2025 Volume 48 Issue 9 Pages 1295-1318
In recent years, radiotheranostics, a theranostic approach utilizing radioisotopes, has been gaining significant attention, primarily in the field of oncology. The success of this technology relies on the design of probes that specifically target desired sites, making the development of effective diagnostic and therapeutic agents a crucial area of research. This review provides an overview of the fundamental concepts and recent advancements in probe design for radiotheranostics, with a particular focus on pharmacokinetics and subcellular localization. Key topics discussed include: (1) multimerization, (2) introduction of the albumin-binding moiety, (3) charge modification, (4) glycosylation, (5) conjugation of cell-penetrating peptides, (6) introduction of the covalent binding moiety, (7) targeting the nucleus, and (8) utilizing drug release properties to control pharmacokinetics and intracellular localization. Through these strategies, we review the optimization and novel design possibilities for probes in radiotheranostics.
In recent years, personalized medicine has rapidly advanced, and theranostics has gained significant attention in oncology. Theranostics is an innovative approach that combines “therapy” and “diagnostics” to maximize therapeutic effects while minimizing side effects by efficiently diagnosing and treating diseases.1) In nuclear medicine, theranostics that utilizes radioisotopes (RIs) is referred to as radiotheranostics, which is expected to play a pivotal role in personalized cancer care.2,3) Radiotheranostics employs RIs that emit imaging (gamma or positron) and/or therapeutic radiation (alpha, beta, or Auger electron), enabling monitoring of drug distribution and the evaluation of therapeutic efficacy during treatment. This approach allows clinicians to tailor treatments to individual patients.4)
A key aspect of radiotheranostics is the design and development of drugs capable of specifically and efficiently delivering radiopharmaceuticals to target tissues. However, precise control over drug pharmacokinetics remains a challenge, particularly in increasing drug accumulation in target tissues while reducing distribution to non-target areas. To address this, “controlled pharmacokinetic drugs for radiotheranostics” have been developed through advanced molecular designs and nanocarrier technologies. These novel technologies are expected to significantly enhance the safety and efficacy of radiotheranostics by improving target specificity.
In addition, due to the short range of α-particles (50–100 µm in tissue), studies have shown that α-particles are more effective within cells than on the surface.5) Furthermore, because Auger electrons (1–20 nm in tissue) have an even shorter range than the size of cells, controlling intracellular localization is crucial for developing drugs for Auger electron therapy. Using membrane-penetrating peptide signals and chemical modifications, it is now possible to create probes that cross the cell membrane and selectively accumulate in specific cell organelles, such as the nucleus and mitochondria.
The success of this technology relies on the design of probes that target specific sites within the tissues and cells. This review will provide an overview of the fundamental concepts and recent advancements in the design and application of probes for radiotheranostics, with a focus on pharmacokinetics and subcellular localization. Key topics discussed will include: (1) multimerization, (2) introduction of the albumin-binding moiety (ABM), (3) charge modification, (4) glycosylation, (5) conjugation of cell-penetrating peptides, (6) introduction of the covalent binding moiety, (7) targeting the nucleus, and (8) utilizing drug release properties to control pharmacokinetics and intracellular localization.
Radiolabeled peptides and ligands are essential tools in nuclear medicine, particularly in the field of radiotheranostics. Despite their broad utility, a major challenge remains enhancing the tumor-specific accumulation and retention of these radiolabeled molecules to achieve high-contrast diagnostic imaging and effective therapeutic outcomes. Among the various strategies investigated, multimerization, such as dimerization and tetramerization, of targeting peptides and ligands has emerged as a highly promising approach.6) This section highlights the fundamental principles, advantages, and recent advancements of employing a multimeric strategy to optimize the pharmacokinetic profiles of radiolabeled peptides and ligands, thereby improving their clinical applicability.
Multimerization involves conjugating multiple targeting moieties into a single molecular structure, thereby increasing their binding valency to target receptors. This approach takes advantage of the clustering effect observed in many tumor-associated receptors, such as integrins (e.g., αvβ3) and somatostatin receptors (SSTRs), which are often overexpressed on cancer cells. The concept is that the increased accumulation of radiolabeled molecules can be achieved through the multivalent effect—the ability of multivalent molecules to bind simultaneously to multiple sites (receptors) on a target, significantly enhancing binding stability compared to monovalent molecules.7) This simultaneous interaction reduces dissociation and improves binding affinity toward the target, leading to greater accumulation and prolonged retention in the tumor.8)
Homodimerization of targeting peptides has been shown to improve tumor uptake effectively. For instance, radiolabeled dimeric cyclic Arg-Gly-Asp (RGD) peptides targeting the αvβ3 integrin receptor exhibit superior binding affinity and tumor accumulation compared with their monomeric counterparts.9,10) Further enhancements in tumor accumulation have been achieved with radiolabeled tetrameric and octameric RGD peptides11,12) (Fig. 1). However, increasing the number of multimeric RGD units can also increase off-target accumulation, which remains a significant challenge and cannot be fully addressed by simply adding more units (Fig. 2).
This research was originally published in JNM.12) ©SNMMI.
(A) Decay-corrected whole-body coronal images of athymic female nude mice bearing U87MG tumor at 30 min and at 1, 2, 6, and 20 h after injection of about 9 MBq of [64Cu]Cu-DOTA-RGD tetramer or [64Cu]Cu-DOTA-RGD octamer. (B) Coronal images of U87MG tumor-bearing mice at 2 h after injection of [64Cu]Cu-DOTA-RGD tetramer or [64Cu]Cu-DOTA-RGD octamer without and with (denoted as “blocking”) coinjection of 10 mg of c(RGDyK) per kilogram of mouse body weight. (C) Decay-corrected whole-body coronal images of c-neu oncomice at 1, 5, and 20 h after injection of about 9 MBq of [64Cu]Cu-DOTA-RGD tetramer or [64Cu]Cu-DOTA-RGD octamer. These mice are 7 mo old, and all have multiple tumors. [64Cu]Cu-DOTA-RGD tetramer and [64Cu]Cu-DOTA-RGD octamer are denoted as “RGD tetramer” and “RGD octamer,” respectively. All images shown are of 5- or 10-min static scans and representative of 3 mice per group. Tumors are indicated by arrows. This research was originally published in JNM.12) ©SNMMI.
The multimeric strategy has also been applied to other tumor target molecules beyond RGD peptides. For instance, Demmer et al. reported a [68Ga]Ga-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated dimeric peptide targeting Cysteine-X-Cysteine chemokine receptor-4 (CXCR4),13) a chemokine receptor family member overexpressed in over 70% of cancers. The dimerization improved target affinity, and the [68Ga]Ga-DOTA-conjugated peptide successfully visualized CXCR4-expressing human xenograft in tumor-bearing mice (Fig. 3). However, the study also revealed high nonspecific uptake in the liver.
Reprinted with permission from.13) © 2011, American Chemical Society.
For prostate-specific membrane antigen (PSMA) ligands—molecules designed to bind specifically to PSMA, a protein highly expressed on prostate cancer cells—Böhnke et al. reported thorium-227-labeled monomeric and multimeric PSMA ligands.14) In vitro assays showed subtle differences in affinity for PSMA between monomers, dimers, trimers, and tetramers, with multimerization improving affinity. However, the monomer exhibited the strongest cellular binding and internalization, while the dimer displayed weaker binding, and the trimer and tetramer showed diminished binding and internalization. In vivo, the monomer and dimer showed similar tumor accumulation and efficacy.
Fibroblast activation protein (FAP) targeting radiolabeled compounds may represent the most successful application of this multimeric strategy. FAP, a type II transmembrane glycoprotein expressed by cancer-associated fibroblasts, is selectively overexpressed in many cancers. Radiolabeled FAP inhibitors (FAPIs) has been developed for molecular imaging and theranostic applications.15–17) Zhao et al. reported a [68Ga]Ga-DOTA-2P(FAPI)2, which demonstrated approximately twice the tumor uptake of 68Ga-labeled FAPI monomer ([68Ga]Ga-FAPI-46) in tumor-bearing mice (Figs. 4, 5). Positron emission tomography (PET)/computed tomography (CT) scans of three cancer patients revealed higher intratumoral uptake of [68Ga]Ga-DOTA-2P(FAPI)2 compared to [68Ga]Ga-FAPI-46 across all tumor lesions18) (Fig. 6). Pang et al. further demonstrated that 68Ga-, 64Cu-, and 177Lu-labeled tetrameric FAPIs had higher uptake, longer retention, and slower clearance than their dimeric and monomeric counterparts, leading to improved therapeutic efficacy in tumor-bearing mice19) (Fig. 7).
This research was originally published in JNM.18) ©SNMMI.
(B) Representative static PET imaging of 68Ga-DOTA-2P(FAPI)2 in HCC-PDX-1 and HCC-PDX-2 with and without simultaneous injection of unlabeled FAPI-46 as competitor 1 h after administration. This research was originally published in JNM.18) ©SNMMI.
Hematoxylin and eosin (H&E) staining and FAP immunohistochemistry staining showed high FAP expression in tumor stroma (×100). This research was originally published in JNM.18) ©SNMMI.
(B) PET images of [68Ga]Ga-FAPI-46, [68Ga]Ga-DOTA-2P(FAPI)2, and [68Ga]Ga-DOTA-4P(FAPI)4 in HT-1080-FAP tumor-bearing mice. (C) Tumor growth curves in U87MG tumor-bearing mice after treatment of [177Lu]Lu-FAPI-46, [177Lu]Lu-DOTA-2P(FAPI)2, and [177Lu]Lu-DOTA-4P(FAPI)4. This research was originally published in JNM.19) ©SNMMI.
The dimerization strategy extends beyond homodimers to include heterodimers, which involve the combination of two different carrier molecules. These heterodimers are designed to bind to different target molecules, thereby improving tumor accumulation, enabling targeting of different tumor types, and allowing for simultaneous accumulation in multiple target tissues.
In recent years, radiolabeled heterodimer FAPI-RGD peptides have shown promise.20–23) Zhao et al. compared [68Ga]Ga-FAPI-RGD with [68Ga]Ga-FAPI-46 PET/CT in seven patients with five types of cancer. The study found that [68Ga]Ga-FAPI-RGD exhibited significantly higher maximum standardized uptake value (SUVmax) and tumor-to-background ratios in metastatic lesions, particularly in lymph node, lung, and bone metastases. Representative PET images are presented in Fig. 8.22)
This research was originally published in JNM.22) ©SNMMI.
Several radiolabeled heterodimer compounds targeting PSMA and gastrin-releasing peptide receptor (GRPR) have also been reported.24–26) GRPR is overexpressed in various cancers, including lung, prostate, and breast cancer, making it a promising target for both imaging and therapy.
PET and single photon emission computed tomography (SPECT) images of [68Ga]Ga-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)-DUPA-RM26 (68Ga-6) and [111In]In-NOTA-DUPA-RM26 (111In-6) in PC3-PIP in PSMA-positive/GRPR-positive tumor-bearing mice are shown in Fig. 9.26) In blocking studies, tumor uptake was partially reduced when only the PSMA- or GRPR-targeting compounds were co-injected. However, significant blocking occurred when both PSMA- and GRPR-targeting compounds were co-injected. Targeting both PSMA and GRPR with heterodimer peptides shows significant potential to enhance diagnostic accuracy and therapeutic efficacy prostate cancer patients.
(B) PET/CT images of PC3-PIP-xenografted mice (PSMA positive/GRPR positive) after iv injection of [68Ga]Ga-NOTA-DUPA-RM26 (68Ga-6) with or without co-injected with PSMA-617 and RM26. (C) SPECT/CT images of PC3-PIP-xenografted mice after iv injection of [111In]In-NOTA-DUPA-RM26 (111In-6) with or without co-injected with PSMA-617 and RM26. Reprinted from26) under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).
Other dual receptor-targeting peptides have been reported to enhance tumor accumulation. These include integrin- and GRPR-targeting heterodimer peptides,27–29) integrin- and epidermal growth factor receptor (EGFR)-targeting heterodimer peptides,30,31) integrin- and neuropeptide Y receptor-targeting heterodimer peptide,32) and integrin- and neuropilin-1-targeting heterodimer peptides.33) These heterodimeric constructs leverage the distinct properties of each receptor to improve tumor localization and retention. In addition to dual tumor receptor-targeting peptides, heterodimeric peptides designed to simultaneously target cancer and bone metastases have also been explored. For instance, RGD-, octreotide-, and PSMA ligand-conjugated oligoaspartic acid derivatives were developed to enhance uptake in both tumor and bone tissues.34–36) SPECT imaging of [67Ga]Ga-DOTA-D11-c(RGDfK) demonstrated high uptake in αvβ3 integrin overexpressed U87MG tumor and bone (Fig. 10). Furthermore, [67Ga]Ga-DOTA-D11-PSMA showed high uptake in the PSMA-positive LNCaP tumor and bone, whereas [67Ga]Ga-DOTA-D11-TATE exhibited no significant accumulation in SSTR-positive tumors. While there are relatively few reports on SSTR-targeted dimeric and multimeric peptides,37) Yim et al. reported that a monomeric octreotide derivative displayed the highest binding affinity (IC50 = 1.3 nM), followed by its dimeric, and tetrameric counterparts (2.5 and 14.0 nM) in a competitive binding assay.38) This suggests that SSTRs may be less suitable for multimerization strategies from an affinity perspective.
Reproduced with permission from.35) © 2015, American Chemical Society.
Overall, while dimerization and multimerization hold promise for improving tumor targeting and the pharmacokinetics of radiolabeled molecules for both diagnostic and therapeutic purposes, these strategies may not be universally applicable to all molecular targets. Moreover, the distance between and linker design is also a critical factor in optimizing binding efficiency and specificity.8)
Albumin, the most abundant plasma protein, exists in blood at a concentration of approximately 700 µM, accounting for 55% of the total plasma proteins with a biological half-life of about 19 d.39–41) Due to its long circulation time and multiple binding sites, albumin has been widely utilized as a drug delivery carrier. For example, fatty acids such as myristic acid have high albumin affinity and have been incorporated into therapeutics such as Levemir®, a long-acting insulin analog with a prolonged circulation half-life, allowing for once-daily subcutaneous injections.40)
Compounds that bind reversibly to albumin with micromolar affinity, such as the myristic acid and other fatty acids, are referred to ABM, and Evans Blue (EB) and 4-(4-iodophenyl)-butyric acid (IPBA) (Fig. 11) have commonly been used in basic research.2,42–44) Introducing an ABM into a probe that has nanomolar affinity for its target can enhance tumor accumulation, as the probe’s affinity for target remains stronger than its affinity for albumin. In nuclear medicine, diagnostic imaging probes should show rapid blood clearance to achieve high tumor-to-blood ratios. However, excessively fast rapid blood clearance may reduce tumor accumulation and retention, thereby diminishing therapeutic efficacy. To address this, numerous studies have focused on incorporating ABMs into radiolabeled probes to increase tumor accumulation and retention, as described below.
Several ABM-modified radiolabeled probes have been developed targeting PSMA, αvβ3 integrin, SSTR, FAP, and folate receptor (FR) —biomarkers frequently overexpressed in ovarian, breast, and brain cancers, while minimally expressed in normal tissues. Most of these probes contain DOTA derivatives, which facilitate coordination with radiometals such as 67/68Ga and 111In for imaging and 90Y, 177Lu and 225Ac for therapeutic application in radiotheranostics.2,43,44)
As PSMA-targeted probes, [90Y]Y/[177Lu]Lu-EB-PSMA-617,45) PSMA-617 with EB as ABM, and [90Y]Y/[177Lu]Lu-IP-PSMA-617,45) [177Lu]Lu-HTK01169,46) and [111In]In/[225Ac]Ac-PNT-DA1,47) PSMA-617 with IPBA as ABM (Fig. 12), exhibited higher blood retention, increased tumor accumulation and retention, and improved therapeutic effects in PSMA-expressing tumor-bearing mice compared to corresponding PSMA-617 without ABM. Figure 13A presents SPECT/CT images of [111In]In-PNT-DA1, illustrating excellent tumor visualization due to its high tumor-to-blood and tumor-to-kidney ratio. Figure 13B depicts tumor growth curves from a therapeutic study comparing [225Ac]Ac-PNT-DA1, [225Ac]Ac-PSMA-DA1, and [225Ac]Ac-PSMA-617. Among these, [225Ac]Ac-PNT-DA1 demonstrated the most potent antitumor effects. DOTA-based chelators require heating for effective complexation with 225Ac. In contrast, N,N′-bis[(6-carboxy-2-pyridil)methyl-4,13-diaza-18-crown-6 (macropa)-based chelators enable rapid complexation at room temperature, offering a more convenient and efficient approach for radiolabeling.48) Recently, [225Ac]Ac-macropa conjugated compounds incorporating one or two ABMs and PSMA-targeting moieties ([225Ac]Ac-mcp-M-alb-PSMA and [225Ac]Ac-mcp-D-alb-PSMA) (Fig. 14) showed prolonged circulation half-lives and increased tumor accumulation compared to corresponding probes without ABM.49) A first-in-human study compared [177Lu]Lu-P17-087 with its ABM-modified derivative, [177Lu]Lu-P17-088, in patients with metastatic castration-resistant prostate cancer (Fig. 15). [177Lu]Lu-P17-087 exhibited earlier tumor uptake and faster kinetics, while [177Lu]Lu-P17-088 demonstrated slower washout and higher tumor retention, suggesting potential advantages for prolonged therapeutic efficacy.50)
Data are expressed as the mean ± standard deviation. Reproduced with permission from.47) © 2023, American Chemical Society.
Several αvβ3 integrin-targeted probes have been developed using ABMs to enhance blood retention, tumor accumulation, and therapeutic efficacy. Notable examples include [64Cu]Cu-NMEB-RGD, [90Y]Y-DMEB-RGD,51) [177Lu]Lu-Palm-3PRGD2,,52) and Ga-DOTA-K([125I]IPBA)-c(RGDfK).53) Each of these RGD-based peptides incorporates different ABMs, i.e., EB, palmitic acid, and IPBA, respectively, to optimize pharmacokinetics (Fig. 16). [64Cu]Cu-NMEB-RGD, [90Y]Y-DMEB-RGD and [177Lu]Lu-Palm-3PRGD2 demonstrated higher blood retention, tumor accumulation and retention, and therapeutic effects in αvβ3 integrin-expressing tumor-bearing mice compared to corresponding RGD peptides without ABM51,52) (Fig. 17). Figure 17A presents PET images of [64Cu]Cu-NMEB-RGD and [64Cu]Cu-NOTA-c(RGDfk), [64Cu]Cu-NMEB-RGD exhibits tumor more clearly due to its higher tumor accumulation. Figure 17B depicts tumor growth curves from a therapeutic study of [90Y]Y-DMEB-RGD and [90Y]Y-DOTA-RGD, [90Y]Y-DMEB-RGD demonstrated the antitumor effects.51) Additionally, the iodine in IPBA was replaced with 125I to create Ga-DOTA-K([125I]IPBA)-c(RGDfK) and further modified to 211At, an α-emitting halogen similar to iodine to form Ga-DOTA-K([211At]APBA)-c(RGDfK) for targeted alpha therapy. Both compounds exhibited similar biodistribution, increased blood retention, and enhanced tumor accumulation and retention compared to the 67Ga-labeled RGD peptide without ABM. Therefore, even if the iodine in IPBA is converted to astatine, it still functions as ABM well enough as IPBA. Importantly, Ga-DOTA-K([211At]APBA)-c(RGDfK) showed dose-dependent tumor growth inhibition, demonstrating the potential of 211At-labeled probes in targeted alpha therapy.53)
Arrows indicate tumor location. (B) Design of therapy protocol and tumor volume after treatment with [90Y]Y-DMEB-RGD and [90Y]Y-DOTA-RGD. This research was originally published in JNM.51) ©SNMMI.
For SSTR-targeted probes using in treating neuroendocrine tumors (NETs), several ABM-modified compounds have been developed, including [177Lu]Lu-GluAB-DOTA-octreotate (DOTATATE), [177Lu]Lu-AspAB-DOTATATE, and [177Lu]Lu/[225Ac]Ac-DOTA-EB-TATE54–56) (Fig. 18). In AR42J tumor-bearing mice, both [177Lu]Lu-GluAB-DOTATATE and [177Lu]Lu-AspAB-DOTATATE demonstrated higher blood retention, with [177Lu]Lu-AspAB-DOTATATE exhibiting increased tumor accumulation and retention compared to [177Lu]Lu-DOTATATE54) (Fig. 19A). In clinical studies involving NET patients, [177Lu]Lu-DOTA-EB-TATE showed enhanced tumor retention and a higher absorbed dose to tumor tissues relative to [177Lu]Lu-DOTATATE in NET patients55) (Fig. 19B). [225Ac]Ac-DOTA-EB-TATE showed superior therapeutic efficacy over [225Ac]Ac-DOTATATE, suggesting potential effectiveness in treating 177Lu-resistant tumors.56)
(Rousseau E, et al. Nucl Med Biol, 66, 10–17 (2018)). (B) (i) Representative whole-body anterior projection images of a 61-y-old male patient with NET liver metastases at 2, 24, 72, 120, and 168 h after intravenous administration of [177Lu]Lu-DOTA-EB-TATE. (ii) Representative whole-body anterior projection images of a 49-y-old male patient with NET liver metastases at 1, 3, 4, 24, and 72 h after intravenous administration of [177Lu]Lu-DOTATATE. This research was originally published in JNM.55) ©SNMMI.
FAP-targeted ligands incorporating IPBA (TEFAPI-06), EB (TEFAPI-07), and fatty acids (FAPI-C12 and FAPI-C16) as ABM (Figs. 20A–D) have also been developed to improve tumor accumulation and therapeutic efficacy over [177Lu]Lu-FAPI-04 (without ABM).57,58) [86Y]Y-TEFAPI-06 and [86Y]Y-TEFAPI-07 exhibited high tumor accumulation and retention, and [177Lu]Lu-TEFAPI-06 and [177Lu]Lu-TEFAPI-07 showed superior therapeutic efficacy over [177Lu]Lu-FAPI-04 (Fig. 21). In FAPI-C12 and FAPI-C16, both using fatty acids as ABMs, FAPI-C16, which has a longer fatty acid chain, demonstrated higher blood retention, increased tumor accumulation and greater therapeutic efficacy.58) Recently, [211At]At-APBA-FAPI (Fig. 20E), incorporating [211At]APBA as an ABM, also showed dose-dependent tumor growth inhibition, highlighting its potential in targeted alpha therapy.59)
(B) Therapy protocol and tumor growth curves after targeted radionuclide therapy in HT-1080-FAP tumor-bearing mice. This research was originally published in JNM.57) ©SNMMI.
FR-targeting radiopharmaceuticals have also been modified with IPBA-based ABMs. A prime example is [177Lu]Lu-cm09 (Fig. 22A) which demonstrated higher tumor accumulation and retention, lower renal accumulation compared to [177Lu]Lu-EC0800 (without ABM), and enhanced therapeutic efficacy60) (Figs. 22B, C).
(B) SPECT/CT images of KB tumor-bearing mice injected with (i) [177Lu]Lu-cm09 and (ii) [177Lu]Lu -EC0800. Accumulation of radioactivity was found in FR-positive tumors (white arrows) and kidneys (yellow arrows). (C) Therapy protocol and tumor growth curves after targeted radionuclide therapy in KB tumor-bearing mice. This research was originally published in JNM.60) ©SNMMI.
While ABM incorporation is a promising strategy to improve tumor accumulation and retention, it may also lead to high blood retention, which can reduce imaging contrast in diagnostic applications and increase side effects in therapeutic settings. To address these concerns, studies have explored replacing ABMs with lower albumin-affinity moieties. For example, ibuprofen, a nonsteroidal anti-inflammatory drug with lower albumin affinity than EB and IPBA, was tested as an ABM in PSMA-617-based pharmaceuticals.61,62) Among these, [177Lu]Lu-SibuDAB (Fig. 23A), which features a positively charged linker between (S)-ibuprofen and PSMA-617, demonstrated faster tumor accumulation, improved tumor-to-blood and kidney area under the curve (AUC) ratio (49 ± 13 and 15 ± 4, respectively), and better pharmacokinetics compared to [177Lu]Lu-Ibu-PSMA (30 ± 5 and 4.6 ± 0.4, respectively) and [177Lu]Lu-RibuDAB (94 ± 16 and 8.8 ± 1.8, respectively).62,63) Further modification involves substituting iodine in IPBA with other functional groups. Among these, replacing iodine with chlorine ([177Lu]Lu-HTK03055) or a methoxy group ([177Lu]Lu-HTK03086) (Fig. 23B) showed higher tumor accumulation and lower blood retention compared to [177Lu]Lu-HTK0302464) (which retains IPBA as the ABM, Fig. 12C).
Another strategy to improve imaging contrast and minimize systemic toxicity involves competitively inhibiting albumin binding by administering IPBA as an albumin-binding inhibitor. In SPECT/CT imaging with [67Ga]Ga-DOTA-K(IPBA)-c(RGDfK) (Fig. 16D), IPBA administration dramatically reduced background radioactivity, leading to clear tumor visualization (Fig. 24). Similarly, the combining Ga-DOTA-K([211At]APBA)-c(RGDfK) (Fig. 16D) with IPBA reduced adverse events while maintaining the therapeutic efficacy.65)
Arrows indicate the site where U-87 MG cells were inoculated. Reprinted from65) under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).
The incorporation of ABMs into radiolabeled probes has proven to be a promising strategy for enhancing tumor accumulation and retention through prolonged blood circulation. However, its use presents a challenge, i.e., elevated blood retention, which can reduce imaging contrast and increase systemic effects. This issue can be potentially resolved by optimizing the ABM affinity and administering an albumin-binding inhibitor.
Charge-modified radiolabeled peptides have been reported to improve the biodistribution, reduce non-organ uptake, and enhance tumor imaging contrast. One effective approach involves the introduction of a peptide linker consisting of three repeats of glutamic acid (E) and histidine (H), denoted as (EH)3, to optimize pharmacokinetics by reducing liver and kidney accumulation. Specifically, PSMA inhibitors with (EH)3 showed a 2.8-fold reduction in kidney and liver uptake while maintaining tumor uptake. In addition, octreotide derivatives exhibited an even greater effect, with a 51-fold reduction in kidney and liver accumulation. These findings highlight the potential of (EH)3 modification to enhance the radiopharmaceutical pharmacokinetics without compromising tumor uptake.66) A similar strategy was applied to PSMA-targeted tracers, such as 68Ga-PSMA-11, by incorporating H/E linkers. This approach reduced non-target organ uptake while maintaining tumor uptake, especially with the (HE)3 linker, which also enhanced tumor-to-background contrast in PET/CT imaging of prostate cancer67) (Fig. 25). Furthermore, highly negatively charged linkers, such as tridentate glutamic acid, further improved renal excretion and minimized background accumulation in the liver, spleen, bone, and blood, leading to superior tumor imaging contrast for PSMA-targeting radiopharmaceuticals.68) For dual-targeted radiolabeled ligands that recognize both PSMA and GRPR, (HE)n linkers (n = 1–3) demonstrated reduced non-target tissue accumulation while maintaining tumor uptake. These dual-targeted tracers could offer high-sensitivity imaging for prostate cancer.69) In particular, modifying the linkers with (HE)3 led to an 11-fold reduction in splenic uptake, significantly improving tumor-specific accumulation and contrast in PET imaging.70) The charge of [68Ga]DOTA-Tyr3-TATE derivatives, modified with glutamic acid or ornithine. affected biodistribution. Neutral or positively charged derivatives increased kidney uptake, leading to undesirable retention. Notably, the negatively charged derivatives, such as [68Ga]Ga-DOTA-Glu-PEG4-TATE, reduced kidney uptake and improved pharmacokinetics. However, this modification also slightly reduced tumor uptake compared to DOTA-TATE71) (Fig. 26). For antibody-based radiopharmaceuticals, modifying the charge of linkers also played a crucial role in optimizing pharmacokinetics. In human Epidermal Growth Factor Receptor 2 (HER2)-targeted affibodies, incorporating a triglutamyl (E3) spacer significantly improved biodistribution.72) These findings demonstrate that optimizing the charge of peptides and proteins, particularly by incorporating negatively charged linkers or spacers, can reduce non-target organ accumulation (e.g., kidneys, liver, spleen) while maintaining or improving tumor uptake, ultimately improving pharmacokinetics and tumor-to-background contrast for cancer imaging and therapy.
© 2017, American Chemical Society.
Reproduced with permission from.71) © 2023, American Chemical Society.
Glycosylation enhances tumor uptake of radiolabeled peptides while reducing nonspecific background accumulation, particularly in the liver. For example, hydrophilic sugar-based linkers in bombesin analogs decrease abdominal tissue accumulation, improving tumor-to-background ratios and reducing liver uptake.73) Research on glycosylated RGD peptides modified with glucose, galactose, maltose, and cellobiose demonstrated improved radiolabeling yields and reduced liver and kidney accumulation. Notably, the maltosyl derivative [18F]Mlt-RGD exhibited excellent tumor-targeting properties and favorable tumor-to-liver ratios, highlighting its potential as a radiotracer for imaging integrin receptors in tumors74) (Fig. 27). The glycosylated somatostatin derivative [18F]FP-Gluc-TOCA demonstrated rapid renal clearance and favorable pharmacokinetics compared with the nonglycosylated [111In]In-diethylenetriaminepentaacetic acid (DTPA)-octreotide, confirming its clinical utility confirmed.75) Schottelius et al. investigated the biodistribution of three glycosylated somatostatin analogs ([125I]Gluc-S-TOCA, [125I]Gal-S-TOCA, and [125I]Mtr-TOCA) in mice. Both [125I]Gluc-S-TOCA and [125I]Gal-S-TOCA exhibited higher water solubility and were predominantly excreted through the kidneys, leading to significantly reduced liver and intestinal accumulation. Among these, [125I]Gluc-S-TOCA showed the highest tumor accumulation.76) These studies highlight the importance of glycosylation in optimizing the pharmacokinetics of peptide-based radiopharmaceuticals, improving tumor targeting while minimizing undesirable background accumulation.
Reproduced with permission from.74) © 2014, American Chemical Society.
Cell-penetrating peptides (CPPs) are promising carriers for low-permeability molecules like antibodies, with ongoing research on peptide- and nanoparticle-based tumor therapy and diagnosis. CPP-conjugated radiotracers have shown potential for in vivo cancer imaging and therapy, with a focus on enhancing radiopharmaceutical tumor accumulation by targeting cell membrane molecules.77) For example, porphyrin and trans-activator of transcription (TAT) peptide conjugate (porphyrin-TAT) demonstrated enhanced cellular uptake and light-dependent toxicity, with its singlet oxygen generation potential indicating its suitability for photodynamic therapy (PDT). While 68Ga labeling improved tumor accumulation, non-target retention posed a challenge for PET imaging.78) Our previous study with 125I-labeled porphyrin derivatives demonstrated prolonged circulation and increased tumor uptake over time,79) suggesting that evaluating the kinetics at later time points could further optimize the utility of 68Ga-labeled TAT-porphyrin derivatives. Stangl et al. developed a CPP-based PET tracer, TPP-PEG24-DFO[89Zr], targeting membrane Hsp70 (mHsp70) for tumor imaging. TPP enabled efficient tumor accumulation with favorable biodistribution, renal clearance, and broad cancer imaging potential.80) Several enzyme-activatable PET or SPECT probes have also been developed for noninvasive protease assessment, where a radiolabeled polyarginine-based CPP and a negatively charged polyanion peptide are linked by an enzyme-cleavable linker, targeting matrix metalloproteinases (MMPs)81) (Fig. 28) and legumain.82) However, peripheral degradation of these probes can complicates their specificity, as intracellular uptake after MMP cleavage can impact both tumor-specific and vascular activation. SVS-1, a cancer-targeting peptide, binds to cancer cells, forms a β-hairpin, and is internalized.83) Our group developed 67Ga-labeled SVS-1 derivatives with arginine or histidine substitutions, achieving high tumor uptake and favorable tumor-to-blood (3.8–8.0) and tumor-to-muscle (3.3–5.0) ratios.84) A 67Ga-labeled SVS-1-RGD fusion peptide exhibited superior tumor accumulation and retention compared to the RGD derivative.85) Despite their high renal accumulation, SVS-1 fusion peptides are promising for cancer targeting and in vivo imaging. Zhao reported a novel Granzyme-targeting Restricted Interaction Peptide (GRIP B) to measure secreted granzyme B activity in vivo using PET. The innovative design enables enzymatic cleavage of 64Cu-labeled GRIP B, releasing a radiolabeled form of Temporin L, which integrates into phospholipid bilayers. This method facilitates noninvasive noninvasive assessment of granzyme B activity in tumor and lymphoid tissues, offering potential applications in human studies.86) Camps et al. developed 111In-labeled nanobodies conjugated with CPP (r9 or penetratin) for EGFR tumor imaging. In vitro, CPP-nanobodies exhibited higher retention in spheroid cores than non-CPP-bound nanobodies. However, SPECT studies revealed that liver accumulation masked tumor uptake of 111In-CPP nanobodies.87) Sauter et al. reported that conjugating the radioiodinated EGFR-targeting monoclonal antibody matuzumab with a TAT peptide tetramer reduced blood concentration while maintaining tumor accumulation, thereby improving the tumor-to-blood ratio and pharmacokinetics.88) Veal et al. investigated whether TAT-modified 111In-labeled antibodies could address this challenge. Using GFP as a model, they demonstrated significant accumulation in H1299 lung cancer cells overexpressing nuclear GFP compared with wild-type cells. In tumor-bearing mice, tumor accumulation correlated with GFP expression. While nonspecific uptake in wild-type cells remains a concern, TAT-modified antibodies showed promise for intracellular biomolecule imaging.89) Similarly, Alakonya et al. utilized TAT-modified 111In-labeled anti-p53 antibodies for SPECT imaging of intracellular p53. 111In-anti-p53-TAT was retained in p53-overexpressing cells but not in p53-deficient ones. SPECT imaging showed uptake in tumors with mutant or wild-type p53 but not in p53-deficient tumors. Imaging in KPC mice confirmed its potential for p53 molecular imaging90) (Fig. 29). In summary, CPP conjugation enhances radiopharmaceutical pharmacokinetics, tumor targeting, and intracellular delivery, despite challenges such as nonspecific uptake and degradation. Advances in CPP-based probes continue to improve cancer imaging and therapy potential.
MMP-2/9-mediated cleavage of the linker releases the polycationic CPP, enabling radionuclide transfer across the cell membrane. This research was originally published in JNM.81) ©SNMMI.
This research was originally published in JNM.90) ©SNMMI.
Covalent drugs are therapeutic agents containing a mildly reactive electrophile, or warhead, that form reversible or irreversible covalent bonds with target protein amino acids, enabling prolonged regulation of protein function.91) Targeted Covalent Inhibitors combine warheads with specific ligands for high selectivity and efficacy, minimizing side effects (Fig. 30A). In this context, radiolabeled covalent drugs represent an innovative approach that enhances drug potency and selectivity through covalent binding to target molecules while leveraging the tracer properties of radioactive isotopes for noninvasive pharmacokinetic evaluation. This method enables detailed analysis of drug distribution and metabolic profiles, facilitating the optimization of stability and binding optimization through molecular design. This section highlights efforts to assess and improve the pharmacokinetics of radiolabeled covalent drugs.
(A) Reproduced with permission from.91) © 2025, American Chemical Society.
Third-generation EGFR-tyrosine kinase (TK) inhibitors (e.g., Olmutinib and Osimertinib), which target mutant EGFR-TK responsible for drug resistance in non-small cell lung cancer (NSCLC), are widely used in clinical cancer therapy. These compounds form irreversible covalent bonds with the cysteine residues near the enzyme’s active site.92) To evaluate NSCLC diagnosis and therapeutic efficacy, radiolabeled ligands derived from these covalent-binding compounds, such as radiohalogenated osimertinib derivatives as cysteine-reactive irreversible covalent radioligands (Fig. 30B), have been developed. These ligands showed greater tumor accumulation compared to surrounding tissues such as muscles.93,94) These ligands also exhibited higher tumor-to-blood and tumor-to-muscle ratios than reversible EGFR-TK-binding molecules, such as radiolabeled Rociletinib derivatives.95) However, comparative studies using structural analogs are lacking, and further research is needed to determine whether reversible or irreversible EGFR-TK binding molecules are better suited for in vivo pharmacokinetics optimization. KRAS G12C mutations are critical targets in cancer. Covalent radiotracers [131I]I-ARS-1620 and [124I]I-Sotorasib were developed for KRAS G12C imaging, showing specificity in vitro and in vivo. However, [131I]I-ARS-1620 accumulated in the liver and intestines, while [124I]I-Sotorasib exhibited poor tumor uptake and high renal accumulation, highlighting the need for improved pharmacokinetics and tumor targeting.96) Transglutaminase 2 (TGase 2), a key enzyme in pathological processes such as fibrosis and tumor growth, remains largely latent in healthy cells but becomes activated under disease conditions. An 18F-labeled derivative of Nε-acryloyllysine piperazide, a covalent inhibitor, Nα-phenylacetyl-Nε-acryloyl-L-lysine-4-(6-[18F]fluoropyridin-2-yl)piperazide, was developed for quantitative profiling of TGase 2 activation in tissues and tumors.97) However, recent in vivo studies revealed that the pharmacokinetic limitations that prevent specific imaging of TGase 2, underscoring the need for further structural optimization.98) Together, these studies underscore the potential of TGase 2-targeting radiopharmaceuticals while emphasizing the need for further structural optimization. Radiolabeled halogenated nitrofuran derivatives, such as [131I]I-NFIP, have been developed as stimulator of interferon genes (STING)-targeting tracers. [131I]I-NFIP demonstrated high specificity for STING (IC50: 7.56 nM) and effective tumor accumulation in mouse models, achieving a tumor-to-muscle ratio of 2.03 ± 0.30 in SPECT imaging, suggesting its potential for in vivo STING visualization in tumors.99) Chang et al. developed the covalent radiotracer [18F]JW199, which targets neutral cholesterol ester hydrolase 1 (NCEH1), a cancer-associated serine hydrolase. [18F]JW199 exhibited high selectivity for NCEH1 (over 1000-fold) and rapidly accumulated in mouse tissues in an NCEH1-dependent manner, enabling detection of NCEH1 activity at the tumor progression site in triple-negative breast cancer. However, significant physiological accumulation in the heart and lungs remains a concern, potentially limiting breast cancer imaging applications.100) Ode et al. reported the development of 211At-labeled aryl azides that selectively reacted with acrolein, a protein produced by cancer cells. This 211At-labeled compound demonstrated significant tumor growth inhibition due to excellent tumor retention via covalent binding with cancer cell organelles in. Further research in animal models and clinical applications is anticipated.101)
Cui et al. developed covalent targeting radiotracers using sulfur (VI) fluoroexchange (SuFEx) chemistry, achieving high tumor specificity and minimal accumulation in normal tissues. Notably, [68Ga]Ga-FAPI-mFS, a noncysteine-reactive irreversible covalent radioligand targeting lysine, tyrosine, and histidine (Fig. 30B), demonstrated excellent stability, enhanced tumor enrichment, and low normal tissues accumulation. FAPI-mFS derivatives radiolabeled with therapeutic radionuclides (90Y, 177Lu, and 225Ac), effectively inhibited tumor growth, suggesting broader applications for radiopharmaceutical development targeting other diseases.102) Larimer et al. developed a PET probe that traps its target through a covalently binds its target through an interaction between an aldehyde and the serine residue in the granzyme B active site. This tracer accumulated in response to granzyme B expression in mouse tumors during immunotherapy.103) However, nonspecific accumulation raised concerns, leading to the development of a noncovalent PET tracer currently in clinical trials for immunotherapy assessment.104,105) Introducing a lower reactivity substitute in place of an aldehyde may improve granzyme B imaging efficiency. Peng et al. developed an irreversibly binding peptidomimetic radioligand, [68Ga]Ga-DOTA-RQAR-kbt, targeting Suppression of Tumorigenicity 14 (ST14), confirming its enhanced tumor accumulation. PET imaging demonstrated that [68Ga]Ga-DOTA-RQAR-kbt exhibited superior tumor specificity and higher accumulation efficiency in ST14-expressing tumors compared to its reversible-binding derivative ([68Ga]Ga-DOTA-RQAR–OH), suggesting its promising potential as a radiotracer106) (Fig. 31). Klauser et al. pioneered the first covalent protein radionuclide therapy using the proximity-enabled reactive therapeutics strategy, incorporating a bioreactive unnatural amino acid (FSY) into a nanobody for irreversible binding to target proteins such as HER2. The radiolabeled covalent nanobody demonstrated enhanced tumor accumulation and retention while maintaining rapid clearance. The 225Ac-conjugated covalent nanobody effectively inhibited tumor growth without causing toxicity to normal tissues. This strategy shifts targeted radionuclide therapy toward covalent binding, improving therapeutic efficacy and enabling broader tumor targeting.107)
Reproduced with permission from.106) © 2025, American Chemical Society.
Radiolabeled covalent drugs hold significant promise in cancer diagnostics and therapy by improving tumor targeting and pharmacokinetics. Despite advancements, challenges such as off-target accumulation and limited efficacy persist. Further molecular design optimizations are key to overcoming these limitations.
The Auger electron is emitted from radionuclides when energy released by an inner-shell electron transition is transferred to another electron, causing its ejection. Auger electrons exhibit high linear energy transfer over a short range (4–26 keV/µm), enabling ultra-specific targeted radionuclide therapy (TRT). To achieve this, probes labeled with Auger electron emitters, including 111In and 125I, must accumulate around target organelles in cancer cells.108–110) This chapter focuses on probes targeting the nuclei of cancer cells.
Several DNA-targeting agents have been investigated as foundations for Auger electron therapy probes, including nucleoside analogs, platinum-based drugs, and anthracyclines. Nucleoside analogs are synthetic compounds structurally similar to natural nucleosides that are incorporated into DNA. 123/125I-5-iodo-2-deoxyuridine (IUdR) has been developed and evaluated. [125I]IUdR primarily accumulated in the nucleus, causing high cytotoxicity in V79 lung fibroblasts,111) with greater cytotoxicity than [131I]IUdR that emit β-particles.112) In rats with intrathecal tumors, intrathecal administration of [125I]IUdR (18.5 MBq) significantly prolonged time-to-paralysis compared with saline-treated controls.113) Additionally, intrathecal injection of four doses of methotrexate before and after administration of [125I]IUdR (1850 MBq) resulted in clinical improvement and a dramatic decrease in cerebrospinal carbohydrate antigen 19.9 levels in a patient with advanced pancreatic cancer and resistant neoplastic meningitis.114) Platinum (Pt)-based drugs primarily bind to guanine bases in DNA.115) Radioactive Pt isotopes, including 191Pt and 195mPt, are attractive for Auger electron therapy, as their decay produces numerous Auger electrons along with γ-rays, facilitating radiotheranostics.116) Cisplatin labeled with 191/195mPt was synthesized, and its therapeutic effects were assessed in vitro and in vivo.117–120) Tumor uptake of 191Pt was visualized using gamma camera imaging.121) Obata et al. demonstrated that 191Pt-labeled Hoechst33258 was more effective than 111In for DNA targeting, leading to extensive DNA damage.116) Anthracyclines such as doxorubicin (DOX) and daunorubicin (DNR) intercalate between DNA base pairs.122,123) Various 125I-labeled DOX and DNR derivatives were synthesized,124) demonstrating in vitro cytotoxicity.125,126) DOX is also clinically used in liposomal formulations, and stimuli-responsive liposomes encapsulating DOX have been developed for tumor therapy.127,128) Thermosensitive liposomes loaded with 125I-labeled DOX derivatives released their contents upon heating, leading to high nuclear accumulation (approximately 60–90%), and cytotoxicity against Colon 26 cancer cells.126) Due to their high tumor accumulation, these liposomal formulations are expected to be effective in vivo.
Poly(ADP-ribose) polymerase (PARP) enzymes play a key role in DNA repair, particularly in the base excision repair pathway. PARP inhibitors bind to the nicotinamide adenine dinucleotide binding site within PARP’s catalytic domain, inhibiting activity.129) Several PARP inhibitor-based probes labeled with 123I, 125I, and 77Br have been developed for Auger electron therapy.130) [123I]MAPi, [125I]PARPi-01, and [123I]CC1, which share similar structures (Fig. 32), were evaluated in model animals. Intratumoral injection of [123I]MAPi resulted in high nuclear uptake and conferred a survival benefit in glioblastoma mouse models.131) However, MDA-MB-231 tumor-bearing mice intravenously injected with [125I]PARPi-01 (8.15 MBq/dose) at four doses over 10-d intervals showed only slightly delayed tumor growth due to low tumor accumulation.132) Intravenous administration of [123I]CC1 (3 MBq) significantly inhibited tumor growth in mice PSN1 tumor-bearing mice,133) but no therapeutic effects were observed in AsPC1 and U87MG cells, which expressed lower levels of PARP. Additionally, radiobrominated Auger-emitting inhibitor targeting PARP-1 have been reported134,135) (Fig. 33). In PC-3 tumor-bearing mice, intravenously injection of 56 MBq of [77Br]Br-WC-DZ ([77Br]RD1) inhibited tumor growth.135) PET and SPECT imaging using 76Br and 123I-labeled PARP-targeting probes have also been performed for radiotheranostics.131–134)
(A, B) 18F-FDG uptake was clearly increased in tumors. On administration of 89Zr-HDL, signal intensity in esophageal tumor increased over time, and focal uptake pattern was clearly visualized at 72 h. (C) Tumor SUVs and target-to-blood pool ratios increased over time, indicating accumulation of 89Zr-HDL particles in tumors. (D) There was no association between 18F-FDG uptake and 89Zr-HDL uptake in tumors. Ao = aorta, He = heart, Li = liver; TBR = target-to-blood pool ratio. This research was originally published in JNM.148) ©SNMMI.
Nuclear localization signals (NLS) are short amino acid sequences recognized by importins, which transport proteins into the nucleus.136) Many radiolabeled antibodies modified with NLS have been developed for Auger electron therapy.109) Modification of 111In-labeled antibodies with NLS enhanced nuclear uptake and increased DNA doble strand breaks, as detected by immunofluorescence staining for γH2AX foci.137–140) In HER2-overexpressing MDA-MB-361 tumor-bearing mice, intravenous injection of 111In-NLS-trastuzumab (9.25 MBq/dose) in two doses at 2-week intervals inhibited tumor growth.141) More efficient nuclear accumulation of NLS-modified antibodies may enhance clinical applicability. Recently, Iizuka et al. developed an anti-cancer antibody modified with a 211At-labeled NLS peptide via a cathepsin B-cleaved linker.142) This nuclear-selective drug delivery strategy is expected to find future applications in Auger electron therapy.
Radiotheranostics with nanoparticles is an emerging field that combines diagnostics and therapeutics, particularly in oncology. Nanoparticles provide a versatile platform for targeted drug delivery, multimodal imaging, and multidisciplinary treatment. Nanotheranostics platforms have been developed in various combinations with diagnostic methods such as magnetic resonance imaging, optical imaging, and photoacoustic imaging, as well as treatments such as chemotherapy, PDT, and photothermal therapy.143) Most nanoparticles can be designed to evade the immune system through modification with polyethylene glycol, enhancing their half-life and accumulation at the tumor site via the enhanced permeability and retention effect.144) Additionally, nanoparticles can be functionalized with peptides, antibodies, and aptamers that specifically target tumor cells or receptors, improving tumor selectivity. Some nanoparticles inherently possess targeting properties; for instance, high density lipoprotein (HDL) and low density lipoprotein (LDL) are known to be taken up via the scavenger receptor class B type I (SR-BI) and LDL receptors, which are highly expressed in certain cancer cells.145–147) 89Zr-labeled HDL-mimetic nanoparticles have also been shown to accumulate in esophageal cancer via SR-BI, exhibiting a distinct accumulation pattern from 18F-FDG, which is commonly used for cancer diagnosis148) (Fig. 33).
Various nanoparticles, including liposomes, polymeric nanoparticles, dendrimers, gold nanoparticles, magnetic nanoparticles, quantum dots, and silica nanoparticles, have been utilized for radiotheranostics.142,149) Biodistribution of these nanoparticles in different organs, such as the lungs, liver, spleen, and kidneys, is dictated by their size, shape, and surface charge.150–152) Most nanoparticles with diameters >20 nm accumulate highly in the liver and spleen.153) Because accumulation in normal tissues reduces diagnostic capability and increases side effects, improving the biodistribution of radiolabeled nanoparticles in the body is a critical challenge. Recent research has focused on utilizing the properties of radiopharmaceuticals encapsulated in nanoparticles to minimize their accumulation in normal tissues. Lee et al. developed an esterase-labile radiotracer-loaded liposome that selectively decreases mononuclear phagocyte system organ uptake by exploiting the differential esterase activity in tumors and other organs.154) In the liver and spleen, radioactivity was rapidly excreted after ester bond cleavage by esterase, whereas the lipophilic radiotracer delivered to the tumor remained intact, exhibiting minimal bond cleavage (Fig. 34). Umeda et al. developed [111In]In-ethylenedicysteine (EC)-carrying liposomes that significantly reduced nonspecific accumulation due to the property of [111In]In-EC, which is more readily excreted from tissues compared to the classically used [111In]In- nitrilotriacetic acid.155) Since most liposomes remained intact in tumors without releasing encapsulated [111In]In-ligand complexes, high tumor accumulation was observed. Additionally, thermosensitive liposomes encapsulating [111In]In-DTPA, which is rapidly excreted from blood upon heating, have been developed.156) These thermosensitive liposomes accumulated in tumors, released [111In]In-DTPA upon heating in the blood, and reduced radioactivity levels in both the blood and normal tissues. Reducing accumulation in normal tissues through the design of these encapsulated drugs is expected to promote radiotheranostics using nanoparticles.
a The schematic illustration of the liposome was drawn by an artist (Seungeun Lee). Reproduced with permission from.154) © 2021 The Authors, American Chemical Society. Lisenced under CC BY-NC-ND 4.0 (https://creativecommons.org/licenses/by-nc-nd/4.0/).
When analyzing the distribution of radiolabeled nanoparticles in the body, it is crucial to accurately track the specific elements of interest. Since the detachment of radionuclides from nanoparticle probes can lead to the loss of target tracking, various methods have been developed to stably label probes.157) A common approach involves coordinating radioactive metal nuclides to nanoparticles via bifunctional chelators. However, α-particle-emitting radionuclides have been desorbed due to the influence of recoil electrons during decay, altering the dynamics of their daughter radionuclides.158) Consequently, various radiolabeling strategies have been developed, including radiochemical doping, cation exchange, cavity encapsulation, direct chemisorption, and neutron activation.157) In addition, it is important to investigate whether nanoparticle distribution changes with dosage, as radiolabeled nanoparticles used in combination with chemotherapy typically require higher dosages. PET/CT imaging with a diagnostic dose of a docetaxel-entrapping polymeric nanoparticle (89Zr-CPC634) accurately reflects accumulation during treatment (Fig. 35), allowing for the prediction of treatment efficacy.159) Although no radiolabeled nanoparticles have yet reached clinical application, precise control over biodistribution is expected to pave the way for their clinical use.
A) Two representative examples of tumor accumulation (upper panel patient 3, lower panel patient 4) at 96 h post-injection showing both on-treatment and diagnostic dose imaging, arrows indicate tumor lesions. B) Quantitative analyses of all visually positive tumors at several time points corrected for urinary excretion (N = 21). Individual tumors are indicated by black dots. Results of Mann–Whitney U test are shown ns = p > 0.05. Reprinted with permission from Miedema IHC et al. PET-CT Imaging of Polymeric Nanoparticle Tumor Accumulation in Patients. Reproduced from Adv. Mater. 2022. © 2022 The Authors. Published by Wiley-VCH GmbH.
Controlled in vivo pharmacokinetics is essential for optimizing the efficacy and safety of radiotheranostics. Strategies such as multimerization, introduction of ABM, charge modification, glycosylation, conjugation of CPPs, introduction of covalent binding moiety, targeting the nucleus, and utilizing drug release properties have shown promise in enhancing tumor accumulation while minimizing off-target distribution. While challenges remain, including the balance between blood clearance and tumor accumulation, advancements in molecular design and targeted delivery techniques continue to refine radiotheranostic applications. Future research should focus on integrating these approaches with emerging technologies to achieve more precise and effective cancer radiotheranostics.
The authors declare no conflict of interest.
