therapy.